Switching p Supplies

36 SWITCHING POWER SUPPLIES

CONT CO NTEN ENTS TS AT A GL GLAN ANCE CE Understanding Switching Supplies Concepts of switching regulation

Connecting the Power Supply AT-style power connections Drive power connections ATX/NLX-style power connections Optional ATX/NLX power connector Voltage tolerances

Troubleshooting Switching Po Troubleshooting Power wer Supplies Tips for power-supply service An example power supply Symptoms

Further Study

Power supplies play a vital role in the operation of PCs and their peripherals—a supply converts commercial ac into various levels of dc that can be used by electronic and electromechanical devices. For the purposes of this book, power supplies are broken broken into three classes: linear (dc) supplies, switching switching (dc) supplies, and high-voltage supplies. Although linear power supplies are popular popular because of their simplicity, they they are inefficient. As a result, linear supplies are typically relegated to low-end applications, such as ac adapters and battery eliminators, eliminators, and are not covered in this edition of the book. On the other hand, switching power supplies are well-entrenched as the primary power source in PC applications. Virtually all all PC and peripheral peripheral designs incorporate incorporate a switching switching supply. This chapter illustrates the operation and troubleshooting approaches for a switching power supply.

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UNDERSTAND UNDER STANDING ING SWITC SWITCHING HING SUPPL SUPPLIES IES

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Understanding Switching Supplies The great disadvantage to linear power supplies supplies is their tremendous waste. At least half of all power provided to a linear supply is literally “thrown away” as heat—most of this waste occurs in the regulator. regulator. Ideally, if just enough energy was was supplied to the regulator to achieve a stable output voltage, regulator waste could be reduced almost entirely and supply efficiency would be vastly improved.

CONCEPTS OF SWITCHING REGULATION REGULATION Instead of throwing away extra input energy, a switching power supply creates a feedback a feedback loop.. Feedback senses the output voltage provided to a load, then switches loop switches the ac primary (or secondary) voltage on or off (as needed) to maintain maintain steady levels at the output. In effect, a switching power supply is constantly turning on and off to keep the output voltage(s) steady. A block diagram of a typical switching power supply is shown in Fig. 36-1. A variety of configurations are possible, but Fig. 36-1 illustrates one classic design. Raw ac line voltage entering the supply is immediately converted to pulsating dc, then filtered to provide a primary dc voltage. voltage. Notice that unlike a linear supply, ac is not transformed before rectification, rectification, so primary dc can easily reach levels that exceed 170 V. Remember that ac is 120 V RMS. Because capacitors charge to the peak voltage ( peak = peak = RMS (( 1.414), dc levels can be higher than your ac voltmeter readings. RMS Remember that high-voltage pulsating dc can be as dangerous as ac line voltage, so treat it with extreme caution.

On start-up, the switching transistor is turned on and off at a high frequency (usually 20kHz to 40kHz), and a long duty duty cycle. The switching transistor transistor acts as a chopper chopper,, which Primary rectifier

ac input voltage

Primary filter

Solid-state switch

Transformer

Secondary rectifier

Secondary filter

Switching pulses Sensing/ switching circuit

FIGURE FIG URE 3636-1 1

Voltage sense signal

Block diagram of a switching power supply.

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S Y S T E M D A T A A N D T R O U B L E S H O O T I N G

1 10 10 6

S WI WI TC TC HI HI NG NG P PO O WE WE R S UP UP PL PL IE IE S

breaks up this primary dc to form chopped dc, dc, which can now be used as the primary signal for a step-down transformer. transformer. The duty cycle of chopped dc will will affect the ac voltage voltage level generated on the transformer’s transformer’s secondary. A long duty cycle means a larger output voltage (for heavy loads) and a short duty cycle means lower output voltage (for light loads). Duty cycle itself refers to the amount of time that a signal is “on,” compared to its overall cycle. The duty cycle is continuously adjusted adjusted by the sensing/switching sensing/switching circuit. You can use an oscilloscope to view switching and chopped dc signals. signals. Figure 36-2 illustrates a more practical representation for a switching supply. Ac voltage produced on the transformer’s secondary winding (typically a step-down transformer) is not a pure sine wave, but it alternates regularly enough to be treated as ac by the remainder of the supply. Secondary voltage is re-rectified re-rectified and re-filtered to form a secondary dc voltage that is actually applied applied to the load. Output voltage is sensed by the sensing/switching circuit, circuit, which constantly adjusts the chopped dc duty cycle. As load increases on the secondary circuit (more current is drawn by the load), output voltage tends to drop. This is perfectly normal, normal, and the same thing happens in every unregulated unregulated supply. However, a sensing circuit detects this voltage drop and increases the switching duty cycle. In turn, the duty cycle for chopped dc increases, increases, which increases the voltage voltage produced by the secondary winding. winding. Output voltage climbs climbs back up again again to its desired desired value. The output voltage is regulated. The reverse will happen as load decreases on the secondary circuit (less current is drawn by the load). A smaller load will will tend to make output voltage voltage climb. Again, the same actions happen in an unregulated supply. The sensing/switching circuit circuit detects this increase in voltage and reduces the switching duty cycle. As a result, the duty cycle for chopped dc decreases and transformer secondary secondary voltage decreases. Output voltage drops back to its desired value. The output voltage voltage remains regulated. regulated. Consider the advantages of a switching switching power circuit. Current is only drawn in the primary circuit when its switching transistor is on, so very little power is wasted in the primary circuit. The secondary circuit will supply just enough power to keep the load voltage constant (regulated), but very little power is wasted by the secondary rectifier, filter, or switching circuit. Switching power supplies can reach efficiencies efficiencies higher than 85% (35%

