JPH11289759A - Overvoltage protective circuit - Google Patents

Abstract

PROBLEM TO BE SOLVED: To suppress undue increase of output level by controlling the output impedance of a switching converter circuit when the output voltage level thereof entering into overvoltage state. SOLUTION: When the output voltage Vout enters into overvoltage state, output voltage of a primary winding, i.e., the voltage at point (a), also increases at time point t0. DC voltage also fluctuates in correspondence with the voltage at point (a). When the DC voltage reaches a threshold level Vth1, a hysteresis comparator outputs a detection signal and based on that signal, the voltage at point (a) decreases from a level corresponding to the threshold level Vth1 shown at the period T2. When the voltage at point (a) reaches a level corresponding to a threshold level Vth2, oscillation frequency is controlled to decrease based on the detection signal. Consequently, the voltage at point (a) increases from a level corresponding to the threshold level Vth2 shown at the period T3.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an overvoltage provided in a switching converter circuit mounted on various electronic devices and provided for protecting the switching converter circuit in response to an overvoltage state of the output voltage. It relates to a protection circuit.

[0002]

2. Description of the Related Art FIG. 3 is a circuit diagram of a so-called PWM (Pulse
1 shows a configuration example of a switching power supply circuit that employs a Width Modulation (control) method. DC voltage source 1 is connected to switching element 21 via primary winding N1 of transformer PIT.
Supplied to The switching element 21 is driven by a switching pulse output from the pulse generation circuit 20 to perform a switching operation.
The alternating voltage obtained in the primary winding N1 is excited in the secondary winding N2. The alternating voltage excited by the secondary winding N2 is converted to a direct current by a half-wave rectifier circuit including a rectifier diode D10 and a smoothing capacitor Co, and is output as an output voltage Vout.

The feedback control circuit 5A outputs a control signal to the pulse generation circuit 20 in accordance with a level change appearing in the output voltage Vout. The pulse generation circuit 20 varies the pulse width of the switching pulse in accordance with the control signal, that is, performs the PWM control, so that the output voltage Vout
It operates to stabilize.

FIG. 6 shows an example of the relationship between the pulse width and the output voltage Vout in a switching power supply circuit employing such a PWM control method. As shown in this figure, in general, in order to increase the output voltage Vout, control is performed so as to increase the pulse width of the switching pulse.
In order to reduce out, control is performed so as to narrow the pulse width of the switching pulse.

[0005] Further, the circuit diagram of FIG.
1 shows a configuration example of a switching power supply circuit employing a frequency modulation method. In this figure, the same parts as those in FIG. 3 are denoted by the same reference numerals. In this case,
Two switching elements 2 and 3 for DC voltage source 1
Are provided in a so-called half-bridge connection. The primary earth side of the switching elements 2 and 3 connected in a half bridge is grounded via a resistor R0. As shown in the figure, a series connection of a capacitor C2 and a resistor R1 is connected in parallel to both ends of the switching element 2, and a series connection of a capacitor C3 and a resistor R2 is connected to both ends of the switching element 3. Connected in parallel. Further, diodes D1 and D2 are connected in parallel to both ends of the switching elements 2 and 3, respectively.

In this case, the connection point between the switching elements 2 and 3 is used as a switching output point. The switching output point includes the inductance of the primary winding N1 including the leakage component of the transformer PIT and the series resonance. Since the series resonance circuit including the capacitor C1 is connected, the switching operation is a current resonance type.

In this case, the primary winding N1 of the transformer PIT
Is output to the secondary winding N2 and the tertiary winding N3. The alternating voltage excited in the secondary winding N2 is converted to a direct current by the rectifier circuit 4 and the capacitor Co (and the resistor R3), and is output as an output voltage Vout. The alternating voltage excited by the tertiary winding N3 is generated as a DC voltage Vd by the diode D3 and the capacitor C4.

The oscillating circuit 40 outputs a pulse signal for driving the switching element 40. In this case, the oscillating circuit 40 is configured as one IC (Integrated Circuit). The oscillation circuit 40 includes a pulse output circuit 41 and an oscillator 42. The IC as the oscillation circuit 40 includes a dead time terminal 43 and a reference terminal 44. In the oscillation circuit 40, a pulse signal according to the frequency generated by the oscillator 42 is generated by the pulse output circuit 41, and the drive circuit 12
Output to In the drive circuit 12, switching pulses S whose polarities are inverted from each other are obtained from the input pulse signal.
1, S2 are generated, and switching elements 2, 3 are respectively generated.
To be applied. With this, the switching element 2,
No. 3 performs the switching operation at the timing of turning on / off alternately.

