JPH09294370A - Power supply circuit using power insulation gate type fet - Google Patents

Abstract

PROBLEM TO BE SOLVED: To reduce the power loss, and to improve the efficiency of a circuit by reducing the simultaneous ON-period of a power MOSFET for rectification and commutation on the secondary side of a transformer for voltage conversion in a power source circuit using the power MOSFET. SOLUTION: In a power source circuit using a power MOSFET of one-stone forward method, an ON period control circuit 24 provided with a commutation control part 18 to control the ON period of an FET 6 with resistors 19,20 to detect the voltage change of a transformer 4 and the potential at a connection point of these resistors as the trigger input and a positive power source 17 is connected to a gate of the power MOSFET 6 for commutation to be connected to the secondary side of the transformer 4. The FET 6 is turned off before the simultaneous ON period of the FETs 5,6 by the commutation control part 18, and during this period, a high speed retification diode 7 is conducted to prevent the power loss during the period, and to improve the efficiency of the circuit.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a power supply circuit using a power insulated gate FET, and more particularly to a synchronous rectification circuit in a one-stone forward type switching power supply using a voltage converting transformer and a power MOSFET as a switching element.

[0002]

2. Description of the Related Art Conventionally, as a switching power supply of this kind, a voltage converting transformer and a power MOSFET are used as in a DC-DC converter, and various devices are devised to stabilize the output voltage. A circuit has been proposed.

FIG. 5 shows a conventional power MOSFE.
It is a power supply circuit diagram using T. As shown in FIG. 5, the power supply circuit using this one-stone forward type power MOSFET includes an electrolytic capacitor 8 connected between a pair of input terminals 1a and 1b, a primary winding and a secondary winding, and A voltage conversion transformer 4 in which one end of a secondary winding is connected to one input terminal 1a, and a switching power MOSFET 3 connected between the other end of the primary winding of the voltage conversion transformer 4 and the other input terminal 1b. A choke coil 9 connected between one end of the secondary winding of the voltage conversion transformer 4 and the output terminal 2a, and a rectification connected between the other end of the secondary winding of the voltage conversion transformer 4 and the output terminal 2b. Power MO
SFET5, one end of secondary winding and power MO for rectification
A capacitor 11 connected between the gates of the SFET 5, a resistor 13 connected between the gate and source of the rectifying power MOSFET 5, a power MOSFET 6 for commutation connected in parallel between one end of the secondary winding and the output terminal 2b, and a high speed Rectifier diode 7 and capacitor 1 connected between the gate of commutation power MOSFET 6 and the other end of the secondary winding
2, a resistor 14 connected between the gate and the source of the commutation power MOSFET 6, a capacitor 10 connected between the output terminals 2a and 2b, and a voltage between the output terminal 2a and the other end of the secondary winding. The error amplifier 25 is connected to the photocoupler 16 driven by the output of the error amplifier 25, and its output is connected to the switching power M.
A feedback control unit 15 that feeds back to the gate of the OSFET 3 is provided. The diodes 3 ', 5', 6'connected between the drain (D) and the source (S) of the power MOSFETs 3, 5, 6 are the internal diodes of the respective FETs, the capacitor 11, the resistor 13 and the capacitor 12 and the resistor 14 are power MOSFETs, respectively.
This is an element that determines the rising or falling time constant of T5 and T6.

Although the feedback control unit 15 is shown here in a simplified manner, the feedback control unit 15 is usually shown.
Is formed of a control IC for exclusive use of a power supply, has an oscillation circuit, a PWM comparator, a power supply unit, etc. inside, and has a starting resistor (not shown), a resistor for determining an oscillation frequency, a capacitor, and an input terminal 1a, The switching power MOSFET 3 is connected between 1b and has a function of controlling on / off of the switching power MOSFET 3 by a comparator output for comparing the output of the photocoupler 16 and the output of the oscillation circuit. That is, the input level of the comparator is changed by the output voltage on the secondary side of the transformer 4, and for example, when the output voltage rises, control is performed so that the ON width of the power MOSFET 3 is narrowed, thereby lowering the output voltage. It is stabilized at.

In short, switching power MOSFE
T3 is supplied with a continuous rectangular wave as its gate signal to repeat the on / off operation.

First, a DC voltage (+ on the 1a side is negative on the 1a side) is always applied to the input terminals 1a and 1b, but when the switching power MOSFET 3 on the primary side is turned on,
The rectifying power MOSFET 5 on the secondary side is biased in the forward direction between its gate and source electrodes and turned on. Therefore, current flows in a closed circuit composed of the voltage conversion transformer 4, the choke coil 9, the capacitor 10, and the rectifying power MOSFET 5, and supplies a DC voltage to the outside via the output terminals 2a and 2b.

