JPH04217859A - Switching power supply - Google Patents

Abstract

PURPOSE:To obtain an ideal constant current drooping characteristic even if DC output voltage reaches close to zero point in the overcurrent region of a switching power supply. CONSTITUTION:A peak detector 20 detects peak current of a current detection signal VS outputted from a current detector 6 and applies a corresponding control voltage VC on an operational amplifier 21 in a current detection signal control circuit 25 where it is compared with an overcurrent reference voltage Vref 2. In a constant current operating region, the control voltage VC is lower than the overcurrent reference voltage Vref 2 to cause a transistor 23 to be turned OFF and a voltage detection signal VB is determined based on the impedance of a phototransistor 10b. In an overcurrent region, the control voltage increases and when it reaches the overcurrent reference voltage Vref 2, base potential of the transistor rises to forcibly cause voltage level drop of the voltage detection signal VB thus controlling the peak value of the current detection signal VA at a constant level.

Description

[Detailed description of the invention]

0001

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a current mode controlled switching power supply device equipped with a constant current feedback loop.

0002

2. Description of the Related Art FIG. 3 shows a conventional example of this type of switching power supply. This switching power supply has a constant current control feedback loop and a constant voltage control feedback loop, and a DC input voltage source 1 has a primary side and a secondary side. A switching element 3 is connected in series to the primary winding, and the voltage induced in the secondary winding of the transformer 2 is rectified by a diode and a capacitor. The voltage is rectified and smoothed by a smoothing circuit 4, and a constant DC output voltage Vo is supplied to a load 5 connected between output terminals +V and -V.

In the constant current control feedback loop, a current pulse having a slope is detected by a current detector 6 consisting of a resistor inserted between the switching element 3 and the DC input voltage source 1, and a level is applied to the current detection signal VS. A current detection signal VA on which a DC voltage VD output from a shift DC power source 7 is superimposed is applied to a non-inverting input terminal of a current comparator 8. On the other hand, the constant voltage control feedback loop includes a resistor 9 and a light emitting diode 10 of a photocoupler 10 between the output terminals.
Connect the series circuit of a and the shunt regulator 11,
An output voltage detection circuit 14 is connected to the reference of the shunt regulator 11 for applying an output detection voltage obtained by dividing the DC output voltage Vo by resistors 12 and 13. and,
The phototransistor 10b of the photocoupler 10 is connected to both ends of a resistor R2 connected in series with a resistor R1 between the reference voltage supply terminal Vref1, and the DC output voltage Vo
According to the difference between the output detection voltage divided by the resistors 12 and 13 and the reference voltage of the shunt regulator 11, the amount of light emitted from the light emitting diode 10a changes, thereby changing the impedance of the phototransistor 10b. The voltage detection signal VB at the connection point between the resistors R1 and R2 is applied to the inverting input terminal of the current comparator 8.

Current comparator 8 receives current detection signal VA.
is compared with the voltage detection signal VB, and the current detection signal VA
When the detection pulse reaches the level of the voltage detection signal VB, a reset signal is supplied from the current comparator 8 to the reset terminal R of the flip-flop circuit 15. In the flip-flop circuit 15, when the clock signal from the oscillation circuit 16 is supplied to the set input terminal S of the flip-flop circuit 15, the output terminal Q is raised to H level, and the reset signal from the current comparator 8 is supplied to the reset input terminal R. When supplied to the current detection signal VA, the output of the output terminal Q is lowered and the current detection signal VA
The pulse conduction width of the switching element 3 is controlled so that the current peak value of the voltage detection signal VB matches the voltage level of the voltage detection signal VB.

[0005] Then, the load 5 becomes overloaded,
When the output terminals +V and -V are short-circuited and the secondary side of the transformer 2 enters an overcurrent state, the DC output voltage Vo reaches a predetermined voltage level or lower, and the light emitting diode 10a of the photocoupler 10 stops emitting light. Phototransistor 10b
Since the impedance of current comparator 8 becomes high,
The voltage level of the voltage detection signal VB applied to VB increases, and VB = (Vref1 x R2)/(R1 + R2
) is fixed.

On the other hand, since the current flowing to the primary side of the transformer 2 also increases due to the overcurrent condition, the voltage of the current detection signal VA increases, and the time required to reach the voltage level of the voltage detection level VB becomes shorter. Therefore, the conduction time of the pulse output from the flip-flop circuit 15 to the switching element 3 is shortened, and the DC output voltage Vo can be rapidly lowered to protect the load 5 and the switching element 3.