The output voltage Vout is supplied to a comparator 30 of a constant voltage control system and is compared with a reference voltage Vref. Then, the output as the comparison result is input to the feedback control circuit 5B. The feedback control circuit 5B generates a control signal based on the output of the comparator 30 and outputs the control signal to the variable current source 31.

[0010] The variable current source 31 is connected to the oscillator 42 in parallel with the resistor R13. , And the oscillator 42
Is connected to a capacitor C12 functioning as a bypass capacitor, for example.

In this case, the amount of current in the variable current source 31 is variably controlled by the control signal of the feedback control circuit 5B, and the oscillator 42 changes its oscillation frequency in accordance with the amount of current in the variable current source 31. It is designed to be variable. Therefore, during normal operation, the output voltage Vout
In this case, the oscillation frequency of the oscillator 42 is variably controlled in accordance with the fluctuation of the oscillation frequency, that is, the switching frequency is controlled to achieve constant voltage control (switching frequency control method).

A brief description of such a switching frequency control method will be given. Here, the power supply circuit shown in FIG. 4 employs a so-called upper side control method as a switching frequency control method. As shown in FIG. 8A, the upper side control method
By changing the switching frequency fs in a frequency range higher than the resonance frequency fa of the series resonance circuit (C1, N1) in which the impedance Z of the out becomes lower,
The impedance Z is variably controlled, thereby achieving a constant voltage. According to such a control method, the higher the switching frequency, the higher the impedance Z, which acts to suppress the output voltage Vout. FIG. 7 shows the relationship between the switching frequency and the output voltage Vout in such an upper side control method. That is, the output voltage Vout decreases as the switching frequency increases, and the output voltage Vout increases as the switching frequency decreases.

As a switching frequency control method, there is also known a lower side control in which the control tends to be opposite to the above-mentioned upper side control. That is, FIG.
As shown in (b), the switching frequency fs is varied in a frequency range lower than the resonance frequency fa of the series resonance circuit. In this case, the impedance Z decreases as the switching frequency fs increases.
The operation is performed so as to increase the output voltage Vout, and conversely, the operation is performed such that the output voltage Vout decreases as the switching frequency fs decreases.

Subsequently, the description returns to FIG. Here, FIG.
Let us consider a case where the output voltage Vout is in an overvoltage state in a case where an overvoltage protection circuit to be described later is omitted as the power supply circuit of FIG. The case where the output voltage Vout becomes an overvoltage is a state where the detection voltage for the constant voltage control is lost or the control loop is opened or the like and the constant voltage control circuit system does not function properly. Naturally, there is a possibility that an overvoltage state will occur even when a condition occurs that exceeds the additional condition or input voltage condition of the performance guarantee range specified in advance as a specification.

Here, considering the operation of the constant voltage control system under the condition that the input voltage source (DC voltage source 1) and the load are constant, from the control tendency of the upper side control system shown in FIG. As can be understood, when the constant voltage control is disabled, the output voltage Vout becomes the maximum value at that time, and the maximum power determined by the conditions of the input voltage source and the load at this time is transmitted. Will be.

For this reason, usually, as a protection against the above-mentioned overvoltage, the output voltage level is directly or indirectly detected, and the oscillation itself of the switching element is stopped. Specifically, for example, the DC voltage level obtained in response to the output voltage on the primary side is detected and compared with a predetermined threshold value, and the detected primary side DC voltage level exceeds the threshold value at a point in time. The output of the switching pulse for driving the switching element is stopped with a time constant.

In the power supply circuit shown in FIG. 4, such an overvoltage protection circuit is formed as follows. As described above, the alternating voltage excited in the tertiary winding N3 of the transformer PIT is converted into the DC voltage Vd by the diode D3 and the capacitor C4. This DC voltage Vd is the primary DC voltage described above. The DC voltage Vd is applied to a Zener diode ZD functioning as a comparator. For example, when the DC voltage Vd exceeds a predetermined threshold, the Zener diode ZD conducts and gives a trigger to the holding circuit unit 45. To be. In general, the holding circuit unit 45 often takes the form of a circuit block formed by a plurality of required components for performing a holding operation. Here, a resistor R11, a capacitor C11 (and a resistor R11) are inserted between the Zener diode ZD and the holding circuit unit 45 in the connection form shown in FIG. To be given.

Then, as described above, the holding circuit section 45
, The transistor Q is made conductive in response to the trigger. Note that the resistor R13 is a base resistor.

