Basic composition and working principle of switching power supply

I. Overview

Power supply is an indispensable component of various electronic equipment, and its performance is directly related to the technical indicators of the electronic equipment and whether it can work safely and reliably. Since the internal key components of the switching power supply operate in a high-frequency switching state, with low power consumption, high conversion rate, and only 20%-30% of the volume and weight of the linear power supply, it has become the mainstream product of regulated power supply. The maintenance of electrical faults in electronic equipment is based on the principle of starting from easy to difficult. Basically, it starts with the power supply. After confirming that the power supply is normal, maintenance of other parts is carried out. Power supply faults account for the majority of electrical faults in electronic equipment. . Therefore, understanding the basic working principles of the power supply and being familiar with its maintenance techniques and common faults will help shorten the repair time of electronic equipment faults and improve personal equipment maintenance skills.

2. Composition of switching power supply

The switching power supply is composed of four major parts: main circuit, control circuit, detection circuit, and auxiliary power supply, as shown in Figure 1.

1. Main circuit

Inrush current limiting: Limit the inrush current on the input side at the moment when the power is turned on.

Input filter: Its function is to filter the clutter existing in the power grid and prevent the clutter generated by the machine from being fed back to the power grid.

Rectification and filtering: directly rectify the grid AC power into smoother DC power.

Inverter: converting rectified DC power into high-frequency alternating current, which is the core part of high-frequency switching power supply.

Output rectification and filtering: Provide stable and reliable DC power supply according to load needs.

2. Control circuit

On the one hand, samples are taken from the output terminal, compared with the set value, and then the inverter is controlled to change its pulse width or pulse frequency to stabilize the output. On the other hand, based on the data provided by the test circuit and identified by the protection circuit, it provides The control circuit performs various protection measures on the power supply.

3. Detection circuit

Provide various running parameters and various instrument data in the protection circuit.

4. Auxiliary power supply

Realize the software (remote) startup of the power supply to provide power for the protection circuit and control circuit (PWM and other chips).

3. Working principle of switching power supply

The switching power supply uses power semiconductor devices as switching elements, and controls the duty cycle of the switching elements to adjust the output voltage by periodically switching on and off. The switching element is repeatedly turned on and off at certain time intervals. When the switch is turned on, the input power Vi provides energy to the load RL through the switch S and the filter circuit. When the switch S is turned off, the energy storage device in the circuit (The circuit composed of L1, C2, and diode D) releases the energy stored when the switch is turned on to the load RL, so that the load receives continuous and stable energy.

VO=TON/T*Vi

VO is the average voltage across the load

TON is the time the switch is turned on each time

T is the duty cycle of the switch on and off

It can be seen from the formula that when the ratio of the switch on time and the working cycle is changed, the average voltage between VO will also change. Therefore, as the load and input power supply voltage change, the ratio of TON and T will be automatically adjusted to keep the output voltage VO constant. Change. Changing the on-time TON and the duty cycle ratio is also changing the duty cycle of the pulse. This method is called "time ratio control" (TimeRationControl, abbreviated as TRC).

According to the TRC control principle, there are three ways:

1. PulseWithModulation (PulseWithModulation, abbreviated as PWM)

The switching period is constant, and the duty cycle is changed by changing the pulse width.

2. Pulse Frequency Modulation (PFM)

The on-pulse width is constant, and the duty cycle is changed by changing the switching operating frequency.

3. Mixing and modulation

Both the conduction pulse width and the switching operating frequency are not fixed and can be changed by each other. It is a mixture of the above two methods.

4. Maintenance skills and common faults of switching power supply

1. Maintenance skills

Maintenance of switching power supply can be divided into two steps:

In case of power outage, "look, smell, ask, and measure"

Look: Open the casing of the power supply, check whether the fuse is blown, and then observe the internal conditions of the power supply. If you find burnt spots or broken components on the PCB of the power supply, you should focus on checking the components here and related circuit components.

Smell: Smell if there is a paste smell inside the power supply, and check if there are any burnt components.

Question: I want to ask how the power supply was damaged and whether any illegal operations were performed on the power supply.

Measurement: Before powering on, use a multimeter to measure the voltage across the high-voltage capacitor. If the fault is caused by the failure of the switching power supply to vibrate or the open circuit of the switch tube, in most cases, the voltage at both ends of the high-voltage filter capacitor is not discharged. This voltage is more than 300 volts, so be careful. Use a multimeter to measure the forward and reverse resistance at both ends of the AC power cord and the charging condition of the capacitor. The resistance value should not be too low, otherwise there may be a short circuit inside the power supply. Capacitors should be able to charge and discharge. Disconnect the load and measure the resistance to ground of each group of output terminals. Normally, the needle of the meter should swing when charging and discharging the capacitor, and the final indication should be the resistance of the bleeder resistor of that circuit.

Power on test

After powering on, observe whether the power supply has burnt fuses or smoke from individual components. If so, cut off the power supply in time for maintenance.

Measure whether there is 300 volt output at both ends of the high-voltage filter capacitor. If not, focus on checking the rectifier diode, filter capacitor, etc.

Measure whether there is output from the secondary coil of the high-frequency transformer. If not, you should focus on checking whether the switch tube is damaged, whether it is oscillating, whether the protection circuit operates, etc. If so, you should focus on checking the rectifier diode, filter capacitor, and three-way stabilizer on each output side. Pressure tube etc.