Then, when the power MOSFET 3 on the primary side is turned off, the rectifying power MOSFET 5 on the secondary side is turned off because its gate and source electrodes are biased in the reverse direction. At this time, the commutation power MOSFET 6
Is forward biased between its gate and source electrodes,
Turn on. Therefore, the energy stored in the choke coil 9 when the rectifying power MOSFET 5 is turned on is
Choke coil 9, capacitor 10, commutation power MO
The output terminal 2a, which is regenerated by the closed circuit composed of SFET6,
A DC voltage is supplied to the outside via 2b.

As described above, the power MOSFET 3 is repeatedly turned on and off to supply constant power to the external circuit.

FIGS. 6 (a) and 6 (b) are a timing diagram and a partially enlarged view of the portion B for explaining the current characteristics of the power MOSFET in FIG. 5, respectively. FIG.
As shown in (a), these current characteristics show the drain current characteristics of the rectifying power MOSFET 5 and the commutating power MOSFET 6, but in the switching power supply circuit of the one-stone forward system, the secondary rectifying circuit constituting the synchronous rectifying circuit is used. When the diversion power MOSFET 6 changes from the ON state to the OFF state, the commutation power MOSFE is generated between the drain and source electrodes of the FET 6 despite the reverse bias state.
By the internal diode 6'built between the source and drain of T6, the internal diode 6 ', the choke coil 9,
There is a reverse recovery period because it forms a closed circuit of the capacitor 10. That is, the rectification power MOSFET 5 and the commutation power MOSFET 6 are turned on at the same time. This, as shown by a dotted line frame B, the drain current I _D of the drain current I _D and FET6 the FET 5 (opposite directions) is meant to flow simultaneously.

Further, as shown in FIG. 6B, the relationship between the source / drain current I _SD and the source / drain voltage V _SD of the FET 6 in the simultaneous ON period B is opposite to each other because the reverse recovery period T2 exists. Have a relationship. Although the current / voltage characteristics of the FET 5 are omitted, the current / voltage characteristics of the FET 5 are the same as those of the FET 6 but are the same. That is, it means that the overlap period in which the FETs 5 and 6 are simultaneously turned on becomes longer.

To this end, the commutation power MOSFET 6
Connects a high-speed rectifier diode 7 in its forward direction and in parallel between its source and drain in order to reduce the loss of T2 at the time of reverse recovery.
During the ON period of 6, the current flows through the internal diode 6 '.
However, even if the internal diode 6'is used, the reverse recovery period T2 is long, so that the power loss during the reverse recovery period increases,
It reduces the efficiency of the circuit. This is shown in FIG.
Means that the overlapping period of the current I _SD flowing from the source to the drain and the voltage V _SD applied between the source and the drain becomes long.

[0012]

The power supply circuit using the conventional power MOSFET described above has two rectifications provided on the secondary side of the voltage conversion transformer, as is apparent from FIG. 6 (b). And commutation power MOSFETs
Is on for a long time. Therefore, a synchronous rectification circuit using two power MOSFETs has a drawback that power loss increases and circuit efficiency decreases.

As a publicly known example for solving such a problem, a switching regulator having a control circuit 25 "for simultaneously driving the secondary power MOSFETs 5" and 6 "and the primary power MOSFET 3" shown in FIG. 7 is proposed. (Japanese Patent Laid-Open No. 60-59973).

In this example, the drive circuit 25 "controls the simultaneous ON periods of the secondary side rectifying and smoothing power MOSFETs 5" and 6 ", but the control method and timing chart thereof are embodied. It has not been done and it is unknown.

Further, although it is specified in the known example that the short circuit period of the MOSFET is almost eliminated, there is actually a reverse recovery period of the internal diode, and how to shorten this reverse recovery period is a major problem. No mention has been made of measures to improve this point.

In FIG. 7, 8 "is the input terminal 1a.
And a smoothing capacitor connected between 1b, 24 ″ is a rectifying element (diode), 26 ″ is a pulse width modulation circuit, 2
Reference numeral 7 "is an oscillating circuit that determines the repetition period of the pulse width modulation circuit 26".

An object of the present invention is to provide a voltage conversion transformer 2
It is an object of the present invention to provide a power supply circuit using a power insulated gate type FET, which shortens the simultaneous ON period of two power insulated gate type FETs provided on the next side to reduce power loss and improve circuit efficiency.

