A synchronous rectification circuit for a switching power supply

Abstract

The present invention discloses a novel synchronous rectification circuit for a switching power supply, including a synchronous rectification chip U1 connected between the secondary coil of a primary transformer T4 of the switching power supply and a secondary power output module. The synchronous rectification chip U1 includes a logic control module, a switch tube Q4 and a wake-up module electrically connected to the logic control module, and a power module for supplying power to the synchronous rectification chip U1. The present invention directly generates a jitter signal through the switch tube Q4, eliminating the need for a dedicated high-resistance power tube to achieve this function. This reduces the number of resistor components in the peripheral circuit, thereby reducing chip layout area and lowering costs. The circuit has a rational design, a simple principle, ease of packaging, low power rectification losses, and strong practicality.

Description

Technical Field

[0001] The present invention relates to the technical field of switching power supplies, and in particular to a novel synchronous rectification circuit of a switching power supply. Background Art

[0002] In a flyback switching power supply, there are two main secondary rectification methods. The first is to use a rectifier diode for rectification, but due to the high conduction voltage drop of the rectifier diode, the power loss is more serious. The second is to use a low on-resistance MOS tube instead of a diode for rectification, which can significantly improve the power conversion efficiency, reduce the power output rectification loss, and there is no diode dead zone voltage.

[0003] Reference Figure 1 As shown in the figure, it is a conventional circuit that uses MOS tubes to rectify the secondary power supply. In the circuit, U1 is a synchronous rectification chip integrating MOS tubes. When the drain terminal of chip U1 changes from a high level to a negative voltage, that is, when the turn-on threshold of chip U1 is reached, the Logic module (logic processing module) will output a high level to turn on MOS tube Q2. At this time, the circuit charges the load resistor RL3 and capacitor C3 through the secondary coil of the transformer. After MOS tube Q2 is turned on, the drain terminal voltage of chip U1 gradually rises. When the predetermined time is reached, the pre-shutdown function of chip U1 starts to work. At this time, the gate voltage of MOS tube Q2 will decrease. When the drain terminal voltage of chip U1 rises to the turn-off threshold of chip U1, the gate of MOS tube Q2 will be pulled low. Then the MOS tube Q2 is turned off. In summary, the secondary power supply can be rectified by controlling the switching state of the MOS tube Q2. In addition, when the chip U1 detects that the output voltage Vout has dropped to a certain threshold (preset), the chip U1 will generate a fixed-frequency pulse (jitter signal or wake-up signal) by controlling the MOS tube Q3, which is sensed into the primary coil through the secondary coil of the transformer to control the primary power tube to turn on, thereby recharging the secondary power supply (load resistor RL3 and capacitor C3). The resistance of the MOS tube Q3 is relatively large. The purpose of using the MOS tube Q3 is to reduce the energy consumed by the output capacitor C3 when generating the jitter signal, thereby preventing the output voltage from being reduced to a lower value. At the same time, the chip U1 adopts a self-powered mode, and the internal LDO module can generate a 5V power supply to supply the internal circuit.

[0004] The above synchronous rectification circuit has the following shortcomings:

[0005] 1) To generate a wake-up signal, chip U1 uses an additional high-voltage power MOS transistor Q3 and a current-limiting resistor R2, which increases the chip U1 area and cost. In addition, chip U1 needs to control two power transistors (MOS transistor Q2 and MOS transistor Q3) separately, making the control method more complicated.

[0006] 2) In order to use the function of MOS tube Q3, chip U1 needs to bind and package an additional det pin, which increases the binding and packaging cost of chip U1;

[0007] 3) For customers using this synchronous rectifier chip U1, an external current-limiting resistor R1 is also required, which increases the customer's component costs and the PCB layout area. Summary of the Invention

[0008] The purpose of the present invention is to provide a novel synchronous rectification circuit for a switching power supply. The circuit directly generates a jitter signal through the switch tube Q4, thereby eliminating the need to design a high-resistance power tube to implement this function. The circuit can reduce the number of resistance components in the peripheral circuit, thereby reducing the chip layout area and reducing costs. The circuit has a reasonable design, a simple principle, is easy to package, has low power supply rectification loss, and is highly practical.

[0009] In order to achieve the above objectives, the following technical solutions are adopted:

[0010] A novel synchronous rectification circuit for a switching power supply includes a synchronous rectification chip U1 connected between the secondary coil of the primary transformer T4 of the switching power supply and a secondary power output module. The synchronous rectification chip U1 includes a logic control module, a switch tube Q4 and a wake-up module electrically connected to the logic control module, and a power module for powering the synchronous rectification chip U1; the logic control module is used to control the switching state of the switch tube Q4 to achieve rectification of the secondary power output module, and the wake-up module is used to detect the output voltage of the secondary power output module, and when the voltage is lower than a preset threshold, the logic control module controls the switch tube Q4 to generate a jitter signal and transmit it to the primary coil of the primary transformer T4 to control the primary power tube of the switching power supply to turn on and thereby charge the secondary power output module.