[0007]

[Problems to be Solved by the Invention] In the above-mentioned prior art, the conduction width of the pulse supplied to the switching element 3 in the overcurrent region cannot be completely reduced to zero due to the latch delay time of the flip-flop circuit 15, etc. Therefore, even if the DC output voltage Vo decreases to near zero, the conduction pulse width of the switching element 3 remains at the predetermined minimum width, and the waveforms of the current detection signal VA and the voltage detection signal VB become as shown in FIG. 4. That is, the load current increases when the pulse width is the minimum width determined by the delay time, etc., and even if the current detection signal VA exceeds the voltage detection signal VB, it becomes impossible to narrow the pulse width and control the output current. At this time, the DC output voltage Vo and the output current Io are as shown in the characteristic line in FIG. The problem is that the short-circuit current on the secondary side of the transformer 2 (see the solid line in Figure 5) increases when the overcurrent droops compared to the broken line (see the broken line), and the load 5 and other circuit elements on the secondary side of the transformer 2 are destroyed. was occurring.

SUMMARY OF THE INVENTION An object of the present invention is to provide a switching power supply device that can obtain ideal constant current drooping characteristics even when the DC output voltage reaches near zero in the overcurrent region.

[0009]

[Means for Solving the Problems] The present invention includes a current detector that detects the current on the primary side of the transformer and a voltage detection circuit that detects the output voltage as a feedback circuit for stabilizing the DC output voltage. , a switching power supply device that compares the current detection signal output from the current detector and the voltage detection signal output from the voltage detection circuit using a current comparator, and controls the conduction pulse width of the switching element based on the comparison result. a peak detection circuit that detects a current peak value of the current detection signal; and a peak detection circuit that matches the current peak value of the current detection signal to the level of an overcurrent reference voltage in an overcurrent region based on the peak detection signal from the peak detection circuit. A voltage detection signal control circuit for controlling the voltage level of the voltage detection signal is connected between the current detection signal line and the voltage detection signal line.

0010

[Operation] With the above configuration, when the current peak value of the current detection signal detected from the current detector increases in the overcurrent region, the voltage detection signal control circuit adjusts the level of the overcurrent reference voltage and the current peak value of the current detection signal. Since the voltage detection signal is controlled to match the current peak value, the output current, which is proportional to the current peak value, remains constant in an overcurrent condition.

[0011]

DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of the present invention will be described below with reference to the accompanying drawings.

FIG. 1 is a circuit configuration diagram of a switching power supply device showing an embodiment of the present invention, and the same parts as those shown in FIG.

As shown in FIG. 1, the anode of a diode 17 is connected to the current detection signal VS line detected by the current detector 6, and a capacitor 18 is connected to the cathode of the diode 17. Peak detection circuit 2 formed by connecting a parallel circuit with resistor 19
0, and also connect the connection point of the resistor 19 and capacitor 18 to the non-inverting input terminal of the operational amplifier 21,
A control voltage VC outputted from the peak detection circuit 20 is applied to the operational amplifier 21. Then, the DC power supply 22 that outputs the overcurrent reference voltage Vref2 is connected to the inverting input terminal of the operational amplifier 21, and the output terminal of the operational amplifier 21 is connected to the base of the transistor 23 whose emitter is grounded. It is the same as FIG. 3 except that the collector is connected to the inverting input terminal of the current comparator 8, which is the voltage detection signal VB line, via the current limiting resistor 24 to form the voltage detection signal control circuit 25.

Next, the operation of the above structure will be explained. When the DC input power source 1 is supplied, a current pulse flowing to the primary side of the transformer 2 by a constant current feedback loop is detected by the current detector 6, and a current detection signal VS is output. In the peak detection circuit 20, the current detection signal VS is rectified by the diode 17, and the capacitor 18 is charged to its peak voltage. This detection signal is applied to the inverting input terminal of the operational amplifier 21 as a control voltage VC obtained by subtracting the voltage drop VF of the diode 17 from the current detection signal Vs. Overcurrent reference voltage Vr
It is compared with ef2, and this compared voltage is supplied to the base of transistor 23.

At this time, in a constant current operation region where the secondary side of the transformer 2 is not in an overcurrent state, the control voltage VC
is lower than the overcurrent reference voltage Vref2, the base potential of the transistor 23 becomes low, and this transistor 23
is in the off state. Therefore, the voltage detection signal VB is determined by the impedance of the phototransistor 10b, which changes based on the amount of light emitted from the light emitting diode 10a of the photocoupler 10, and the voltage detection signal VB and the current detection signal VA detected by the current detector 6 The current comparator 8 compares the pulse width of the switching element 3 based on the comparison result.