In this case, the collector of the transistor Q is
The oscillation circuit 40 is connected to the dead time terminal 43, and the emitter is connected to the reference terminal 44. Here, when the transistor Q does not conduct, the oscillation circuit 40 compares the oscillation frequency signal of the oscillator 42 with a voltage value (threshold level) for determining a predetermined pulse width to thereby provide a switching drive signal. It operates to generate a pulse signal.
Becomes conductive and the dead time terminal 43 and the reference terminal 4
When the collector current and the emitter current flow to each of the terminals 4 and both terminals are set to the same potential, the voltage value for determining the pulse width is raised to the IC reference voltage level of the oscillation circuit 40. As a result, for example, the threshold level is at a level higher than the oscillation frequency signal level. As a result. The drive pulse is stopped, and the switching operation of the switching elements 2 and 3 is also stopped.

FIG. 5 shows a power supply circuit which operates using an auxiliary power supply as an operation power supply and employs a circuit configuration for performing overvoltage protection by controlling on / off of a switch provided in a path capable of cutting off an input voltage source. An example of the configuration will be described. In this figure, the same parts as those in FIG. In the power supply circuit shown in this figure, an AC power supply 50 is rectified and smoothed by a bridge rectifier circuit 51 and a smoothing capacitor Ci to obtain a DC voltage, which is input to a switching converter unit 52. Switching converter section 5
2 is provided with an insulating transformer for interrupting the input DC voltage by its switching operation and transmitting the switching output to the secondary side. In addition, a relay switch RL-SW whose on / off operation is controlled by a relay (electromagnetic relay) RL is inserted in the line of the AC power supply 50. The relay switch RL-SW is turned on (closed) during normal operation. The output of the switching converter section 52, that is, the alternating voltage on the secondary side is supplied to the rectifier diode Do and the smoothing capacitor Co.
(And the resistor R3) and output voltage Vout
Is output as

The configuration of the overvoltage protection circuit system is as follows. The transistor Q shown in FIG.
Operate as an operating power supply. In this case, the auxiliary power supply 60 is a transformer TR for transmitting the alternating voltage excited by the AC power supply 50 from the primary side to the secondary side.
S, and a rectifier circuit D20 for rectifying the alternating voltage obtained on the secondary side of the transformer TRS. The DC voltage obtained by the rectifier circuit D20 is used as an auxiliary power supply voltage, and is actually supplied to each required functional circuit unit. Supplied to. The collector of the transistor Q is connected to a line of the auxiliary power supply voltage via the relay RL. The diode D21 is connected to the relay R
L are provided in parallel to both ends.

In the case of the configuration shown in FIG. 5, the configurations of the comparator 30 and the feedback control circuit 5B shown in FIG. 4 are omitted, but the basic concept is the same. However, in this case, the output voltage Vout is directly detected by the Zener diode ZD as a comparator. Then, when the output voltage Vout exceeds the predetermined threshold value, first, the holding circuit unit 45 is triggered.
Here, a time constant determined by the resistors R11 and C11 is given to the output of the Zener diode ZD. In this case, a time constant circuit including a resistor R14 and a capacitor C12 is inserted between the holding circuit unit 45 and the base of the transistor Q. The insertion of such a time constant circuit is provided for the purpose of preventing malfunction of the overvoltage protection circuit system due to, for example, the influence of internal / external noise.

Here, as described above, the holding circuit unit 45
Is turned on, the relay RL is turned on when the transistor Q is turned on, and the relay switch RL-S which has been turned on is turned off (open state). If the relay switch RL-S is turned off, the input of the AC power supply 50 is cut off, so that the switching operation in the switching converter unit 52 is stopped.

Note that, with respect to the base of the transistor Q,
The terminal Tm connected via the base resistor R13 and the resistor R15 is, for example, a terminal to which an on / off signal for driving the relay RL is applied independently of the overvoltage protection operation described above.

[0025]

As shown in FIG. 5, in a voltage protection circuit using a switch by an electromagnetic relay, for example, control is not directly performed on a switching pulse, and a voltage protection circuit system is provided. It has been found that a considerable delay time occurs in the time until the switching operation is actually stopped due to factors such as a time constant. This is shown in the timing chart of FIG. FIG.
As shown in FIG. 9A, when the output voltage Vout exceeds the threshold level Vth of the Zener diode ZD, a trigger is applied to the holding circuit unit 45, and FIG.
As shown in (b), the voltage V1 across the holding circuit unit 45 is obtained.
Even if the output voltage Vout drops, as shown in FIG. 9A, after waiting for the delay time Td from this point,
The operation is such that a decrease in the number is observed. In this case, the output voltage Vout rises to a considerable value during the delay time Td, and a reliable overvoltage protection operation cannot always be obtained.