[0011] Furthermore, the synchronous rectification chip U1 further includes an oscillation circuit module connected between the wake-up module and the logic control module, and a gate drive module connected between the logic control module and the gate of the switch tube Q4.

[0012] Furthermore, the drain of the switch tube Q4 is connected to the secondary same-name terminal of the secondary coil of the primary transformer T4 via the drain pin of the synchronous rectifier chip U1, and the source of the switch tube Q4 is grounded.

[0013] Furthermore, the secondary power output module includes an energy storage capacitor C5 and a load resistor RL4; one end of the energy storage capacitor C5 is connected to the secondary opposite-name end of the secondary coil of the primary transformer T4, and the other end of the energy storage capacitor C5 is connected to the source of the switch tube Q4; the load resistor RL4 is connected in parallel with the energy storage capacitor C5 and grounded.

[0014] Furthermore, the novel synchronous rectification circuit of the switching power supply further includes an energy storage capacitor C6, and the power module is grounded via the energy storage capacitor C6.

[0015] Furthermore, the novel synchronous rectification circuit of the switching power supply also includes a voltage divider resistor R3 and a voltage divider resistor R4 connected in series; the secondary like-name ends of the secondary coil of the primary transformer T4 are grounded in sequence through the voltage divider resistors R3 and R4, and the wake-up module is connected to the common connection end of the voltage divider resistors R3 and R4 through the vref pin of the synchronous rectification chip U1.

[0016] By adopting the above scheme, the beneficial effects of the present invention are:

[0017] The jitter signal is directly generated by the switch tube Q4, so there is no need to design a high-resistance power tube to implement this function. The number of resistance components in the peripheral circuit can be reduced, thereby reducing the chip layout area and reducing costs. The design is reasonable, the principle is simple, the packaging is convenient, the power rectification loss is low, and the practicability is strong. BRIEF DESCRIPTION OF THE DRAWINGS

[0018] Figure 1 It is a synchronous rectification circuit diagram of the prior art;

[0019] Figure 2 A synchronous rectification circuit diagram of the present invention;

[0020] Figure 3 This is a principle block diagram of the synchronous rectification chip of the present invention;

[0021] Figure 4 Figure 1 shows various waveform changes generated by various modules of the circuit in one embodiment of the present invention (the waveform changes after the primary power tube is turned on are not shown);

[0022] Figure 5 This is a diagram showing various waveform changes generated by various modules of the circuit in one embodiment of the present invention (showing the waveform changes after the primary power tube is turned on);

[0023] Figure 6 FIG. 1 is a simplified circuit diagram of one embodiment of the present invention.

[0024] The accompanying drawings illustrate:

[0025] 1—Synchronous rectifier chip; 2—Secondary power output module;

[0026] 11—Logic control module; 12—Wake-up module;

[0027] 13—power supply module; 14—oscillation circuit module;

[0028] 15—Gate drive module. DETAILED DESCRIPTION

[0029] The present invention is described in detail below with reference to the accompanying drawings and specific embodiments.

[0030] Reference Figures 2 to 6 As shown, the present invention provides a novel synchronous rectification circuit for a switching power supply, including a synchronous rectification chip U1 connected between the secondary coil of the primary transformer T4 of the switching power supply and the secondary power output module 2, the synchronous rectification chip U1 including a logic control module 11, a switch tube Q4 and a wake-up module 12 electrically connected to the logic control module 11, and a power module 13 for powering the synchronous rectification chip U1; the logic control module 11 is used to control the switching state of the switch tube Q4 to achieve rectification of the secondary power output module 2, and the wake-up module 12 is used to detect the output voltage of the secondary power output module 2, and when the voltage is lower than a preset threshold, the logic control module 11 controls the switch tube Q4 to generate a jitter signal and transmit it to the primary coil of the primary transformer T4 to control the primary power tube of the switching power supply to turn on and thereby charge the secondary power output module 2.

