Упражнение 64. Переведите текст

PC Power Supply Repair

The low-voltage section of the power supply is a very simple rectifier, L-section filter design which provides four output voltages. Key to the success of this design is a multiple secondary power transformer. There is a 5-volt winding and a 12-volt winding. In high-power supplies (250 watts and larger), there are usually two five-volt windings that are paralleled for higher output current—yet treated as a single winding.

Each winding has a grounded center tap to permit full wave rectification using just two diodes (full wave bridge rectifiers need four diodes). The direction of the rectifiers determines the polarity of the output voltage. Common cathodes are positive, and common anodes are negative.

Because of its high current requirements, the +5-volt rectifier is usually an array of parallel Schottky diodes in a single package that mounts on a heat sink. The 5-volt output is often derived from the 12-volt rectifier via an IC regulator (typically an LM7905 equivalent) rather than from the five-volt transformer winding. The output of the rectifiers is filtered first by an inductor, called a choke, then by a heavyduty electrolytic capacitor. In some designs, the five-volt line is double-filtered to reduce ripple by cascading two L-section filters on the output. Invariably, a bleeder resistor is placed across the output to discharge the capacitors after power off. The most common cause of low-voltage failure is a shorted rectifier. If one blows, so does its companion, which forces you to replace them as a package deal. Second on the hit list is a shorted capacitor, which usually does less overall damage. Most of the time, the failure is limited to just one output line, but there’s no guarantee. The first step is to locate the shorted components. For this operation you need access to the bottom side of the printed circuit board.

Be careful not to damage other components in the process. For example, twisting and turning the board too many times can cause attached wires to break loose. Then you have to first locate the affected parts on the circuit board. An ohmmeter is a good way to probe suspected areas for shorted devices.

It’s virtually impossible to tell the difference between a shorted diode and a shorted capacitor without removing one or the other. Since the rectifier is the most likely culprit and the easier to remove (the electrolytics are glued in place), you can start there. The +5-volt and +12-volt diodes are most likely nestled inside a transistor case mounted on a heat sink. The bigger one is the +5-volt rectifier, and the smaller one is the +12-volt rectifier. The negative-voltage rectifiers are individual diodes typically in a DO-41 case.

With the suspect rectifier or diodes in hand, do a resistance check of the defective voltage output line again. If the reading is within the normal range, trash the old part or parts and replace with new. (If the new diodes come in an axial-lead package, typically DO-41, solder them on the trace side of the circuit board instead of the component side. It’s a lot easier.) If the output still shows a short, yank the electrolytic and check again. If the output is still shorted, make sure you’re pulling the right teeth. Exact replacement parts always cost more than generics, so go with the generic. You can get “universal” replacements from GE, RCA, and Philips ECG. Unfortunately, they’re almost as expensive as the original. For the +5-volt rectifier, I recommend the MBR series from General Instruments and Motorola (available from Digi-Key and Allied Electronics, respectively). The +12-volt rectifier is a dual Schottky device that’s available from several vendors, and generally sells for a buck or two. The negative voltage rectifiers must be fast recovery diodes, like a 1N4933. Replacement electrolytic capacitors are as close as your local Radio Shack. When the voltage line has a three-terminal IC voltage regulator, check the resistance between both the input and the output to ground. If only the output pin is shorted, the output capacitor is bad. If only the input pin is shorted, the rectifiers are bad. If both are shorted, the chance is both the diodes and the IC are shorted. To verify this theory, remove the IC and check the resistance again. If it reads okay, replace the semiconductors. The re-placement for the -5-volt IC voltage regulator is an LM7905.

If the new fuse blows when you apply power, there’s a problem in the high-voltage section. We know this because the low-voltage section has an automatic shutdown circuit that reacts a lot faster than the fuse; that is, a low-voltage problem disables the power supply long before the fuse has time to blow. That doesn’t necessarily mean the low-voltage outputs are okay, because failure of the -12-volt line can cause cascading damage that goes all the way back to the high-voltage section.

The high-voltage section is divided into two parts: the high-voltage power supply and the switching circuit. Most high-voltage failures occur in the switching circuit. If the fuse has a “mirrored” look to it, you can bet the farm that at least one of the two switching transistors is shorted. Typically they perish as a couple. These transistors are mounted on the heat sink(s) closest to the two largest electrolytic capacitors. With the red probe of a VOM on the collector of the first transistor, check the collector-to-emitter resistance, then the collector-to-base resistance. If a short is found, replace both the transistor and the damper diode that’s across its emitter-collector. You can use a Motorola MJE13009 for the power transistor and a 1N4937 for the damper diode.

You should also replace the low-value resistor that’s in series with the transistor’s base. This resistor is often used as a fusible link that goes puff when the switcher fails. Its purpose is to protect other components in the chain from harm. If the resistor is burned beyond recognition, you can replace it with any 1/4-watt resistor with a value of 1 to 10 ohms (the exact value isn’t important). Sometimes, though, even the fusible isn’t fast enough to prevent damage. So before installing the new parts, it’s wise to check out the pulse shaper network (typically a resistor-diode-capacitor combination) associated with the base circuit, too. A quick way to test all three components at once is to treat the network like a single diode, checking it as a whole for shorts and opens.

Now repeat the procedure for the second switching transistor. The high-voltage supply is a simple voltage doubler with an output of about 300 volts. While this section rarely fails on its own, a shorted switching transistor can wipe out the bridge rectifier in an instant. Check the AC input for shorts, and replace the entire bridge if a short is found. Bridges can be either discrete diodes or a large, rectangular module, and you can find suitable replacements from Radio Shack. There’s probably a one-ohm resister in line with the AC input that needs to be checked, too. On the outside chance that one of the doubler capacitors is shorted, do a resistance check of each. When powered from a 220-volt AC power source, the capacitors serve as voltage dividers to provide an artificial ground. Consequently, the capacitance and ESR (equivalent series resistance) values of the capacitors are critical when operating from a 220-volt line and have to be evenly matched, otherwise the switching voltages will be uneven. As electrolytics age, both the capacitance and ESR changes. If the mismatch is too great, one voltage could exceed the limits of the switching transistor, which can start parts a-poppin’. You can check the voltage balance with a VOM. Always replace both capacitors, not just one, and use a good grade capacitor, like the Panasonic TSU series.