JP2023021706A - Switching power supply and synchronous rectification control circuit - Google Patents

Abstract

To provide a switching power supply capable of protecting input terminals of an IC of the synchronous rectification control circuit against a negative surge of a secondary transformer, and the synchronous rectification control circuit.SOLUTION: A resistance RDET is provided between one end T21 of a secondary transformer T2 and a detection terminal DET of a synchronous rectification circuit 5. The emitter of a bipolar transistor Q1 is connected to the detection terminal DET. A current mirror circuit 52 detects a current IRDET flowing from the emitter of the bipolar transistor Q1 to the resistance RDET. A comparison circuit 53 compares the current IRDET detected by the current mirror circuit 52 with a first current (I1+I2) or a second current I1, and performs ON/OFF control over a power MOS transistor MPW2.SELECTED DRAWING: Figure 2

Description

The present invention relates to switching power supplies and synchronous rectification control circuits.

A flyback converter using a transformer is known as the switching power supply described above. By intermittently supplying input voltage to the primary transformer, the flyback converter transfers energy to the secondary transformer, rectifies it with a diode on the secondary transformer side, smoothes it with a smoothing capacitor, and converts it to the required DC voltage. do. Also, it has been proposed to use a power MOS transistor instead of the diode provided in the secondary transformer.

As a synchronous rectification control circuit for controlling the on/off of this power MOS transistor, the voltage at one end of the secondary transformer is input, the input voltage is detected, and when the detected voltage becomes a negative voltage, the power MOS transistor is activated. A switch that turns on has been proposed (Non-Patent Document 1).

However, when a large negative surge occurs at one end of the secondary transformer, a negative voltage is applied to the terminals of the synchronous rectification control circuit IC, causing problems such as malfunction.

Maxim Integrated Products, Inc., ``Secondary-Side Synchronous MOSFET Driver for Flyback Converters'', [online], 2015, [searched on December 28, 2020], Internet < URL: https://datasheets.maximintegrated. com/en/ds/MAX17606.pdf>

SUMMARY OF THE INVENTION The present invention has been made in view of the above circumstances, and its object is to provide a switching power supply and a synchronous rectification control circuit that can protect the input terminal of the IC of the synchronous rectification control circuit from the negative surge of the secondary transformer. It is in.

In order to achieve the above object, a switching power supply and a synchronous rectification control circuit according to the present invention are characterized by the following [1] to [4].
[1]
a primary transformer and a secondary transformer for stepping up or stepping down an input voltage;
a first MOS transistor that turns on and off the input voltage supplied to the primary transformer;
a second MOS transistor for rectifying the output of the secondary transformer;
A flyback type switching power supply device comprising a synchronous rectification control circuit that controls on/off of the second MOS transistor,
A resistor provided between one end of the secondary transformer and an input terminal of the synchronous rectification control circuit,
The synchronous rectification control circuit is
a first bipolar transistor having an emitter connected to the input terminal;
a second bipolar transistor whose bases are connected to the first bipolar transistor, whose emitter is connected to the ground, and whose base is connected to the collector;
a current detection circuit that detects current flowing from the emitter of the first bipolar transistor to the resistor;
A detection current detected by the current detection circuit is compared with a first current, and when the detection current becomes larger than the first current, the second MOS transistor is turned on, and the second MOS transistor is turned on. After the transistor is turned on, the detected current is compared with a second current smaller than the first current, and the second MOS transistor is turned off when the detected current becomes smaller than the second current. and a comparison circuit for
Must be a switching power supply.
[2]
In the switching power supply device according to [1],
The current detection circuit is a switching power supply device having a current mirror circuit to which a current flowing from the emitter of the first bipolar transistor is input.
[3]
In the switching power supply device according to [2],
The comparison circuit is
a first current source connected to the output of the current mirror circuit;
a second current source connected to the output of the current mirror circuit and connected in parallel with the first current source;
a switch element connected in series with the second current source and connected in parallel with the first current source;
a comparator for comparing the output of the current mirror circuit and the connection point voltage of the first current source with a reference voltage;
the switch element is turned on and off according to the output of the comparator;
Must be a switching power supply.
[4]
In a synchronous rectification control circuit that controls on/off of a second MOS transistor that rectifies a secondary transformer provided in a flyback type switching power supply,
an input terminal to which the other end of the resistor whose one end is connected to one end of the secondary transformer is connected;
a second bipolar transistor whose bases are connected to the first bipolar transistor and whose emitter is grounded;
a current detection circuit that detects current flowing from the emitter of the first bipolar transistor to the resistor;
A detection current detected by the current detection circuit is compared with a first current, and when the detection current becomes larger than the first current, the second MOS transistor is turned on, and the second MOS transistor is turned on. After the transistor is turned on, the detected current is compared with a second current smaller than the first current, and the second MOS transistor is turned off when the detected current becomes smaller than the second current. and a synchronous rectification control circuit.

