The switchmode power supply (SMPS)

Until the 1970s or so, most consumer electronic equipment used a basic power transformer/rectifier/filter capacitor type of power supply for converting the AC line into the various voltages needed by internal circuitry. Even regulation was present only where absolutely needed - the high voltage supplies of color TV sets, for example. Remember those old TVs with boat anchor type power transformers? (Of course, if you recall those, you also recall the fond days of vacuum tube sets and the corner drugstore with a public tube tester!)

Switchmode supplies had been commonplace in military and avionic equipment long before they found their way into consumer electronics. I have some DC-DC and DC-AC converter modules from a Minuteman I missile from around 1962 as one example. I suppose that the cost of the switching transistors wasn't as big a deal with a $100 million missile as a $300 TV (even in 1960s dollars).

Nowadays, all TVs, monitors, PCs; most laptop and camcorder power packs; many printers, fax machines, and VCRs; and even certain audio equipment like portable CD players use this technology to reduce cost, weight, and size.

Switchmode power supply repair

Unlike PC system boards where any disasters are likely to only affect your pocketbook, power supplies, especially line connected switchmode power supplies (SMPSs) can be dangerous. Read, understand, and follow the set of safety guidelines provided later in this document whenever working on line connected power supplies as well as TVs, monitors, or other similar high voltage equipment.

Having said that, repairing a power supply yourself may in fact be the only economical option. It is very common for service centers to simply replace the entire power supply board or module even if the problem is a 25 cent capacitor. It may simply not pay for them to take the bench time to diagnose down to the component level. Many problems with switchmode power supplies are easy to find and easy and inexpensive to fix. Not all, but surprisingly many.

This document will provide you with the knowledge to deal with a large percentage of the problems you are likely to encounter with the common small switchmode power supplies found in many types of consumer electronic equipment including PCs, printers, TVs, computer monitors, and laptop or camcorder power packs. It will enable you to diagnose problems and in many cases, correct them as well. With minor exceptions, specific manufacturers and models will not be covered as there are so many variations that such a treatment would require a huge and very detailed text. Rather, the most common problems will be addressed and enough basic principles of operation will be provided to enable you to narrow the problem down and likely determine a course of action for repair. In many cases, you will be able to do what is required for a fraction of the cost that would be charged by a repair center - assuming they would even bother.

Should you still not be able to find a solution, you will have learned a great deal and be able to ask appropriate questions and supply relevant information if you decide to post to sci.electronics.repair. It will also be easier to do further research using a repair text such as the ones listed at the end of this document. In any case, you will have the satisfaction of knowing you did as much as you could before taking it in for professional repair. With your new-found knowledge, you will have the upper hand and will not easily be snowed by a dishonest or incompetent technician.

Most Common Problems

The following probably account for 95% or more of the common SMPS ailments:

• Supply dead, fuse blown - shorted switchmode power transistor and other semiconductors, open fusable resistors, other bad parts. Note: actual cause of failure may be power surge/brownout/lightning strikes, random failure, or primary side electrolytic capacitor(s) with greatly reduced capacity or entirely open - test them before powering up the repaired unit.

• Supply dead, fuse not blown - bad startup circuit (open startup resistors), open fusable resistors (due to shorted semiconductors), bad controller components.

• One or more outputs out of tolerance or with excessive ripple at the line frequency (50/60 Hz) or twice the line frequency (100/120 Hz) - dried up main filter capacitor(s) on rectified AC input.

• One or more outputs out of tolerance or with excessive ripple at the switching frequency (10s of kHz typical) - dried up or leaky filter capacitors on affected outputs.

• Audible whine with low voltage on one or more outputs - shorted semiconductors, faulty regulator circuitry resulting in overvoltage crowbar kicking in, faulty overvoltage sensing circuit or SCR, faulty controller.

• Periodic power cycling, tweet-tweet, flub-flub, blinking power light - shorted semiconductors, faulty over voltage or over current sensing components, bad controller.

In all cases, bad solder connections are a possibility as well since there are usually large components in these supplies and soldering to their pins may not always be perfect. An excessive load can also result in most of these symptoms or may be the original cause of the failure. And don't overlook the trivial: a line voltage select switch in the wrong position or between positions (possibly by accident when moving the supply, particularly with PCs), or damaged.

Repair or replace

Some manufacturers have inexpensive flat rate service policies for power supplies. If you are not inclined or not interested in doing the diagnosis and repair yourself, it may be worthwhile to look into these. In some cases, $25 will get you a replacement supply regardless of original condition. However, this is probably the exception and replacements could run more than the total original cost of the equipment - especially as in the case of most TVs and many computer monitors, where the power supply is built onto the main circuit board.

Nothing really degrades in a switchmode power supply except possibly the electrolytic capacitors (unless a catastrophic failure resulted in a total meltdown) and these can usually be replaced for a total cost of a few dollars. Therefore, it usually makes sense to repair a faulty supply assuming it can be done reasonably quickly (depending on how much you value your time and the down time of the equipment) and, of course, assuming that the equipment it powers is worth the effort. Most replacement parts are readily available and kits containing common service components are also available for many popular power supplies (such as those found in some terminals, MacIntosh and other Apple computers, various brands of video monitors, and some TVs and VCRs).

Where an exact replacement power supply is no longer available or excessively expensive, it may be possible to simply replace the guts if space allows and the mounting arrangement is compatible. For example, for an older full size PC tower, the original power supply may be in a non-standard box but the circuit board itself may use a standard hole configuration such that an inexpensive replacement may be installed in its place.

Alternatively, many surplus electronics distributors have a wide selection of power supplies of all shapes, sizes, output voltages, and current capacities. One of these may make a suitable replacement for your custom supply with a lot less hassle than attempting to repair your undocumented original. It will likely be much newer as well with no end-of-life issues like dried up electrolytic capacitors to deal worry about. Of course, you must know the voltage and current maximum current requirements of each of the outputs in order to make a selection.

For the specific case of SMPSs for standard computers (PC, Macs, workstations, servers), it often doesn't make sense to spend much time or money on repair. The cost of replacement of power supplies for PCs in particular is so low, that just buying a new power supply may be the best course of action. Furthermore, the risk of a faulty repair causing expensive or fatal damage to the mainboard and peripherals including total loss of all data stored on disk, makes repair a risky endeavor unless thorough testing can be performed before installation. However, it won't hurt to check for obvious problems like bad connections. Put the dead one aside and considering trying to repair it if there isn't anything better to do. Realistically, this will be never. :)