JP5270300B2 - Switching converter - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a switching converter which can be constituted of a main switching element of low breakdown voltage without increasing switching loss at the time of normal operation. <P>SOLUTION: A switching element is connected to a charging/discharging resistor of the main switching element in parallel. The switching element is set in an on-state in a period when an output voltage detecting means detecting output voltage is in the normal operation state. When the output voltage detecting means detects a short circuit state of output, the switching element is set to an off-state. <P>COPYRIGHT: (C)2010,JPO&amp;INPIT

Description

  The present invention relates to a switching converter that protects a main switching element when an output is short-circuited without increasing switching loss during normal operation.

  Conventionally, a constant power drooping type is generally known as an overcurrent protection circuit for primary side detection in a switching converter. However, even when this conventional constant power drooping type overcurrent protection circuit is adopted, switching elements, etc. are required during transient operation such as output voltage rise, overcurrent, and output short-circuit (dead short). However, there is a problem that the electric parts used must have a large current rating so as not to break.

Therefore, in order to solve the above problems, an overcurrent protection circuit has been proposed in which the output capacity at the time of start-up and dead short is reduced, and the switching element and the like are reduced in price and improved in safety ( For example, see Patent Document 1.)
JP-A-9-163730

  As shown in FIG. 5, the circuit described above includes a light receiving unit of the photocoupler PC2 in parallel with the R10 side of the resistors R10 and R11 that divide the reference voltage VREF of the comparator IC1 of the constant power overcurrent detection circuit of the primary side forward circuit. Is connected in series with the resistor R12 in series, the reference terminal of the shunt regulator IC2 is connected between the voltage divider resistors R13 and R14 of the secondary output voltage, and the current limiting resistor R15 and the light emission of the photocoupler PC2 are connected between the output voltage terminals. And the shunt regulator IC2 are connected. Thus, a two-stage constant power drooping operation is realized through the reduced protection characteristic.

  However, even if the above-described overcurrent protection circuit is used, when the output is short-circuited, the main switching element performs the switching operation with excessive current flowing until the operation stops. A surge voltage is generated in the main switching element during operation. For this reason, for example, when the main switching element is an FET, it is necessary to provide a margin for the drain-source breakdown voltage of the FET. However, when a high breakdown voltage FET is selected, there is a problem that the ON resistance increases and the efficiency of the converter decreases.

  Further, in the conventional switching converter, as shown in FIG. 4, a resistor R1 is connected to the gate of the FET (Q1) which is the main switching element, and the switching speed of the FET (Q1) is adjusted using the resistor R1. I was going. That is, according to this circuit configuration, by increasing the constant of the resistor R1, the switching speed of the FET (Q1) is slowed down, and the voltage applied between the drain and source of the FET (Q1) is reduced when the output is short-circuited. Can do.

  However, if the constant of the resistor R1 is increased, the switching speed is reduced even during normal operation. Therefore, as shown at points “A”, “A ′”, “B”, and “B ′” in FIG. Even during normal operation, the surge voltage applied to the FET (Q1) is reduced as in the case of an output short-circuit, so that the efficiency of the switching converter is lowered.

  Accordingly, the present invention has been made in view of the above-described problems, and an object of the present invention is to provide a switching converter that can be configured with a main switching element having a low withstand voltage without increasing switching loss during normal operation. To do.

  The present invention proposes the following items in order to solve the above-described problems.

(1) The present invention is a switching converter that protects a main switching element of a switching converter when an output is short-circuited, the switching element is connected in parallel with the charge / discharge resistance of the main switching element, and an output voltage is predetermined. In a period that is equal to or higher than the threshold voltage , the switching element is turned on, assuming that the switching element is in a normal operation state, and in a period in which the output voltage is less than the threshold voltage , the switching element is turned off, assuming that the output is in a short circuit state. The switching converter characterized by doing is proposed.

According to the invention, to connect the switching element in parallel with the discharge resistance of the main switching device, during a period the output voltage is the threshold voltage or more, as in the normal operation state, the switching element is turned on, the output voltage In a period less than the threshold voltage , the switching element is turned off, assuming that the output is in a short circuit state. Therefore, only when the output is short-circuited, the switching element is turned off, and the switching speed of the main switching element can be reduced by the charge / discharge resistance, and the surge voltage applied to the main switching element can be suppressed. In the normal operation state, the switching element is turned on and the charge / discharge resistance is short-circuited, so that an increase in switching loss during the normal operation can be prevented.

