I. Switching Power Supply Circuit Principle
So, what is the working principle of a switching power supply circuit?
The working process of a switching power supply is quite easy to understand. In a linear power supply, the power transistor operates in linear mode. Unlike a linear power supply, a PWM switching power supply operates the power transistor in both on and off states. In these two states, the product of volt-amperes applied to the power transistor is very small (when on, the voltage is low and the current is high; when off, the voltage is high and the current is low). The product of volt-amperes on the power device is the loss generated on the power semiconductor device.
Compared to linear power supplies, PWM switching power supplies operate more efficiently by "chopping," which transforms the input DC voltage into pulse voltages with an amplitude equal to the input voltage. The duty cycle of these pulses is adjusted by the power supply's controller. Once the input voltage is chopped into an AC square wave, its amplitude can be increased or decreased using a transformer. Increasing the number of secondary windings in the transformer increases the output voltage. Finally, these AC waveforms are rectified and filtered to obtain the DC output voltage.
The main purpose of the controller is to maintain a stable output voltage, and its operation is very similar to that of a linear controller. That is, the controller's function blocks, voltage reference, and error amplifier can be designed to be the same as those of a linear regulator. The difference is that the output of the error amplifier (error voltage) passes through a voltage-to-pulse-width converter unit before driving the power transistor.
Switching power supplies have two main operating modes: forward converter and boost converter. Although the arrangement of their components is very similar, their operating processes are quite different, and each has its advantages in specific applications.
II. Methods for Troubleshooting Switching Power Supplies
1. Dummy load method
When repairing switching power supplies, 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 apply a dummy load to the main voltage output terminal of the switching power supply for testing. The reason for using a dummy load is that during the off-state of the switching transistor, the energy stored in the primary winding of the switching transformer needs to be released to the secondary side. Without a dummy load, the energy stored in the switching transformer has nowhere to be released, which can easily lead to the switching transistor breaking down and being damaged. 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 light bulb or a high-power resistor of around 300Ω should be selected as the dummy load; if the output voltage is below 30V, a light bulb used in automobiles/motorcycles or a 600Ω-1kΩ high-power resistor can be selected as the dummy load.
2. Short-circuit method
Parallel switching power supplies generally use direct sampling voltage regulation control circuits with optocouplers. When the output voltage is high, the short-circuit method can be used to distinguish the fault range.
The short-circuit troubleshooting process is as follows: First, short-circuit the two pins of the photosensitive receiver tube of the optocoupler. This is equivalent to reducing the internal resistance of the photosensitive receiver tube. If the main voltage still does not change, it indicates that the fault is after the optocoupler (on the primary circuit side of the switching transformer). Conversely, the fault is in the circuit before the optocoupler.
It should be noted that the short-circuit method should be performed selectively and only after familiarity with the circuit; blind short-circuiting should be avoided to prevent exacerbating the fault. Furthermore, from a safety perspective during maintenance, the load circuit should be disconnected before short-circuiting.
3. Series bulb method
The so-called series bulb method involves removing the fuse from the input circuit and connecting a 60W/220V light bulb in series across the fuse. When AC power is applied, if the bulb is very bright, it indicates a short circuit. Because the bulb has a certain resistance (approximately 500Ω for a 60W/220V bulb, referring to thermal resistance), it acts as a current limiter. This allows for a direct assessment of the circuit fault by observing the bulb's brightness; furthermore, the bulb's current-limiting effect prevents immediate damage to components in the short-circuited circuit. Once the short circuit is cleared, the bulb's brightness will naturally decrease. Finally, the bulb can be removed and the fuse replaced.