On the other hand, when the secondary side of the transformer 2 enters an overcurrent state, the current detection signal Vs from the current detector 6 increases, and the control voltage VC applied to the non-inverting input terminal of the operational amplifier 21 increases. The overcurrent control voltage Vre
When the voltage rises to the same value as f2, the base potential of the transistor 23 rises, and the transistor 23 operates to lower the voltage of the inverting input VB of the current comparator 8 via the resistor 24.

In this circuit, the current comparator 8
Although it is not possible to control the minimum pulse width due to delays in latching and the like, the operational amplifier 21 can control the current peak value, that is, the short circuit current, to be constant.

As described above, in this embodiment, the voltage level of the voltage detection signal VB is determined by the output voltage detection circuit 14 that detects the DC output voltage Vo, the control signal VC output from the peak detection circuit 20, and the overcurrent reference voltage. The light emitting diode 10 of the photocoupler 10 is controlled by the output of the operational amplifier 21 which compares the voltage with Vref2, and the DC output voltage Vo reaches a predetermined voltage level or less in the overcurrent operation region.
Even if the light emission of a is no longer performed, the voltage level of the voltage detection signal VB is forcibly lowered, and control is continued such that the current peak value Vc of the current detection signal VA matches the level of the overcurrent reference voltage Vref2. Therefore, the conduction pulse width of the switching element 3 can be narrowed without being affected by the latch delay of the flip-flop circuit 15, so that an ideal constant current drooping characteristic in which the output current Io does not increase can be obtained, and the transformer There is no risk that the load 5 or each circuit element will be destroyed due to a large current flowing to the secondary side of the circuit 2.

Note that the present invention is not limited to the above-mentioned embodiments, and various modifications can be made within the scope of the gist of the present invention. For example, the constant current control feedback loop can be applied to various types, and the current detector may use a current transformer instead of a resistor. Furthermore, although an FET is shown as a switching element, a transistor may also be used as the switching element, and a driver circuit for the switching element may be inserted between the switching element and the flip-flop circuit if necessary. Furthermore, an operational amplifier may be used in place of the shunt regulator in the output voltage detection circuit to compare and amplify the output detection voltage and the reference voltage.

[0020]

Effects of the Invention The present invention includes a current detector for detecting the current on the primary side of the transformer and a voltage detection circuit for detecting the output voltage as a feedback circuit for stabilizing the DC output voltage. In the switching power supply device, the current detection signal output from the detector and the voltage detection signal output from the voltage detection circuit are compared by a current comparator, and the conduction pulse width of the switching element is controlled based on the comparison result. a peak detection circuit for detecting a current peak value of the current detection signal; and a peak detection circuit configured to match the current peak value of the current detection signal to the level of an overcurrent reference voltage in an overcurrent region based on the peak detection signal from the peak detection circuit. By connecting a voltage detection signal control circuit that controls the voltage level of the voltage detection signal between the current detection signal line and the voltage detection signal line, even if the DC output voltage reaches near zero in the overcurrent region, , it is possible to provide a switching power supply device that can obtain ideal constant current droop characteristics.

[Brief explanation of the drawing]

FIG. 1 is a circuit configuration diagram showing one embodiment of the present invention.

FIG. 2 is a waveform diagram of a current detection signal and a voltage detection signal in an overcurrent region, showing an embodiment of the present invention.

FIG. 3 is a circuit configuration diagram showing a conventional example.

FIG. 4 is a waveform diagram of a current detection signal and a voltage detection signal in an overcurrent region showing a conventional example.

FIG. 5 is a waveform diagram showing characteristics of DC output voltage and output current in a conventional example.

[Explanation of symbols]

2 Transformer 3 Switching element 6 Current detector 8 Current comparator 14 Output voltage detection circuit (voltage detection circuit) 20 Peak detection circuit 25 Voltage detection signal control circuit

Claims (1)

[Claims] 1. A feedback circuit for stabilizing the DC output voltage includes a current detector that detects the current on the primary side of the transformer and a voltage detection circuit that detects the output voltage, and the output from the current detector is In a switching power supply device, a current detection signal output from the voltage detection circuit is compared with a voltage detection signal output from the voltage detection circuit by a current comparator, and the conduction pulse width of the switching element is controlled based on the comparison result. a peak detection circuit for detecting a current peak value; and a voltage detection signal configured to match the current peak value of the current detection signal to the level of an overcurrent reference voltage in an overcurrent region based on the peak detection signal from the peak detection circuit. 1. A switching power supply device, characterized in that a voltage detection signal control circuit for controlling a voltage level of the switching power supply is connected between the current detection signal line and the voltage detection signal line.