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Switching Power Supply

A power supply is an electrical device that converts the electric current that comes from a power source to the voltage value necessary for powering a load, like a motor or an electronic device. There are two main designs for power supplies: a linear power supply and a switching power supply.

Linear: A linear power supply designs use a transformer to step down the input voltage. Then the voltage is rectified and turned into a direct current voltage, which is then filtered to improve the waveform quality. Linear power supplies use linear regulators to maintain a constant voltage at the output. These linear regulators dissipate any extra energy in the form of heat.

Switching: A switching power supply design is a newer methodology developed to solve many of the problems associated with linear power supply design, including transformer size and voltage regulation. In switching power supply designs, the input voltage is no longer reduced; instead, it’s rectified and filtered at the input. Then the voltage goes through a chopper, which converts it into a high-frequency pulse train. Before the voltage reaches the output, it’s filtered and rectified once again.

Switching AC/DC power supplies offer increased performance for a fraction of the size, which is what has made them so popular. The downside is that their circuits are significantly more complex, and they require more precise control circuits and noise cancellation filters. But is is currently the most efficient way of transforming AC power to DC power. The power is converted in three stages:

1. Input rectification: This process takes the AC mains voltage and converts it into a DC rectified wave using a diode bridge. A capacitor is added at the output of the bridge to reduce the ripple voltage.

2. Power factor correction (PFC): Because of the nonlinear current in the rectifier, the harmonic content of the current is quite large. There are two ways two resolve this. The first is passive PFC, using a filter to dampen the effect of the harmonics, but it is not very efficient. The second option, called active PFC, uses a switching boost converter to make the current waveform follow the input voltage shape. Active PFC is the only method of designing a power converter that meets current standards of size and efficiency.

3. Isolation: Switching power supplies can be isolated or non-isolated. A device is isolated when the power supply’s input and output are not physically connected. Isolation is done through the use of transformers, which galvanically isolate the two halves of the circuit. However, transformers can only transfer electric power when there is a variation in current, so the rectified DC voltage is cut up into a high-frequency square wave, which is then transferred to the secondary circuit, where it is rectified again and finally transmitted to the output. There are many different aspects to consider when designing a switching power supply, especially related to safety, performance, size, weight, etc. The control circuits for switching power supplies are also more complex than in linear power supplies.
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