Twenty questions and answers on basic power supply knowledge

1. What is the basic working principle of power supply?

A: It converts the input higher alternating current voltage (AC) into the lower direct current voltage (DC) required for the PC to work by running high-frequency switching technology.

2. What is the working process of the power supply?

A: When the mains electricity enters the power supply, it first passes through the choke coil and capacitor filter to remove high-frequency clutter and interference signals, and then passes through rectification and filtering to obtain high-voltage DC. Then the DC is converted into high-frequency pulsating DC through the switching circuit, and then sent to the high-frequency switching transformer for voltage reduction. Then the high-frequency AC part is filtered out, so that the final output is relatively pure low-voltage DC for use by the computer.

3. What is the main function of EMI circuit?

A: The function of the EMI circuit is to filter out various interference signals coming from the power grid and prevent the high frequency generated by the power switch circuit from interfering with the power grid. EMI is an important part of CCC certification.

4. What is a high voltage rectifier filter circuit?

A: The high-voltage rectifier filter circuit consists of a rectifier bridge and two high-voltage electrolytic capacitors. Its function is to convert 220V AC mains power into 300V DC power.

5. What are the common types of high-voltage electrolytic capacitors?

A: High-voltage electrolytic capacitors are what we usually call large capacitors. There are usually two of them. Because of their high withstand voltage, they are very large. According to capacity, high-voltage electrolytic capacitors are generally 330uf, 470uf, 680uf, 820uf, 1000uf, 1200uf, etc. The withstand voltage is generally 200V and the temperature is 85 degrees.

6. What is the principle of the switching circuit?

Answer: The principle of the switching circuit is that the switch tube and the PWM (Pulse Width Modulation) control chip form an oscillation circuit to generate high-frequency pulses. The high-voltage DC generated by the high-voltage rectifier and filter circuit is converted into high-frequency pulse DC, which is sent to the main transformer for voltage reduction and converted into low-frequency pulse DC.

7. What is the principle of low voltage rectifier filter circuit?

Answer: After the low-frequency pulsed DC is rectified by a diode, it is filtered by an electrolytic capacitor, so that the output is a stable current of different voltages. Since the voltage here is already very low, although the capacitor capacity is large, usually 1000uf, 2200uf, etc., it does not require a very high withstand voltage value, so the capacitor volume is very small.

8. What is the function of the auxiliary circuit?

Answer: 300V DC becomes pulse current through the auxiliary power switch tube, and outputs two sets of AC voltage through the auxiliary power transformer. One is rectified and stabilized by a three-terminal voltage regulator, and outputs +5VSB, which is added to the motherboard as standby voltage; the other is rectified and filtered, and outputs auxiliary 20V power supply to supply PWM and other chips. With the auxiliary circuit, the computer can realize software startup and shutdown.

9. What is PFC?

Answer: PFC (Power Factor Correction) is mainly used to characterize the efficiency of electronic products in using electric energy. The higher the power factor, the higher the efficiency of using electric energy. All computer power supplies that have passed the CCC certification must have a PFC circuit. The position is after the second layer of filtering and before the full-bridge rectifier circuit. There are two types of PFC, one is passive PFC (also called passive PFC) and the other is active PFC (also called active PFC).

10. What are the characteristics of active PFC?

Answer: The active PFC input voltage can be from 90V to 270V; the power factor is higher than 0.99, and it has the advantages of low loss and high reliability; it can be used as an auxiliary power supply without the need for an auxiliary power transformer; the output DC voltage ripple is very small, so the power supply using active PFC does not need to use a large-capacity filter capacitor.

11. What are the characteristics of passive PFC?

Answer: Passive PFC generally uses an inductor compensation method to improve the power factor by reducing the phase difference between the fundamental current and voltage of the AC input. The power factor of passive PFC is not very high, only reaching 0.7~0.8, and the heat generated is relatively large.

