A primary-side feedback type circuit of a flyback power supply

Abstract

The application discloses a primary side feedback type circuit of a flyback power supply, which comprises a control circuit, a transformer, a main power switch tube, a primary side power supply circuit, a primary side output voltage feedback circuit and a starting circuit, and aims at solving the problems that the existing primary side feedback transformer is difficult to realize miniaturization due to the existence of an auxiliary winding, the miniaturization of the power supply is difficult, and the parasitic current existing in the original auxiliary winding also has an influence on the feedback signal.

Description

Technical Field

[0001] This invention relates to the field of switching power supply technology, specifically a primary-side feedback circuit for a flyback power supply. Background Technology

[0002] In low-power switching power supplies, flyback primary-side control loops have been widely used in chargers, LED drivers and other fields due to their advantages such as fewer hardware components, simple topology, and low cost.

[0003] In the existing primary-side control, an auxiliary winding needs to be added to the transformer to detect the output voltage after the MOSFET is turned off when the inductor current is not applied. This allows the selection of the MOSFET's on-time in the next cycle, thus enabling alternating operation in different modes to improve the power system's load efficiency and standby power consumption under light load.

[0004] The added auxiliary winding serves two purposes: firstly, to provide power to the chip; secondly, it may affect the coupling between the primary and secondary windings in the transformer during actual operation, or, in small-sized power supplies, the limited frame size prevents the selection of smaller magnetic cores, thus hindering miniaturization. Thirdly, without the auxiliary winding, the feedback signal required by the control chip cannot be obtained through it. Therefore, a feedback signal needs to be added from the secondary output loop. However, feedback from the secondary side requires an optocoupler and a secondary detection circuit, significantly increasing the system complexity of the original flyback circuit. Furthermore, the lifespan of the optocoupler also affects the lifespan of the power supply.

[0005] Therefore, designing a primary-side feedback circuit for a flyback power supply to reduce the size of the transformer, allowing for the selection of a smaller magnetic core, and addressing the issue that the sampling signal of the original auxiliary winding is easily affected by noise, especially the parasitic leakage current generated by the coil, which can also interfere with the feedback signal, have become urgent technical problems for those skilled in the art. Summary of the Invention

[0006] The purpose of this invention is to provide a primary-side feedback circuit for a flyback power supply to solve the problems mentioned in the background art.

[0007] To achieve the above objectives, the present invention provides the following technical solution:

[0008] A primary-side feedback circuit for a flyback power supply includes a main control circuit, a transformer T1, a main power switch Q1, a primary-side power supply circuit, a primary-side output voltage feedback circuit, and a startup circuit. One end of the primary winding of the transformer T1 is connected to a resistor R1 and an HVDC voltage. The other end of the primary winding of the transformer T1 is connected to the anode of a diode D1 and the drain of the main power switch Q1. The gate of the main power switch Q1 is connected to the main control circuit. The source of the main power switch Q1 is connected to a resistor R2. The other end of the resistor R2 is connected to a resistor R3 and the main control circuit. The other end of the resistor R3 is grounded. The cathode of the diode D1 is connected to the primary-side power supply circuit and the primary-side output voltage feedback circuit. The primary-side power supply circuit is connected to the other end of the resistor R1.

[0009] As a further technical solution of the present invention: the main control circuit includes one of a power management chip and a wireless SOC module, which is used to control the power supply to achieve constant voltage or constant current output.

[0010] As a further technical solution of the present invention: the power management chip includes a power supply pin, an output voltage feedback sampling pin, an output current sampling pin, and a switch drive pin.

[0011] As a further technical solution of the present invention: the wireless SOC module includes a switching transistor driver module, an ADC input module and a power supply module.

[0012] As a further technical solution of the present invention: the primary power supply circuit can be used to convert the electrical energy stored in the leakage inductance coupled out of the primary coil of the transformer into 2~20V low-voltage DC power according to the working state of the transformer, so as to power the main control circuit.

[0013] As a further technical solution of the present invention: the primary-side output voltage feedback circuit can detect the voltage on the drain of the power main switch transistor according to the working state of the power main switch transistor, and feed back the critical voltage point here to the output voltage feedback sampling pin in the main control circuit, thereby controlling the power management chip or wireless SOC module in the main control circuit to turn on the main power switch transistor and switch to the next working state.

