Its function is to convert a voltage level into the voltage or current required by the user through different architectures. The input of a switching power supply is mostly AC power (such as mains power) or DC power, while the output is mostly devices that require DC power, such as personal computers. The switching power supply performs the voltage and current conversion between the two.
Unlike linear power supplies, switching power supplies utilize transistors that primarily switch between fully on (saturation) and fully off (cutoff) modes. Both modes feature low dissipation. While the transition between modes involves higher dissipation, the time is very short, resulting in energy savings and less heat generation. Ideally, the switching power supply itself does not consume electrical energy. Voltage regulation is achieved by adjusting the on and off times of the transistors. Conversely, linear power supplies consume electrical energy because the transistors operate in the amplification region during output voltage generation. The high conversion efficiency of switching power supplies is a major advantage. Furthermore, due to their high operating frequency, smaller and lighter transformers can be used, making switching power supplies smaller and lighter than linear power supplies. If efficiency, size, and weight are primary considerations, switching power supplies are superior to linear power supplies. However, switching power supplies are more complex, with internal transistors switching frequently. If the switching current is not properly managed, it can generate noise and electromagnetic interference that could affect other devices. Additionally, without specific design features, the power factor of a switching power supply may be low.
Circuit composition of switching power supply
The main circuitry of a switching power supply consists of an input electromagnetic interference (EMI) filter, a rectifier filter circuit, a power conversion circuit, a PWM controller circuit, and an output rectifier filter circuit. Auxiliary circuits include input over/under voltage protection circuits, output over/under voltage protection circuits, output overcurrent protection circuits, and output short-circuit protection circuits.
The circuit block diagram of a switching power supply is as follows:
The principle and common circuits of input circuits
1. Principle of AC input rectifier and filter circuit:
① Lightning protection circuit: When lightning strikes and generates high voltage that is fed into the power grid, the circuit consisting of MOV1, MOV2, MOV3, F1, F2, F3, and FDG1 provides protection. When the voltage across the varistor exceeds its operating voltage, its resistance decreases, causing the high voltage energy to be dissipated in the varistor. If the current is too large, F1, F2, and F3 will burn out the protection circuit downstream.
② Input Filtering Circuit: The double π-type filter network composed of C1, L1, C2, and C3 mainly suppresses electromagnetic noise and spurious signals from the input power supply, preventing interference to the power supply and also preventing high-frequency noise generated by the power supply itself from interfering with the power grid. When the power supply is turned on, C5 needs to be charged. Due to the large instantaneous current, adding RT1 (thermistor) effectively prevents surge current. Because all the instantaneous energy is consumed by RT1, after a certain period of time, as the temperature rises, the resistance of RT1 decreases (RT1 is a negative temperature coefficient element). At this point, the energy consumed is very small, and the subsequent circuits can operate normally.
③ Rectifier and Filter Circuit: After the AC voltage is rectified by BRG1, it is filtered by C5 to obtain a relatively pure DC voltage. If the capacitance of C5 decreases, the output AC ripple will increase.
2. Principle of DC input filter circuit:
① Input Filtering Circuit: The double π-type filter network composed of C1, L1, and C2 mainly suppresses electromagnetic noise and spurious signals from the input power supply, preventing interference to the power supply and also preventing high-frequency noise generated by the power supply itself from interfering with the power grid. C3 and C4 are safety capacitors, and L2 and L3 are differential-mode inductors. ② R1, R2, R3, Z1, C6, Q1, Z2, R4, R5, Q2, RT1, and C7 form an anti-surge circuit. At the moment of startup, Q2 is not conducting due to the presence of C6, and the current forms a loop through RT1. When the voltage on C6 reaches the regulated value of Z1, Q2 conducts. If C8 leaks or there is a short circuit in the subsequent circuit, the voltage drop across RT1 increases at the moment of startup, Q1 conducts, and Q2 does not conduct due to the lack of gate voltage. RT1 will burn out in a very short time to protect the subsequent circuit.
