JP2002272096A - Dc-dc converter - Google Patents

Abstract

PROBLEM TO BE SOLVED: To prevent current in a main switch from saturating by surely discharging residual energy in a transformer in a DC-DC converter. SOLUTION: The DC-DC converter has a transformer 3 having primary, secondary, and auxiliary coils N1, N2, and N3, a main switching transistor Q1 connected to the primary coil N1 in series, a rectifier/filter circuit connected to the secondary coil N3, and a control circuit 2 for carrying out PWM control of the main switching transistor Q1 by the output voltage of the rectifier/filter circuit. The auxiliary coil N3 is connected to local power supply means (D3 and C3) that utilize excitation energy of the transformer 3 for supplying a local power supply. Also, the auxiliary coil is connected to forced drive circuits (R1, R2, and Q4) that forcedly turn off a main switch 1 even if a signal for turning on the main switch 1 is outputted from the control circuit 2 when the generation of voltage due to the residual energy is detected.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a DC-DC converter that can be used for various electronic devices (mobile phones, PHS phones, game machines, personal computers, etc.).

[0002]

2. Description of the Related Art A conventional example will be described below.

§1: Description of Conventional Example 1 FIG. 4 shows a circuit diagram of Conventional Example 1. Conventional example 1 is an example of a forward type DC-DC converter. This circuit comprises a constant voltage circuit 1, a control circuit 2, and transistors Q2 and Q3.
, A transformer 3, a main switching transistor Q1, capacitors C1, C2, diodes Dr, D1, D2, a coil L1, an input terminal T1,
T2 and output terminals T3, T4, and the like.

The transistor Q1 is an N-channel MOS-FET (MOS type field effect transistor),
Q2 and Q3 are bipolar transistors. Also,
In this example, a DC input voltage + V _in is applied to the input terminal T1, operated by connecting to the GND input terminal T2.

The transformer 3 has a primary winding N1 and
The transformer 3 includes a secondary winding N2 and a reset winding Nr.
Main switching transistor Q in series with the primary winding N1 of
1 (main switch) is connected. The capacitor C1 is connected between the input terminals T1 and T2,
2 is connected between the output terminals T3 and T4.

[0006] The control circuit 2 receives the voltage of the capacitor C2 and applies a PWM to the main switching transistor Q1.
This is a circuit that performs control (PWM: pulse width modulation). The transistors Q2 and Q3 are connected to the PW output from the control circuit 2.
The driver is turned on / off by the M pulse and controls the gate voltage of the main switching transistor Q1. The constant voltage circuit 1 is composed of a series regulator or the like, generates a DC constant voltage V _cc (for example, DC 10 V) from an input voltage + V _in (for example, DC 50 V), and locally generates the DC voltage to the control circuit 2 and the drivers (transistors Q 2 and Q 3). It is supplied as power.

A circuit composed of diodes D1, D2, a coil L1, and a capacitor C2 connected to the secondary winding N2 of the transformer 3 is a rectifying / smoothing circuit. The output of this rectifying / smoothing circuit is obtained from output terminals T3, T4. It is supposed to be. In this case, the voltage of the output terminal T3 is + V _out, the voltage of the output terminal T4 is a -V _out.

The control circuit 2 receives the output voltage of the rectifying / smoothing circuit (the voltage between the output terminals T3 and T4),
It is configured to perform PWM control on the main switching transistor Q1 via drivers (Q2, Q3).

In this case, in the driver, when the PWM pulse output from the control circuit 2 is at a high level, the transistor Q2 is turned on and Q3 is turned off, so that a high-level voltage (approximately + _Vcc ) is applied to the gate of the main switching transistor Q1. When the PWM pulse is at a low level, the transistor Q2 is turned off and Q3 is turned on to apply a low-level voltage (approximately GND potential) to the gate of the main switching transistor Q1.

The main switching transistor Q1
When a high-level voltage is applied to the gate of the switch, the main switching transistor Q1 is turned on, and when a low-level voltage is applied, the main switching transistor Q1 is turned off.