[0018]

According to the present invention, an electrolytic capacitor connected between a pair of input terminals, a primary winding, and a secondary winding are provided, and one end of the primary winding is connected to the pair of input terminals. A voltage conversion transformer connected to one of the input terminals; a switching power insulated gate FET connected between the other end of the primary winding of the voltage conversion transformer and the other of the pair of input terminals; A choke coil connected between one end of the secondary winding of the voltage conversion transformer and one of a pair of output terminals, and the other end of the secondary winding of the voltage conversion transformer and the other of the pair of output terminals. A rectified power insulated gate FET connected between the
A first capacitor connected between the one end of the secondary winding and the gate of the rectifying power insulated gate FET, and a commutating power connected in parallel between one end of the secondary winding and the other of the output terminals. Insulated gate type FET and high speed rectification diode, and a second connected between the pair of output terminals
Capacitor, an error amplification unit connected to the one of the pair of output terminals and the other end of the secondary winding, and connected via a photocoupler driven by the output of the error amplification unit, and its output In a power supply circuit using a power insulated gate type FET that supplies a constant voltage between the output terminals, and a feedback control unit that feeds back to the gate of the switching power insulated gate type FET. An ON period for detecting a voltage change on the secondary winding side and controlling an ON period of the commutation power insulated gate FET between both ends of the secondary winding and the gate of the commutation power insulated gate FET. A power supply circuit using a power insulated gate FET characterized by connecting a control circuit can be obtained.

[0019]

DESCRIPTION OF THE PREFERRED EMBODIMENTS Next, embodiments of the present invention will be described with reference to the drawings.

FIG. 1 shows a power supply circuit using a power MOSFET according to the first embodiment of the present invention. As shown in FIG. 1, a power supply circuit using this power MOSFET is similar to the power supply circuit of the conventional example shown in FIG.
And the primary winding of the voltage conversion transformer 4 and the electrolytic capacitor 8 are connected, and a rectifying power MOS is similarly provided on the secondary side.
The FET 5, the commutation power MOSFET 6, the high speed diode 7, and the capacitor 10 are connected. Further, from the secondary side including the output, the error amplifier 25, the photocoupler 1
Switching power MOS provided on the input side via 6
A feedback controller 15 for controlling the FET 3 is provided. Note that these are the same as the above-mentioned conventional example, and a detailed description thereof will be omitted.

In addition to these, in the present embodiment, in addition to these, a voltage change on the secondary winding side is detected between both ends of the secondary winding of the voltage conversion transformer 4 and the gate of the commutation power MOSFET 6 and used for commutation. The ON period control circuit 24 for controlling the ON period of the power MOSFET 6 is connected.
The ON period control circuit 24 mainly includes resistors 19 and 20 for detecting a voltage change on the secondary winding side, and the resistors 19 and 20.
The commutation control unit 18 that changes the output voltage according to the voltage at the connection point of 20, the resistors 19 and 20, and the commutation control unit 1
A positive power supply 17 for regulating the potential of 8 and a time constant adjusting resistors 21 and 22 and a capacitor 23 are further provided. The resistor 22 and the capacitor 23 are elements that regulate the output voltage of the commutation control unit 18. The commutation control unit 18 also includes an inverter INV that inverts the potential of the output terminal 2b, an AND circuit that ANDs the potential of the connection point of the resistors 19 and 20 and the output of the inverter INV.
The output of this AND circuit causes the power MOSFET for commutation.
6 and a flip-flop FF for outputting the control potential, and a CMOS logic IC is used in this embodiment.

In short, this embodiment is based on the commutation power MO.
On the gate electrode of the SFET 6, components 19 and 20 for detecting the voltage change on the secondary side of the voltage conversion transformer 4 in the preceding stage by resistance division, and a commutation control unit 18 for controlling the on period of the commutation power MOSFET 6 are provided. This is because the commutation controller 18 is connected to the positive power source 17 for driving.

2 (a) and 2 (b) are a timing diagram and an enlarged view of a portion A for explaining the current characteristic of the power MOSFET and the potential at each point in FIG. 1, respectively. As shown in FIG. 2 (a), these characteristics are
The drain current I _{D of} ET5 and ET6, the potential VA at the connection point of the resistors 19 and 20, and the gate potential VB of the power MOSFET 6 are shown, and in FIG. T1A and power MOSFET 5,
6 represents a simultaneous ON period in which the transistors are simultaneously ON.

Further, the current I _SD in FIG. 2B represents the source-drain current of the commutation power MOSFET 6, the voltage V _SD represents the source-drain voltage of the commutation power MOSFET 6, and the commutation controller 18 is added. By doing so, the overlap time of the current I _SD and the voltage V _SD is shortened, and thus the simultaneous ON period is shortened.

