Switching power supply and synchronous rectification control circuit

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

[Technical field]

[0001] The present invention relates to a switching power supply and a synchronous rectification control circuit. [Background technology]

[0002] A flyback converter using a transformer as the switching power supply is known. A flyback converter transmits energy to a secondary transformer by intermittently supplying an input voltage to a primary transformer, where the energy is rectified by a diode and smoothed by a smoothing capacitor to convert the voltage into the required DC voltage. It has also been proposed to use a power MOS transistor instead of the diode in the secondary transformer.

[0003] A synchronous rectification control circuit that controls the on / off of this power MOS transistor has been proposed, which inputs the voltage at one end of the secondary transformer, detects the input voltage, and turns on the power MOS transistor when the detected voltage becomes negative (Non-Patent Document 1).

[0004] 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. [Prior art documents] [Non-patent literature]

[0005] [Non-Patent Document 1] Maxim Integrated Products,Inc.,"Secondary-Side Synchronous MOSFET Driver for Flyback Converters",[online],2015,[Retrieved December 28, 2020],Internet<URL:https: / / datasheets.maximintegrated.com / en / ds / MAX17606.pdf> Summary of the Invention [Problem to be solved by the invention]

[0006] The present invention has been made in consideration of the above-mentioned circumstances, and an object of the present invention is to provide a switching power supply and a synchronous rectification control circuit that can protect the input terminal of an IC of a synchronous rectification control circuit from a negative surge of a secondary transformer. [Means for solving the problem]

[0007] In order to achieve the above-mentioned 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 synchronous rectification control circuit that controls the on / off of the second MOS transistor. In a flyback type switching power supply device, 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 includes: a first bipolar transistor having an emitter connected to the input terminal; a second bipolar transistor having a base connected to the first bipolar transistor, an emitter connected to ground, and a base connected to a collector; a current detection circuit that detects a current flowing from the emitter of the first bipolar transistor to the resistor; a comparison circuit which compares a detection current detected by the current detection circuit with a first current, turns on the second MOS transistor when the detection current becomes larger than the first current, and compares the detection current with a second current smaller than the first current after the second MOS transistor is turned on, and turns off the second MOS transistor when the detection current becomes smaller than the second current. It is a switching power supply. [2] In the switching power supply device according to [1], The current detection circuit has a current mirror circuit to which a current flowing from the emitter of the first bipolar transistor is input. It is a switching power supply. [3] In the switching power supply device according to [2], The comparison circuit includes: a first current source connected to an output of the current mirror circuit; a second current source connected to an output of the current mirror circuit and connected in parallel with the first current source; a switch element connected in series to the second current source and connected in parallel to the first current source; a comparator that compares a voltage at a connection point between the output of the current mirror circuit and the first current source with a reference voltage; The switch element is turned on and off in response to an output of the comparator. It is a switching power supply. [4] A synchronous rectification control circuit for controlling on / off of a second MOS transistor that rectifies a secondary transformer provided in a flyback type switching power supply device, an input terminal to which one end of a resistor having the other end connected to one end of the secondary transformer is connected; a second bipolar transistor having a base connected to the first bipolar transistor and an emitter connected to ground; a current detection circuit that detects a current flowing from the emitter of the first bipolar transistor to the resistor; a comparison circuit which compares a detection current detected by the current detection circuit with a first current, turns on the second MOS transistor when the detection current becomes larger than the first current, and compares the detection current with a second current smaller than the first current after the second MOS transistor is turned on, and turns off the second MOS transistor when the detection current becomes smaller than the second current. It is a synchronous rectification control circuit. Effect of the Invention

[0008] 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 an IC of the synchronous rectification control circuit from a negative surge of a secondary transformer.

[0009] The present invention has been briefly described above. Furthermore, the details of the present invention will be further clarified by reading the following description of the embodiment of the present invention (hereinafter, referred to as "embodiment") with reference to the accompanying drawings. [Brief description of the drawings]

[0010] [Figure 1] FIG. 1 is a circuit diagram showing an embodiment of a flyback converter as a switching power supply of the present invention. [Diagram 2] FIG. 2 is a circuit diagram showing details of the synchronous rectification control circuit shown in FIG. [Diagram 3] 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. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0011] Specific embodiments of the present invention will be described below with reference to the accompanying drawings.