The operation of the above circuit is as follows. The input terminal T2 is connected to GND, and an input voltage + V is applied to the input terminal T1.
The application of _in, by the input voltage + V _in, generates a voltage V _cc from the constant voltage circuit 1, applied to the control circuit 2 and the transistor Q2, Q3. The transistors Q2 and Q3 are turned on / off by the PWM control of the control circuit 2, and the main switching transistor Q1 is turned on / off accordingly.

Further, as the main switching transistor Q1 is turned on / off, the primary winding of the transformer 3 is excited, and a voltage is induced in the secondary winding N2. Therefore, when the diode D1 is forward-biased by the voltage induced in the secondary winding N2, the secondary winding N2 → the diode D2
A current flows through a path of 1 → coil L1 → capacitor C2 → secondary winding N2 to charge the capacitor C2.

Thereafter, when the voltage polarity of the secondary winding N2 is reversed, the diode D1 is reverse-biased, so that no current flows through the diode D1 and the electromagnetic energy (residual magnetic flux) stored in the coil L1. As a result, the coil L1 →
A current flows through the path of the capacitor C2 → the diode D2 → the coil L1, and charges the capacitor C2. Thereafter, each time the voltage polarity of the secondary winding N2 changes, the above operation is repeated.

At this time, the voltage of the capacitor C2 is input to the control circuit 2, and the control circuit 2 outputs the PWM
A pulse is output and the transistor Q
2. The main switching transistor Q1 is turned on / off (by controlling the on / off interval) by turning on / off the driver composed of Q3.

In the above operation, when the main switching transistor Q1 is turned on, the input terminal T1 → primary winding N1 → main switching transistor Q1 → input terminal T2
A current flows through the (GND) path and excites the primary winding N1. At this time, an induced voltage is also generated in the reset winding Nr.

Next, when the main switching transistor Q1 is turned off, the induced voltage of the reset winding Nr is reversed, and the input terminal T2 → the diode Dr → the reset winding N
A current flows through a path from r to the input terminal T1. With this operation, the excitation energy (residual energy) of the transformer 3 is released to the input terminal side by the reset winding Nr.

§2: Description of Conventional Example 2 A circuit diagram of Conventional Example 2 is shown in FIG. This example is an example in which a local power supply is created using an auxiliary winding of a transformer in a forward DC-DC converter similar to the first conventional example.

As shown in FIG. 5, the transformer 3 does not have the reset winding Nr of the conventional example 1, but has an auxiliary winding N3. Then, a series circuit of a diode D3 and a capacitor C3 is connected between both terminals of the auxiliary winding N3, and the terminal voltage of the capacitor C3 is used as a local power supply (voltage: _Vcc ) to control the control circuit 2 and the driver (Q2 , Q3). Except for the auxiliary winding N3 and the local power supply, the configuration is the same as that of the conventional example 1, and the description is omitted.

§3: Description of operation according to waveform chart of conventional example 2 FIG. 6 shows a waveform chart of each part when the output of the conventional example 2 is short-circuited. FIG.
3A shows the output waveform (PWM pulse waveform) of the control circuit 2, FIG. 3B shows the gate voltage waveform of the main switching transistor Q1, and FIG.
(Voltage waveform at point P), FIG. D shows a current waveform of the main switching transistor Q1, and t0 to t7 show respective timings.

In the circuit shown in FIG.
3. When T4 is short-circuited due to an abnormal condition of the load or the like, the waveform of each part is as shown in FIG. In FIG. 6, the main switching transistor Q1 is turned on during the timings t0 to t1, t2 to t3, t4 to t5, and t6 to t7. Transistor Q1 is off.

When the output is short-circuited, the control circuit 2 outputs a PWM pulse as shown in FIG. Then, the gate voltage of the main switching transistor Q1 is controlled by a drive circuit including the transistors Q2 and Q3 so as to have a voltage waveform as shown in FIG.

For this reason, the main switching transistor Q
1 is turned on / off by the gate voltage and a current flows. In this case, the main switching transistor Q1
The voltage waveform (voltage waveform between the drain and the source) becomes as shown in FIG. C, and the current flowing through the main switching transistor Q1 gradually increases due to the residual energy of the transformer 3, and the current waveform becomes as shown in FIG. Become.

