A synchronous rectification slow start circuit

Abstract

The present invention discloses a synchronous rectification slow start circuit, including a comparison circuit, a trigger circuit and a drive conversion circuit, wherein the comparison circuit input is connected to a drive signal, the comparison circuit output is connected to the trigger circuit input, the trigger circuit output is connected to the drive conversion circuit input, the drive conversion circuit output outputs a slowly unfolded drive pulse wave, the trigger circuit includes a trigger U2, the trigger U2 input IN is connected to the comparison circuit output, the trigger U2 SET pin is connected to a ground resistor R3, the trigger U2 DIV pin is connected to the power supply terminal through a voltage divider circuit composed of a resistor R4 and a resistor R5, and the trigger U2 output OUT is connected to the drive conversion circuit input through a coupling circuit composed of a resistor R6 and a capacitor C1 in parallel. The present invention can effectively prevent the current backflow caused by the high duty cycle of the conventional synchronous rectification signal, and realize the synchronous rectification soft start.

Description

Technical Field

[0001] The invention relates to the technical field of synchronous rectification, and in particular to a synchronous rectification slow start circuit. Background Art

[0002] With the development trend of modern electronic technology towards high speed and high frequency, the development trend of power modules is bound to be towards lower voltage and higher current. The switching loss and conduction voltage drop loss of the power rectifier have become important factors in the power loss of the power supply. In order to improve efficiency and reduce losses, the use of synchronous rectification technology has become an inevitable means for low-voltage and high-current power modules. Synchronous rectification circuits are widely used and can significantly improve the conversion efficiency of power modules. When the synchronous rectification circuit is lightly loaded or unloaded, the inductor current is discontinuous. At this time, if the synchronous rectification is still turned on, the inductor current will be forced to connect, and reverse current will be generated in the secondary. This will generate excess circulating current in the primary and secondary of the transformer, increase no-load and light-load losses, and may also generate very high voltage spikes in the secondary when connected in parallel, increasing the probability of damage to secondary components.

[0003] In a circuit that uses synchronous rectification on the secondary side of the power supply output, when the output voltage has residual voltage and the power is turned on again, if there is no Oring circuit or the Oring circuit does not react in time, a relatively large backflow current will be generated, causing damage to the primary MOS or the secondary MOS. Summary of the invention

[0004] In order to solve the above problems, the present invention provides a synchronous rectification slow start circuit, including a comparison circuit, a trigger circuit and a drive conversion circuit, wherein the input end of the comparison circuit is connected to the drive signal, the output end of the comparison circuit is connected to the input end of the trigger circuit, the output end of the trigger circuit is connected to the input end of the drive conversion circuit, and the output end of the drive conversion circuit outputs a slowly unfolding drive pulse wave, the trigger circuit includes a trigger U2, the input end IN of the trigger U2 is connected to the output end of the comparison circuit, the SET pin of the trigger U2 is connected to a ground resistor R3, the DIV pin of the trigger U2 is connected to the power supply end through a voltage divider circuit composed of a resistor R4 and a resistor R5, and the output end OUT of the trigger U2 is connected to the input end of the drive conversion circuit through a coupling circuit composed of a resistor R6 and a capacitor C1 in parallel.

[0005] Preferably, the trigger circuit also includes a voltage-dividing resistor R7 and a diode D1 for shaping the output signal of the trigger circuit, wherein one end of the resistor R7 is connected to the output end of the trigger circuit and the other end is grounded, and one end of the diode D1 is connected to the output end of the trigger circuit and the other end is grounded.

[0006] Preferably, the comparison circuit comprises a comparator U1, a positive input pin of the comparator U1 is connected to a power supply via a voltage divider circuit formed by a resistor R1 and a resistor R2, and a negative input pin of the comparator U1 is connected to a driving signal.

[0007] Preferably, the drive conversion circuit includes a comparator U3 and a comparator U4, the positive input pin of the comparator U3 is connected to the secondary synchronization signal SA1 through a coupling circuit composed of a resistor R8, a resistor R9, a capacitor C2 and a diode D2, the positive power pin of the comparator U3 is connected to the positive pole of the power supply, the negative power pin of the comparator U3 is grounded, and the negative input pin of the comparator U3 is connected to the output end of the trigger circuit; the positive input pin of the comparator U4 is connected to the secondary synchronization signal SB1 through a coupling circuit composed of a resistor R10, a resistor R11, a capacitor C3 and a diode D3, the positive power pin of the comparator U4 is connected to the positive pole of the power supply, the negative power pin of the comparator U4 is grounded, and the negative input pin of the comparator U4 is connected to the output end of the trigger circuit.