[0012] The flyback converter 1 shown in FIG. 1 is a switching power supply device. It operates by converting a DC input voltage V IN The primary and secondary transformers T1 and T2 step up or step down the voltage, and the DC output voltage V is output from the output terminal OUT. OUT The output is as follows:

[0013] Flyback converter 1 is connected to the input capacitor C IN The primary transformer T1, the secondary transformer T2, and the input voltage V supplied to the primary transformer T1 IN A power MOS transistor M as a first MOS transistor that turns on and off PW1 and a power MOS transistor M as a second MOS transistor for rectifying the current flowing through the secondary transformer T2. PW2 and the output capacitor C OUT It is equipped with the following:

[0014] The above-mentioned power MOS transistor M PW1 Turn on the power MOS transistor M PW2 When the input voltage V IN is supplied to the primary transformer T1, current is supplied to the primary transformer T1, and energy is stored in a core (not shown) around which the primary and secondary transformers T1 and T2 are wound. Next, the power MOS transistor M PW1 Turn off the power MOS transistor M PW2 When the power MOS transistor M is turned on, the energy stored in the core is output from the secondary transformer T2. PW1 Turn on the power MOS transistor M PW2 This is repeated. The voltage output from the secondary transformer T2 is OUT The DC output voltage V OUT will be output as:

[0015] Next, the input capacitor C IN , primary transformer T1, secondary transformer T2, power MOS transistor M PW1 , M PW2 , output capacitor C OUTThe detailed configuration of the input capacitor C IN is the input voltage V IN It is connected between a pair of input terminals IN to stabilize the voltage and eliminate noise. One of the pair of input terminals IN is connected to ground.

[0016] The primary transformer T1 has one end T 11 is the MOS transistor M PW1 and sense resistor R S The other end is connected to ground via T 12 The input terminal IN and the input capacitor C IN The secondary transformer T2 is connected to the connection point of one end T 21 is the power MOS transistor M PW2 It is connected to one of the pair of output terminals OUT via 22 is connected to the other of the pair of output terminals OUT.

[0017] Power MOS transistor M PW1 The power MOS transistor M is composed of NMOS type field effect transistors. PW1 The drain is connected to one end T of the primary transformer T1. 11 The source is connected to the sense resistor R S The power MOS transistor M is connected to ground via a power supply, and its gate is connected to a control circuit 3, which will be described later. PW1 When the input voltage V IN is supplied, and the other end T 12 From one end T 11 Current flows toward the power MOS transistor M PW1 When the input voltage V is turned off, the input voltage V supplied to the primary transformer T1 IN is cut off, and the current flowing through the primary transformer T1 is also cut off.

[0018] Power MOS transistor M PW2 The power MOS transistor M is composed of NMOS type field effect transistors. PW2 The drain is connected to one end T of the secondary transformer T2.21 The power MOS transistor M PW2 When the power supply is turned on, the energy stored in the core of the secondary transformer T2 is 21 From the other end T 22 The current flows towards

[0019] Output capacitor C OUT is connected between a pair of output terminals OUT. In more detail, the output capacitor C OUT One end is connected to the connection point between the secondary transformer T2 and the output end, and the other end is connected to the power MOS transistor M PW2 and is connected to the connection point of the output terminal OUT.

[0020] The flyback converter 1 also includes a power MOS transistor M PW1 By controlling the on / off duty of the output voltage V OUT In order to keep S The power supply circuit includes voltage dividing resistors R1 and R2, a photocoupler PC1, a resistor R3, a shunt regulator 2, and a control circuit 3.

[0021] Sense resistor R S is the power MOS transistor M PW1 The resistors R1 and R2 are connected in series between the other (positive side) of the output terminal OUT and the ground, and the output voltage V OUT The detection voltage V OUTS is supplied to the shunt regulator 2, which will be described later.