Power conversion circuit
1. Working principle of a MOSFET: The most widely used insulated gate field-effect transistor is the MOSFET (Metal-Oxide-Semiconductor Transistor), which operates by utilizing the electroacoustic effect on the semiconductor surface. It is also called a surface field-effect device. Because its gate is in a non-conductive state, the input resistance can be greatly increased, reaching up to 10⁵ ohms. The MOSFET controls the drain current by changing the amount of induced charge on the semiconductor surface through the gate-source voltage. 2. Common schematic diagrams:
3. Working Principle: R4, C3, R5, R6, C4, D1, and D2 form a buffer connected in parallel with the switching MOSFET to reduce voltage stress on the switching transistor, reduce EMI, and prevent secondary breakdown. When the switching transistor Q1 is turned off, the primary winding of the transformer is prone to voltage and current spikes. These components work together to effectively absorb these spikes. The peak current signal measured from R3 participates in the duty cycle control of the current operating cycle, thus limiting the current for the current operating cycle. When the voltage across R5 reaches 1V, the UC3842 stops working, and the switching transistor Q1 immediately turns off. R1 and the junction capacitances CGS and CGD in Q1 together form an RC network. The charging and discharging of the capacitors directly affect the switching speed of the switching transistor. If R1 is too small, it is prone to oscillation and electromagnetic interference; if R1 is too large, it will reduce the switching speed of the switching transistor. Z1 typically limits the GS voltage of the MOSFET to below 18V, thereby protecting the MOSFET. The gate voltage of Q1 is a sawtooth wave. The larger its duty cycle, the longer the conduction time of Q1, and the more energy the transformer stores. When Q1 is off, the transformer releases energy through D1, D2, R5, R4, and C3, simultaneously achieving magnetic field reset and preparing for the next energy storage and transfer. The IC constantly adjusts the duty cycle of the sawtooth wave at pin 6 based on the output voltage and current, thereby stabilizing the overall output current and voltage. C4 and R6 form a spike voltage absorption circuit.
4. Push-pull power conversion circuit: Q1 and Q2 will be turned on alternately.
5. Power conversion circuit with drive transformer: T2 is the drive transformer, T1 is the switching transformer, and TR1 is the current loop.
Output rectifier filter circuit
1. Forward rectifier circuit:
T1 is a switching transformer, with its primary and secondary terminals in phase. D1 is a rectifier diode, D2 is a freewheeling diode, and R1, C1, R2, and C2 form a peak clipping circuit. L1 is a freewheeling inductor, and C4, L2, and C5 form a π-type filter.
2. Flyback rectifier circuit:
T1 is a switching transformer with its primary and secondary terminals out of phase. D1 is a rectifier diode, and R1 and C1 form a peak clipping circuit. L1 is a freewheeling inductor, R2 is a dummy load, and C4, L2, and C5 form a π-type filter.
3. Synchronous rectifier circuit:
Working principle: When the upper end of the transformer secondary winding is positive, the current flows through C2, R5, R6, and R7, turning on Q2 and forming a circuit loop. Q2 is a rectifier diode. Q1 is cut off due to reverse bias. When the lower end of the transformer secondary winding is positive, the current flows through C3, R4, and R2, turning on Q1, which is a freewheeling diode. Q2 is cut off due to reverse bias. L2 is a freewheeling inductor, and C6, L1, and C7 form a π-type filter. R1, C1, R9, and C4 form a peak-shaving circuit.
