A typical circuit for a flyback switching power supply is shown in the figure below. The term "single-ended" in the circuit refers to the fact that the core of the high-frequency converter operates only on one side of the hysteresis loop. The term "flyback" refers to the fact that when the switching transistor VT1 is turned on, the induced voltage in the primary winding of the high-frequency transformer T is positive at the top and negative at the bottom, and the rectifier diode VD1 is in the off state, storing energy in the primary winding.
When the switching transistor VT1 is turned off, the energy stored in the primary winding of transformer T is rectified by the secondary winding and VD1 and filtered by capacitor C before being output to the load.
Flyback switching power supplies are a type of low-cost power supply circuit, with an output power of 20-100 W. They can output different voltages simultaneously and have good voltage regulation. The only drawback is the relatively large output ripple voltage and poor external characteristics, making them suitable for relatively fixed loads.
The maximum reverse voltage that the switching transistor VT1 used in the flyback switching power supply can withstand is twice the circuit's operating voltage, and the operating frequency is between 20 and 200 kHz.
The working process of a counter-current switching power supply can be simply divided into four steps:
Step 1 Rectification
When the power supply receives 220V AC mains power at its input, it passes through a rectifier bridge, converting the 220V AC to 310V DC.
Step 2 Inverter
The core component in a switching power supply is the controller, which controls the power switching transistors to repeatedly switch 310V DC power into high-frequency AC power, typically ranging from tens to hundreds of kHz. This high-frequency AC power is then stepped down by a transformer. Because AC power has a very high frequency, the size and cost of the transformer can be reduced while increasing conversion efficiency. This is why switching power supplies are less expensive than traditional power frequency transformers.
Step 3 Secondary Rectification
High-frequency alternating current (AC) is reduced in amplitude after passing through a transformer, and then needs to be converted into direct current (DC) through a rectifier circuit. The converted DC is the direct output of the power supply.
Step 4 Feedback
Feedback is the most complex and technically challenging aspect of counter-current switching power supplies and various other types of switching power supplies. Experienced power supply engineers need to repeatedly adjust it to optimize parameters such as efficiency and response speed. The purpose of feedback is to allow the controller to promptly detect fluctuations or disturbances in the output voltage and make appropriate adjustments to quickly stabilize the output voltage. Ideally, a power supply should quickly return to the target voltage value when faced with disturbances, achieving stability, accuracy, and speed.
Below is the schematic diagram of a counter-current switching power supply circuit with 100-240VAC input and 14V 10A DC output. The principle of each part of the schematic diagram is explained below.
1. Circuit before the GBP410 rectifier bridge.
F1: Fuse, used to protect the circuit when the current is too high.
RT1: NTC thermistor, negative temperature coefficient, to prevent inrush current during switching.
MOV1: Varistor, used to suppress overvoltage in the input circuit.
L1: Common-mode inductor, used for common-mode interference in EMC.
CX1:X safety capacitor, used to filter differential mode interference.
R1, R2, R3, R4: Resistors in the filter circuit. Series connection of resistors divides the voltage, keeping the voltage across them within their withstand voltage range. Parallel connection of resistors primarily increases the discharge current, while also considering the resistor's power rating.
LF2: Two-stage common-mode inductor; CX2: X-rated safety capacitor, used to filter differential-mode interference.
II. Filtering circuit after the GBP410 rectifier bridge
1. Two GBP410 rectifier bridges are connected in parallel for current amplification and heat dissipation.
2. C1, EC1, EC2, and EC3 are filter capacitors.
3. L3 is a filter inductor, and the resistor R5 connected in parallel with the inductor is used as the inductor's leakage circuit to discharge current during power-on and power-off.
4. CY1 and CY2 are Y capacitors. (CY2 is located at the bottom right of the schematic diagram.)
III. WT6629 Switching Circuit Section
1. D1, D2, R6, R10, and C4 form an RCD snubber circuit. D6 is a TVS diode to prevent excessively high peak voltages from damaging capacitor C4. R6 is used to limit the discharge current. D1 and D2 are connected in parallel for current amplification and heat dissipation.