[0014] As a further technical solution of the present invention: the main power switch Q1 is a MOS transistor.

[0015] Compared with the prior art, the beneficial effects of the present invention are:

[0016] 1. The primary-side feedback circuit used in this invention does not require auxiliary windings for feedback, and it also does not require auxiliary windings to power the main control circuit, which greatly reduces the size of the transformer, improves the coupling between the primary and secondary sides of the transformer, and the circuit is simple and low in cost.

[0017] 2. This feedback circuit supports switching between inductor current critical conduction mode and intermittent conduction mode, and can also achieve frequency hopping operation in critical mode, thus improving the stability of the system. Attached Figure Description

[0018] Figure 1 This is a module circuit diagram of a primary-side feedback circuit for a flyback power supply according to the present invention.

[0019] Figure 2 This is a circuit diagram of a primary-side feedback circuit for a flyback power supply according to the present invention. Detailed Implementation

[0020] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0021] Please see Figure 1 Example 1: A primary-side feedback circuit for a flyback power supply, including a power supply pin, an output voltage feedback sampling pin, an output current sampling pin, and a switching transistor drive pin;

[0022] The power management chip includes at least a power supply pin, an output voltage feedback sampling pin, an output current sampling pin, and a switching transistor drive pin.

[0023] Transformer T1, connected to the main control circuit, consists of a primary winding circuit and a secondary winding circuit. The primary winding is used to provide power to the secondary circuit and to provide power to the primary circuit. The secondary winding is used to provide DC constant current or constant voltage output.

[0024] The primary-side power supply circuit, connected to the main control circuit and the primary-side winding circuit, is used to convert the electrical energy stored in the leakage inductance coupled from the primary-side coil of the transformer into 2~20V low-voltage DC power to power the main control circuit. It consists of diode D2, inductor L2, switching transistor Q2, capacitors C1, C2, C3, diode D3, and Zener diode Z1.

[0025] The specific operation of the primary power supply circuit is as follows: Please refer to... Figure 2When transformer T1 stores energy on its primary side, the anode voltage of diode D2 is less than its cathode voltage, and diode D2 is not conducting. At this time, no electrical energy is transmitted to the power supply pin of the control chip, so capacitors C2 and C3 continuously supply power to the control chip. When transformer T1 is in the energy release stage, the anode voltage of D2 is greater than its cathode voltage, and diode D2 conducts. At this time, the energy stored in the leakage inductance of the transformer is transmitted to inductor L2 through D2 to prevent the chip from being damaged by a large current. Switch Q2 is also conducting at this time (its conducting state is opposite to that of switch Q1). At this time, electrical energy is used to charge capacitor C1 through switch Q2. After capacitor C1 is fully charged, it continues to charge capacitors C2 and C3. The output voltage can be adjusted by adjusting the ratio of capacitor C1 to capacitors C2 and C3, and finally the energy in the leakage inductance is converted into the electrical energy required by the chip.

[0026] The primary-side output voltage feedback circuit is connected to the main control circuit and the primary-side winding circuit, and is used to transmit the state of the output voltage to the output voltage feedback sampling pin in the main control circuit.

[0027] The specific operation of the primary-side output voltage feedback circuit is as follows: Please refer to... Figure 2 The primary voltage feedback circuit is used to detect when the inductor current in the primary winding of the transformer crosses zero when the control chip is operating in intermittent conduction mode or critical conduction mode. It consists of diode D4, Schottky diode D5, capacitor C4, Zener diode Z2, resistor R5, and resistor R4.