Further, even in the case of controlling the switching pulse as in the overvoltage protection circuit described with reference to FIG. 4, the delay time due to the time constant given to the voltage protection circuit system described above can be ensured. Will exist.

Here, the time constant given to the voltage protection circuit system is not only the one provided as the configuration of the detection circuit itself, but also a time constant circuit intentionally inserted for the purpose of preventing malfunction due to external / internal noise. Sometimes it is. The malfunction here means that the detection voltage or the transmission signal system is disturbed by the noise component and reacts sensitively to perform the overvoltage protection operation even though the output voltage is not actually in an overvoltage state. As a result, the oscillation operation of the switching pulse is stopped.

Here, an operation example when it is assumed that the time constant in the voltage protection circuit system is set very short is shown in a timing chart of FIG. Here, FIG.
FIG. 10B shows a voltage Vg as a trigger signal of the holding circuit unit 45, and FIG. 10C shows a voltage V1 across the holding circuit unit 45 in the circuit shown in FIG. I have. For example, as shown in FIG. 10A, when the detection voltage Vd does not exceed the threshold level Vth because the output voltage has not actually risen, some noise is mixed in the detection voltage Vd. As shown in FIG.
In this case, when a certain degree of change appears, this becomes a trigger of the holding circuit unit 45, and the holding circuit unit 45 operates.
That is, the holding circuit unit 45 becomes conductive, and the level of V1 drops as shown in FIG. 10C, and the transistor Q becomes conductive. As a result, the pulse output from the pulse output circuit 41 is stopped even during the steady operation.

As can be seen from the above description, the ideal overvoltage protection operation detects an increase in the output voltage as quickly as possible, without applying a voltage stress to the output load, and receiving a noise disturbance. After making it difficult, in the actual overvoltage protection operation, the operation is made such that the output stop state can be reliably maintained.

When a current resonance type converter as shown in FIG. 4 is provided, the switching pulse output is suddenly stopped at the start of the protection operation as shown in the waveform diagram of FIG. When the operation starts such that the pulse width of the pulse (S1 or S2) is narrowed, the current flowing through the switching element at this time causes a discontinuous current mode as shown in FIG. Transient switching loss will increase. Therefore, it is preferable that the pulse generation is finally stopped by suppressing or increasing the switching frequency to 0 while suppressing the peak value of the drive pulse of the switching element.

[0031]

SUMMARY OF THE INVENTION In order to solve the above-mentioned problems, the present invention provides an output load circuit which can suppress the output voltage in response to the output voltage level corresponding to the overvoltage. The purpose of the present invention is to prevent the application of excessive voltage. Still another object is to prevent an increase in switching loss at the start of the protection operation.

Therefore, as an overvoltage protection circuit for protecting the switching converter circuit when the output voltage level of the switching converter circuit becomes overvoltage, the output voltage level of the switching converter circuit is set to a predetermined first voltage. When the level reaches the level, the first impedance control for increasing the output impedance of the output voltage of the switching converter circuit, and the output voltage level of the switching converter circuit decreases from the first level to a predetermined second level. An overvoltage protection circuit comprising impedance control means for performing a second impedance control for reducing an output impedance of an output voltage of the switching converter circuit from an output impedance obtained by the first impedance control when the output voltage of the switching converter circuit is reached; To do It was.

Further, in contrast to the above configuration, the switching operation of the switching converter circuit is stopped when a predetermined time has elapsed from when the output voltage level of the switching converter circuit is set to an overvoltage state. Switching operation stopping means is provided.

According to the configuration of the present invention, when the state in which the output voltage level of the switching converter circuit is overvoltage is detected, the output voltage level is switched between the first level and the second level. To be maintained. If the switching operation stopping means according to the above configuration is provided for this configuration, the state where the output voltage level is maintained between the first level and the second level as described above is obtained. It is possible to operate so that the state in which the switching operation is stopped after being continued for a certain period of time is maintained.

[0035]

FIG. 1 is a circuit diagram showing a configuration example of a switching power supply circuit having an overvoltage protection circuit according to an embodiment of the present invention. The switching power supply circuit shown in this figure includes a separately excited current resonance type converter in which two switching elements are half-bridge connected. For the constant voltage control, a switching frequency control method is adopted. Parts that can be regarded as the same as those of the switching power supply circuit of FIG. 4 are denoted by the same reference numerals, and description thereof may be omitted as appropriate.