According to the present invention, it is possible to provide a switching power supply and a synchronous rectification control circuit that can protect the input terminal of the IC of the synchronous rectification control circuit from the negative surge of the secondary transformer.

The present invention has been briefly described above. Furthermore, the details of the present invention will be further clarified by reading the following detailed description of the invention (hereinafter referred to as "embodiment") with reference to the accompanying drawings. .

FIG. 1 is a circuit diagram showing one embodiment of a flyback converter as a switching power supply of the present invention. FIG. 2 is a circuit diagram showing details of the synchronous rectification control circuit shown in FIG. FIG. 3 is a time chart of the voltage at one end of the secondary transformer and the gate voltage of the power MOS transistor _MPW2 .

Specific embodiments relating to the present invention will be described below with reference to each drawing.

A flyback converter 1 as a switching power supply device shown in FIG. 1 boosts or steps down a DC input voltage V _IN supplied to an input terminal IN by primary and secondary transformers T ₁ and T ₂ to convert the DC voltage from an output terminal OUT to a DC voltage. output voltage _VOUT .

The flyback converter 1 includes an input capacitor C _IN , a primary transformer T ₁ , a secondary transformer T ₂ , and a power MOS transistor _{MPW 1} as a first MOS transistor for turning on and off an input voltage V _IN supplied to the primary transformer T ₁ . , a power MOS transistor _MPW2 as a second MOS transistor for rectifying the current flowing through the secondary transformer _T2 , and an output capacitor _COUT .

When the power MOS transistor _MPW1 is turned on and the power MOS transistor _MPW2 is turned off, the input voltage _VIN is supplied to the primary transformer _T1 , current is supplied to the primary transformer _T1 , and the primary and secondary transformers _T1 , _T2 are wound around the core (not shown). Next, when the power MOS transistor _MPW1 is turned off and the power MOS transistor _MPW2 is turned on, the energy stored in the core is output from the secondary transformer _T2 . Next, the power MOS transistor _MPW1 is turned on again, the power MOS transistor _MPW2 is turned off, and this is repeated. The voltage output from the secondary transformer _T2 is smoothed by the output capacitor _COUT and output as a DC output voltage _VOUT .

Next, detailed configurations of the above-described input capacitor C _IN , primary transformer T ₁ , secondary transformer T ₂ , power MOS transistors M _PW1 and M _PW2 , and output capacitor C _OUT will be described. The input capacitor C _IN is connected between the pair of input terminals IN to stabilize the input voltage V _IN and to eliminate noise. One of the pair of input terminals IN is connected to the ground.

The primary transformer _T1 has one end _T11 connected to the ground via a MOS transistor _MPW1 and _a sense resistor _RS , which will be described later, and the other end _T12 not connected to the ground. connected to the dots. One end _T21 of the secondary transformer _T2 is connected to one of a pair of output ends OUT via a power MOS transistor _MPW2 , which will be described later, and the other end _T22 is connected to the other of the pair of output ends OUT.

