It replaces a PWM switching power supply with a working frequency of 20 kHz, which can save energy. It is known as the 20 kHz revolution in the history of power supply technology development.
As ULSI chips continue to shrink in size, power supplies are much larger than microprocessors. Many electronic devices need a smaller and lighter power supply, such as aerospace, submarine, military switching power supplies, and battery-operated portable electronic devices such as portable calculators, mobile phones, etc.
Therefore, requirements of small size and lightweight are imposed on the switching power supply, including the volume and weight of magnetic components and capacitors. In addition, the switching power supply requirements are higher efficiency, better performance and higher reliability.
It is quite easy to understand the working process of the switching power supply. In a linear power supply, the power transistor is operating in a linear mode. The PWM switching power supply, unlike a linear power supply, allows the power transistor to flip between on and off states.
The volt-ampere product applied to the power transistor is always small in both states the voltage is low and the current is large when turned on; the voltage is high and the current is small when turned off. The product of volt-ampere on a power device is the loss produced on a power semiconductor device.
Compared with linear power supplies, PWM switching power supplies work more efficiently by "chopper", which is to convert the input DC voltage into a pulse voltage with an amplitude equal to the input voltage amplitude. The duty ratio of the pulse is regulated by the controller of the switching power supply. Once the input voltage clamps into an AC square wave, its amplitude can be raised or lowered by the transformer. The number of voltage groups of the output can be increased by increasing the number of secondary windings of the transformer.
Finally, a DC output voltage is obtained after these AC waveforms are rectified and filtered. The main purpose of the controller is to ensure that the output voltage is stable, and its working process is very similar to a linear controller.
This means that the controller's functional block voltage reference and error amplifier designed to be identical to a linear regulator. They differ in that the output of the error amplifier error voltage passes through a voltage pulse conversion unit before driving the power transistor.
Switching power supplies have two main modes of operation: forward conversion and boost conversion. Although the arrangement of the various parts differs little, the working process varies greatly and they have different advantages in specific situations.
The advantage of the forward converter is that the output voltage has a lower ripple peak than the boost converter, and can output relatively high power.
The forward converter can provide several kilowatts of power. Because the boost converter has a high peak current, it is only suited for applications with a maximum power of W.
These converters use the smallest components in all topologies, making them popular in low- to medium-power applications. Figure 1. The main circuit of the switching power supply is composed of an input electromagnetic interference filter EMI , a rectification and filtering circuit, a power conversion circuit, a PWM controller circuit, and an output rectification and filtering circuit.
The auxiliary circuit has an input over-voltage protection circuit, an output over-voltage protection circuit, an output over-current protection circuit, and output short-circuits protection circuit. The circuit block diagram of the switching power supply is as follows:. Figure 2. Figure 3. Schematic of Input Filter, Rectifier Circuit. When the voltage applied across the varistor exceeds its operating voltage, its resistance decreases. So the high-voltage energy is consumed on the varistor.
If the current is too large, F1, F2, and F3 will burn and protect the next circuit. The C5 should be charged when the power is switched on. Due to the high instantaneous current, adding RT1 thermistor efficiently prevents the surge current.
Because the instantaneous energy is utilized by the resistor RT1, the resistance of RT1 drops after a given time as the temperature rises RT1 is the negative temperature coefficient component. The energy consumption is quite low at this period, and the subsequent circuit can operate normally. If the capacity of C5 becomes smaller, the output AC ripple will increase. Figure 4. DC Input Filter Circuit. L2 and L3 are differential mode inductors, whereas C3 and C4 are safety capacitors.
Because of the presence of C6, Q2 does not conduct at the start and the current forms a loop through RT1. When the voltage on C6 is charged to the controlled value of Z1, Q2 turns on.
If the C8 leakage or the subsequent circuit is short-circuited, the voltage drop created by the current on RT1 increases at the start, and Q1 is turned on such that Q2 is not turned on without the gate voltage, and RT1 will burn out quickly to protect the subsequent circuit. At present, the most widely used insulated gate field effect transistor is a MOSFET MOS transistor , which works by utilizing the electroacoustic effect of the semiconductor surface and is also known as surface field-effect devices.
Since its gate is non-conducting, the input resistance can be greatly improved up to ohms. The MOS transistor uses the magnitude of the gate-source voltage to change the amount of induced charge on the semiconductor surface, thereby controlling the drain current.
Figure 5. Power Conversion Circuit. When the switch tube Q1 is turned off, the transformer's primary winding easily produces spike voltage and spike current. These components, when combined, can effectively absorb the spike voltage and current. The current peak signal measured from R3 is used to control the duty ratio of the current working cycle and hence represents the current limit of the current working cycle.
When the voltage on R5 reaches 1V, the UC stops operating and switch tube Q1 immediately switches off. If R1 is too small, oscillation and electromagnetic interference will be very large; if R1 is too large, the switching speed of the switching tube will be reduced. Q1's gate-controlled voltage is a saw-toothed wave.