2. Q1 is the switching transistor, C3 is used to absorb leakage inductance spikes, RS1, RS2, and RS3 are current sampling resistors used for overcurrent protection. R21 and C8 form an RC filter circuit. Why are RS1, RS2, and RS3 connected in parallel, resulting in an unusual sampling resistor?
3. R24, R23, R22, and T4 form the gate drive of the switching transistor. T4 provides a fast turn-off path when Q1 is turned off. R24 is the current-limiting resistor when turned off. R22 is the gate pull-down resistor.
4. The 4-pin connector of the WT6629 is used to set the frequency, so R25 is used to set the frequency. The formula is: f = 1560 / R25 (KR), and this resistance value has a range requirement: between 24-31KR.
5. The 5-pin connector of the WT6629 is used for temperature detection. The datasheet does not explicitly state this, but from the table, we can see that 0.8V is the recovery voltage. Therefore, 0.65V means that it will activate when the voltage drops below 0.65V. The NTC resistor connected in series from the 5-pin connector to ground will decrease in resistance as the temperature rises, resulting in a drop in voltage across the resistor, which is consistent with the expected behavior.
6. In the schematic, VCC is the transformer voltage for the secondary coil, supplying power to the WT6629. The secondary coil is rectified by D3 (half-wave), filtered by C6 and EC4, and R7 and R8 form a leakage inductance discharge circuit. D4, R15, EC5, and C9 then perform another RC filter. D4 is an isolation diode. 7. R16 and C5 are connected in series and then in parallel with D3 to absorb reverse high voltage and protect the diode; otherwise, the diode is easily damaged by breakdown.
IV. Rectifier Output Circuit
1. Diodes U5 and U4 are connected in parallel and then combined with the capacitor to form a half-wave rectifier filter. The two diodes are connected in parallel to increase current and dissipate heat. The capacitor and resistor connected in parallel with the diodes are to absorb the reverse voltage spike. The reverse voltage spike is caused by the parasitic capacitance and leakage inductance of the diodes.
V. Feedback Loop
1. The WT431 is not used for voltage regulation, but rather as a voltage threshold switch. R28 and R29 divide the voltage and connect it to pin R of the WT431. Pin R is compared with the internal Vref. R26 limits the current of the optocoupler. R27 and R26 simultaneously power the WT431. 1) If the voltage at pin R is higher than 2.5V, the comparator output is high, the transistor conducts, points K and A are short-circuited, and pins 1 and 2 of the optocoupler PC817 conduct. 2) If the voltage at pin R is lower than 2.5V, the comparator output is low, the transistor is off, points K and A are open-circuited, and pins 1 and 2 of the optocoupler PC817 are open-circuited. A switching power supply is a power supply device that converts alternating current (AC) to direct current (DC). Its working principle is to use switching devices to periodically connect and disconnect the input power supply, and by changing the circuit topology and control method, to achieve stable regulation of the output voltage and current.
Flyback switching power supplies are a common topology. Their basic principle is to transform the input voltage into the required output voltage through the periodic switching of switching devices and transformers.
2. Basic Components
A flyback switching power supply mainly consists of a switching transistor, an output inductor, a transformer, a filter, a voltage regulation control circuit, and a feedback circuit.
2.1 Switching transistor
The switching transistor is one of the most basic components in a flyback switching power supply. Its function is to periodically switch the input power supply on and off. Commonly used switching transistors include MOSFETs and IGBTs, and their selection should be based on the actual application requirements.
2.2 Output Inductor
Output inductors are designed to filter and stabilize the output current, primarily smoothing out current ripple. They typically consist of a coil and a magnetic core, with common core materials including ferrite and high-performance magnetic materials.