Voltage stabilizing loop principle
1. Feedback circuit schematic:
2. Working principle:
When the output U0 increases, after being divided by sampling resistors R7, R8, R10, and VR1, the voltage at pin ③ of U1 increases. When it exceeds the reference voltage at pin ② of U1, pin ① of U1 outputs a high level, turning on Q1. This causes the LED in optocoupler OT1 to light up, and the phototransistor to conduct. Consequently, the potential at pin ① of UC3842 decreases, thus changing the duty cycle of the output at pin ⑥ of U1, decreasing U0. When the output U0 decreases, the voltage at pin ③ of U1 decreases. When it falls below the reference voltage at pin ② of U1, pin ① of U1 outputs a low level, Q1 does not conduct, the LED in optocoupler OT1 does not light up, the phototransistor does not conduct, and the potential at pin ① of UC3842 increases. This changes the duty cycle of the output at pin ⑥ of U1, increasing U0. This cycle repeats, thus stabilizing the output voltage. Adjusting VR1 can change the output voltage value.
The feedback loop is a crucial circuit affecting the stability of a switching power supply. Issues such as faulty or leaky feedback resistors and capacitors, or poor soldering, can lead to self-oscillation, manifesting as abnormal waveforms, oscillations under no-load and full-load conditions, and unstable output voltage.
Short circuit protection circuit
1. When the output terminal is short-circuited, the PWM control circuit can limit the output current to a safe range. It can implement the current limiting circuit in a variety of ways. When the power current limiting does not work when short-circuited, an additional part of the circuit must be added.
2. There are generally two types of short-circuit protection circuits. The following diagram shows a low-power short-circuit protection circuit, and its principle is briefly described below:
When the output circuit is short-circuited, the output voltage disappears, optocoupler OT1 does not conduct, and the voltage at pin 1 of UC3842 rises to approximately 5V. The voltage division between R1 and R2 exceeds the reference voltage of TL431, causing it to conduct. The VCC potential at pin 7 of UC3842 is pulled low, and the IC stops working. After UC3842 stops working, the potential at pin 1 disappears, TL431 does not conduct, the potential at pin 7 of UC3842 rises, and UC3842 restarts, repeating the cycle. When the short circuit disappears, the circuit can automatically return to normal operating conditions.
3. The following diagram shows a medium-power short-circuit protection circuit. Its principle is briefly described below:
When the output is short-circuited, the voltage at pin ① of UC3842 rises. When the potential at pin ③ of U1 is higher than that at pin ②, the comparator flips, outputting a high potential at pin ① to charge C1. When the voltage across C1 exceeds the reference voltage at pin ⑤, pin ⑦ of U1 outputs a low potential, and the voltage at pin ① of UC3842 drops below 1V. UCC3842 then stops working, and the output voltage becomes 0V. This cycle repeats until the short circuit disappears, at which point the circuit operates normally. R2 and C1 are the charging/discharging time constants; incorrect resistance values will render the short-circuit protection ineffective.
4. The following diagram shows a common current-limiting and short-circuit protection circuit. Its working principle is briefly described below:
When the output circuit is short-circuited or overcurrent occurs, the primary current of the transformer increases, the voltage drop across R3 increases, the voltage at pin 3 rises, the duty cycle of the output at pin 6 of UC3842 gradually increases, and when the voltage at pin 3 exceeds 1V, UC3842 shuts down and has no output.
5. The following diagram shows a protection circuit that uses a current transformer to sample the current. It has low power consumption, but high cost and relatively complex circuit. Its working principle is briefly described below:
If the output circuit is short-circuited or the current is too high, the voltage induced in the secondary coil of TR1 will be higher. When the voltage at pin 3 of UC3842 exceeds 1 volt, UC3842 will stop working. This cycle repeats until the short circuit or overload disappears and the circuit recovers on its own.
VII. Output Current Limiting Protection
The diagram above shows a common output current limiting protection circuit. Its working principle is briefly described in the diagram: When the output current is too large, the voltage across RS (manganese copper wire) rises, the voltage at pin ③ of U1 is higher than the reference voltage at pin ②, the output voltage at pin ① of U1 is high, Q1 is turned on, the optocoupler undergoes photoelectric effect, the voltage at pin ① of UC3842 decreases, the output voltage decreases, thereby achieving the purpose of output overload current limiting.