  (2) In the switching converter according to (1), the main switching element is an FET, the charge / discharge resistor is connected to the gate of the FET, and the value of the charge / discharge resistor is a short-circuit state of the output Therefore, a switching converter having a large value enough to sufficiently reduce the switching speed is proposed.

  According to the present invention, the value of the charge / discharge resistance is a value large enough to sufficiently reduce the switching speed in the output short-circuit state. Therefore, the switching speed of the main switching element when the output is short-circuited can be sufficiently reduced, and the surge voltage applied to the main switching element can be sufficiently suppressed.

  (3) The present invention proposes a switching converter according to (1) or (2), wherein the switching converter is a forward converter.

  According to the present invention, it is possible to reduce the switching speed of the main switching element by connecting the switching element in parallel with the charging / discharging resistor of the main switching element and turning off the switching element when the output is short-circuited. become. Thereby, there is an effect that the surge voltage applied to the main switching element when the output is short-circuited can be suppressed.

  In addition, during normal operation, the switching element is turned on and the charging / discharging resistor is short-circuited, so that an increase in switching loss during normal operation can be prevented.

  Furthermore, since the surge voltage applied to the main switching element when the output is short-circuited can be suppressed, it is possible to select a main switching element having a lower withstand voltage than that of the conventional one, so that the cost of the entire switching converter can be reduced. effective.

  Hereinafter, embodiments of the present invention will be described with reference to the drawings. Note that the constituent elements in the present embodiment can be appropriately replaced with existing constituent elements and the like, and various variations including combinations with other existing constituent elements are possible. Therefore, the description of the present embodiment does not limit the contents of the invention described in the claims.

<Configuration of switching converter>
The configuration of the switching converter according to the present invention will be described with reference to FIGS. 1 and 2. In this embodiment, a forward converter will be described as an example of a switching converter.

  As shown in FIG. 1, the switching converter according to the present embodiment includes a main switching element Q1, a charge / discharge resistor R1, a switch SW1, a power conversion output transformer T1, a rectifying switching element Q2, and a commutation switching. The device includes an element Q3, an output choke coil L3, a smoothing capacitor C1, a synchronous rectification drive circuit 10, a pulse width control circuit 20, a switching drive circuit 30, and an output voltage detection unit 40.

  Between the DC input terminals, the primary winding L1 of the transformer T1 and the main switching element Q1 are connected in series. As the main switching element Q1, an FET (electrolytic effect transistor) or the like may be used. The gate of the main switching element Q1 is connected to the output terminal of the switching drive circuit 30 via a switch SW1 and a charge / discharge resistor R1 connected in parallel.

  The switching drive circuit 30 is connected to the output terminal of the pulse width control circuit 20, and a rectifying switching element Q2 and a commutation switching element Q3 are connected in series between the secondary windings of the transformer T1. . Each of the switching elements Q2 and Q3 is composed of an FET, and each output terminal of the synchronous rectification drive circuit 10 is connected to each gate. A Schottky barrier diode may be used in place of the switching elements Q2 and Q3.

  Further, the output terminal of the pulse width control circuit 20 is connected to the synchronous rectification drive circuit 10, and the synchronous rectification circuit is configured by the rectification and commutation switching elements Q 2 and Q 3 and the synchronous rectification drive circuit 10. A choke coil L3 and a smoothing capacitor C1 are connected in series between the drain and source of the switching element Q3 for commutation, and both ends of the smoothing capacitor C1 are connected to a load. Further, the output of the output voltage detector 40 is connected to the control terminal of the switch SW1.

<Operation of switching converter>
Hereinafter, the operation of the switching converter during normal operation and when the output is short-circuited will be described.
During normal operation, the switch SW1 is turned on, and the DC input voltage Vin is changed to a high frequency voltage by the on / off operation of the main switching element Q1 driven by the switching drive circuit 30 through a resistance value substantially equal to the on resistance of the switch SW1. Converted.

  The high-frequency voltage is converted by the transformer T1, and a pulse positive voltage B having a predetermined frequency is output from the secondary winding L2. The pulse positive voltage B is rectified by complementary on / off operations of the switching elements Q2 and Q3 driven by the synchronous rectification driving circuit 10. This is the rectification action by the synchronous rectification circuit.

  The rectified output obtained between the drain and source of the commutating transistor Q3 is smoothed by the smoothing circuit of the choke coil L3 and the smoothing capacitor C1, and the DC output Vout supplied to the load is obtained from both ends of the smoothing capacitor C1.

  As shown in FIG. 2, the synchronous rectification drive circuit 10 generates and outputs drive signals X and Y for the switching elements Q2 and Q3 in synchronization with the control signal A of the pulse width control circuit 20. That is, the rectifying switching element Q2 is turned on when the main switching element Q1 is turned on, and the rectifying switching element Q2 is turned off when the main switching element Q1 is turned off.