In the switching power supply circuit shown in FIG. 1, two switching elements 2 and 3 are half-bridge connected and inserted between the positive line of the DC voltage source 1 and the primary side ground. Although not shown here, DC voltage source 1
Is obtained, for example, by rectifying and smoothing a commercial AC power supply by a rectifying and smoothing circuit. Further, as the switching elements 2 and 3, for example, a MOS-FET (Field Effect
Transistor) can be used, and a bipolar transistor can also be used.

For example, diodes D1 and D2 forming a current path when the switching element is off are connected in parallel to both ends of the switching elements 2 and 3, respectively. As shown in the figure, a series connection of a capacitor C2 and a resistor R1 and a series connection of a capacitor C3 and a resistor R2 are connected in parallel to both ends of the switching elements 2 and 3, respectively.

When the switching elements 2 and 3 are connected by half-bridge coupling, these switching elements 2 and 3
The connection point 3 is a switching output point. One end of the series resonance capacitor C1 is connected to the switching output point. The other end of the series resonance capacitor C1 is grounded from the resonance inductance L1 to the primary side ground through a parallel connection of the primary winding N1 of the transformer PIT and the excitation inductance L2. That is, the switching output is supplied to the series connection circuit of [series resonance capacitor C1]-[resonance inductance L1]-[parallel connection circuit of primary winding N1 / excitation inductance L2]. This series connection circuit is determined by the capacitance of the series resonance capacitor C1 and the inductance of the parallel connection circuit of [primary winding N1 / excitation inductance L2] obtained including the leakage inductance of the transformer PIT or the external resonance inductance. Is formed.

In this case, the transformer PIT is an insulated transformer configured to be loosely coupled to obtain a leakage inductance component, and mainly transmits a switching output obtained on the primary side to the secondary side. In this case, the primary winding N1 and the secondary winding N
2, and a tertiary winding N3 are wound.

The oscillation circuit 11 outputs an oscillation frequency signal for determining the switching frequency to the drive circuit 12. The drive circuit 12 outputs two pulse signals S1 and S2 having inverted polarities as pulse signals having a frequency according to the input oscillation frequency signal.
It is applied as a drive pulse for the switching elements 2 and 3.
Thus, the switching elements 2 and 3 repeat the switching operation at the timing of being turned on / off alternately, so that the DC voltage source 1 is turned on and off. At this time, the current flowing through the above-described series resonance circuit (series resonance capacitor C1, resonance inductance L1, primary winding N1, excitation inductance L2) and capacitors C2 and C3 is changed according to the switching operation.
Transmits necessary energy to the load connected to the secondary side of the transformer PIT. Thereby, a current resonance type operation can be obtained as a switching converter.

On the secondary side of the transformer PIT, the alternating voltage excited by the secondary winding N2 is converted into a direct current by the rectifier circuit 4 and the smoothing capacitor Co to generate an output voltage Vout, which is supplied to a load (not shown). Is done.

The constant voltage control system during the normal operation of the power supply circuit shown in this figure is formed by including the feedback control circuit 5. Here, the following description will be made assuming that the upper side control method (see FIGS. 7 and 8) is adopted as the constant voltage control method.

The feedback control circuit 5 receives the output voltage Vout, generates a control signal corresponding to the level change of the output voltage Vout, and supplies the control signal to the oscillation circuit 11. The oscillation circuit 11 operates so as to vary the oscillation frequency of the oscillation frequency signal to be output to the drive circuit 12, for example, according to the level of the control signal.

Here, since the upper side control system is adopted, when a control signal corresponding to the increase in the output voltage Vout is input to the drive circuit 12,
It operates to increase the oscillation frequency. As a result, as described with reference to FIGS. 7 and 8A, the switching frequency fs is increased and is separated from the resonance frequency fa, so that the impedance of the output voltage is increased and the level of the output voltage Vout is suppressed. Will be.

On the other hand, when a control signal corresponding to the decrease in the output voltage Vout is input, control is performed so as to lower the oscillation frequency. As a result, the impedance of the output voltage is reduced by lowering the switching frequency, and the level of the output voltage Vout is increased. The constant voltage control is performed by such an operation.

With respect to the basic configuration described so far,
In this embodiment, an overvoltage protection circuit system configured as follows is provided. In the present embodiment, as the overvoltage protection circuit system, there are provided a voltage level maintaining system including the hysteresis comparator 6 and an oscillation operation stopping system for stopping the oscillation operation of the oscillation circuit 11 with a certain waiting time.