This is because when the output terminal is short-circuited due to a load abnormality, the on-time of the main switching transistor Q1 extends to the maximum on-duty, the excitation energy of the transformer increases, and it cannot be consumed by the local power supply or the like.
Excitation energy remains in the transformer 3.

In this state, when the next ON time of the main switching transistor Q1 comes, the remaining amount of the excitation energy (residual energy) further increases, and finally the transformer 3 is saturated, and the current of the main switching transistor Q1 is reduced. The saturation rapidly increases (see timing t6 to t7). This phenomenon becomes more pronounced when the input voltage is high.

[0025]

The above-mentioned prior art has the following problems.

(1): Forward type DC-DC of Conventional Example 1
In the case where the constant voltage circuit 1 is not provided in the converter, the power supplied to the control circuit 2 and the drive circuit (transistors Q2 and Q3) of the main switching transistor Q1 is a local power supply obtained by stepping down the input voltage applied to the input terminal with a resistor or the like. Supplied from

In this case, when the input voltage is high, the control circuit 2
Or the maximum rated voltage of the drive circuit (transistors Q2 and Q3) of the main switching transistor Q1 may be exceeded, and it is necessary to separately provide a low-voltage power supply.
As a result, the cost and the size of the product are increased.

(2) Therefore, as in the prior art 1, a constant voltage circuit 1 composed of a series regulator or the like is provided as the constant voltage power supply, and a control circuit 2 for performing PWM control from the constant voltage circuit 1 and a main switching circuit are provided. Power was supplied to the driver (Q2, Q3) of the transistor Q1.

However, when a constant voltage is generated from the input voltage by a series regulator and used as a local power supply (power supply for the control circuit 2 and driver) as in the constant voltage circuit 1, the series regulator has a large resistance and a large loss. turn into. For example, the input voltage V _in at the input terminal T1, the current consumption of the control circuit 2, I _cc, power loss I _cc × V _in next to the control circuit 2, greater power loss and the input voltage V _in is greater turn into.

(3): In order to improve the above-mentioned drawback, the auxiliary winding N3 is provided in the transformer 1 as in the conventional example 2, and the voltage induced in the auxiliary winding N3 by the residual energy of the transformer 3 is used. The local power supply (or auxiliary power supply) has been used.

However, when the output terminal is short-circuited due to a load abnormality, as described in the conventional example 2, the on-time of the main switching transistor Q1 extends to the maximum on-duty, the excitation energy of the transformer 3 increases, and the local The power cannot be consumed by the power supply (auxiliary power supply) or the like, and the excitation energy remains in the transformer 3.

In this state, when the next ON time of the main switching transistor Q1 comes, the residual amount of the excitation energy further increases, and finally the transformer 3 saturates, and the current of the main switching transistor Q1 sharply increases (when the current is excessive). Separate)
However, elements such as the main switching transistor Q1 may be destroyed.

The present invention solves such a conventional problem and prevents the saturation of the current flowing through the main switch by performing the PWM control of the main switch while reliably discharging the residual energy of the transformer. The purpose is to:

[0034]

According to the present invention, there is provided a transformer having a primary winding, a secondary winding, and an auxiliary winding, and a transformer connected in series to the primary winding. A DC-DC converter comprising a switch, a rectifying / smoothing circuit connected to the secondary winding, and a control circuit for performing PWM control on the main switch, wherein the auxiliary winding is A forced drive circuit is provided for forcibly turning off the main switch when a signal for turning on the main switch is output from the control circuit when detecting that a voltage due to residual energy is generated in the line.

Further, the forcible driving circuit includes a plurality of resistors for dividing the voltage of the auxiliary winding, and a transistor which is turned on / off by the voltage divided by the resistor. Further, the excitation energy utilizing means is a local power supply connected to the auxiliary winding, the control circuit,
A power supply for a circuit including a driver for driving the main switch on / off by a PWM pulse output from the control circuit.

Further, the transformer includes a second auxiliary winding, the rectifying / smoothing circuit includes a synchronous rectifying element, and the exciting energy utilizing means includes the second auxiliary winding. The rectifier was configured to be driven.

(Operation) When the output of the DC-DC converter is short-circuited, the control circuit outputs a PWM pulse. However, when the output terminal is short-circuited due to a load abnormality, the on-time of the main switch extends to the maximum on-duty. The excitation energy of the transformer increases and cannot be consumed by a local power supply or the like, and the excitation energy remains in the transformer (residual energy is generated).

