Nowadays, most peripherals use switching power supplies for voltage transformation. Although switch mode power supplies have the characteristics of small size, high work efficiency, and good voltage stabilization effect, they are directly connected to the mains power supply, and changes in mains voltage and surges can cause damage to the switch mode power supply. The circuit of a switching power supply is quite complex, and many enthusiasts are helpless when it comes to power supply damage. In fact, as long as we have a certain understanding of it, repairing it is not difficult.

The principle of switch mode power supply is roughly the same. Here, we take the example of the no voltage output of the switch mode power supply that comes with the HP3748 printer to explain the working principle and fault detection methods of the switch mode power supply.

Understand the working principle

If we want to learn how to troubleshoot switch mode power supplies, we need to have an understanding of their working principles and which components are prone to damage. When the mains power is input from the input terminal, it first reaches the L-type or π - type filtering circuit composed of capacitors and inductors for filtering, in order to eliminate surge voltage and interference signals in the mains power and improve the quality of the power supply. At the same time, there is also a fuse connected in series at the input end of the mains power supply. When a short-circuit fault occurs in the power supply, the fuse will melt to prevent the fault from expanding. Moreover, most switch mode power supplies now also have varistors at their input terminals. When the voltage is normal, this resistor has an infinite resistance value and does not affect the operation of the circuit. Once the voltage is too high, the varistor will short-circuit, causing the current through the fuse to increase and the fuse to melt, avoiding damage to other components caused by high voltage.

After filtering, the AC power is rectified by a diode bridge rectifier circuit and a high voltage large capacity capacitor to generate a high voltage DC voltage of 300V. The voltage is then reduced by a resistor and stabilized by a simple voltage regulator before being sent to an oscillation control circuit to generate an oscillation signal. The generated oscillation signal is amplified by a power source oscillator and combined with a high-frequency transformer to be converted into a low voltage AC voltage. After another rectification and filtering process, the low voltage AC voltage can generate various low voltage DC power that can be used for equipment. In addition, there is a voltage sampling feedback circuit at the main voltage output terminal, which feeds back the current voltage to the oscillation control circuit. Once the main voltage drifts due to load changes, the oscillation control circuit will change the oscillation pulse width to ensure the stability of the output voltage. At the same time, when the load is short circuited, the sampling feedback signal will also promptly notify the oscillation control circuit to stop the voltage output, avoiding damage to the power supply due to overload.