First, the configuration and operation of the voltage level maintaining system will be described. The alternating voltage excited in the tertiary winding N3 of the transformer PIT is converted into a DC voltage Vd by a half-wave rectifier circuit including a rectifier diode D3 and a capacitor C4. The DC voltage Vd has a level corresponding to the output voltage Vout. It is assumed to have. The DC voltage Vd is input to the non-inverting input of the hysteresis comparator 6, and is compared with the reference voltage Vref1. In the hysteresis comparator 6, a threshold level based on the reference voltage Vref1 corresponds to a threshold level Vth1 at which the DC voltage Vd can exceed the threshold level and a threshold level at which the DC voltage Vd once drops from the state exceeding the threshold level Vth1. There are two threshold levels, threshold level Vth2.

Here, it is assumed that the output voltage Vout has entered an overvoltage state. That is, the above-described constant voltage control system does not function, and the output voltage Vout rises to the maximum level determined by the circuit constant. In this case, as the output voltage Vout rises, the voltage at the point a, which is the output of the primary winding N1, is also changed at the time t as shown in FIG.
It rises after 0 (at the point of entering the overvoltage state). Further, the DC voltage Vd also fluctuates corresponding to the voltage at the point a.

Thus, the hysteresis comparator 6
When the DC voltage Vd reaches the threshold level Vth1, a detection signal indicating that this is detected is output. Based on this detection signal, the oscillation circuit 11 controls the oscillation frequency to be higher, so that the voltage at the point a corresponds to the threshold level Vth1 as shown in a period T2 in FIG. It is an operation of descending from the level to be performed. That is, the output voltage Vout tends to be suppressed.

When the voltage at point a, which has started falling from the level corresponding to the threshold level Vth1, reaches the level corresponding to the threshold level Vth2, the hysteresis comparator 6 determines that the DC voltage Vd has reached the threshold level Vth2. The detection signal shown in FIG. Based on this detection signal, the oscillation circuit 11 controls to lower the oscillation frequency. As a result, the voltage at the point a is increased from the level corresponding to the threshold level Vth2 as shown in a period T3 in FIG. 2A. That is, the output voltage V
out tends to be raised.

In this way, after time t0 when the overvoltage state is started, the voltage at point a (that is, output voltage V
(out), an operation of repeatedly rising / falling within a range corresponding to threshold level Vth1-threshold level Vth2 is obtained.

Next, the operation of the oscillation operation stop system will be described. In the oscillation stop system, first, a comparator 7 is provided. The comparator 7 compares the DC voltage Vd with the reference voltage Vref2. Here, the reference voltage Vr
As the threshold level Vth3 determined by ef2, as shown in FIG. 2A, a level lower than the above-described threshold levels Vth1 and Vth2 is set.

The comparator 7 outputs a detection signal obtained when the DC voltage Vd reaches the threshold level Vth3 as a control signal for turning on the operation of the current source 8. As a result, a predetermined amount of current flows from the current source 8 whose operation has been stopped, and
This current charges the capacitor C5.
By this operation, the voltage b across the capacitor C5 is obtained.
As shown in FIG. 2B, the voltage level at the point gradually increases after time t1 according to a time constant determined by, for example, the capacitor C5 and the resistor R4.

The voltage at the point b is input to the comparator 7 and compared with the reference voltage Vref3. This reference voltage Vref3 is determined corresponding to a threshold level Vth4 shown in FIG. When the voltage at point b reaches the threshold level Vth4 at the time t2 when the period T1 has elapsed from the time t1, the comparator 7 outputs, for example, an H level detection signal to the set terminal of the RS flip-flop 10. Output.

In response to the detection signal input to the set terminal, the RS flip-flop 10
The output signal S3 at the H level is output to the oscillation circuit 11 as shown in FIG. When the H level output signal S3 from the RS flip-flop 10 is obtained, the oscillation circuit 11 operates to stop the output of the oscillation frequency. Thus, at time t2, the output of the switching pulses S1 and S2 in the drive circuit 12 is also stopped, and the switching operation of the switching elements 2 and 3 is stopped. After the time point t2, the voltage at the point a sharply drops as shown in FIG. Here, even if the comparator 9 detects that the voltage at the point b has become lower than the reference voltage Vref3 due to the voltage drop at the point a, the output signal S3 output from the RS flip-flop 10 is The H level will be maintained.

As can be seen from the above description, in the present embodiment, when an overvoltage state occurs, the level of the output voltage Vout is first within a certain predetermined range by the operation of the voltage level maintaining system. In addition, by controlling the oscillation pulse to stop the switching pulse after a predetermined standby time by the operation of the oscillation operation stop system, the circuit is protected from overvoltage.