Therefore, the voltage of the auxiliary winding remains high due to the residual energy of the transformer. In such a case, when the forcible drive circuit detects that a voltage due to residual energy is generated in the auxiliary winding, the control circuit forcibly switches the main switch even if a signal for turning on the main switch is output from the control circuit. Control to turn off.

For this reason, the main switch is forcibly driven to the off state, so that no current flows through the primary winding of the transformer.
At this time, the control circuit is supplied with power from a local power supply created by using the voltage of the auxiliary winding of the transformer, continues to operate, and consumes residual energy of the transformer.

As described above, the forced drive circuit operates and keeps the main switch off until there is no residual energy in the transformer. When the residual energy disappears and the voltage of the auxiliary winding becomes approximately 0 V, the forcible drive circuit stops operating, and the main switch can be turned on / off by a PWM pulse from the control circuit.

Then, the PW output from the control circuit is
The main switch is turned on at the rising timing of the M pulse. By doing so, the PWM control of the main switch can be performed while reliably discharging the residual energy of the transformer, and the transformer does not saturate, and the current flowing through the main switch does not become excessive.

Further, the exciting energy utilizing means consumes the exciting energy of the transformer by driving the synchronous rectifier with the voltage induced in the second auxiliary winding. Therefore, also in this case, since the residual energy of the transformer is reliably released as in the above case, the current flowing through the main switch does not become excessive.

[0043]

Embodiments of the present invention will be described below in detail with reference to the drawings.

§1: Description of Configuration of DC-DC Converter FIG. 1 shows a circuit diagram of the DC-DC converter. In this example, the control circuit 2 turns on the main switching transistor Q1 when detecting that a voltage due to residual energy is generated in the auxiliary winding N3 of the transformer 3 in the forward type DC-DC converter of the conventional example 2. This is an example of a DC-DC converter provided with a forced drive circuit for forcibly turning off the main switching transistor Q1 even when a signal is output. Therefore, the configuration other than the forced drive circuit is the same as that of the second conventional example. Specifically, it is as follows.

As shown in FIG. 1, the DC-DC converter includes a primary winding N1, a secondary winding N2, and an auxiliary winding N3.
, A main switching transistor Q1 (N-channel MOS-FET) connected in series to the primary winding N1 of the transformer 3, and a secondary winding N2 of the transformer 3.
And a rectifying / smoothing circuit (a circuit composed of diodes D1 and D2, a coil L1 and a capacitor C2) connected to the rectifying / smoothing circuit and inputting the output voltage (voltage of the capacitor C2) to perform PWM control on the main switching transistor Q1. A control circuit 2 is provided.

The auxiliary winding N3 of the transformer 3 is connected to a local power supply circuit (a circuit including a diode D3 and a capacitor C3) for supplying local power using the excitation energy of the transformer 3. Furthermore, transformer 3
, The main switching transistor Q1 is forcibly turned off even if a signal for turning on the main switching transistor Q1 is output from the control circuit 2 when it is detected that a voltage due to residual energy is generated in the auxiliary winding N3. A forced drive circuit is provided.

In this case, the forcible drive circuit is driven on / off by resistors R1 and R2 for dividing the voltage of the auxiliary winding N3 and a voltage (voltage at the connection point between the resistors R1 and R2) divided by the resistors. Transistor Q4. The local power supply includes a control circuit 2 and a driver (Q2, Q2) for driving the main switching transistor Q1 on / off by a PWM pulse output from the control circuit 2.
The power supply of the circuit including 3).

§2: Description of operation The operation of the DC-DC converter is as follows. It connects the input terminal T2 to GND, the upon application of a DC input voltage + V _in the input terminal T1, by the input voltage + V _in,
The DC-DC converter starts operating, and an output voltage is obtained from the rectifying / smoothing circuit (D1, D2, L1, capacitor C2). In this case, the voltage of the output terminal T3 is + V _out ,
The voltage at the output terminal T4 is -V _out. Specifically, the operation of this DC-DC converter is as follows.