If the power supply stops as soon as it is started, the power supply is in a protection state. You can directly measure the voltage of the PWM chip protection input pin. If the voltage exceeds the specified value, it means that the power supply is in a protection state. You should focus on checking the cause of the protection.

2. Common faults

blown fuse

Generally, a blown fuse indicates there is a problem with the internal wiring of the power supply. Since the power supply operates at high voltage and high current, fluctuations and surges in the grid voltage will cause the current in the power supply to increase instantaneously and cause the fuse to blow. The focus should be to check the rectifier diode, high-voltage filter electrolytic capacitor, inverter power switch tube, etc. at the power input end to check whether these components have breakdown, open circuit, damage, etc. If the fuse is indeed blown, you should first check the various components on the circuit board to see if the appearance of these components is burned and whether the electrolyte has overflowed. If the above conditions are not found, use a multimeter to measure whether the switch tube has a breakdown or short circuit. . Special attention should be paid to this: when a component is found to be damaged, you must not turn it on directly after replacing it. This is very likely to damage the replaced component because other high-voltage components are still faulty. Be sure to conduct a comprehensive inspection of all high-voltage components in the above circuit. Only after checking and measuring can the fault of the blown fuse be completely eliminated.

No DC voltage output or unstable voltage output

If the fuse is intact, there will be no DC voltage output at all levels under load. This situation is mainly caused by the following reasons: open circuit or short circuit in the power supply, failure of the overvoltage and overcurrent protection circuit, auxiliary power supply failure, oscillation circuit not working, overload of the power supply, rectifier diode in the high-frequency rectifier filter circuit Breakdown, filter capacitor leakage, etc. After measuring the secondary components with a multimeter and ruling out the breakdown of the high-frequency rectifier diode and the load short circuit, if the output is zero at this time, it is certain that the control circuit of the power supply is faulty. If there is some voltage output, it means that the front-end circuit is working normally, and the fault lies in the high-frequency rectifier and filter circuit. The high-frequency filter circuit mainly consists of a rectifier diode and a low-voltage filter capacitor to form a DC voltage output. The breakdown of the rectifier diode will cause the circuit to have no voltage output, and the leakage of the filter capacitor will cause output voltage instability and other faults. Damaged components can be detected by statically measuring the corresponding components with a multimeter. Example: There is no 24-volt DC output after a certain 24-volt DC motor power supply is powered on. Open the power supply casing and observe that the fuse is not blown and that the circuit board has no obvious burnt spots or cracked components. Measure the AC input terminal when the power is not supplied. The resistance value and DC output terminal resistance are normal, and important components such as switch tubes, rectifier bridges, and rectifier tubes are normal. Therefore, it is judged that there is no possibility of serious internal short circuit, and the protection circuit is estimated to be activated. After checking that this switching power supply uses the U3842 PWM control chip, and searching for relevant information, we found that when the 3-terminal voltage of the U3842 chip is higher than 1 volt, the internal current-sensitive comparator outputs a high level, which resets the PWM latch and causes the output closure. The power-on measurement shows that terminal 3 of U3842 is higher than 1 volt, and terminal 6 has no output. After checking the relevant circuit, it is found that the voltage regulator tube D2 has broken down, as shown in Figure 3, so PC1 is turned on, causing terminal 3 of U3842 to be at a high level, so 6 There is no output at the terminal, the switch tube does not work, and there is no DC output on the DC side. Replace the voltage regulator tube D2 of the same model and the fault is resolved.

Poor power load capacity

Poor power supply load capacity is a common fault, which usually occurs in old-fashioned or long-working power supplies. The main reasons are aging of components, unstable switching tubes, and lack of timely heat dissipation. You should focus on checking whether the Zener diode is hot and leaking, the rectifier diode is damaged, the high-voltage filter capacitor is damaged, etc. For example: Our factory's near-red laser spectrometer (VECTOR 22) cannot complete the self-test and alarms after starting up, and the mainboard indicator light keeps flashing. After inspection, the DC 5V power supply for the main board of the spectrometer is only about 2.3 volts. Disconnect the load of the 5V DC power supply and measure the 5V DC power supply again. At this time, there is 5V. It is preliminarily judged that the load capacity of the 5V DC power supply is poor, so we disassemble it. The power supply casing is inspected. Since there is a DC 5V output when there is no load, the focus is on checking the output rectifier circuit on the secondary coil side. Connect the 5V power supply to a dummy load and power on for measurement. It is found that 1 and 2 of the three-way voltage regulator 7805 are The voltage between pins is 5.2 volts, but there is 2.3 volts left between pins 2 and 3, as shown in Figure 4. Therefore, it is judged that the performance of the three-way voltage regulator tube 7805 has deteriorated. Replace the three-way voltage regulator tube 7805 to solve the problem.

5. Conclusion

At present, switching power supply is widely used in almost all electronic equipment due to its small size, light weight and high efficiency. It is an indispensable power supply method for the rapid development of today's electronic information industry. As an equipment maintenance professional, it is necessary to understand the basic working principles of switching power supplies, master its maintenance skills, and be familiar with its common faults. This will help reduce the maintenance costs of electronic equipment, shorten its fault maintenance time, and improve its own skill level.