The power MOS transistor _MPW1 is composed of an NMOS type field effect transistor. The power MOS transistor _MPW1 has a drain connected to one end _T11 of the primary transformer _T1 , a source connected to the ground via a sense resistor _RS described later, and a gate connected to the control circuit 3 described later. When the power MOS transistor _MPW1 is turned on, the input voltage V _IN is supplied to the primary transformer _T1 , and current flows from the other end _T12 to the one end _T11 . When the power MOS transistor _MPW1 is turned off, the input voltage _VIN supplied to the primary transformer _T1 is cut off, and the current flowing through the primary transformer _T1 is also cut off.

The power MOS transistor _MPW2 is composed of an NMOS type field effect transistor. The power MOS transistor _MPW2 has a drain connected to one end _T21 of the secondary transformer _T2 and a source connected to one of the output terminals OUT. When the power MOS transistor _MPW2 is turned on, the energy stored in the core causes current to flow through the secondary transformer _T2 from one end _T21 to the other end _T22 .

An output capacitor C _OUT is connected between the pair of output terminals OUT. More specifically, the output capacitor _COUT has one end connected to the connection point between the secondary transformer _T2 and the output terminal, and the other end connected to the connection point between the power MOS transistor _MPW2 and the output terminal OUT.

In addition, _the flyback converter 1 controls the on/off duty of the power MOS transistor _MPW1 to keep the _output voltage _{VOUT constant} . It comprises a PC ₁ , a resistor R ₃ , a shunt regulator 2 and a control circuit 3 .

The sense resistor _RS is connected between the source of the power MOS transistor _MPW1 and the ground, converts the current flowing through the primary transformer _T1 into a sense voltage Vs, and supplies the sense voltage Vs to the control circuit 3, which will be described later. The voltage dividing resistors R ₁ and R ₂ are connected in series between the other (positive side) of the output terminal OUT and the ground, and supply the detection voltage V _OUTS obtained by dividing the output voltage V _OUT to the shunt regulator 2 described later. .

The photocoupler PC ₁ is composed of a light-emitting element 41 that emits light when a current is supplied, and a light-receiving element 42 that conducts when the light-emitting element 41 emits light. The light emitting element 41, the resistor _R3 , and the shunt regulator 2, which constitute the photocoupler _PC1 , are connected in series between the other (positive side) of the output terminals and the ground. The shunt regulator 2 adjusts the current flowing through the resistor _R3 so that the detected voltage V _OUTS becomes the target voltage. The current flowing through resistor _R3 is supplied to light emitting element 41 . As a result, a current flows through the light receiving element 42 in accordance with the current flowing through the resistor _R3 . This current is supplied to the control circuit 3 as a feedback voltage VFB that is voltage-converted by a resistor (not shown). The control circuit 3 turns on and off the power MOS transistor _MPW1 based on the comparison between the sense voltage Vs and the feedback voltage VFB, thereby setting the output voltage _VOUT to the target voltage. As a specific control method of the control circuit 3 for turning on and off _the power MOS transistor _MPW1 , a PWM control method or, although not shown in _FIG . There is a control method of self-excited switching and quasi-resonant switching using the output voltage of the auxiliary winding by adding .

The flyback converter 1 also includes a resistor _RDET for controlling on/off of the power MOS transistor _MPW2 , and a synchronous rectification control circuit 5. FIG. The resistor _RDET is a resistor for converting the voltage on the one end _T21 side of the secondary transformer _T2 into a current. The resistor R _DET is connected between the connection point of the secondary transformer T ₂ and the drain of the power MOS transistor _{MPW 2} and the detection terminal DET (FIG. 2) as the input terminal of the synchronous rectification control circuit 5 .

The synchronous rectification control circuit 5 is composed of an IC, and turns on the power MOS _transistor M _PW2 after one end _T21 of the secondary transformer _T2 is switched from positive voltage to negative voltage to turn on the one end _T21 of the secondary transformer T2. This circuit turns off the power MOS transistor MPW2 at the timing when the voltage of M-- _PW2 rises and becomes close to 0V. The synchronous rectification control circuit 5 includes a clamp circuit 51, a current mirror circuit 52 as a current detection circuit, a comparison circuit 53, and a driver circuit 54, as shown in FIG.