[0008] Preferably, the trigger U2 is a monostable trigger.

[0009] The beneficial effect of the present invention is that it can effectively prevent the current backflow caused by the high duty cycle of the conventional synchronous rectification signal, thereby realizing the synchronous rectification soft start. BRIEF DESCRIPTION OF THE DRAWINGS

[0010] Figure 1 This is a principle block diagram of the present invention;

[0011] Figure 2 is a circuit diagram of the present invention;

[0012] Figure 3 is the initial SA / SB driving signal;

[0013] Figure 4 is the reference signal V2 after trigger shaping;

[0014] Figure 5 It is the signal of SA / SB after filtering and shaping;

[0015] Figure 6 This is the final output synchronous rectification startup waveform. DETAILED DESCRIPTION

[0016] See also Figure 1-2The present invention proposes an embodiment, a synchronous rectification slow start circuit, comprising a comparison circuit, a trigger circuit and a drive conversion circuit, wherein the input end of the comparison circuit is connected to a drive signal, the output end of the comparison circuit is connected to the input end of the trigger circuit, the output end of the trigger circuit is connected to the input end of the drive conversion circuit, and the output end of the drive conversion circuit outputs a slowly unfolded drive pulse wave, i.e., it has a slow start function, the trigger circuit comprises a monostable trigger U2, the input end of the monostable trigger U2 is connected to the output end of the comparison circuit, the SET pin of the monostable trigger U2 is connected to one end of a resistor R3, and the resistor The other end of R3 is grounded, and the DIV pin of the monostable trigger U2 is respectively connected to the resistor R4 and the resistor R5, the other end of the resistor R4 is grounded, the other end of the resistor R5 is connected to the power input end of the monostable trigger U2, the power input end of the monostable trigger U2 is connected to the positive pole of the power supply +5V, the output end of the monostable trigger U2 is connected to one end of the resistor R6 and one end of the capacitor C1, the output end of the monostable trigger U2 outputs a high voltage V1, the resistor R6 and the capacitor C1 are arranged in parallel, the other end of the resistor R6 and the other end of the capacitor C1 are connected to the output end of the trigger circuit, and the GND pin of the monostable trigger U2 is grounded.

[0017] Specifically, the trigger circuit also includes a resistor R7 and a diode D1. One end of the resistor R7 is connected to the output end of the trigger circuit, and the other end is grounded. One end of the diode D1 is connected to the output end of the trigger circuit, and the other end is grounded. The diode D1 is used to control the signal output by the trigger circuit to the drive conversion circuit to be a positive pulse signal. It can be understood that the resistor R6 and the resistor R7 jointly determine the amplitude of the output signal of the trigger circuit.

[0018] Specifically, the comparison circuit includes a comparator U1, wherein the positive input pin of the comparator U1 is connected to a resistor R1 and a resistor R2 respectively, the other end of the resistor R1 is connected to a power supply, and the other end of the resistor R2 is grounded. It can be understood that the resistors R1 and R2 are voltage-dividing resistors of the comparator U1, and a reference value is set for the comparator U1 through the voltage-dividing resistor; the positive power pin of the comparator U1 is connected to the positive electrode of the power supply +5V, the negative power pin of the comparator U1 is grounded, and the negative input pin of the comparator U1 is connected to the secondary side drive signal SA or SB, and the secondary side drive signal SA or SB is a secondary side drive signal output by a digital single-chip microcomputer or an analog PWM chip. The initial signal can be referred to. Figure 3 .

[0019] Specifically, the drive conversion circuit includes a comparator U3 and a comparator U4, wherein the positive phase input pin of the comparator U3 is respectively connected to a resistor R8 and a resistor R9, the other end of the resistor R8 is connected to a secondary side synchronization signal SA1, the other end of the resistor R9 is respectively connected to a diode D2 and a capacitor C2, the other end of the diode D2 is connected to the secondary side synchronization signal SA1, the other end of the capacitor C2 is grounded, the positive side power supply pin of the comparator U3 is connected to a positive electrode of a power supply +5V, the negative side power supply pin of the comparator U3 is grounded, and the negative phase input pin of the comparator U3 is connected to an output end of a trigger circuit; the positive phase input pin of the comparator U4 is respectively connected to a resistor R10 and a resistor R11, the other end of the resistor R10 is connected to a secondary side synchronization signal SB1, the other end of the resistor R11 is respectively connected to a diode D3 and a capacitor C3, the other end of the diode D3 is connected to the secondary side synchronization signal SB1, the other end of the capacitor C3 is grounded, the positive side power supply pin of the comparator U4 is connected to a positive electrode of a power supply +5V, the negative side power supply pin of the comparator U4 is grounded, and the negative phase input pin of the comparator U4 is connected to an output end of a trigger circuit.