2.3 Transformer
The transformer is one of the core components of a flyback switching power supply. Its main function is to transform the input and output voltages and provide electrical isolation. A transformer typically consists of an iron core and coils, with the coils divided into main coils and auxiliary coils, used to transfer energy and implement feedback control.
2.4 Filter
Filters are used to filter high-frequency noise and harmonics generated by switches to ensure output voltage stability and ripple levels. Commonly used filters include capacitor filters and inductor filters, and their structure and parameters should be selected according to actual requirements.
2.5 Voltage Regulator Control Circuit
A voltage regulator circuit is designed to stabilize the output voltage. Its main components include a flyback power supply control chip, a feedback element, and an error amplifier. The voltage regulator circuit uses the feedback signal to control the switching time and frequency of the switching transistor to maintain a stable output voltage.
2.6 Feedback Circuit
A feedback circuit is used to acquire and process the feedback signal of the output voltage and compare it with a reference voltage to regulate the output voltage. A feedback circuit typically consists of a feedback resistor, a reference voltage source, and an error amplifier.
3. Working Principle
The working principle of a flyback switching power supply can be divided into two cycles: the turn-on cycle and the turn-off cycle.
3.1 Conduction Period
During the conduction cycle, the switching transistor is in the on state, and the energy from the input power supply is transferred to the load through the transformer. When the switching transistor is on, the inductor stores the energy of the input current and transfers it to the output terminal, at which point the output voltage rises. The flyback power supply control chip generates a control signal by comparing the difference between the output voltage and the reference voltage, thereby controlling the on-time and frequency of the switching transistor.
3.2 Shutdown Cycle
During the shutdown cycle, the switching transistor is in the off state, and energy from the input power supply is no longer transferred to the load. At this time, the energy storage compensation capacitor in the inductor discharges, and the output voltage begins to drop. Simultaneously, the electrical energy in the energy storage inductor is also fed back to the control chip through the auxiliary coil to control the on-time and frequency of the switching transistor.
By continuously switching between the on and off cycles, the flyback switching power supply converts the input voltage into the required stable output voltage.
4. Design Considerations
When designing a flyback switching power supply, the following key points should be noted:
4.1 Voltage Regulation Control
Voltage regulation is one of the keys to improving the performance of flyback switching power supplies. The voltage regulation control circuit should possess characteristics such as fast response, high precision, and strong stability. Common voltage regulation control methods include fixed-frequency PWM control and variable-frequency pulse width modulation control.
4.2 Output Inductor and Filter Design
The design of the output inductor and filter directly affects the output performance of a flyback switching power supply. The output inductor should have a sufficient inductance value to reduce output current ripple, while the filter should have appropriate parameters to reduce ripple levels.
4.3 Transformer Design
Transformer design is a crucial step in flyback switching power supply design, as it affects the efficiency and stability of output voltage and current conversion. The transformer design should determine the turns ratio, wire diameter, and core parameters of the main and auxiliary coils based on specific requirements, and involve rigorous and reasonable calculations.
4.4 Switch Selection and Drive Design
The selection of a switching transistor should consider factors such as its on-resistance, losses, and switching capability to meet the requirements of output current and frequency. The driving circuit of the switching transistor should have reasonable waveforms and power to ensure stable operation of the switching transistor.
4.5 Feedback Circuit Design
The design of the feedback circuit is related to the accuracy and stability of the voltage regulation control. It is usually implemented by means of resistor voltage division or current sampling, and should have high precision and anti-interference ability.
5. Summary
Flyback switching power supplies are a commonly used design approach, offering advantages such as high efficiency, small size, and high reliability. Through proper design and optimization, they can achieve input voltage transformation and stable output voltage regulation. Key considerations when designing a flyback switching power supply include voltage regulation control, output inductor and filter design, transformer design, selection of switching transistors and driver design, and feedback circuit design to ensure the power supply's performance and stability.
