Basic theory of Switching supplies

The incoming ac voltage is rectified and filtered to produce a high-voltage dc. A low-current, high-voltage bridge rectifier (that may not require a heat sink) can be used, as opposed to the linear bridge.

A power transistor_a metal-oxide semiconductor field-effect transistor (MOSFET)_is connected in series with the transformer. The MOSFET serves as an on-off switch and switches at a preset frequency. While the MOSFET is switching, the magnetic field in the transformer is building and collapsing, allowing energy to transfer to the secondary side.

The magnetic energy received by the secondary windings of the transformer is then full-wave-rectified and reconstructed into the proper dc level. A sample of parameters (Vout, Iload, etc.) can be sent back to the primary side to serve as input to the pulse-width modulator (PWM). The PWM circuit modifies the length of time that the MOSFET is switched "on" in order to maintain output regulation. For example, in a switching power supply producing 12Vdc and powering a 3A load, an increase in the load to 4A causes the output voltage to drop slightly. The feedback circuit detects the voltage drop and passes it to the PWM, which turns the MOSFET on for a longer period (i.e., it increases the duty cycle), causing more magnetic energy to transfer to the secondary side until the output voltage reaches its predetermined value.

Switching frequency for a switching power supply usually ranges between 30kHz and 150kHz, but it can be much higher. Frequency for linear power supplies is the same as the line frequency (60Hz in North America). Switching frequency selection depends on the application for which the power supply is designed. Because high frequency switching occurs at fo, harmonics are generated at 3fo, 5fo, 7fo.... The selection of the frequency has to be such that none of these harmonics will interfere with the load. With power supplies for two-way radios, for example, the switching frequency should be selected so as not to interfere with VHF, UHF or the intermediate frequencies (IF) used in the radios.

More on Power Supplies:

• Transformers Power Supplies
• Switching Transformers
• Linear Supplies
• Switching Supplies
• Advantages and Disadvantages