  The commutation switching element Q3 is supplied with an inverted on / off signal Y with respect to the main switching element Q1 and the rectifying switching element Q2. Thus, the rectifying switching element Q2 and the commutation switching element Q3 alternately perform on / off operations.

  That is, during normal operation, the switch SW1 is turned on, and the main switching element Q1 is driven by the switching drive circuit 30 through a resistance value approximately equal to the on-resistance of the switch SW1, so that the switching of the main switching element Q1 is performed. Speed does not decrease. Therefore, it is possible to prevent the switching loss of the switching converter from decreasing.

  On the other hand, when the output is short-circuited, the output voltage decreases. The output voltage detection unit 40 compares the output voltage with a predetermined threshold value, and when the output voltage falls below this threshold value, determines that the output is in a short-circuited state so that the switch SW1 is turned off. A signal is given to the control terminal of the switch SW1.

  That is, when the output is short-circuited, the switch SW1 is forcibly turned off by the output voltage detection unit 40, so that the main switching element Q1 is switched by the switching drive circuit 30 via the resistance value of the charge / discharge resistor R1. Driven. Here, since the resistance value of the charging / discharging resistor R1 is large enough to sufficiently reduce the switching speed of the main switching element Q1, by sufficiently reducing the switching speed of the main switching element Q1, the resistance value of FIG. As indicated by points “C” and “C ′”, the surge voltage generated between the drain and source of the main switching element Q1 can be suppressed only when the output is short-circuited.

  Therefore, according to the present embodiment, during normal operation, the switch SW1 is turned on to suppress a decrease in switching speed of the main switching element Q1, thereby preventing a decrease in switching loss of the switching converter. On the other hand, when the output is short-circuited, the switch SW1 is forcibly turned off, and the main switching element Q1 is connected via the charging / discharging resistor R1 having a value large enough to sufficiently reduce the switching speed of the main switching element Q1. By performing the switching drive, the switching speed of the main switching element Q1 can be sufficiently reduced, and the surge voltage generated between the drain and source of the main switching element Q1 can be suppressed.

  Note that the present invention is not limited to the above-described embodiments, and various modifications and applications are possible without departing from the spirit of the present invention. For example, in the present embodiment, the example in which the output short circuit is performed by monitoring the output voltage has been described. However, the current value on the primary side or the secondary side of the transformer may be monitored regardless of this. .

  In the present embodiment, the forward converter is exemplified as the switching converter. However, the present invention is not limited to this and may be a switching converter of another form, and can be applied to an insulating type and a non-insulating type.

It is a figure which shows the structure of the forward converter which concerns on this embodiment. It is a timing chart which shows the control algorithm of the synchronous rectification drive circuit which concerns on this embodiment. It is the figure which compared the surge voltage waveform in the switching element which concerns on this embodiment, and the surge voltage waveform in the conventional switching element. It is a figure which shows the structure of the forward converter in a prior art example. It is the figure which showed the surge voltage waveform which generate | occur | produces in the switching element in the conventional forward converter of FIG. It is a figure which shows the structure of the forward converter in a prior art example.

Explanation of symbols

Q1 ... Main switching element Q2 ... Switching element Q3 ... Switching element SW1 ... Switch C1 ... Smoothing capacitor R1 ... Charge / discharge resistor T1 ... Transformer L3 ... Choke coil Ra /・ ・ Resistance R10 ... Resistance R11 ... Resistance R12 ... Resistance R13 ... Resistance R14 ... Resistance R15 ... Resistance PC1 ... Photocoupler PC2 ... Photocoupler IC1 ... Comparator IC2 ... Shunt regulator 10 ... Synchronous rectifier circuit 20 ... Pulse width control circuit 30 ... Switching drive circuit 40 ... Output voltage detector

Claims (3)

A switching converter that protects the main switching element of the switching converter when the output is short-circuited,
A switching element is connected in parallel with the charge / discharge resistor of the main switching element, and in a period in which the output voltage is equal to or higher than a predetermined threshold voltage , the switching element is turned on as a normal operation state,
In the period when the output voltage is less than the threshold voltage , the switching element is turned off, assuming that the output is in a short circuit state.   The main switching element is an FET, the charge / discharge resistance is connected to the gate of the FET, and the value of the charge / discharge resistance is large enough to sufficiently reduce the switching speed in a short-circuit state of the output. The switching converter according to claim 1.   The switching converter according to claim 1 or 2, wherein the switching converter is a forward converter.

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