When the local power supply (voltage V _cc ) is applied to the control circuit 2 and the drivers (Q2, Q3), the control circuit 2
Is the output voltage of the rectifying and smoothing circuit (the voltage of the capacitor C2)
Is used to perform PWM control. Then, the PWM pulse output from the control circuit 2 causes the driver transistor Q
2, Q3 is turned on / off to apply a drive signal to the gate of the main switching transistor Q1.

Thus, the main switching transistor Q1 is turned on / off via the driver by the PWM pulse output from the control circuit 2. Accordingly, when the main switching transistor Q1 is turned on,
When an exciting current flows through the primary winding N1 of the transformer 3 and the main switching transistor Q1 is turned off, the primary winding N1
The current flowing through is cut off. Thereafter, such an operation is repeated.

As described above, when the main switching transistor Q1 is turned on / off as the drivers (Q2, Q3) are turned on / off by the PWM control of the control circuit 2, the winding of the transformer 3 is turned on. The line is excited and induces a voltage in the secondary winding N2. When the diode D1 is forward-biased by the voltage induced in the secondary winding N2, a current flows through the secondary winding N2 → diode D1 → coil L1 → capacitor C2 → secondary winding N2. The capacitor C2 is charged.

Thereafter, when the voltage polarity of the secondary winding N2 is reversed, the diode D1 is reverse-biased, so that no current flows through the diode D1 and the current is stored in the coil L1. → Capacitor C
A current flows through a path of 2 → diode D2 → coil L1,
The capacitor C2 is charged. Thereafter, each time the voltage polarity of the secondary winding N2 changes, the above operation is repeated.

The capacitor C2 charged in this way
Is input to the control circuit 2, and a PWM pulse is output from the control circuit 2 in accordance with this voltage, and the transistors Q2 and Q3 are turned on / off by the PWM pulse, thereby turning on / off the main switching transistor Q1. Drive (drive while controlling ON / OFF intervals).

In the above operation, when the main switching transistor Q1 is turned on, the input terminal T1 → primary winding N1 → main switching transistor Q1 → input terminal T2
A current flows through the (GND) path to excite the primary winding N1. At this time, an induced voltage is also generated in the tertiary winding N3.

Next, when the main switching transistor Q1 is turned off, the polarity of the induced voltage of the tertiary winding N3 is reversed. Then, when a voltage is induced in the tertiary winding N3, a current half-wave rectified by the half-wave rectifier circuit including the diode D3 and the capacitor C3 flows through the capacitor C3, and charges the capacitor C3. The capacitor C generated at this time
3 is used as a local power supply and is supplied to the control circuit 2 and the drivers (Q2, Q3).

The induced voltage of the tertiary winding N3 is equal to the resistance R
1 and R2, and the transistor Q4 is turned on / off according to the divided voltage (the voltage at the connection point of the resistors R1 and R2). In this case, the resistance R
If the voltage (or potential) at the connection point between R1 and R2 is larger than a certain value, the transistor Q4 turns on and the control circuit 2
Is forced to substantially the GND potential (low-level potential) so that the main switching transistor Q1 is not turned on.

The voltage at the connection point between the resistors R1 and R2 is
If less than a certain value, the transistor Q4 is turned off,
The main switching transistor Q1 is turned on / off by a PWM pulse output from the control circuit 2 (drive during normal operation).

§3: Description of operation by waveform FIG. 2 shows a waveform diagram of each part when the output of the circuit shown in FIG. 1 is short-circuited. 2, A is a control circuit output waveform (PWM pulse waveform), B is an auxiliary winding voltage waveform, C is a main switching transistor Q1 (main switch) gate voltage waveform, and D is a main switching transistor Q1. (Main switch) voltage waveform (voltage waveform between drain and source),
E shows the current waveform of the main switching transistor Q1. Further, t0 to t7 indicate each timing.

The timings t0 to t1, t2 to t
3, a signal for turning on the main switching transistor Q1 is output from the control circuit 2 during t4 to t5 and t6 to t7, and the main switching transistor Q1 is turned off during timing t1 to t2, t3 to t4, and t5 to t6. A signal is output.

