Unlike linear power supplies, switching power supplies utilize transistors that primarily switch between fully on (saturation region) and fully off (cutoff region) modes. Both modes are characterized by low dissipation. While the transition between modes involves higher dissipation, the time is very short, thus saving energy and generating less waste heat. Ideally, the switching power supply itself does not consume electrical energy. Voltage regulation is achieved by adjusting the on and off times of the transistors. Conversely, in linear power supplies, the transistors operate in the amplification region during the output voltage generation process, consuming electrical energy themselves. The high conversion efficiency of switching power supplies is a major advantage. Furthermore, because switching power supplies operate at high frequencies, they can use smaller, lighter transformers, resulting in smaller size and lighter weight compared to linear power supplies.
If efficiency, size, and weight are key considerations, switching power supplies are better than linear power supplies. However, switching power supplies are more complex, as their internal transistors switch frequently. If the switching current is not properly managed, it can generate noise and electromagnetic interference that could affect other devices. Furthermore, without special design, the power factor of a switching power supply may not be high.
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 volt-ampere product across 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 volt-ampere product across 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.
The working principle of a switching power supply is:
1. The AC power input is rectified and filtered into DC;
2. By controlling the switching transistor with a high-frequency PWM (Pulse Width Modulation) signal, that DC current is applied to the primary winding of the switching transformer;
3. A high-frequency voltage is induced on the secondary side of the switching transformer, which is then rectified and filtered to supply the load.
4. The output section feeds back to the control circuit through a certain circuit to control the PWM duty cycle in order to achieve stable output.
When AC power is input, it generally passes through a choke coil or similar device to filter out interference from the power grid, and also to filter out interference from the power supply to the power grid.
At the same power, the higher the switching frequency, the smaller the size of the switching transformer, but the higher the requirements for the switching transistor;
The secondary winding of a switching transformer can have multiple windings or a single winding with multiple taps to obtain the desired output;
Generally, additional protection circuits should be added, such as no-load and short-circuit protection; otherwise, the switching power supply may be burned out.