By performing the overvoltage protection operation in this manner, first, the state of the overvoltage is quickly detected, and in response to this, the rise of the output voltage Vout is suppressed. Then, under the state where the above-described operation is maintained, the switching operation is stopped after waiting for a certain period of time, so that stress is not applied to the output load. In the case of a converter,
The loss in the switching element at the start of the overvoltage protection operation does not need to be increased.

In the present embodiment, when the output voltage Vout returns and returns to the normal operation within the period T1, the RS flip-flop 10 is set and the output of the H level is performed. And comparator 9
In this case, the output is switched to an L-level detection signal indicating that the voltage at point b has become lower than the reference voltage Vref3 (threshold level Vth4), so that the oscillation operation of the oscillation circuit 11 is not stopped. As a result, a malfunction as the overvoltage protection circuit is prevented.

In the above description, the case where the upper side control system is adopted has been described. However, in the case where the power supply circuit shown in FIG. 1 adopts the lower side control system according to the configuration of the overvoltage protection circuit system described above. Also, the overvoltage protection operation can be performed.

When the lower side control method is adopted, the oscillation circuit is varied so as to lower the oscillation frequency, that is, the switching frequency is reduced in order to realize the operation in the period T2 in FIG. It is sufficient to lower the output voltage Vout by increasing the impedance. In addition, in order to realize the operation in the period T3 in FIG. 2, the oscillation circuit is varied so as to increase the oscillation frequency, and the switching frequency is changed. By increasing the voltage, the output voltage Vout may be increased by reducing the impedance.

Further, instead of the resonance type converter as shown in FIG. 1, a PWM adopting the basic configuration as shown in FIG.
The configuration of the overvoltage protection circuit system according to the present embodiment is also applicable to a switching power supply circuit based on a control method. In this case, in order to realize the operation in the period T2 of FIG. 2, as the operation of the voltage level maintaining system, the operation of expanding the pulse width of the PWM pulse for switching drive in the period T2 is performed. In order to realize the operation in the period T3, the operation of reducing the pulse width of the PWM pulse is performed in the period T3. Then, when the period T1 is reached, the output of the PWM pulse may be stopped as the operation of the oscillation operation stop system.

The configuration of the overvoltage protection circuit according to the present invention is not limited to the configuration described above, and various changes can be made. For example, in the configuration shown in FIG. 1, the level of the output voltage Vout Is indirectly detected by the detection voltage Vd obtained on the primary side, but a configuration in which the output voltage Vout is input and detected, for example, may be adopted.

[0063]

As described above, according to the present invention, when the output voltage level of the switching converter circuit is in an overvoltage state, the first level and the second level are controlled by controlling the output impedance of the switching converter circuit. And the output voltage level is maintained. As a result, for example, it is possible to quickly detect an overvoltage state without substantially depending on the time constant of the overvoltage protection circuit system, and to suppress an excessive increase in the output voltage level. In the present invention, when controlling the output impedance as described above, for example, in the case of a configuration that employs an upper side control or a lower side control method in a switching frequency control method as a constant voltage control method of a switching converter circuit. In this case, the output voltage level may be maintained between the first level and the second level by controlling the switching frequency. In the case of a configuration employing the PWM control method, What is necessary is just to comprise so that the pulse width of a drive pulse may be changed. In this manner, the overvoltage protection operation for maintaining the output voltage level between the first level and the second level is realized by using the operation principle of the constant voltage system adopted by the switching converter. Is possible,
No particularly difficult technology is required.

In parallel with the above operation, the output of the switching drive pulse is stopped at a point in time when a predetermined time has elapsed since the output voltage level of the switching converter circuit was detected to be in an overvoltage state. Thus, by employing a configuration in which the switching operation of the switching converter circuit is stopped, stress on the output load due to overvoltage is prevented. In particular, when the switching converter circuit is of the current resonance type, the phenomenon that the pulse width of the switching drive pulse is reduced at the start of the overvoltage protection operation does not occur.
An effect is obtained that an increase in loss in the switching element is also prevented.

[Brief description of the drawings]

FIG. 1 is a circuit diagram showing a configuration of a switching power supply circuit provided with an overvoltage protection circuit according to an embodiment of the present invention.

FIG. 2 is a waveform chart showing an operation of a main part of the switching power supply circuit shown in FIG.

FIG. 3 is a circuit diagram showing a configuration of a switching power supply circuit as a conventional example.

FIG. 4 is a circuit diagram showing a configuration of a switching power supply circuit as a conventional example.

FIG. 5 is a circuit diagram showing a configuration of a switching power supply circuit as a conventional example.

