Teach you how to repair switching power supply - Solutions - Huaqiang

Probe domestic switch needle KG-300K needle head diameter is 3.0mm normally open switch needle

First, we need to know how the switching power supply works. The power supply first turns the high voltage alternating current (220V) through the full bridge diode to become a high voltage fluctuating direct current, and then becomes a relatively smooth high voltage direct current after being filtered by the capacitor.

At this time, the control circuit controls the high-power switching tube to send the high-voltage direct current to the primary of the high-frequency transformer in batches according to a certain high-frequency frequency. Next, the stepped high-frequency low-voltage alternating current outputted from the secondary coil is converted into a direct current of a low voltage and a strong current capable of operating the load by rectifying and filtering. Among them, the control circuit is an indispensable part. It can effectively monitor the voltage value at the output and send a signal to the power switch tube to control the voltage up and down. In the computer switching power supply, the fault rate is the highest because the power input part operates in a high voltage and high current state; the rectifier diode, the protection diode, and the high power switch transistor of the output DC part are relatively easy to be damaged; and then the pulse width modulator Feedback and protection section. The following is the maintenance method for the general section:
First, in the case of power failure, "looking, smelling, asking, cutting"
note! : Before powering up, use a multimeter to measure the voltage across the high voltage capacitor first. If the switching power supply does not vibrate or the open circuit of the switch is faulty, in most cases, the voltage across the high-voltage filter capacitor is not discharged. This voltage is more than 300 volts. If you accidentally touch it, you must let it You have an unforgettable memory!
Since the power supply is exposed to 220V high voltage, the human body is in danger of being exposed to voltages above 36V. Therefore, under the possible conditions, try to check whether there is obvious short circuit or component damage in the power-off state. First, open the outer casing of the power supply, check whether the fuse is blown, and then observe the internal condition of the power supply. If the component on the PCB board of the power supply is found to be broken, the component should be inspected. Generally speaking, this is the main cause of the failure; Is there any smell inside, check if there are burnt components; ask if the power supply is damaged, and whether the power supply is operated illegally. This is necessary for repairing any equipment. After the initial inspection, the power supply should be tested more deeply.
Use a multimeter to measure the forward and reverse resistance of both ends of the AC power line and the charging condition of the capacitor. If the resistance value is too low, it indicates that there is a short circuit inside the power supply. When normal, the resistance should reach 100 kΩ or more. The capacitor should be able to charge and discharge if damaged. , the performance is that the resistance of the AC power line is low at both ends, which is short-circuited, otherwise it may be the breakdown of the switch.
Then check the DC output section. Disengage the load and measure the ground resistance of each group's output. When normal, the needle should have capacitor charging and discharging swing. The last indication should be the resistance of the bleeder resistor of the circuit. Otherwise, most of them are caused by reverse breakdown of the rectifier diode.
Second, the power-on test can pass the above test, it can be powered on. This is the key, and you need to have some experience, electronic foundation and maintenance skills. Generally speaking, it is important to check the input end of the power supply, the switching transistor, the power protection circuit, and the output voltage and current of the power supply. If the power supply is stopped, the power supply is in the protection state, and the voltage of the PWM chip protection input pin can be directly measured. If the voltage exceeds the specified value, the power supply is in the protection state, and the cause of the protection should be checked. Due to exposure to high voltages, it is recommended that friends who do not have an electronic foundation should be careful.
Third, common faults 1. Fuse blown In general, a blown fuse indicates a problem with the internal wiring of the power supply. Due to the high voltage and high current of the power supply, the fluctuations and surges of the power grid will cause the current in the power supply to increase instantaneously and the fuse will be blown. The key points should be to check the rectifier diode of the power input terminal, high-voltage filter electrolytic capacitor, inverter power switch tube, etc., check whether these components have breakdown, open circuit, damage, etc. If the fuse is really blown, you should first look at the various components on the board to see if the appearance of these components is burnt and there is no electrolyte spill. If the above situation is not found, use a multimeter to measure the presence or absence of a breakdown short circuit.

2. No DC voltage output or unstable voltage output

If the fuse is intact, there is no output of DC voltage at all levels under load. This situation is mainly caused by the following reasons: open circuit, short circuit phenomenon in the power supply, overvoltage, overcurrent protection circuit failure, the oscillation circuit does not work, the power supply load is too heavy, the rectifier diode in the high frequency rectifier filter circuit is broken down, Filter capacitor leakage, etc. After measuring the secondary component with a multimeter and eliminating the breakdown of the high-frequency rectifier diode and the short-circuit of the load, if the output is zero at this time, it is sure that the control circuit of the power supply has failed.
3. Poor power load capacity Poor power load capacity is a common fault. It is usually found in old-fashioned or long-running power supplies. The main reason is that the components are aging, the switching tube is unstable, and there is no heat dissipation in time. It is important to check whether the Zener diode is hot or leaky, the rectifier diode is damaged, and the high voltage filter capacitor is damaged.
Maintenance example 1. A computer ATX power supply, no voltage output, no noise in the power supply.
This is a typical feature of a power supply overload or no load. Check each component carefully, and check the rectifier diodes, switch tubes, etc. After careful inspection, it was found that the surface of a rectifier diode 1N4007 was blackened and the board was burnt black. Looking for the same type of diode to replace, using a multimeter is really a breakdown. Connected to the power supply, the fan does not turn, the hum is still. With a multimeter, the +12V output is only +0.2V, and +5V is only 0.1V. This means that the power supply is self-protected when the component is broken down. Measure the primary and secondary switch tubes and find that one of the primary switch tubes is damaged. Replace it with the same type of switch tube. The fault is removed and everything is normal.
General section: The above inspection has taken a detour. Before the power is turned on, the switch tube should be measured for damage.
Maintenance Example 2. Without a click, a fuse is burned on the last fuse.
As the fuse continually blows, the search range is reduced. There are only three possibilities: 1. Rectifier bridge breakdown; 2. Large electrolytic capacitor breakdown; 3. Primary switch tube breakdown. The rectifier bridge of the power supply is typically a discrete four rectifier diodes or a combination of four diodes. It is normal to remove the rectifier bridge by the amount. The large electrolytic capacitor is also normal after the test is removed. Pay attention to the positive and negative poles when soldering back. The last possibility is that there is only a switch. The primary of this power supply has only one high-power switch. Remove the amount and actually puncture, find the same type of switch tube, the problem is solved.

In fact, repairing the power supply is not difficult. Generally, the power supply damage can be attributed to the fuse blown, the rectifier diode is damaged, the filter capacitor is open or broken, the switch tube is broken down, and the power supply is self-protected. Because the circuit of the switching power supply is simple, the fault type is small. It is easy to determine the location of the fault. As long as there is enough electronic basic knowledge, look at the relevant technical articles, do more hands-on, usually pay attention to the accumulation of experience, power failure can be easily repaired.