Hereinafter, the circuit operation will be described more specifically with reference to FIGS. 1 and 2.

First, when the power MOSFET 3 as a switching element provided on the primary side of the transformer 4 is turned on, the rectifying power MOSFET connected to the secondary side thereof.
The gate and source electrodes of No. 5 are forward biased and turned on. Therefore, the transformer 4 for voltage conversion, the choke coil 9, the capacitor 10, the power MOSFET 5 for rectification
A current flows through the closed circuit consisting of, and power is supplied to the external circuit via the output terminals 2a and 2b. At this time, the resistor 19,
The potential of the connection point VA of 20 becomes high level, but the output (VB) of the commutation control unit 18 becomes low (0) level because it passes through the AND circuit and the flip-flop FF. Therefore, the drive circuit 24 including the commutation controller 18
Does not drive the commutation power MOSFET 6.

Next, when the power MOSFET 3 on the primary side is turned off, the gate and source electrodes of the power MOSFET 5 for rectification on the secondary side are biased in the opposite direction, so that the power MOSFET 5 for rectification is turned off, and the connection point. VA
Potential also drops to 0 level. That is, this point becomes the trigger point for the commutation control unit 18. Further, contrary to the rectifying power MOSFET 5, the commutating power MOSFET
Since the output of the commutation controller 18 forward biases between the gate and source electrodes of T6, the commutation power MO
SFET6 turns on. Therefore, power MOSF for rectification
The energy stored in the choke coil 9 when the ET 5 is turned on is regenerated in a closed circuit composed of the choke coil 9, the capacitor 10 and the commutation power MOSFET 6, and is supplied to the external circuit from the output terminals 2a and 2b.

During this regeneration period, the rectifying power MOSFET 5 is utilized by utilizing the voltage change of the voltage converting transformer 4.
The point at which is turned off is detected from the voltage across the voltage conversion transformer 4. That is, the commutation power MOSFET 6 is turned on by inputting the potential (low level) at the point VA to the commutation control unit 18 as a trigger signal. When the commutating power MOSFET 6 is on,
The constants of the resistor 22 and the capacitor 23 connected to the commutation control unit 18 are set so that the “turn-off time” of the FET 6 (time when the FET 6 is completely turned off) <“dead time” is set, and the reverse recovery period T1 is set. Time (ie power M
When the OSFETs 5 and 6 are turned on simultaneously, the commutation power M
The OSFET 6 is turned off. Therefore, no current flows in the internal diode 6'of the commutation power MOSFET 6, and the high-speed rectifying diode 7 is turned on,
The power loss during this period can be reduced, and the efficiency of the circuit can be improved.

FIG. 3 shows a power supply circuit using a power MOSFET according to the second embodiment of the present invention. In FIG.
In the first embodiment, an ON period control circuit (delay circuit) 24 for controlling the ON period of the commutation power MOSFET 6 is provided with a resistor 20, a capacitor 24 ', and a DIAC (Diode AC).
Switch) 25 ', and SCR (Silicon)
Controlled Rectifier) 26.

Next, the operation of the embodiment shown in FIG. 3 will be described with reference to FIG.

The basic operation of the secondary side is the same as that of the first embodiment, and the operation of the ON period control circuit (delay circuit) 24 will be described.

During the regeneration period in which the rectifying power MOSFET 5 is off and the commutating power MOSFET 6 is on, the point b on the secondary side of the voltage conversion transformer 4 is a positive voltage, and the charge is accumulated in the capacitor 24 'through the resistor 20. To be done. The voltage at the middle point c of the resistor 20 and the capacitor 24 is DI
When the voltage exceeds the breakover voltage of AC25 ', DIA
C25 'turns on, supplies a trigger current to the gate of SCR26, and turns on SCR26. The anode and cathode of the SCR 26 are connected to the gate and source of the commutation power MOSFET 6. When the SCR 26 is turned on, the gate-source voltage of the commutation power MOSFET 6 becomes almost 0 V, and the commutation power MOSFET 6 is turned off. As described above, the ON period of the commutation power MOSFET 6 can be set by the time constants of the resistor 20 and the capacitor 24 ', and the reverse recovery time can be shortened as in the first embodiment.

This embodiment is different from the first embodiment in that the resistor 20, the capacitor 24 ', the DIAC 25', and the SCR 26 are provided.
Since it does not use an IC for control as in the first embodiment, a stabilizing power supply circuit such as a three-terminal regulator is not required, and further miniaturization can be achieved.