Principle of output overvoltage protection circuit
The function of the output overvoltage protection circuit is to limit the output voltage to a safe range when it exceeds the design value. When the internal voltage regulation loop of the switching power supply malfunctions or when improper user operation causes output overvoltage, the overvoltage protection circuit protects against damage to downstream equipment. The most commonly used overvoltage protection circuits include the following:
1. Thyristor trigger protection circuit:
As shown in the diagram above, when the output Uo1 increases, the Zener diode (Z3) breaks down and conducts, triggering the control terminal of the silicon controlled rectifier (SCR1), thus turning on the SCR1. When the voltage Uo2 is short-circuited to ground, the overcurrent protection circuit or short-circuit protection circuit will activate, stopping the entire power supply circuit. When the output overvoltage is eliminated, the trigger voltage at the control terminal of the SCR1 is discharged to ground through R, and the SCR1 returns to its off state.
2. Optocoupler protection circuit:
As shown in the diagram above, when Uo experiences overvoltage, the Zener diode breaks down and conducts, generating current that flows to ground via the optocoupler (OT2) R6. This causes the optocoupler's LED to light up, thus turning on the optocoupler's phototransistor. The base of Q1 is energized and conducts, reducing the voltage at pin 3 of the 3842, shutting down the IC and stopping the entire power supply. Uo becomes zero, and this cycle repeats continuously.
3. Output Voltage Limiting Protection Circuit: The output voltage limiting protection circuit is shown in the figure below. When the output voltage increases, the Zener diode and optocoupler conduct, and the base of Q1 is driven by the voltage, causing it to conduct. The voltage of UC3842③ increases, and the output voltage decreases. The Zener diode does not conduct, and the voltage of UC3842③ decreases, causing the output voltage to increase. This cycle repeats until the output voltage stabilizes within a certain range (depending on the Zener diode's voltage regulation value).
4. Output overvoltage lockout circuit:
The working principle of Figure A is as follows: when the output voltage Uo increases, the Zener diode conducts, the optocoupler conducts, and the base of Q2 is energized and conducts. Due to the conduction of Q2, the base voltage of Q1 decreases and also conducts. The Vcc voltage, through R1, Q1, and R2, keeps Q2 conducting, and pin ③ of UC3842 remains high and stops working. In Figure B, when UO increases, the voltage at pin ③ of U1 increases, and pin ① outputs a high level. Due to the presence of D1 and R1, pin ① of U1 always outputs a high level, Q1 always conducts, and pin ① of UC3842 remains low and stops working. Positive feedback?
Power factor correction circuit (PFC)
1. Schematic diagram:
2. Working Principle: The input voltage passes through an EMI filter composed of L1, L2, and L3. BRG1 rectifies the voltage, sending one path to the PFC inductor and the other path, after being divided by R1 and R2, to the PFC controller as a sample of the input voltage. This sample is used to adjust the duty cycle of the control signal, i.e., changing the on and off times of Q1, thus stabilizing the PFC output voltage. L4 is the PFC inductor; it stores energy when Q1 is on and releases energy when Q1 is off. D1 is the startup diode. D2 is the PFC rectifier diode, and C6 and C7 filter the voltage. One path of the PFC voltage is sent to the subsequent circuit, and the other path, after being divided by R3 and R4, is sent to the PFC controller as a sample of the PFC output voltage, used to adjust the duty cycle of the control signal and stabilize the PFC output voltage.
Switching power supplies are widely used in industrial automation control, military equipment, scientific research equipment, LED lighting, industrial control equipment, communication equipment, power equipment, instruments and meters, medical equipment, semiconductor refrigeration and heating, air purifiers, electronic refrigerators, LCD displays, LED lamps, communication equipment, audio-visual products, security monitoring, LED light strips, computer cases, digital products and instruments, and other fields.