In the circuit shown in FIG.
3, when T4 is short-circuited due to an abnormal state of the load or the like, the waveform of each part is as shown in FIG. When the output is short-circuited, the control circuit 2 outputs a PWM pulse having a waveform as shown in FIG.
In this case, if the input voltage is high and the output terminal is short-circuited due to a load abnormality, the on-time of the main switching transistor Q1 extends to the maximum on-duty, the excitation energy of the transformer increases, and the local power supply (auxiliary power supply) And the excitation energy remains in the transformer 3 (generation of residual energy).

Therefore, the voltage of the auxiliary winding N3 continues to be high from the timing t1 due to the residual energy of the transformer 3 as shown in FIG. B, the transistor Q4 is turned on from the timing t1, and the transistors Q2 and Q2 are turned on.
3 is maintained at a substantially GND potential, and the transistor Q2
Is turned off, the transistor Q3 is turned on, and the gate of the main switching transistor Q1 is set to a substantially GND potential (about 0).
V).

For this reason, the main switching transistor Q
1 is forcibly turned off, and the primary winding N of the transformer 3 is
No current is applied to 1. At this time, the control circuit 2
The power is supplied from a local power supply created by using the voltage of the tertiary winding N3, and the operation is continued.
Consumes residual energy.

As described above, the transistor Q4 is driven on until the residual energy of the transformer 3 is exhausted, and the main switching transistor Q1 is kept off. Then, after the time t3, the residual energy starts to disappear, and when the voltage of the auxiliary winding N3 becomes substantially 0 V,
Transistor Q4 is turned off, and PW from control circuit 2
The main switching transistor Q1 can be turned on / off by the M pulse.

Then, P output from the control circuit 2
At the timing t4 when the WM pulse rises, the transistor Q2 is turned on and the transistor Q3 is turned off, and a high-level voltage is applied to the gate of the main switching transistor Q1 as shown in FIG.
Turns on.

Therefore, a current having a waveform as shown in FIG. E flows from the timing t4 to the main switching transistor Q1, and the voltage waveform (voltage waveform between the drain and the source) of the main switching transistor Q1 becomes substantially 0 V as shown in FIG. . Thereafter, the same operation is repeated.

(Description of Another Example) §1: Description of Configuration of Another Example FIG. 3 shows a circuit diagram of a DC-DC converter of another example.
In this example, a second auxiliary winding N3 is provided in the transformer 3, and transistors Q5 and Q6 constituting a synchronous rectifier for synchronous rectification in a rectifying and smoothing circuit on the secondary side of the transformer 3.
(N-channel MOS-FET). In this case, the auxiliary winding N3
And the induced voltage drives the transistor Q5 of the synchronous rectifier. Specifically, it is as follows.

As shown in FIG. 3, the DC-DC converter comprises a primary winding N1, a secondary winding N2, and an auxiliary winding N3.
A transformer 3 having an N4 and a primary winding N1 of the transformer 3;
The main switching transistor Q1 (N
Channel MOS-FET) and a synchronous rectifier circuit (transistors Q5, Q5) connected to the secondary winding N2 of the transformer 3.
6, circuit consisting of coil L1, capacitors C2 and C5)
And a transistor Q constituting a commutation element of the synchronous rectifier circuit
5 (N3, D1)
2, a resistor R12).

A local power supply circuit (D1) for supplying local power to the auxiliary winding N4 of the transformer 3
3, a circuit including D14 and C3). Further, when it is detected that a voltage due to residual energy is generated in the auxiliary winding N4 of the transformer 3, even if the control circuit 2 outputs a signal for turning on the main switching transistor Q1, the main switching transistor Q1 is forcibly turned off. Is provided with a forced drive circuit that is turned off.

This forced drive circuit includes a transistor Q1
1, Q12, Q13, resistors R5, R6, etc., capacitor C
6. It is composed of a diode D11 and the like. The local power supply (power supply using the DC voltage of the capacitor C3) includes the control circuit 2 and the drivers (Q2, Q3) for driving the main switching transistor Q1 on / off by the PWM pulse output from the control circuit 2. It is the power supply of the circuit including.

In the circuit having the above configuration, when the main switching transistor Q1 is turned off from on, the diode D1 is turned off.
2 is turned on, the exciting energy stored in the transformer 3 is used to charge the input capacitance of the transistor Q5, and the energy consumed at this time becomes the residual energy consumption of the transformer 3.