The clamp circuit 51 is a circuit that clamps the voltage of the detection terminal DET to around 0V. The clamp circuit 51 has a bipolar transistor Q ₁ as a first bipolar transistor, a bipolar transistor Q ₂ as a second bipolar transistor, and a bias current source 511 . The bipolar transistor _Q1 has an emitter connected to the detection terminal DET and a base connected to the base of the bipolar transistor _Q2 .

The bipolar transistor _Q2 has its emitter connected to the ground (0 V) and its collector connected to the power supply Reg. , and the base and collector are connected. Power supply Reg. is a power supply that outputs a desired voltage from the power supply VDD. As described above, the bases of the bipolar transistors Q ₁ and Q ₂ are connected to each other, and the emitter-base voltages of the bipolar transistors Q ₁ and _{Q 2} are almost equal. can be clamped.

Also, when the voltage at one end _T21 of the secondary transformer _T2 becomes negative and becomes lower than the emitter of the bipolar transistor _Q1 , the current IR _DET flows from the emitter of the bipolar transistor _Q1 to the resistor _RDET . The current IR _DET has a value corresponding to the voltage at one end _T21 of the secondary transformer _T2 , and the resistor R _DET converts the voltage at one end _T21 of the secondary transformer _T2 into a current.

The current mirror circuit 52 is a circuit that receives the current IR _DET flowing from the emitter of the bipolar transistor _Q1 to the resistor _RDET , copies the input current IR _DET , and supplies the detected current to the comparison circuit 53, which will be described later. The current mirror circuit 52 has transistors M ₁ and M ₂ . The transistors M ₁ and M ₂ are composed of PMOS type field effect transistors. The transistor _M1 has a drain connected to the collector of the bipolar transistor _Q1 and a source connected to the power supply Reg. , and the gate and drain are connected in common.

The transistor _M2 has its gate and source connected to the gate and source of the transistor _M1 , respectively, and its drain is supplied to a comparison circuit 53 which will be described later. With the above configuration, the current IR _DET flows to the drain of the transistor M ₁ , the drain current of the transistor M ₁ , that is, the current IR _DET is copied to the drain current of the transistor M ₂ and supplied to the comparison circuit 53 .

The comparison circuit 53 compares the current IR _DET supplied from the current mirror circuit 52 with the first current (I ₁ +I ₂ ) or the second current I ₁ and supplies the result to the driver circuit 54 . This circuit turns on and off the power MOS transistor _MPW2 .

The comparison circuit 53 has a first current source 531, a second current source 532, a transistor _M3 as a switching element, and a comparator _COMP1 . A first current source 531 is connected between the drain of transistor _M2 , the output of current mirror circuit 52, and ground. A first current source 531 is a current source that outputs a current _I1 . A second current source 532 is connected between the drain of transistor _M2 and ground and is connected in parallel with the first current source 531 . A second current source 532 is a current source that outputs a current _I2 .

Transistor _M3 consists of an NMOS type field effect transistor. Transistor _M3 is connected in parallel to first current source 531 in series with second constant current source 532 between the drain of transistor _M2 and ground. Specifically, the connection point between the drain of the transistor _M2 and the first current source 531 is connected to the drain of the transistor _M3 , and the second current source 532 is connected to the source.

The comparator COMP ₁ has an inverting input connected to a connection point between the transistor M ₂ and the first current source 531 and a non-inverting input supplied with the reference voltage V _ref . The output of comparator COMP ₁ is also connected to the gate of transistor M ₃ and driver circuit 54 . The driver circuit 54 outputs a gate signal corresponding to the output of the comparator _COMP1 from the driver terminal DRV and supplies it to the gate of the power MOS transistor _MPW2 .