[0028] Furthermore, the primary-side output voltage feedback circuit also has two operating modes. When the switch Q1 is turned on, the anode voltage of diode D4 is lower than its cathode voltage, so diode D4 is not turned on. At this time, the voltage on the voltage feedback pin comes from the voltage division of resistors R4 and R5. This voltage value is greater than the reference voltage inside the voltage feedback pin. At this time, the control chip turns off the switch Q1 within its controlled duty cycle and enters the next stage. When the switch Q1 is turned off, the primary winding of the transformer is in a discharging state. The voltage at the drain of the switch Q1 is induced by the reflected voltage of the secondary winding, the HVDC voltage, and the voltage at the secondary winding. The voltage spikes caused by the parasitic capacitance of the switching transistor Q1 and the leakage inductance of the transformer are composed of the following: The voltage on the drain of the switching transistor Q1 gradually decreases, and this voltage can also serve as a reference for the gradual increase of the induced current in the secondary coil. When the voltage on the drain of the switching transistor Q1 drops to the valley point, the inductance current of the primary coil of the transformer is zero. After continuously entering a negative value, it returns to zero, causing the cathode voltage of diode D4 to be less than the anode voltage. Diode D4 conducts, and the voltage of the voltage feedback pin is quickly pulled down to zero. At this time, the chip turns on the switching transistor Q1 to charge the primary coil of the transformer, and diode D4 is turned off, completing one cycle.

[0029] The startup circuit is connected to the main control circuit, primary winding circuit, primary power supply circuit, and primary output voltage feedback circuit. It is used to power on the main control circuit and drive the switching transistor to operate the transformer T1 at the moment the power is turned on.

[0030] The power management chip can also be replaced with a wireless SOC module to achieve the same function. The wireless SOC module includes a switching transistor driver module, an ADC input module, and a power supply module. The ADC input module includes a linear compensator, an error amplifier, and an analog-to-digital converter. This can also be considered as another alternative solution in this embodiment.

[0031] The working steps are as follows: The power supply circuit and the power control chip U1 output a synchronous PWM signal. When the PWM signal received by the power control chip changes from full load to light load, the power supply circuit also receives the PWM signal synchronously, causing the voltage of the power control chip to gradually decrease. When the duty cycle of the PWM signal is 100%, the voltage of the power supply circuit drops to 0.1~0.8V below the undervoltage start-up threshold of the power control chip and triggers the undervoltage protection function of the power control chip, causing some circuits inside the chip to be shut down to achieve low power consumption and shut down the secondary output. At this time, the voltage of the power supply pin of the power control chip just triggers the conduction voltage drop threshold of the first NMOS transistor. At this time, the startup circuit supplies power to the power control chip and the third energy storage capacitor. The power control chip releases the undervoltage protection function and enters normal operation. When the voltage of the third energy storage capacitor gradually drops to the undervoltage start-up threshold of the power control chip, the power control chip enters undervoltage protection.

[0032] Example 2:

[0033] The difference from Embodiment 1 is that the fifth resistor R5, the sixth resistor R6, the seventh resistor R7, and the fifth transistor Q5 in the linear regulator are removed, and the sixth resistor R6, the seventh resistor R7, and the fourth transistor Q4 are added to the FB pin of the power control chip U1. Its working principle is that when the fourth transistor Q4 is fully on, the voltage divider resistor is at its lowest, resulting in the lowest voltage detected by the FB pin of the power control chip U1. This causes the power control chip U1 to enter overvoltage protection and shut down the output. Specifically, a resistor divider circuit consisting of a fourth transistor Q4, a sixth resistor R6, and a seventh resistor R7 is connected in parallel on the FB pin of the power control chip. The transistor receives a PWM signal synchronized with the power control chip. When the PWM signal received by the power control chip changes from full load to light load, the voltage detected by the FB pin continuously decreases. When the PWM signal is at 100% duty cycle, the voltage of the FB pin triggers the overvoltage protection threshold of the chip. The chip shuts off the output and waits for the dimming signal to change again to reset the overvoltage protection.

[0034] It will be apparent to those skilled in the art that the present invention is not limited to the details of the exemplary embodiments described above, and that the invention can be implemented in other specific forms without departing from its spirit or essential characteristics. Therefore, the embodiments should be considered in all respects as exemplary and non-limiting, and the scope of the invention is defined by the appended claims rather than the foregoing description. Thus, all variations falling within the meaning and scope of equivalents of the claims are intended to be included within the present invention. No reference numerals in the claims should be construed as limiting the scope of the claims.

