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Rectifier Technologies outlines switchmode power supply repair

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Rectifier Technologies  on switchmode power supply repair:

Also called switching power supplies and sometimes chopper controlled power supplies, SMPSs use high frequency (relative to 50/60 Hz) switching devices such as Bipolar Junction Transistors (BJTs), MOSFETs, Insulated Gate Bipolar Transistors (IGBTs), or Thyristors (SCRs or triacs) to take directly rectified line voltage and convert it to a pulsed waveform.

Most small SMPSs use BJTs or MOSFETs. IGBTs may be found in large systems and SCRs or triacs are used where their advantages (latching in the on state and high power capability) outweigh the increased complexity of the circuitry to assure that they turn off properly (since except for special Gate Turn Off (GTO) thyristors, the gate input is pretty much ignored once the device is triggered and the current must go to zero to reset it to the off state.)

The input to the switches is usually either 150-160 VDC after rectification of 115 VAC, or 300-320 VDC after doubling of 115 VAC or rectification of 220-240 VAC. Up to this point, there is no line isolation as there is no line connected (large, bulky, heavy) power transformer.

A relatively small high frequency transformer converts the pulsed waveform into one or more output voltages which are then rectified and filtered using electrolytic capacitors and small inductors in a pi configuration C-L-C, or for outputs that are less critical, just a capacitor.

This high frequency transformer provides the isolation barrier and the conversion to generate the multiple voltages often provided by a SMPS.

Feedback is accomplished across the isolation barrier by either a small pulse transformer or opto-isolator. The feedback controls the pulse width or pulse frequency of the switching devices to maintain the output constant.
Since the feedback is usually only from the primary output, regulation of the other outputs, if any, is usually worse than for the primary output. Also, because of the nature of the switching designs, the regulation even of the primary output is usually not nearly as good both statically and dynamically as a decent linear supply.

DC-DC converters are switchmode power supplies without the line input rectification and filtering. They are commonly found in battery operated equipment like CD players and laptop computers. They have similar advantages to SMPSs in being compact, light weight, and highly efficient.

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