§2: Description of Operation of Another Example The operation of the DC-DC converter shown in FIG. 3 is as follows. The basic operation is the same as that of the DC-DC converter shown in FIG.

[0072] connects the input terminal T2 to GND, the upon application of a DC input voltage + V _in the input terminal T1, by the input voltage + V _in, DC-DC converter starts to operate,
The rectifying / smoothing circuits (Q5, Q6, L1, C2, C5) perform a synchronous rectification operation, and an output voltage is obtained from the capacitor C2.

The control circuit 2 and the drivers (Q2, Q2)
When power is applied to 3), the control circuit 2 sets the capacitor C3
To perform PWM control. The transistors Q2 and Q3 of the driver are turned on / off by the PWM pulse output from the control circuit 2, and a drive signal is applied to the gate of the main switching transistor Q1.

Thus, the main switching transistor Q1 is turned on / off via the driver by the PWM pulse output from the control circuit 2. Accordingly, when the main switching transistor Q1 is turned on,
When an exciting current flows through the primary winding N1 of the transformer 3 and the main switching transistor Q1 is turned off, the primary winding N1
The current flowing through is cut off. Thereafter, such an operation is repeated.

As described above, when the main switching transistor Q1 is turned on / off as the drivers (Q2, Q3) are turned on / off by the PWM control of the control circuit 2, the winding of the transformer 3 is turned on. The line is excited and a voltage is induced in the secondary winding N2 and the auxiliary windings N3, N4. Therefore, the rectifying and smoothing circuit connected to the secondary side of the transformer 3 performs a synchronous rectification operation, and the current at that time is smoothed by the coil L1 to charge the capacitor C2.

In this case, the main switching transistor Q
When transistor 1 turns on, transistor Q6 turns on and transistor Q5 turns off, so that winding N2 → C2 → L1 → Q
Current flows through the path of 6 → N2, and charges the capacitor C2. Next, when the main switching transistor Q1 is turned off, the voltage of the winding N3 becomes the opposite polarity, a current flows through the path of N3 → the floating capacitance of Q5 → diode D12 → N3, and Q5 is turned on. Further, the voltage of the winding N2 becomes the opposite polarity, and Q6 is turned off. Therefore, due to the electromagnetic energy stored in L1, a current flows through the path of L1, Q5, C2, and L1 to charge the capacitor C2. Thereafter, the same operation is repeated.

In the above operation, when the main switching transistor Q1 is turned on, the input terminal T1 → the primary winding N1 → the main switching transistor Q1 → the input terminal T2
A current flows through the (GND) path to excite the primary winding N1. At this time, an induced voltage is also generated in the auxiliary winding N4.

At this time, the capacitor C3 is charged by the induced voltage of the auxiliary winding N4, and the control circuit 2 and the driver (Q2,
Supply local power to Q3). The auxiliary winding N4
Transistors Q11, Q12, Q13
Is turned on / off, and when the transistor Q13 is turned on, the output side of the control circuit 2 is forcibly set to a substantially GND potential (low level potential) and the main switching transistor Q1 is turned on.
Is not turned on. Also, the transistor Q1
When the switch 3 is turned off, the main switching transistor Q1 is turned on / off by a PWM pulse output from the control circuit 2 (drive during normal operation).

That is, when the excitation energy remains in the transformer 3, when the diode D11 is turned on by the voltage induced in the auxiliary winding N4, the transistor Q11 is turned off, the transistor Q12 is turned off, and the transistor Q13 is turned on. When the transistor Q13 is turned on, the output side of the control circuit 2 is forcibly set to the GND potential (low-level potential) so that the main switching transistor Q1 is not turned on (Q1: off).

When no exciting energy remains in the transformer 3, the diode D11 turns off, the transistor Q11 turns on, the transistor Q12 turns on, and the transistor Q13 turns off. And the transistor Q1
When the switch 3 is turned off, the main switching transistor Q1 is turned on / off by a PWM pulse output from the control circuit 2 (drive during normal operation).

