1. dummy load method
When repairing the switching power supply, in order to distinguish whether the fault lies in the load circuit or the power supply itself, it is often necessary to disconnect the main load, and add a dummy load to the main voltage output of the switching power supply to test the machine, as shown in Figure 4-1. The reason why the dummy load is connected is because the energy stored in the primary winding of the switching transformer will be released to the secondary side during the cut-off period of the switching tube. Tube breakdown damage. Regarding the dummy load, it should be selected according to the output voltage (or power) of the switching power supply. Generally speaking, if the output voltage is above 100V, a 40-100W bulb or a high-power resistor of about 300Q should be selected as the dummy load; If the output voltage is below 30V, you can choose a light bulb or a 600Ω～lkΩ high-power resistor for the dummy load used in the car/motorcycle.
2. short circuit method
Parallel switching power supplies generally use a direct sampling voltage regulator control circuit with a photocoupler. When the output voltage is high, the short-circuit method can be used to distinguish the fault range.
The process of the short-circuit repair method is: first short-circuit the two feet of the photosensitive receiver tube of the photocoupler, which is equivalent to reducing the internal resistance of the photosensitive receiver tube. side of the primary circuit). Conversely, the fault is in the circuit before the optocoupler.
It should be noted that the short-circuit method should be carried out in a targeted manner on the basis of familiarity with the circuit, and cannot be blindly short-circuited to avoid expanding the fault. In addition, from the safety point of view of maintenance, the load circuit should be disconnected before the short circuit.
3. series bulb method
The so-called series light bulb method is to remove the fuse (fuse) of the input circuit and use a 60W/220V light bulb in series at both ends of the fuse. When the AC power is connected, if the light bulb is very bright, it means that the circuit has a short circuit. Since the bulb has a certain resistance, such as a 60W/220V bulb, its resistance is about 500Ω (referring to thermal resistance), so it plays a certain current limiting role. In this way, on the one hand, the failure of the circuit can be roughly judged by the brightness of the light bulb; Until the short circuit fault is eliminated, the brightness of the bulb will naturally dim, and finally remove the bulb and replace the fuse.
4. Substitution method
The substitution method is divided into component-level substitution and board-level substitution.
Component-level replacement refers to replacing suspected components with normal components. If the switching power supply works normally after the replacement, it means that the replaced components are damaged. In the switching power supply, some components can be directly judged by a multimeter, such as resistance; some are difficult to judge, such as the power control chip. Therefore, for components that are not easy to judge, if there is a problem with them during maintenance, it is recommended to replace them with correct components to improve maintenance efficiency.
Board-level replacement refers to the overall replacement of a part of the switching power supply or power supply circuit of the whole machine. This maintenance method is mainly used for the maintenance of large-area components in the switching power supply or when the switching power supply has difficult failures. The characteristics of this maintenance method are: thorough troubleshooting and high maintenance efficiency, but the cost is relatively high.