Next, the operation of the synchronous rectification control circuit 5 having the above configuration will be described with reference to FIG. First, when the power MOS transistor _MPW1 is on, one end _T21 of the secondary transformer _T2 becomes a positive voltage. At this time, the current IR _DET becomes 0, and the current IR _DET <first current (I ₁ +I ₂ ). As a result, the inverting input of the comparator _COMP1 becomes equal to or lower than the reference voltage _Vref , so that the comparator _COMP1 outputs an H level signal. When the H level signal is output from the comparator _COMP1 , the driver circuit 54 supplies the L level gate voltage to turn off the power MOS transistor _MPW2 . Also, when the H signal is output from the comparator COMP ₁ , the transistor M ₃ is turned on, the second current source 532 is connected, and the comparator COMP ₁ receives the current IR _DET and the first current (I ₁ +I ₂ ) is performed.

_{Next, when the power MOS transistor MPW1 is} turned off, as shown in the part surrounded by the dashed _line in _{FIG .} current flows towards As a result, one end _T21 of the secondary transformer _T2 drops to a negative voltage, and current IR _DET >first current ( _I1 + _I2 ). When the current IR _DET >the first current (I ₁ +I ₂ ), the inverting input of the comparator COMP ₁ becomes high and becomes equal to or higher than the reference voltage V _ref , and the comparator COMP ₁ outputs an L level signal.

When an L level signal is output from the comparator _COMP1 , a H level gate voltage is supplied from the driver circuit 54 to turn on the power MOS transistor _MPW2 . When the power MOS transistor _MPW2 is turned on, the voltage at one end _T21 of the secondary transformer _T2 rises slightly due to the voltage drop across the ON resistance of the power MOS transistor _MPW2 . In addition, when an L level signal is output from the comparator _COMP1 , the transistor _M3 is turned off, the second current source 532 is disconnected, and the comparator _COMP1 generates a current between the current IR _DET and the second current _I1. A comparison is made.

After that, the energy stored in the core is output via the power MOS transistor _MPW2 . As the energy decreases, the voltage at one end _T21 of the secondary transformer _T2 approaches zero. As a result, the current IR _DET becomes smaller, and when the current IR _DET < the second current I ₁ , the inverting input of the comparator COMP ₁ becomes low and becomes equal to or lower than the reference voltage V _ref , and the comparator COMP ₁ outputs an H level signal. be done. When the H level signal is output from the comparator _COMP1 , the driver circuit 54 supplies the L level gate voltage to turn off the power MOS transistor _MPW2 .

Even after the power MOS transistor _MPW2 is turned off, the energy stored in the core is transferred to the _secondary transformer T2 through the body diode of the power MOS transistor _MPW2 as shown in the part surrounded by the two-dot chain line in FIG. current flows from one end _T21 to the other end _T22 . As a result, the voltage at one end _T21 of the secondary transformer _T2 drops slightly. After that, when the energy stored in the core finishes being released, the voltage at one end _T21 of the secondary transformer _T2 rises. Then, the power MOS transistor _MPW1 is turned on, and one end _T21 of the secondary transformer _T2 holds a positive voltage.

According to the embodiment described above, the resistor _RDET is connected between one end _T21 of the secondary transformer _T2 and the detection terminal DET, and the emitter of the bipolar transistor _Q1 is connected to the detection terminal DET. As a result, even if one end _T21 of the secondary transformer _T2 becomes a negative voltage, the detection terminal DET is held near 0 V by the bipolar transistor _Q1 , and current only flows through the resistor _RDET . It is possible to prevent a negative surge from being applied. That is, the detection terminal DET of the IC of the synchronous rectification control circuit 5 can be protected from the negative surge of the secondary transformer _T2 .

Further, according to the above-described embodiment, the comparator circuit 53 compares the current IR _DET detected by the current mirror circuit 52 with the first current (I ₁ +I ₂ ) or the second current I ₁ to accurately Power MOS transistor _MPW2 can be turned on and off.

Further, according to the above-described embodiment, by detecting the current IR _DET using the current mirror circuit 52, the current can be detected with a simple configuration IR _DET .

Further, according to the above-described embodiment, the current mirror circuit 52 copies the current IR _DET flowing through the resistor R _DET and supplies it to the comparison circuit 53, and the comparison circuit 53 compares the currents IR _DET , I ₁ +I ₂ , I ₁ with each other. , the power MOS transistor M-- _PW2 can be on/off controlled with a simple configuration using the current IR-- _DET flowing through the resistor R-- _DET .