The longer the Q1 conduction time is when the duty ratio is higher, the more energy the transformer retains. At the same time, it achieves the purpose of magnetic field reset, which is ready for the transformer's next storage and transmission of energy.
The IC adjusts the duty ratio of the saw-shaped wave on pin 6 based on the output voltage and current, thereby stabilizing the machine's output current and voltage. C4 and R6 are spike voltage absorption loops. Figure 6. Push-pull Power Conversion Circuit. Q1 and Q2 will turn on in turn.
Figure 7. Power Conversion Circuit with Drive Transformer. T2 is the drive transformer, T1 is the switching transformer, and TR1 is the current loop. Figure 8. Forward Rectifier Circuit. T1 is a switching transformer whose phase of primary and secondary poles are in phase. Figure 9.
Flyback Rectifier Circuit. T1 is a switching transformer with opposite phases of the primary and secondary poles. D1 is a rectifier diode, and R1 and C1 are Despiking circuits. Figure Synchronous Rectifier Circuit. Working Principle: When the upper end of the transformer's secondary is positive, the current causes Q2 to turn on via C2, R5, R6, and R7; the circuit forms the loop, and Q2 is the rectifier.
Because of the reverse bias, the gate Q1 is turned off. When the lower end of the transformer's secondary is positive, the current causes Q1 to turn on via C3, R4, and R2, and Q1 is a freewheeling tube. Because of the reverse bias, the gate Q2 is turned off. Despiking circuits are R1, C1, R9, and C4. Schematic of Voltage Feedback Loop Circuit. When it surpasses the reference voltage of pin 2 of U1, pin 1 of U1 outputs a high level, turning on Q1 and the optocoupler OT1 LED, the phototransistor, and the potential of pin 1 of the UC, causing the output duty ratio of pin 6 of U1 to fall and U0 to be decreased.
When the output U0 decreases, the voltage of pin 3 of U1 decreases. When it is lower than the reference voltage of pin 2 of U1, pin 1 of U1 outputs a low level, Q1 does not conduct, the optocoupler OT1 LED does not emit light and the phototransistor does not conduct.
The potential of pin 1 of the UC rises high, thus changing the output duty cycle of pin 6 of U1 to increases and U0 decreases. Repeatedly, the output voltage is kept stable. Adjusting VR1 can change the output voltage value. The feedback loop is an important circuit that affects the stability of the switching power supply. Feedback resistor capacitance error, leakage, virtual soldering and so on will produce self-oscillation.
The fault phenomenon is waveform abnormality, empty or full load oscillation, output voltage instability and so on. It has several methods for implementing the current limiting circuit.
Only another part of the circuit will be added if the power limiting current does not operate when it is short-circuited. The following figure shows a low-power short-circuit protection circuit. Short-circuit Protection Circuit. The principle is as follows:. When the output circuit is short-circuited, the output voltage disappears, the optocoupler OT1 is not switched on, the voltage of UC pin 1 rises to around 5V, and the voltage division of R1 and R2 exceeds the TL reference and causes it to turn on.
When UC fails, the potential of pin 1 vanishes and TL does not switch on. The potential of UC pin 7 rises, and the UC restarts and restarts again and again.
When the short circuit is removed, the circuit will immediately resume normal operation. Medium Power Short-circuit Protection Circuit. When the output is short-circuited, the voltage of UC pin 1 rises, and the potential of U1 pin 3 is greater than that of pin 2. The comparator's pin 1 generates a high potential to charge C1. Pin 7 of U1 produces a low potential when the voltage across C1 exceeds the reference voltage of pin5. When the output voltage falls below 0V, the circuit restarts.
When the short circuit is removed, the circuit resumes normal operation. R2 and C1 are charge and discharge time constants, and when the resistance value is incorrect, the short circuit protection does not work.
Current Limiting and Short-circuit Protection Circuit. Its working principle is briefly described as follows:. The output duty ratio of pin 6 of UC is gradually incre ased. When the voltage of pin 3 exceeds 1V, the UC is turned off and has no output.
It has low power consumption, but high cost and a complicated circuit. A Protection Circuit. The working principle is as follows:. If the output circuit is short-circuited or the current is too large, the voltage induced by the TR1 secondary coil will be higher. When pin 3 of UC exceeds 1 volt, the UC stops working and repeats.
When the short circuit or overload disappears, the circuit recovers itself. Output Current Limiting Protection Circuit. The circuit seen in the diagram above is a standard output current limiting protection circuit. Its operation is depicted in the diagram above: When the output current is too high, the voltage across RS manganese copper wire rises, and the voltage at pin 3 of U1 exceeds the reference voltage at pin 2.
Pin 1 of U1 generates a high voltage, Q1 is activated, and the optocoupler exhibits a photoelectric effect. The voltage on UC pin 1 is reduced, as is the output voltage, fulfilling the goal of output overload current limitation.
