1. The power supply voltage of the secondary load is 0V. There is no response after the inverter is powered on, there is no indication on the operation display panel, and the 24V and 10V voltages of the measurement and control terminals are 0V. Check that the charging resistance or pre-charging circuit of the main circuit is correct, which can be identified as a switching power supply fault. The overhaul process is as follows:
1. First, use the resistance measurement method to measure whether the switch Q1 has a breakdown short circuit, and whether the current sampling resistance R4 has an open circuit. The fragile component of the circuit is the switch tube, and when it is damaged, the resistance value of R4 becomes larger or the circuit is broken due to the impact. The G-pole series resistor and vibration chip PC1 of Q1 are often damaged by strong electrical shock and must be replaced together; Check whether there is a short circuit in the load circuit and sweep it.
2. Replace the damaged parts, or there are short-circuit components that have not been detected, you can power on to check and further determine whether the fault is in the vibration circuit or the voltage stabilization circuit.
How to view:
a. Check whether the starting resistor R1 is open first. After normal, the 18V DC power supply is directly sent to the 7th and 5th pins of the UC3844 to power on the vibration circuit alone. Measure that there should be a 5V voltage output for the 8 pins; Pin 6 should have a voltage output of about 1V. It is clarified that the vibration circuit is fundamentally normal, and the fault is in the voltage stabilization circuit;
If the 8 pins have 5V voltage output, but the 6 pins are 0V, check the 8 and 4 pins with R and C timing components and the 6 pins of the peripheral circuit;
If the voltage of pin 8 and pin 6 is 0V, the UC3844 vibrating chip is broken and replaced.
b. Power on UC3844 alone, short circuit PC2 input side, if the circuit vibrates, clarify the fault in the peripheral circuit of PC2 input side; The circuit still does not vibrate, check the output side circuit of PC2.
2. The switching power supply presents intermittent vibration, and the "hiccups" or "squeaks, squeaks" can be heard, or the "hiccups" sound cannot be heard, but the operation display panel is on and off. This is a typical fault feature of overcurrent maintenance circuit action caused by overload of power supply due to abnormal load circuit. The abnormal rise of the load current causes a large fluctuation of the excitation current of the primary winding, and a voltage signal of more than 1V is formed in the current sampling resistor R4, so that the internal current detection circuit of UC3844 starts to control and the circuit stops vibrating; The overcurrent signal on R4 disappears, and the circuit vibrates from the beginning, and so on, and the power supply vibrates intermittently.
How to view:
a. Measure the resistance value of the power supply circuit C4 and C5, if there is a short-circuit straight-through phenomenon, it may be a short circuit of the rectifier diode D3 and D4; Observe the appearance of C4 and C5 for drum tops, liquid spray and other phenomena, and remove them for testing if necessary; There is no abnormality in the power supply circuit, and it may be a short-circuit component in the load circuit;
b. Check that the power supply circuit is abnormal, power on, and use the sweeping method to sweep each power supply one by one. If the fan power supply terminal is unplugged, the switching power supply works normally, and the operation display panel is normal, the 24V cooling fan has been damaged; If the +5V power supply connector is unplugged or the power supply copper foil is blocked, and the switching power supply works normally, the +5V load circuit has damaged components.
3. The power supply voltage of the load circuit is too high or too low. The vibration circuit of the switching power supply is normal, and the problem lies in the voltage stabilization circuit.
The output voltage is too high, and the components of the regulator circuit are damaged or inefficient, resulting in a lack of reaction voltage fluctuations. How to view:
a. Connect a 10k resistor at the output end of PC2, and the output voltage will drop back. It is clarified that the voltage regulator circuit on the output side of PC2 is normal, and the fault lies in the circuit of PC2 itself and the input side.
b. The 500Ω resistor is connected in parallel on R7, and the output voltage drops significantly. It is clarified that the optocoupler PC2 is excellent, and the fault is that PC3 is inefficient or PC3 external resistor element variable. Otherwise, PC2 is bad.
If the load supply voltage is too low, there are three possible problems: 1. The load is too heavy, which makes the output voltage drop; 2. The voltage regulator circuit component is defective, resulting in excessive voltage response signal; 3. The switch tube is inefficient, so that the circuit (switching transformer) is lacking in energy exchange.
View and repair methods:
a. Release the load circuits of the power supply branch one by one (attention!) Do not disconnect the load circuit by opening the power supply rectifier, especially the +5V power supply circuit with a voltage regulation response signal! The disappearance of the reaction voltage signal will cause the output voltage of each channel to rise abnormally, and the load circuit will be burned in large pieces!) Determine whether the voltage drops due to excessive load; For example, after blocking a certain power supply, the circuit rises back to the normal value, indicating that the switching power supply itself is normal, and the load circuit is checked; The output voltage is low, look at the regulator circuit.
b. Check the resistive element R5-R10 of the voltage stabilization circuit, and there is no variable value; Replace PC2 and PC3 one by one, if it is normal, it is clear that the replacement element is inefficient and the internal resistance becomes larger.
c. If the replacement of PC2 and PC3 is invalid, the fault may be the inefficiency of the switch tube, or the problem of the switch and excitation circuit, and the inefficiency of the internal output circuit of UC3844 will not be removed. Replace high-quality switch tube, UC3844.
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