[0020] When either the secondary drive signal SA or SB appears, the comparator U1 outputs a high level pulse to trigger the monostable trigger U2, and the monostable trigger U2 outputs a high level V1 with a relatively long duration. The resistors R3, R4 and R5 jointly determine the duration of the high level. The high level voltage is shaped after passing through the resistor R6, the capacitor C1, the resistor R7 and the diode D1 to become a waveform V2 with a step linear decline, which can be referred to Figure 4 When the driving signal SA or SB exists, V1 always maintains a high level output. When the monostable trigger U2 outputs a high level, V2=V1, and then V2 is slowly discharged through R7 to V2=V1*R7 / (R6+R7).

[0021] The shaped signal is sent to the negative phase input pin of the comparator U3 and the alarm U4 as a reference signal for slow start. The secondary synchronization signal SA1 is filtered and shaped by the coupling circuit composed of resistor R8, resistor R9, diode D2 and capacitor C2, and then sent to the positive phase input pin of the comparator U3. The secondary synchronization signal SB1 is filtered and shaped by the coupling circuit composed of resistor R10, resistor R11, capacitor C3 and diode D3, and then sent to the positive phase input pin of the comparator U4. The signals after filtering and shaping of the secondary synchronization signal SA1 or the secondary synchronization signal SB1 can be referred to. Figure 5 The slow start reference signal is compared with the secondary side synchronization signal SA1 and the secondary side synchronization signal SB1 after filtering and shaping, and finally a slowly unfolded driving pulse wave is output, that is, with a slow start function, which can be referred to Figure 6 , effectively preventing the current backflow caused by the high duty cycle of the conventional synchronous rectification signal, and realizing the synchronous rectification soft start.

[0022] Those skilled in the art should understand that the present invention is not limited to the above-mentioned embodiments, and the above-mentioned embodiments and descriptions are only for explaining the principles of the present invention. Without departing from the spirit and scope of the present invention, modifications and changes made by those skilled in the art shall be within the scope of protection of the claims attached to the present invention without departing from the spirit and scope of the present invention.

Claims

1. A synchronous rectification slow start circuit, characterized in that: It includes a comparison circuit, a trigger circuit and a drive conversion circuit, wherein the input end of the comparison circuit is connected to a drive signal, the output end of the comparison circuit is connected to the input end of the trigger circuit, the output end of the trigger circuit is connected to the input end of the drive conversion circuit, and the output end of the drive conversion circuit outputs a slowly unfolding drive pulse wave, the trigger circuit includes a trigger U2, the input end IN of the trigger U2 is connected to the output end of the comparison circuit, the SET pin of the trigger U2 is connected to a ground resistor R3, the DIV pin of the trigger U2 is connected to a power supply end through a voltage divider circuit composed of a resistor R4 and a resistor R5, and the output end OUT of the trigger U2 is connected to the input end of the drive conversion circuit through a coupling circuit composed of a resistor R6 and a capacitor C1 in parallel; The trigger circuit also includes a voltage-dividing resistor R7 and a diode D1 for shaping the output signal of the trigger circuit, wherein one end of the resistor R7 is connected to the output end of the trigger circuit and the other end is grounded, and one end of the diode D1 is connected to the output end of the trigger circuit and the other end is grounded; The drive conversion circuit includes a comparator U3 and a comparator U4, wherein the positive phase input pin of the comparator U3 is connected to the secondary side synchronization signal SA1 through a coupling circuit composed of a resistor R8, a resistor R9, a capacitor C2 and a diode D2, the positive side power supply pin of the comparator U3 is connected to the positive pole of the power supply, the negative side power supply pin of the comparator U3 is grounded, and the negative phase input pin of the comparator U3 is connected to the output end of the trigger circuit; the positive phase input pin of the comparator U4 is connected to the secondary side synchronization signal SB1 through a coupling circuit composed of a resistor R10, a resistor R11, a capacitor C3 and a diode D3, the positive side power supply pin of the comparator U4 is connected to the positive pole of the power supply, the negative side power supply pin of the comparator U4 is grounded, and the negative phase input pin of the comparator U4 is connected to the output end of the trigger circuit.

2. A synchronous rectification slow start circuit as claimed in claim 1, characterized in that: The comparison circuit includes a comparator U1 , a positive input pin of the comparator U1 is connected to a power supply via a voltage divider circuit formed by a resistor R1 and a resistor R2 , and a negative input pin of the comparator U1 is connected to a driving signal.

3. A synchronous rectification slow start circuit as claimed in claim 1, characterized in that: The trigger U2 is a monostable trigger.

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