[0022] The photocoupler PC1 is composed of a light-emitting element 41 that emits light when a current is supplied thereto, and a light-receiving element 42 that becomes conductive when the light-emitting element 41 emits light. The light-emitting element 41, resistor R3, and shunt regulator 2 that constitute the photocoupler PC1 are connected in series between the other output terminal (positive side) and ground. The shunt regulator 2 detects a detection voltage V OUTSThe current flowing through resistor R3 is adjusted so that the sense voltage Vs becomes the target voltage. The current flowing through resistor R3 is supplied to light-emitting element 41. As a result, a current corresponding to the current flowing through resistor R3 flows through light-receiving element 42. This current is converted into a feedback voltage VFB by a resistor (not shown) and supplied to control circuit 3. Based on a comparison between this sense voltage Vs and feedback voltage VFB, control circuit 3 controls the power MOS transistor M PW1 By turning on and off the output voltage V OUT The target voltage can be set to the power MOS transistor M PW1 Specific control methods for the control circuit 3 that turns on and off include a PWM control method and a self-excited switching or quasi-resonant switching control method in which an auxiliary winding (not shown in FIG. 1) is added to the primary transformer T1 and the secondary transformer T2 and the output voltage of the auxiliary winding is utilized.

[0023] The flyback converter 1 also includes a power MOS transistor M PW2 Resistor R for controlling on / off of DET and a synchronous rectification control circuit 5. DET is one end T of the secondary transformer T2. 21 This is a resistor that converts the voltage on the side into a current. Resistor R DET is the secondary transformer T2 and the power MOS transistor M PW2 and a detection terminal DET (FIG. 2) serving as an input terminal of the synchronous rectification control circuit 5.

[0024] The synchronous rectification control circuit 5 is composed of an IC, and one end T 21 After switching from a positive voltage to a negative voltage, the power MOS transistor M PW2 Turn on the secondary transformer T2. 21 When the voltage of the power MOS transistor M PW2 The synchronous rectification control circuit 5 includes a clamp circuit 51, a current mirror circuit 52 serving as a current detection circuit, a comparison circuit 53, and a driver circuit 54, as shown in FIG.

[0025] The clamp circuit 51 is a circuit that clamps the voltage of the detection terminal DET to about 0 V. The clamp circuit 51 has a bipolar transistor Q1 as a first bipolar transistor, a bipolar transistor Q2 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.

[0026] The bipolar transistor Q2 has an emitter connected to ground (0V), a collector connected to the power supply Reg. via a bias current source 511, and a base and collector connected together. The 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 Q1 and Q2 are connected together, and the emitter-base voltages of the bipolar transistors Q1 and Q2 are approximately equal, so that the emitter of the bipolar transistor Q1 can clamp the detection terminal DET to approximately 0V.

[0027] In addition, one end T of the secondary transformer T2 21 When the voltage becomes negative and becomes lower than the emitter of the bipolar transistor Q1, the resistor R DET Current IR DET A current IR flows. DET is one end T of the secondary transformer T2. 21 The value depends on the voltage of the resistor R DET is one end T of the secondary transformer T2 21 Converts the voltage into a current.

[0028] The current mirror circuit 52 is connected from the emitter of the bipolar transistor Q1 to a resistor R DET The current flowing through DET is input, and the input current IR DETand supplies the resultant current as a detection current to a comparison circuit 53, which will be described later. The current mirror circuit 52 has transistors M1 and M2. The transistors M1 and M2 are configured from PMOS type field effect transistors. The drain of the transistor M1 is connected to the collector of the bipolar transistor Q1, the source is connected to a power supply Reg., and the gate and drain are commonly connected.

[0029] The gate and source of the transistor M2 are connected to the gate and source of the transistor M1, respectively, and the drain is supplied to a comparison circuit 53, which will be described later. DET flows into the drain of the transistor M1, and the drain current of the transistor M1, i.e., the current IR DET is copied to the drain current of transistor M2 and supplied to comparison circuit 53.

[0030] The comparison circuit 53 receives the current IR supplied from the current mirror circuit 52. DET The result is supplied to the driver circuit 54 to drive the power MOS transistor M. PW2 This is a circuit that turns the above on and off.