[0035] Furthermore, it should be understood that although this specification describes embodiments, not every embodiment contains only one independent technical solution. This narrative style is merely for clarity. Those skilled in the art should consider the specification as a whole, and the technical solutions in each embodiment can also be appropriately combined to form other embodiments that can be understood by those skilled in the art.

Claims

1. A primary-side feedback circuit for a flyback power supply, comprising a main control circuit, a transformer T1, a main power switch Q1, a primary-side power supply circuit, a primary-side output voltage feedback circuit, and a startup circuit, characterized in that, One end of the primary winding of transformer T1 is connected to resistor R1 and HVDC voltage. The other end of the primary winding of transformer T1 is connected to the anode of diode D1 and the drain of main power switch Q1. The gate of main power switch Q1 is connected to the main control circuit. The source of main power switch Q1 is connected to resistor R2. The other end of resistor R2 is connected to resistor R3 and the main control circuit. The other end of resistor R3 is grounded. The cathode of diode D1 is connected to the primary power supply circuit and the primary output voltage feedback circuit. The primary power supply circuit is connected to the other end of resistor R1. The main power switch Q1 is a MOSFET; The main control circuit includes a power management chip or a wireless SOC module, used to control the power supply to achieve constant voltage or constant current output. The power management chip includes a power supply pin, an output voltage feedback sampling pin, an output current sampling pin, and a switch drive pin. The switch drive pin is connected to the gate of the main power switch Q1, the power supply pin is connected to the primary power supply circuit, and the output voltage feedback sampling pin is connected to the primary output voltage feedback circuit. The primary-side power supply circuit includes diode D2, inductor L1, switch Q2, capacitors C1, C2, C3, diode D3, and Zener diode Z1. The conduction state of switch Q2 is opposite to that of the main power switch Q1. When transformer T1 is in the energy release stage, diode D2 is turned on, and the energy stored in the leakage inductance of the transformer is transferred to inductor L1 through D2. The electrical energy charges capacitor C1 through switch Q2. After capacitor C1 is fully charged, it continues to charge capacitors C2 and C3, and finally converts the energy in the leakage inductance into 2~20V low-voltage DC power to power the power supply pins of the main control circuit. The primary-side output voltage feedback circuit includes diode D4, Schottky diode D5, capacitor C4, Zener diode Z2, resistor R5, and resistor R4. It is used to detect the zero-crossing point of the inductor current in the primary winding of the transformer when the control chip is operating in intermittent conduction mode or critical conduction mode. Specifically, when switch Q1 is on, the anode voltage of diode D4 is lower than its cathode voltage, so diode D4 is not conducting. The voltage on the voltage feedback pin comes from the voltage division of resistors R4 and R5. When switch Q1 is off, the primary winding of the transformer is in a discharging state, and the voltage on the drain of switch Q1 gradually decreases. When the voltage on the drain of switch Q1 drops to its lowest point, the inductor current in the primary winding of the transformer is zero, causing the cathode voltage of diode D4 to be less than the anode voltage, thus diode D4 conducts. The voltage on the voltage feedback pin is quickly pulled down to zero and fed back to the main control circuit through the output voltage feedback sampling pin. At this time, the chip turns on switch Q1 to charge the primary winding of the transformer, and diode D4 is turned off, realizing the switching between critical conduction mode and intermittent conduction mode.

2. The primary-side feedback circuit of a flyback power supply according to claim 1, characterized in that, The wireless SOC module includes a switching transistor driver module, an ADC input module, and a power supply module.

3. The primary-side feedback circuit of a flyback power supply according to claim 1, characterized in that, The primary power supply circuit can convert the electrical energy stored in the leakage inductance coupled from the primary coil of the transformer into 2~20V low-voltage DC power to power the main control circuit, depending on the transformer's operating state.

4. The primary-side feedback circuit of a flyback power supply according to claim 1, characterized in that, The primary-side output voltage feedback circuit can detect the voltage on the drain of the main power switch transistor according to its operating state, and feed back the critical voltage point to the output voltage feedback sampling pin in the main control circuit. This will then control the power management chip or wireless SOC module in the main control circuit to turn on the main power switch transistor and switch to the next operating state.

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