[0034]

As described above, the power M of the present invention is
A power supply circuit using an OSFET is a commutation circuit that turns off a commutation power MOSFET and conducts a high-speed rectification diode before a simultaneous ON period of a rectification power MOSFET and a commutation power MOSFET connected to a secondary side of a voltage conversion transformer. By providing the control unit, it is possible to reduce the power loss during this period and to improve the efficiency of the circuit.

Further, when the ON period control circuit is composed of a resistor, a capacitor, a DIAC, and an SCR, a stabilizing power supply circuit is not required, and there is an effect that the size can be further reduced.

[Brief description of drawings]

FIG. 1 is a power MOSFET according to a first embodiment of the present invention.
It is a block diagram of a power supply circuit using T.

FIG. 2 is a timing chart for explaining current characteristics of the power MOSFET in FIG. 1 and potentials at respective points.

FIG. 3 is a power MOSFE according to a second embodiment of the present invention.
It is a block diagram of a power supply circuit using T.

FIG. 4 is a timing chart for explaining the operation of the power supply circuit shown in FIG.

FIG. 5 is a block diagram of a power supply circuit using a conventional power MOSFET.

6 is a timing chart for explaining current characteristics of the power MOSFET in FIG.

FIG. 7 is a block diagram of another conventional example.

[Explanation of symbols]

 1a, 1b input terminal 2a, 2b output terminal 3 switching power MOSFET 3 ', 5', 6'internal diode 4 voltage conversion transformer 5 rectification power MOSFET 6 commutation power MOSFET 7 high-speed rectification diode 8 electrolytic capacitor 9 choke coil 10, 11 Capacitor 15 Feedback control unit 16 Photocoupler 17 Positive power supply 18 Commutation control unit 19, 20 Voltage detection resistor 24 ON period control circuit 24 'Capacitor 25 Error amplifying unit 25' DIAC 26 SCR

Claims (4)

[Claims] 1. A voltage conversion device comprising an electrolytic capacitor connected between a pair of input terminals, a primary winding and a secondary winding, and one end of the primary winding being connected to one of the pair of input terminals. A transformer, a switching power insulated gate FET connected between the other end of the primary winding of the voltage conversion transformer and the other of the pair of input terminals, and the secondary winding of the voltage conversion transformer. And a choke coil connected between one end of the output terminal and a pair of output terminals, and a rectifying power insulated gate connected between the other end of the secondary winding of the voltage conversion transformer and the other of the pair of output terminals. -Type FET and a first connection between the one end of the secondary winding and the gate of the rectifying power insulated gate FET
, A power insulated gate FET for commutation and a high-speed rectifying diode connected in parallel between one end of the secondary winding and the other of the output terminals, and a second capacitor connected between the pair of output terminals. And an error amplifier connected to the one of the pair of output terminals and the other end of the secondary winding, and connected via a photocoupler driven by the output of the error amplifier, and the output thereof is A power supply circuit using a power insulated gate type FET for supplying a constant voltage between the output terminals, comprising a feedback control unit for feeding back to the gate of the switching power insulated gate type FET. A power insulated gate for commutation, which detects a voltage change on the secondary winding side between both ends of a secondary winding and a gate of the commutated power insulated gate FET. Power circuit using a power insulated gate FET, characterized in that connecting the on-period control circuit for controlling the ON period of the FET. 2. The on-period control circuit includes a resistance unit that detects a voltage change on the secondary winding side, a commutation control unit that changes an output voltage according to a voltage of the resistance unit, the resistance unit and the 2. A power supply circuit using a power insulated gate FET according to claim 1, which comprises a positive power supply that regulates the potential of the commutation control unit. 3. The commutation control unit ANDs an inverter that inverts the potential of the other end of the output terminal, a coupling potential of the resistance means, and an output of the inverter AN.
The power insulated gate type F according to claim 2, comprising a D circuit and a flip-flop that outputs a control potential of the commutation power insulated gate type FET by the output of the AND circuit.
Power supply circuit using ET. 4. The on-period control circuit is connected to a series connection circuit of a resistor and a capacitor connected between both ends of the secondary winding of the voltage conversion transformer, and a connection point of the resistor and the capacitor. A first terminal and a second terminal, and has a breakover voltage. When the voltage at the first terminal becomes equal to or higher than the breakover voltage, the device is turned on and a trigger current is supplied to the second terminal. D to output
An IAC, a gate connected to the second terminal, a cathode connected to the other of the pair of output terminals, and an anode connected to the gate of the commutation power insulated gate FET. When a trigger current is supplied to the gate, it turns on and the power insulated gate FET for commutation
The power insulated gate according to claim 1, wherein the power insulated gate FET for commutation has an ON period determined by a time constant of a series connection circuit of the resistor and the capacitor. Type power circuit using FET.