According to the above-described embodiment, the comparison circuit 53 makes the compared current variable by turning on and off the transistor _M3 connected in series with the second current source 532 . Thus, with a simple configuration, the first current (I ₁ +I ₂ ) and the second current I ₁ are compared when the power MOS transistor _MPW2 is on and when it is off. can vary between

It should be noted that the present invention is not limited to the above-described embodiments, and can be modified, improved, etc. as appropriate. In addition, the material, shape, size, number, location, etc. of each component in the above-described embodiment are arbitrary and not limited as long as the present invention can be achieved.

According to the above-described embodiment, the two current sources 531 and 532 are used to variably set the currents to be compared, but the present invention is not limited to this. An adjustable current source may be used to provide a variable current.

1 Flyback converter (switching power supply)
5 synchronous rectification control circuit 52 current mirror circuit (current detection circuit)
53 comparison circuit 531 first current source 532 second current source COMP ₁ comparator DET detection terminal (input terminal)
_M3 transistor (switch element)
_MPW1 power MOS transistor (first MOS transistor)
_MPW2 power MOS transistor (second MOS transistor)
_Q1 bipolar transistor (first bipolar transistor)
_Q2 bipolar transistor (second bipolar transistor)
T ₁ primary transformer T ₂ secondary transformer R _DET resistor V _IN input voltage V _ref reference voltage I ₁ +I ₂ first current I ₁ second current

Claims (4)

a primary transformer and a secondary transformer for stepping up or stepping down an input voltage;
a first MOS transistor that turns on and off the input voltage supplied to the primary transformer;
a second MOS transistor for rectifying the output of the secondary transformer;
A flyback type switching power supply device comprising a synchronous rectification control circuit that controls on/off of the second MOS transistor,
A resistor provided between one end of the secondary transformer and an input terminal of the synchronous rectification control circuit,
The synchronous rectification control circuit is
a first bipolar transistor having an emitter connected to the input terminal;
a second bipolar transistor whose bases are connected to the first bipolar transistor, whose emitter is connected to the ground, and whose base is connected to the collector;
a current detection circuit that detects current flowing from the emitter of the first bipolar transistor to the resistor;
A detection current detected by the current detection circuit is compared with a first current, and when the detection current becomes larger than the first current, the second MOS transistor is turned on, and the second MOS transistor is turned on. After the transistor is turned on, the detected current is compared with a second current smaller than the first current, and the second MOS transistor is turned off when the detected current becomes smaller than the second current. and a comparison circuit for
switching power supply. The switching power supply device according to claim 1,
The switching power supply device, wherein the current detection circuit includes a current mirror circuit to which a current flowing from the emitter of the first bipolar transistor is input. In the switching power supply device according to claim 2,
The comparison circuit is
a first current source connected to the output of the current mirror circuit;
a second current source connected to the output of the current mirror circuit and connected in parallel with the first current source;
a switch element connected in series with the second current source and connected in parallel with the first current source;
a comparator for comparing the output of the current mirror circuit and the connection point voltage of the first current source with a reference voltage;
the switch element is turned on and off according to the output of the comparator;
switching power supply. In a synchronous rectification control circuit that controls on/off of a second MOS transistor that rectifies a secondary transformer provided in a flyback type switching power supply,
an input terminal to which the other end of the resistor whose one end is connected to one end of the secondary transformer is connected;
a second bipolar transistor whose bases are connected to the first bipolar transistor and whose emitter is grounded;
a current detection circuit that detects current flowing from the emitter of the first bipolar transistor to the resistor;
A detection current detected by the current detection circuit is compared with a first current, and when the detection current becomes larger than the first current, the second MOS transistor is turned on, and the second MOS transistor is turned on. After the transistor is turned on, the detected current is compared with a second current smaller than the first current, and the second MOS transistor is turned off when the detected current becomes smaller than the second current. and a synchronous rectification control circuit.

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