12. How is the software power on/off function of the power supply realized?

A: The software power on/off function of the power supply is realized through the PW-OK circuit. When in standby mode, PW-OK outputs a zero-level power self-test signal to the host, and the host stops working and is in standby mode. After the controlled startup, PW-OK delays for several hundred milliseconds after the output voltage of the switching power supply stabilizes, jumping from 0 level to +5V, and outputs a high-level signal to the host. This signal is equivalent to the AT power supply.

The host starts the system after detecting the PW-OK signal that the power supply is intact. If the mains power is cut off or the host is shut down during operation, the PW-OK output signal disappears several hundred milliseconds earlier than the +5V output voltage of the ATX switching power supply, notifying the host to trigger the system to automatically shut down before the power is cut off, to prevent the hard disk head from being scratched when the power is suddenly cut off because it has no time to move to the landing area.

13. What is conducted interference?

A: Conducted interference is a concept used to measure the size of electronic interference signals sent by electronic products to the entire power grid during operation. All electronic products will send interference signals to the power grid when using electricity. If the interference signal is too large, it will affect the power quality of the entire power grid, thereby interfering with the normal operation of other electrical appliances. Therefore, most countries have a rigid regulation on the conducted interference index of electronic products, prohibiting the production and sale of products with excessive conducted interference.

14. What are the more important items in power supply testing?

Answer: The main ones include cross load, surge, input voltage, ripple noise, output short circuit, over power, conversion efficiency, power factor, response time, timing, noise, conducted radiation, leakage current, high and low temperature test, etc.

15. What is surge current?

A: Inrush current refers to the peak current that flows into the power supply device when the power is turned on. Since the input filter capacitor charges quickly, the peak current is much larger than the steady-state input current. The power supply should limit the surge level that the AC switch, rectifier bridge, fuse, and EMI filter components can withstand. With repeated switching loops, the AC input voltage should not damage the power supply or cause the fuse to blow.

16. What is conversion efficiency?

A: When the power supply is working, part of the electrical energy is converted into heat and lost. Therefore, the power supply must minimize the heat loss. The conversion efficiency is the percentage of output power divided by input power. The 1.3 version of the power supply requires a minimum conversion efficiency of 70% under full load. The 2.0 version has increased the recommended conversion efficiency to 80%.

17. What is the difference between power factor and conversion efficiency?

A: Although both power factor and conversion efficiency refer to the utilization rate of power supply, there is a big difference between them. Simply put, the loss caused by power factor is borne by the power department, while the loss of conversion efficiency is borne by the user. It can be seen that power factor, EMI, etc. are all protections for the national power grid.

18. What is rated power?

A: Rated power refers to the maximum load of a power supply under stable and continuous operation. Rated power represents the real load capacity of a power supply. For example, if the rated power of a power supply is 300W, it means that when it works continuously for 24 hours a day and 365 days a year, the sum of all loads cannot exceed 300W. But in fact, power supplies have a certain degree of redundancy. For example, a power supply with a rated power of 300W can still work stably and normally at 310W. However, try not to exceed the rated power, otherwise it may cause the power supply or other computer components to burn out due to overcurrent.

19. What is overpower protection?

A: In addition to the rated power, there is another data called "overload protection", or "OPP" in English. Overload protection means that when the load of the power supply continues to rise and reaches a certain point, the power supply will automatically cut off to prevent overcurrent from damaging the power supply or other components of the computer. The OPP value is usually about 1.3 times the rated power. Some manufacturers set the OPP too high, which is actually unsafe. There will be an interval between the rated power and the OPP. For example, the rated power of the new cold drill is 300W, and the OPP is 370W. Then, the area between 300-370W is a "blind zone." If you stay in this interval for too long (usually it can be

Lasts for tens of seconds), it is likely to cause the power supply or other components of the computer to burn out.

20. What is the principle of temperature-controlled power supply?

A: The temperature control circuit is mainly realized by thermistors. When the power supply starts working, the fan supply voltage is 7V. When the temperature inside the power supply rises, the thermistor resistance decreases, the voltage gradually increases, and the fan speed also increases. In this way, the temperature inside the casing can be kept at a low level. When the load is very light, a silent effect can be achieved. When the load is heavy, heat dissipation can be guaranteed.