[0031] Among them, the synchronous rectifier chip U1 also includes an oscillation circuit module 14 connected between the wake-up module 12 and the logic control module 11, and a gate drive module 15 connected between the logic control module 11 and the gate of the switch tube Q4; the drain of the switch tube Q4 is connected to the secondary same-name end of the secondary coil of the primary transformer T4 through the drain pin of the synchronous rectifier chip U1, and the source of the switch tube Q4 is grounded; the secondary power output module 2 includes an energy storage capacitor C5 and a load resistor RL4; one end of the energy storage capacitor C5 is connected to the secondary opposite-name end of the secondary coil of the primary transformer T4, and the energy storage The other end of the capacitor C5 is connected to the source of the switching tube Q4; the load resistor RL4 is connected in parallel with the energy storage capacitor C5 and is grounded; the new synchronous rectification circuit of the switching power supply also includes an energy storage capacitor C6, and the power module 13 is grounded through the energy storage capacitor C6; the new synchronous rectification circuit of the switching power supply also includes a voltage divider resistor R3 and a voltage divider resistor R4 connected in series; the secondary like-name ends of the secondary coil of the primary transformer T4 are grounded in sequence through the voltage divider resistor R3 and the voltage divider resistor R4, and the wake-up module 12 is connected to the common connection end of the voltage divider resistor R3 and the voltage divider resistor R4 through the vref pin of the synchronous rectification chip U1.

[0032] Working principle of the present invention:

[0033] Continue to refer to Figure 2-6As shown, when working, the primary signal of the switching power supply is transmitted to its secondary coil through the primary coil of the primary transformer T4, and the synchronous rectifier chip U1 directly controls the switching state of the switch tube Q4 to achieve rectification of the secondary power output module 2; specifically, when the primary power tube of the switching power supply is turned off, the current flows through the body diode of the switch tube Q4, and the drain end (drain pin) of the synchronous rectifier chip U1 detects a negative voltage. When this negative voltage reaches the turn-on threshold set inside the synchronous rectifier chip U1, the gate of the switch tube Q4 is pulled high, and the switch tube Q4 is turned on; as the d The voltage at the rain end gradually increases. When it reaches the shutdown threshold of the synchronous rectifier chip U1, the gate of the switch tube Q4 will be pulled low, and the switch tube Q4 will be turned off. In summary, by controlling the switching state of the switch tube Q4, the rectification of the secondary power output module 2 can be achieved. When the wake-up module 12 detects that the output voltage of the secondary power output module 2 is lower than the preset threshold (preset voltage), a pulse with a fixed frequency will be generated, thereby controlling the switch tube Q4 to generate a jitter signal, which is transmitted to its primary coil through the secondary coil of the primary transformer T4, thereby controlling the primary power tube to turn on to charge the secondary power output module 2.

[0034] Specifically, the synchronous rectifier chip U1 mainly includes a logic control module 11, a switch tube Q4, a wake-up module 12, a power module 13, an oscillation circuit module 14 and a gate drive module 15; among them, the power module 13 is connected to a filter capacitor C6 via the SVCC pin of the synchronous rectifier chip U1, which mainly supplies power to the chip, and if it detects that the synchronous rectifier chip U1 has not reached the starting voltage or is undervoltage during operation, it will generate an undervoltage lockout signal; the built-in oscillation circuit module 14 does not require any peripheral circuit settings, and under certain conditions (when the wake-up module 12 detects that the output voltage of the secondary power output module 2 is lower than a preset threshold), it will generate a fixed frequency signal, which is controlled by the logic control module 11 to generate a jitter signal for the switch tube Q4 and transmitted to the primary coil of the primary transformer T4; the logic control module 11 is used to process the signals generated by each module inside the synchronous rectifier chip U1, and generate a signal to control the opening or closing of the switch tube Q4, and transmit it to the gate drive module 15. The gate drive module 15 is used to amplify the signal and thereby control the opening or closing of the switch tube Q4.

[0035] Specifically, refer to Figure 4-5 As shown ( Figure 4 The waveform changes after the primary power tube is turned on are not shown. Figure 4 The waveform on the right side of the middle dotted line is not processed. The waveform on the right side of the dotted line is as follows Figure 5As shown), when the wake-up module 12 detects that the output voltage of the secondary power output module 2 is lower than the preset threshold and the gate end of the synchronous rectifier chip U1 has no output, it will control the oscillation circuit module 14 to generate a signal with a fixed frequency and pulse width (as shown). Figure 4 WK waveform), the synchronous rectifier power switch Q4 is turned on (such as Figure 4 driver waveform), and then generate a jitter signal (such as Figure 4 drain waveform), is transferred to the primary coil of the primary transformer T4 through the secondary coil (e.g. Figure 4 When the primary side detects the jitter signal, it turns on the primary side power tube (such as Figure 4 Primary MOS waveform in the figure), thereby charging the secondary power output module 2.