FIG. 6 is an explanatory diagram illustrating a relationship between a pulse width of a switching drive pulse and an output voltage in the switching power supply circuit illustrated in FIG. 3;

FIG. 7 shows a case where an upper side control method is adopted.
FIG. 4 is an explanatory diagram illustrating a relationship between an oscillation frequency of a switching drive pulse and an output voltage.

FIG. 8 is an explanatory diagram showing a relationship between a switching frequency and an output impedance in an upper side control system and a lower side control system.

9 is a timing chart showing an overvoltage protection operation of the switching power supply circuit shown in FIG.

10 is a timing chart showing an operation example when the time constant of the overvoltage protection circuit of the switching power supply circuit shown in FIG. 4 is short.

11 is a waveform chart showing an operation of the switching element at the start of the overvoltage protection operation of the switching power supply circuit shown in FIG.

[Explanation of symbols]

Reference Signs List 1 DC voltage source, 2, 3 switching element, 4 rectifier circuit, 5, 5A, 5B feedback control circuit, 6 hysteresis comparator, 7 comparator, 8 current source, 9 comparator, 10 RS flip-flop, 11 oscillation circuit, 12 drive circuit , Co smoothing capacitor, C1
Resonant capacitor, C2, C3, C4, C5 capacitor, D1, D2 diode, D3 rectifier diode,
N1 primary winding, N2 secondary winding, N3 tertiary winding, L1
Resonance inductance, L2 excitation inductance, P
IT transformer, R1, R2, R3, R4 resistance, RL
-SW relay switch, RL relay

 ────────────────────────────────────────────────── ─── Continuing on the front page (72) Inventor Yoshinori Kobayashi 2-2-1 Otemachi, Chiyoda-ku, Tokyo New Otemachi Building Shindengen Kogyo Co., Ltd. (72) Inventor Takeshi Keni Tokyo 2-2-1 New Otemachi Building Shindengen Kogyo Co., Ltd. (72) Inventor Kenji Horiguchi 2-2-1 Otemachi Chiyoda-ku, Tokyo New Otemachi Building Shindengenkogyo Co., Ltd.

Claims (5)

[Claims] An overvoltage protection circuit for protecting the switching converter circuit when the output voltage level of the switching converter circuit is overvoltage, wherein the output voltage level of the switching converter circuit is a predetermined first voltage. The first impedance control for increasing the output impedance of the output voltage of the switching converter circuit, and the output voltage level of the switching converter circuit decreases from the first level to a predetermined first level. And an impedance control means for performing a second impedance control for reducing the output impedance of the output voltage of the switching converter circuit from the output impedance obtained by the first impedance control when reaching the level of 2. Having Overvoltage protection circuit according to claim. 2. The switching converter according to claim 1, further comprising: a resonance circuit configured to operate the switching converter in a resonance type.
In a control range of a frequency range higher than a resonance frequency of the resonance circuit, a constant frequency control of an output voltage is performed by changing a switching frequency according to an output voltage level of the switching converter circuit. Then, the impedance control means performs the first impedance control operation by increasing the switching frequency, and lowers the switching frequency from the switching frequency obtained by the first impedance control. The overvoltage protection circuit according to claim 1, wherein the overvoltage protection circuit is configured to perform the second impedance control. 3. The switching converter circuit according to claim 1, further comprising a resonance circuit for causing the operation of the switching converter circuit to be of a resonance type, and an output of the switching converter circuit within a control range of a frequency range lower than a resonance frequency of the resonance circuit. The impedance control means is configured to perform the constant voltage control of the output voltage by changing the switching frequency according to the voltage level, and the impedance control means reduces the first switching frequency by decreasing the switching frequency. 2. The apparatus according to claim 1, wherein the second impedance control is performed by performing impedance control and increasing the switching frequency higher than a switching frequency obtained by the first impedance control. 3. Overvoltage protection circuit. 4. The switching converter circuit is configured to perform constant voltage control of an output voltage by varying a pulse width of a drive signal applied to a switching element performing a switching operation. The impedance control means performs the first impedance control by controlling the pulse width of the drive signal to be narrower, and sets the pulse width of the drive signal to be smaller than the pulse width obtained by the first impedance control. 2. The overvoltage protection circuit according to claim 1, wherein the second impedance control is performed by controlling the impedance to be wider. 3. 5. An output of a switching drive pulse for causing the switching converter circuit to perform a switching operation at a point in time when a predetermined time has elapsed from when the output voltage level of the switching converter circuit is set to an overvoltage state. The overvoltage protection circuit according to claim 1, further comprising: a switching operation stopping unit that operates to stop the switching operation.