[0031] The comparison circuit 53 has a first current source 531, a second current source 532, a transistor M3 as a switch element, and a comparator COMP1. The first current source 531 is connected between the drain of the transistor M2, which is the output of the current mirror circuit 52, and the ground. The first current source 531 is a current source that outputs a current I1. The second current source 532 is connected between the drain of the transistor M2 and the ground, and is connected in parallel to the first current source 531. The second current source 532 is a current source that outputs a current I2.

[0032] The transistor M3 is an NMOS type field effect transistor. The transistor M3 is connected between the drain of the transistor M2 and the ground, in series with the second constant current source 532, and in parallel with the first current source 531. Specifically, the drain of the transistor M3 is connected to the connection point of the drain of the transistor M2 and the first current source 531, and the source of the transistor M3 is connected to the second current source 532.

[0033] The comparator COMP1 has an inverting input connected to the connection point of the transistor M2 and the first current source 531, and a non-inverting input connected to a reference voltage V ref The output of the comparator COMP1 is connected to the gate of the transistor M3 and the driver circuit 54. The driver circuit 54 outputs a gate signal according to the output of the comparator COMP1 from the driver terminal DRV, and drives the power MOS transistor M PW2 The gate is supplied with

[0034] Next, the operation of the synchronous rectification control circuit 5 having the above-mentioned configuration will be described with reference to FIG. PW1 When the secondary transformer T2 is turned on, 21 becomes a positive voltage. At this time, the current IR DET becomes 0, and the current IR DET <First current (I1 + I2). Therefore, the inverting input of the comparator COMP1 is equal to the reference voltage V ref When the comparator COMP1 outputs a high-level signal, the driver circuit 54 supplies a low-level gate voltage to the power MOS transistor M PW2 When the comparator COMP1 outputs a high signal, the transistor M3 turns on, the second current source 532 is connected, and the current IR DET A comparison is made between the first current (I1+I2) and the first current (I1+I2).

[0035] Next, the power MOS transistor M PW1When the power MOS transistor M PW2 Through the body diode of the secondary transformer T2, 21 From the other end T 22 This causes a current to flow from one end T 21 becomes negative, and the current IR DET >First current (I1+I2). Current IR DET When the first current (I1 + I2) is reached, the inverting input of comparator COMP1 becomes high and the reference voltage V ref As a result, the comparator COMP1 outputs an L-level signal.

[0036] When the comparator COMP1 outputs an L-level signal, the driver circuit 54 supplies an H-level gate voltage to the power MOS transistor M PW2 is turned on. Power MOS transistor M PW2 When the power MOS transistor M PW2 The voltage drop across the on-resistance of the secondary transformer T2 21 When the comparator COMP1 outputs an L-level signal, the transistor M3 turns off, the second current source 532 is disconnected, and the current IR DET A comparison is made with the second current I1.

[0037] Then, the power MOS transistor M PW2 As the energy decreases, the secondary transformer T2 outputs the energy stored in the core via the 21 The voltage at approaches 0. This causes the current IR DET becomes smaller, and the current IR DET When the second current I1 occurs, the inverting input of the comparator COMP1 goes low and the reference voltage V ref When the comparator COMP1 outputs a high-level signal, the driver circuit 54 supplies a low-level gate voltage to the power MOS transistor M PW2will be turned off.

[0038] Power MOS transistor M PW2 Even after the power MOS transistor M is turned off, the energy stored in the core is PW2 Through the body diode of the secondary transformer T2, 21 From the other end T 22 Therefore, the current flows from one end T of the secondary transformer T2 to 21 The voltage at one end T of the secondary transformer T2 drops slightly. After that, when the energy stored in the core is released, 21 The voltage of the power MOS transistor M PW1 is turned on, and one end T of the secondary transformer T2 21 maintains a positive voltage.

[0039] According to the above-described embodiment, one end T of the secondary transformer T2 21 and the detection terminal DET. DET The emitter of the bipolar transistor Q1 is connected to the detection terminal DET. 21 Even if the voltage at the detection terminal DET becomes negative, the bipolar transistor Q1 keeps the detection terminal DET near 0 V, and the resistor R DET Therefore, it is possible to prevent a negative surge from being applied to the detection terminal DET. That is, it is possible to protect the detection terminal DET of the IC of the synchronous rectification control circuit 5 from a negative surge in the secondary transformer T2.

