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OWER SUPPLY REPAIR

Copyright November, 1994 by Randy Fromm

We live in a "disposable society." Disposable razors, lighters, pens and
a myriad of other items are a part of our daily life. Many operators
consider power supplies to be disposable as well. Their rumored
inability to be repaired and relatively low cost create the impression of
disposability.

This is not the case,however. I repair lots of power supplies and it's
really quite easy and practical. In fact, a repaired supply may end up
having a longer life expectancy than a new one, as you'll see.

Switching Power Supply Operation

Modern power supplies are known as "switching regulator power supplies."
In most switching supplies, the 110 volt AC input is first rectified by
two diodes and filtered by a pairof capacitors. This creates two high-
voltage sources; one positive and the other negative.

A pair of transistors is then used to switch these high voltage supplies
across the primary winding of a transformer. This switching action is
very fast. A typical switching speed is around 40,000 cycles per second
or 40 kilohertz. An integrated circuit is commonly used to control thetransistors. This IC not only controls the speed at which the
transistors are switched, but also controls the amount of time that each
transistor is energized. The output voltage of the power supply is
determined by the "on" time of the transistors. If the transistors are
keep on for a longer period of time, the output voltage of the supply
will rise, while shorter times lower the output voltage.This is known
as "pulse-width modulation."

The output of the transformer (which is now alternating current) is then
rectified by special high-speed diodes to change it back to direct
current. This output is not pure DC however, and requires extensive
filtering to remove the high-frequency "noise" that is generated by the
rapid switching action of the transistors. Filtering isaccomplished by
using a combination of coils (also known as "chokes") and capacitors.

The output voltage of the power supply is regulated by feeding some of
the output back to the integrated circuit that controls the switching
transistors. If the output voltage is too low, the IC allows the
transistors to remain energized for a longer period of time, raising the
voltage. An output voltage that istoo high signals the IC to cut back
on the transistors, lowering the output voltage.

Power Supply Failures

I have found that there are only a small handful of components that fail
in switching regulator power supplies. The most common failure is the
switching transistors themselves. The transistors short-circuit, causing
massive amounts of current to be drawn across the transformer andblowing
the fuse.

Transistor failure is often caused by bad capacitors. It is extremely
common to find output filter capacitors that are swollen or leaking. Any
capacitor that appears to be bad should be replaced. To prevent a
recurrence of this all-to-common failure, output filter capacitors should
be replaced with special "low ESR" (Equivalent Series Resistance)
capacitors. Thesecapacitors are specifically designed to handle the
rigors of filtering in a switching supply. Most power supply
manufacturers do not install low ESR capacitors as original equipment
because they are somewhat more expensive that conventional capacitors.
However, it is well worth the money to use them as replacement components
as they will greatly extend the life of the power supply in thefield.
When I work on a power supply, I replace all the output filter capacitors
with low ESR caps regardless of whether they appear to be good or bad.
Since a service call costs far more than the capacitors, it's a prudent
thing to do.

Diode failure is another common problem. There are quite a few diodes
in a switching supply and failure of any one of them will cause the
supply to blow the...
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