HOW TO FIX SWITCHING POWER SUPPLIES

What Can Go Wrong
Controlling those high-frequency pulses, with their fast rise and fall times, is tougher and stresses the components a lot more than did low-speed linear circuits. The fast pulses with high-voltage peaks punch holes in transistors’ substrates, and the rapid charging and discharging of filter capacitors wears those out too.

Consequently, switchers fail significantly more often than do linear supplies. Nonetheless, very few products still use the old technology; switchers are everywhere, from computer supplies to little AC adapters and chargers for cameras and cell phones.


How to Fix One
Most switchers fail from bad electrolytic capacitors, blown rectifiers or a dead chopper transistor. Look at the capacitors first. Any bulges? Change them. Leakage? Change them. Anything at all unusual about their appearance? Change them!

Checking the rectifiers is easy enough if they’re separate diodes. When you have a bridge rectifier, with all four diodes in one package, each diode must be tested as if it were a separate part. Take a look at the bridge rectifier diagram.

With all power disconnected and the big electrolytic near the bridge discharged, desolder the bridge from the board and use your DMM’s diode function to test each diode in it. You should see around 0.7 volts drop at each diode in the forward direction and an open circuit in the reverse direction, as with any silicon diode.

If you find an open or a short in any of the diodes, replace the bridge. The chopper is the big transistor, probably heatsinked, on the primary side of the transformer. Some choppers are bipolar transistors, but most are power MOSFETs. If the fuse is blown, it’s a good bet the chopper has shorted out. The transistor can fail open, too, in which case the fuse might still be good.

The transistor may have shorted and then opened, and the fuse may or may not have survived the momentary overcurrent. It’s an old technician’s anecdote that transistors are there to protect fuses! Check the chopper using the out-of-circuit techniques.

If you have an isolation transformer, you can do some powered tests before pulling parts. Check the voltage across the big cap on the primary side of the supply, near the chopper. Remember that you can’t use circuit ground on this side. The negative terminal of the cap will be your reference point, where you’ll connect the meter’s black lead. You should see at least 300 volts. If it’s much less, suspect a bad bridge rectifier. If it’s zero, the fuse is probably blown, which could mean a bad bridge, a shorted cap or a bad chopper.

It’s best not to try to scope the chopper directly, as the voltages are very high. The safer approach is to scope the secondary side of the transformer, using normal circuit ground. Many switchers have multiple taps on the secondary winding.

Any of them will do, as long as it’s not the one connected to circuit ground. If the chopper is running, you’ll see pulses at a significantly lower voltage than what’s on the other side. They won’t be tiny, though. Expect anything from 10 to perhaps 60 volts from the baseline to the peak. No pulses? She ain’t running.

If the chopper is good but isn’t running, suspect the pulse-width modulator (PWM) chip or the regulation circuitry near the output. Open zener diodes on the secondary side can allow the output voltage to rise too high, activating protection circuitry and shutting down the PWM, or even tripping the crowbar, deliberately blowing the fuse. No pulses, no chopper, no operation.

If the supply is running but not putting out proper power, caps on the secondary side are the primary suspects. Scope them. If you see much of anything but DC on an electrolytic that has one lead going to ground, change it. Either its capacitance has declined, its ESR has risen, or both. If you change the cap but the waveform still looks noisy, look for a leaky diode feeding the cap.

Finally, remember that most switchers will shut down if output current demand exceeds their safe limits. Some may blow their fuses for the same reason. A short somewhere else in the machine may be pulling too much current and causing the supply to act like it’s broken.

ELECTROSCOPE DEFINITION BASICS AND TUTORIALS

WHAT IS AN ELECTROSCOPE?

The Electroscope.  It has been shown experimentally that an electric charge can be detected because it attracts light objects such as pith balls, bits of paper, etc.

Any device used for detecting electric charges is called an electroscope.  In its simplest form, an electroscope consists of a pith ball hanging on the end of a silk thread.  By touching it with a body of a known charge, you have an instrument that can detect charged bodies and that can indicate the type of charge (polarity).


To illustrate, if you touch the pith ball with a glass rod, which has been rubbed with silk, you charge the pith ball positively.  Any other charged body that is brought near the pith ball will repel it if the body is positive or attract it if the body is negative.  The force of repulsion or attraction indicates the strength of the field surrounding the charged bodies.

A better and more sensitive device is the leaf electroscope shown in figure 1-7. It is two thin sheets of metal foil (usually gold or aluminum) called leaves, supported by a wire or stem whose ends pass through a block of sealing wax or insulating material to a metal ball or cap.

The leaves are usually sealed in a glass container to prevent air currents and moisture from affecting the instrument. The sensitivity of the instrument depends on several factors, the main two being the thickness and the type of material the leaves are made of.

If the ball receives either a positive or a negative charge, it causes the leaves to spread apart.  The leaves spread because like charges repel.  When a charge of positive electricity is placed on the leaves, the spread of the leaves will increase when the ball is approached by a positively charged body.  On the other hand, a negatively charged body brought near the ball or cap will decrease the spread.

You can place a charge on the leaves by bringing a charged body near, but without making physical contact with, the ball.  This is charging by induction.

As soon as you remove the charged body, the electroscope is no longer charged unless you provided some means for it to gain or to lose some electrons while the charge was being induced.  You can do this by connecting a wire from the electroscope to some neutral conducting object, such as ground.

Then, if a charged body is brought near the electroscope, electrons can leave if the charge is negative or enter if the charge is positive.  If the wire is disconnected before the charged body is removed, the electroscope will remain charged oppositely to the charge that induced it.

This is charging by conduction because the electroscope comes into direct contact with the charged body.