[0040] According to the embodiment described above, the comparison circuit 53 detects the current IR detected by the current mirror circuit 52. DET By comparing the first current (I1+I2) or the second current I1, the power MOS transistor M PW2 can be turned on and off.

[0041] According to the above-described embodiment, the current mirror circuit 52 is used to generate the current I DET By detecting the IR, it is easy to configureDET The current can be detected by

[0042] Furthermore, according to the above-described embodiment, the resistor R DET The current flowing through DET is copied by the current mirror circuit 52 and supplied to the comparison circuit 53, and the comparison circuit 53 outputs a current IR DET , I1+I2, I1 are compared with each other, and the resistance R DET The current flowing through DET By using this, a power MOS transistor M PW2 can be controlled to be on or off.

[0043] According to the embodiment described above, the comparison circuit 53 varies the current to be compared by turning on and off the transistor M3 connected in series to the second current source 532. This allows the power MOS transistor M PW2 The current to be compared when the transistor is turned on and when the transistor is turned off can be changed between the first current (I1+I2) and the second current I1.

[0044] The present invention is not limited to the above-described embodiment, and can be appropriately modified, improved, etc. In addition, the material, shape, size, number, arrangement location, etc. of each component in the above-described embodiment are arbitrary as long as the present invention can be achieved, and are not limited.

[0045] According to the embodiment described above, the current to be compared is variably set using the two current sources 531 and 532, but the present invention is not limited to this. The current may be variably set using a current source that can adjust the current. [Explanation of symbols]

[0046] 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 COMP1 comparator DET Detection terminal (input terminal) M3 transistor (switch element) M PW1 Power MOS transistor (first MOS transistor) M PW2 Power MOS transistor (second MOS transistor) Q1 Bipolar Transistor (First Bipolar Transistor) Q2 bipolar transistor (second bipolar transistor) T1 transformer primary T2 secondary transformer R DET resistance V IN Input voltage V ref Reference Voltage I1+I2 First current I1 Second current

Claims

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 synchronous rectification control circuit that controls the on / off of the second MOS transistor. In a flyback type switching power supply device, 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 includes: a first bipolar transistor having an emitter connected to the input terminal; a second bipolar transistor having a base connected to the first bipolar transistor, an emitter connected to ground, and a base connected to a collector; a current detection circuit that detects a current flowing from the emitter of the first bipolar transistor to the resistor; a comparison circuit which compares a detection current detected by the current detection circuit with a first current, turns on the second MOS transistor when the detection current becomes larger than the first current, and compares the detection current with a second current smaller than the first current after the second MOS transistor is turned on, and turns off the second MOS transistor when the detection current becomes smaller than the second current. Switching power supply.

2. 2. The switching power supply device according to claim 1, The current detection circuit has a current mirror circuit to which a current flowing from the emitter of the first bipolar transistor is input. Switching power supply.

3. 3. The switching power supply device according to claim 2, The comparison circuit includes: a first current source connected to an output of the current mirror circuit; a second current source connected to an output of the current mirror circuit and connected in parallel with the first current source; a switch element connected in series to the second current source and connected in parallel to the first current source; a comparator that compares a voltage at a connection point between the output of the current mirror circuit and the first current source with a reference voltage; The switch element is turned on and off in response to an output of the comparator. Switching power supply.

4. A synchronous rectification control circuit for controlling on / off of a second MOS transistor that rectifies a secondary transformer provided in a flyback type switching power supply device, an input terminal to which one end of a resistor having the other end connected to one end of the secondary transformer is connected; a second bipolar transistor having a base connected to the first bipolar transistor and an emitter connected to ground; a current detection circuit that detects a current flowing from the emitter of the first bipolar transistor to the resistor; a comparison circuit which compares a detection current detected by the current detection circuit with a first current, turns on the second MOS transistor when the detection current becomes larger than the first current, and compares the detection current with a second current smaller than the first current after the second MOS transistor is turned on, and turns off the second MOS transistor when the detection current becomes smaller than the second current. Synchronous rectification control circuit.

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