[0036] Continue to refer to Figure 5 As shown, when the primary power tube is turned on (such as Figure 5 The signal at the drain end of the waveform will change accordingly (e.g. Figure 5 Drain waveform), when the primary power tube (PrimaryMOS) is turned off, the body diode of the switch tube Q4 is turned on. At this time, the synchronous rectifier chip U1 detects the negative voltage at the drain end and turns on the switch tube Q4 (as shown in the figure). Figure 5 (driver waveform in the figure).

[0037] like Figure 1 As shown, in the synchronous rectification circuit of the switching power supply in the prior art, another MOS tube Q3 is used to generate a jitter signal in order to prevent the MOS tube Q2 from being directly turned on, which will absorb too much energy from the energy storage capacitor C3, making the output voltage at the Vout terminal lower. Therefore, a MOS tube Q3 is added, and a resistor R2 is connected in series, and connected to the drain terminal through the det pin of the chip to reduce the energy loss on the energy storage capacitor C3 when the jitter signal is generated; in the present invention, through rigorous theoretical calculation and test verification, the same MOS tube, namely the switch tube Q4, is successfully used to generate a jitter signal, thereby eliminating the need for additional power tubes and achieving the purpose of simplifying the circuit. The theoretical calculation process is as follows, and the simplified circuit is as follows Figure 6 As shown in the figure, the switch tube Q4 is simplified to a resistor R according to its on-resistance, the secondary coil of the primary transformer T4 is simplified to an inductor L, and the energy storage capacitor C5 of its secondary power output module 2 is simplified to C. The current consumed by its resistor R is:

[0038]

[0039] The energy consumed by the resistor R during time t1 is:

[0040]

[0041] From the above formula, it can be seen that the energy lost in a fixed time is not only related to the internal resistance of the switch tube Q4, but also to the coil parameters of the primary transformer T4. In this design, the internal resistance of the switch tube Q4 and the coil parameters of the primary transformer T4 are fixed. To reduce energy loss, the minimum effective t1 time is used. The turn-on time of the switch tube Q4 used in this design is 800ns, and the turn-on period is 60μs. By controlling the conduction time of the switch tube Q4 to reduce the loss, the circuit is simpler, the number of components used is reduced, and the cost is reduced.

[0042] The above are only preferred embodiments of the present invention and are not intended to limit the present invention. Any modifications, equivalent substitutions and improvements made within the spirit and principles of the present invention should be included in the scope of protection of the present invention.

Claims

1. A synchronous rectification circuit for a switching power supply, comprising a synchronous rectification chip U1 connected between the secondary coil of a primary transformer T4 of the switching power supply and a secondary power output module, characterized in that: The synchronous rectifier chip U1 includes a logic control module, a switch tube Q4 and a wake-up module electrically connected to the logic control module, and a power module for supplying power to the synchronous rectifier chip U1; the logic control module is used to control the switching state of the switch tube Q4 to achieve rectification of the secondary power output module, and the wake-up module is used to detect the output voltage of the secondary power output module, and when the voltage is lower than a preset threshold, the logic control module controls the switch tube Q4 to generate a jitter signal and transmit it to the primary coil of the primary transformer T4, so as to control the primary power tube of the switching power supply to turn on and thereby charge the secondary power output module; The synchronous rectification chip U1 further includes an oscillation circuit module connected between the wake-up module and the logic control module, and a gate drive module connected between the logic control module and the gate of the switch tube Q4; The synchronous rectification circuit of the switching power supply also includes a voltage divider resistor R3 and a voltage divider resistor R4 connected in series; the secondary like-name ends of the secondary coil of the primary transformer T4 are grounded in sequence through the voltage divider resistors R3 and R4, and the wake-up module is connected to the common connection end of the voltage divider resistors R3 and R4 via the vref pin of the synchronous rectification chip U1.

2. The synchronous rectification circuit of the switching power supply according to claim 1, characterized in that: The drain of the switch tube Q4 is connected to the secondary same-name terminal of the secondary coil of the primary transformer T4 via the drain pin of the synchronous rectifier chip U1, and the source of the switch tube Q4 is grounded.

3. The synchronous rectification circuit of the switching power supply according to claim 2, wherein: The secondary power output module includes an energy storage capacitor C5 and a load resistor RL4; one end of the energy storage capacitor C5 is connected to the secondary opposite-name terminal of the secondary coil of the primary transformer T4, and the other end of the energy storage capacitor C5 is connected to the source of the switch tube Q4; the load resistor RL4 is connected in parallel with the energy storage capacitor C5 and grounded.

4. The synchronous rectification circuit of the switching power supply according to claim 3, characterized in that: The synchronous rectification circuit of the switching power supply further includes an energy storage capacitor C6, and the power module is grounded via the energy storage capacitor C6.

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