Repairing Switched Mode Power Supplies by Erich VK5HSE

Repair SM PS

Switched mode power supplies work by rectifying alternating mains current and then driving a high frequency transformer with pulses of this rectified current.

The high frequency transformer produces a high frequency output which is then rectified and smoothed by filter capacitors.

The advantages of this approach are that a small, lightweight transformer can be used instead of a big iron cored mains frequency transformer, and very little standby current is required when no load is being drawn.

Two of the major downsides are that the high frequency pulses of current are rich in harmonics which can produce broadband RF noise, and that the ripple in the DC output of the high frequency transformer output can cause heating in the smoothing capacitors used to condition the DC output before it makes it way to the electronics being powered.

In addition to the ripple currents, switch mode power supplies are often very compactly constructed, and sometimes lack airflow, causing even more heating of the components.

Failure of the smoothing capacitors is a common mode of failure in the switch mode power supplies which are becoming almost ubiquitous in consumer electronics, computers and radios.

This short tutorial documents the process of identifying and replacing smoothing capacitors which have failed in service in a switch mode power supply. This is by no means a comprehensive guide to repairing power supplies, but it will give you enough information to resurrect a lot of the failed switched mode power supplies you come across.

This is the device being repaired, a DVD HDD recorder which was looking very lonely one night at an Adelaide Hills Amateur Radio Society meeting.

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http://www.ahars.com.au/htm/hb_switchmodePSRepair.html

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Repair SM PS

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