FIXING THE STUBBORN SWITCHER, PART II
Last time we got the preliminaries out of the way. This month I'll try and give some concrete suggestions on getting that stubborn switcher back up and running.
First of all, open the thing up and make a really close physical inspection. You're looking for carbonized parts, blown circuit board traces (which can range from traces completely blown away from the board to hairline cracks that are almost invisible), signs of excessive heat, tiny cracks in solder pads surrounding component leads, cold solder joints, and signs of cooking and corrosion. Be sure to look closely at power resistors for signs of cookage. Be extra vigilant around the following: electrolytic capacitors (signs of outgassing and outright leakage, bulges in the can or perished rubber seals around the positive leads); power transistors (carbon traces, broken leads, carbonizing of insulating washers and holes punched through insulating washers; and input and output connections (look for heat fatigue: cold solder joints, corroded connector pins, and hairline cracks in solder connections). Use your nose as well as your eyes. Trouble is much more likely to be found in areas that get warm in normal use--the high current areas of the supply, for instance.
If you can't find physical evidence, it's time to start thinking about the circuit and how it's supposed to work. Let's examine what the power supply is and is not doing:
1) "It's dead, Jim!" No output voltage, no input current. Obviously you should look for blown fuses, on either the input or output side. Is voltage getting to the input filter cap? If not, look at the input rectifier bridge and components in that area. Usually the switching transistor stage consists of one or more power MOSFETs. Look on the gate terminal. Are pulses getting to the MOSFETs? If yes, then the power transistors may be cooked. If no, get back to the power supply controller. If it's not generating pulses, it may be dead or it may be shut down, internally or externally (time to check out those data sheets), or it may be that its "supervisory power supply" is not working. You should be so lucky!
2) "Call the fire department." Very high input current, low or no output. Well, here you're likely looking for a short in the input or output loop. A current-limited variable voltage supply instead of the regular input connection can be handy here. And disconnect the load from the output. When you run up the input voltage, does the input current rush up right away, or only after you've gotten close to the nominal input level? If right away, look for shorted parts in the input loop; if the troubles start only once the controller wakes up and starts pulsing the power transistors, the problem is likely on the output side. Have a good look at those power MOSFETs, and don't forget the spike suppressing diodes that surround them. Or shorts in the output loop elsewhere--if there's an overvoltage crowbar, you should check to see if it's acting prematurely. If the crowbar circuit is controlled by a zener diode, be especially suspicious. If the trouble's in the load, try running the supply with minimum input voltage/current and feeling parts in the load for hot spots. Careful! Even if there's no high voltage, the hot parts can remove skin from your fingertips in a most distressing and painful manner!
3) "Darn thing works until a load is connected." A very common problem. The supply appears to work properly (input and output voltages in the right neighbourhood), but the supply cannot provide its specified load current. Look for one of two faults: either the protective circuitry is shutting down the controller too soon (overcurrent sensors tripping too easily), or the output filter capacitor has dried out and gone partially open. The few inches of wire between the power supply and the load may represent enough inductance at the switcher's operating frequency to prevent proper operation--location of that output filter cap, close to the power supply, may be crucial for the proper operation of the supply.
With a little patience and perseverance, even the most recalcitrant switcher can be brought to bay. Happy hunting!