When the output voltage exceeds the designed value, the output overvoltage protection circuit limits the output voltage to a safe value. When the switching power supply's internal voltage regulation loop fails or the output overvoltage phenomena are produced by the user's improper operation, the overvoltage protection circuit protects to prevent harm to the power equipment of the subsequent circuit.
The most common overvoltage protection circuits are as follows:. When the output circuit is short-circuited or over-current, the primary current of the transformer increases, the voltage drop across R3 increases, and the voltage of pin 3 rises. Thyristor Trigger Protection Circuit. The voltage of Uo2 is short-circuited to the ground, and the overcurrent protection circuit or the short circuit protection circuit will work to stop the operation of the entire power supply circuit.
When the output overvoltage phenomenon is eliminated, the control terminal trigger voltage of the thyristor is discharged to the ground through R, and the thyristor is restored to the off state. Photoelectric Coupling Protection Circuit.
As shown above, when Uo has an overvoltage phenomenon, the Zener diode breaks through and conducts current through the optocoupler OT2 R6 to the ground, and the LED of the photocoupler lights, thereby making the phototransistor of the photocoupler on.
The base of Q1 is electrically turned on, and pin 3 of is reduced so that the IC is turned off and the operation of the entire power supply is stopped. Uo is zero, and the cycle is repeated. The output voltage limiting protection circuit is as shown in the figure below.
When the output voltage rises, the Zener diode and the optocoupler turn on, and the Q1 base turns on with a driving voltage. The voltage of UC rises, the output decreases, and the Zener tube does not conduct. The voltage of UC is lowered and the output voltage is raised. Repeatedly, the output voltage will stabilize within a range depending on the voltage of the regulator.
Output Voltage Limiting Protection Circuit. Output Overvoltage Lockout Circuit. When the output voltage Uo rises, the Zener diode and optocoupler are switched on, and the base of Q2 is electrically turned on, as shown in Figure 19 a. Because Q2 is switched on, the base voltage of Q1 is dropped and turned on as well, and the Vcc voltage keeps Q2 on all the time via R1. R2 and Q1. Pin 3 of the UC is always high and the device stops working.
In Figure 19 b , UO rises, and the voltage on U1's pin 3 rises. Because D1 and R1 are present, Pin 1 always produces a high level, and Pin 1 of U1 always outputs a high level.
Q1 is always on, and UC pin 1 is always low and quits working. Schematic Diagram. Power Factor Correction Circuit. Working P rinciple. In other words, it is divided by R1 and R2 and then supplied to the PFC controller as an input voltage sampling for adjusting the duty ratio of the controlled signal, i. The PFC voltage is passed on to the next circuit. It is divided by R3 and R4 and then given to the PFC controller as an input voltage sampling for adjusting the duty ratio of the control signal and stabilizing the PFC output voltage in another method.
Working Principle. The fundamentals of input over-voltage and under-voltage protection of AC input and DC input switching power supply are extremely similar. The protection circuit's sampling voltage is derived from the input filtered voltage. If the sampling voltage exceeds the reference voltage of pin 2, pin 1 of the comparator outputs a high signal, causing the main controller to shut down and the power supply to shut down. This document was uploaded by user and they confirmed that they have the permission to share it.
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It eliminates your headache in solving many different types of power supplies problems. Check out the advantages again in solving the below problems: When there is lightning strike on the power supply, usually there will be some bad components. In some cases the power side components could burn beyond recognition.
If there is no schematic diagram or same model of power supply to compare to locate the original parts value, it would be tough to repair the power supply. This modification method can truly save the cost. Save time- Sometime we spent too much time in troubleshooting the power supplies. Easy to follow- It can be done in less than 20 to 25 minutes if you follow my steps.
The most important thing is the parts are quite cheap and you can check out where to buy the parts at page He told me someone had tried repair it before but unsuccessful. The moment I opened the cover I saw the power IC was missing and I do not know what the original part number was. The power FET shorted and part of the current sense resistor outer layer also came off see the photo below and next page.
Even if you could find the power IC you still have to proceed to buy the parts, replace the power FET and check all the primary side components and make sure they are all good. This also will not guarantee the power supply will come back to life after it had been badly repaired by someone.
Precious time will be lost doing a repair work that had been done by others. In my opinion the best solution is to modify with a universal external power module onto it. How this can be done? Check the photos in the next page and read the step by step explanations.
If you do not have a Blue Ring tester no worries because usually the primary winding of power transformers are quite robust and rarely have problem. Besides the above components, you are also required to check on the corresponding components in the power supply especially the semiconductors like diode, zener diode and transistor if have and made sure they are good.
Assuming all of the components mentioned above tested good, you now can begin to install the universal external power module. Apply some heat compound to the module before fitting it to the heatsink.
See the photo in the next page. Before you turn the power supply On solder a watt light bulb on the fuse points so that if there is any error in fixing the wires, the power module would not blow. If the power module blows you will waste your precious time and have to redo everything again. Note: Please refer to page 26 on how to connect the bulb.
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