§3: Other explanations (1): Although the main switching transistor Q1 is constituted by an N-channel MOS-FET, the present invention is not limited to such an example, and a bipolar transistor and a P-channel MOS-FET are used. Any switching element can be used.

(2) The voltage dividing resistors constituting the compulsory driving circuit are not limited to the two resistors R1 and R2, and can be implemented by dividing the voltage using an arbitrary number of three or more resistors.

[0083]

As described above, the present invention has the following effects.

In the DC-DC converter described in the second conventional example, when the input voltage is high, when the output terminal is short-circuited due to a load abnormality, the on-time of the main switch is extended to the maximum on-duty, and the excitation energy of the transformer 3 is increased. Increases, and is no longer consumed by local power (auxiliary power), etc.
Excitation energy remains in the transformer.

In this state, when the next ON time of the main switch comes, the residual amount of the excitation energy further increases, and
Finally, the current of the main switch is saturated, and there is a possibility that elements such as the main switch may be destroyed.

Accordingly, the present invention provides a DC-DC converter which, when detecting that a voltage due to residual energy is generated in an auxiliary winding, outputs a signal for turning on a main switch from a control circuit. A forced drive circuit for forcibly turning off the main switch is provided.

Then, while the main switch is forcibly turned off by the operation of the compulsory driving circuit, the residual energy of the transformer is consumed by the control circuit or the driver operated by the local power supply, so that it can be completely discharged. . Therefore, the transformer does not saturate and the current flowing through the main switch does not become excessive, so that the main switch can be prevented from being destroyed.

Further, the excitation energy utilizing means consumes the excitation energy of the transformer by driving the synchronous rectifier with the voltage induced in the second auxiliary winding. Therefore,
Also in this case, since the residual energy of the transformer is reliably discharged, the saturation of the transformer does not occur, and the current flowing through the main switch does not become excessive, so that the main switch can be prevented from being destroyed.

[Brief description of the drawings]

FIG. 1 is a circuit diagram of a DC-DC converter according to an embodiment of the present invention.

FIG. 2 is a waveform diagram of each part when the output of the circuit shown in FIG. 1 is short-circuited.

FIG. 3 shows another example of DC-D according to the embodiment of the present invention.
It is a circuit diagram of a C converter.

FIG. 4 is a circuit diagram of Conventional Example 1.

FIG. 5 is a circuit diagram of a second conventional example.

FIG. 6 is a waveform diagram of each part when the output is short-circuited in Conventional Example 2.

[Explanation of symbols]

Reference Signs List 1 constant voltage circuit 2 control circuit 3 transformer Q1 main switching transistor Q2 to Q6, Q11, Q12, Q13 transistor R1 to R6, R12 resistor C1 to C6 capacitor L1 coil D1 to D3, D11, D12, D13, Dr diode N1 Transformer Primary winding N2 Secondary winding of transformer N3, N4 Auxiliary winding of transformer Nr Reset winding of transformer T1, T2 Input terminal T3, T4 Output terminal

Claims (4)

[Claims] A transformer having a primary winding, a secondary winding, and an auxiliary winding; a main switch connected in series to the primary winding; and a rectifying / smoothing circuit connected to the secondary winding. And a control circuit for performing PWM control on the main switch, and a DC connected to an excitation energy utilization unit of the transformer.
-In the DC converter, when detecting that a voltage due to residual energy is generated in the auxiliary winding, the main switch is forcibly turned off even if a signal for turning on the main switch is output from the control circuit. DC having a forced drive circuit
-DC converter. 2. The forcible driving circuit includes a plurality of resistors for dividing a voltage of the auxiliary winding, and a transistor which is turned on / off by the voltage divided by the resistor. Item 6. The DC-DC converter according to Item 1. 3. The excitation energy utilization means is a local power supply connected to the auxiliary winding, and the control circuit and a driver for driving the main switch on / off by a PWM pulse output from the control circuit. 3. A power supply for a circuit including:
C-DC converter. 4. The transformer includes a second auxiliary winding, the rectifying / smoothing circuit includes a synchronous rectifier, and the exciting energy utilizing means includes the second auxiliary winding, and the synchronous voltage is induced by the induced voltage. The DC-DC converter according to claim 1, wherein the DC-DC converter drives a rectifier.