Switching transistor Q



ac Input voltage

 dc Output voltage

High-frequency switching pulses

FIGURE FIGURE 36-2 36-2

Sensing/ switching circuit

Voltage sense

Simplified diagram of a switching power supply.

CONNEC CONNECTIN TING G THE POWER POWER SUPPLY SUPPLY

1107 1107

dc Output voltage

ac Input voltage

Choke

Fuse

 Switch regulating IC Primary filter

FIGUR FIGURE E 36-3 36-3

 Output filter

Simplified schematic of an IC-based switching power supply.

more efficient than most comparable comparable linear supplies). More efficiency means less heat is generated by the supply, so components can be smaller and packaged more tightly. Unfortunately, switching switching supplies have several disadvantages. First, switching supplies tend to act as radio transmitters. transmitters. Their 20kHz to 40kHz operating frequencies frequencies can wreak havoc radio and television reception, not to mention the circuitry within the PC or peripheral itself. This is why you will see most switching supplies somehow somehow covered or shielded in a metal casing. It is crucially important important that you replace any shielding removed removed during your repair. Strong ElectroMagnetic Strong ElectroMagnetic Interference (EMI) can easily disturb the operation of a logic circuit. Second, the output voltage will always contain some amount of high-frequency ripple. In many applications, this this is not enough noise to present interference interference to the load. In fact, most of the noise is filtered out in a carefully designed supply. Finally, a switching supply often contains more components and is more difficult to troubleshoot than a linear supply. This is often outweighed by the smaller, lighter packaging of switching supplies. Today, sensing and switching functions can be fabricated right onto an integrated circuit. IC-based switching circuits circuits allow simple, inexpensive circuits circuits to be built (Fig. 36-3). Notice how similar this looks versus a linear supply. Ac line voltage is transformed (usually stepped down), then it is rectified and filtered filtered before reaching a switch-regulating IC. The IC chops dc voltage at a duty cycle cycle that will provide adequate adequate power to the load. Chopped dc from the switching regulator is filtered by the combination of choke and output filter ca pacitor to reform a steady dc signal signal at the output. The output voltage is sampled back at the IC, which constantly adjusts the chopped dc duty cycle.

Connecting the Power Supply PC power supplies operate the motherboard directly, as well as a number of internal drives. This part of the chapter presents presents the typical connection schemes schemes for AT, ATX, and NLX power supplies, and highlights the major signals.

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AT-STYLE POWER CONNECTIONS The AT-style power supply is largely considered to be the classic connection scheme for IBM-compatible PCs. PCs. An AT-style supply provides four voltages to the motherboard (+5 Vdc, –5 Vdc, +12 Vdc, and –12 Vdc) through a series of two 6-pin connectors (Fig. 36-4). You might notice that several wires are used for Ground and other voltage signals, such as +5 Vdc. There is no difference between these similarly similarly colored wires—the wires—the extra wires are provided simply because the additional wire is needed to help carry the required current. If you can’t remember the orientation of P8 and P9 connectors, just remember that the “black ends” of each connector go together.

The only discrete “signal” in the AT-style power connector is the Power Good (PwrGood or Good or PG) PG) signal. This signal is typically typically tied to the CPU’s CPU’s Reset pin. When the PC is first powered up, this signal is logic 0 and the CPU is forced into a continuous reset mode. After the power supply is stable (usually about 0.5 seconds from the time you flip the power switch), this signal signal rises to a logic 1. This releases the Reset, Reset, and the CPU can begin processing, which starts the boot process.

DRIVE POWER CONNECTIONS The internal drives of the PC (e.g., floppy drives, hard drives, CD-ROM drives, etc.) must also be powered. Because drives are electromechanical electromechanical devices that typically demand a substantial amount of current, they are powered directly from the power supply, rather than from their respective interfaces. interfaces. Drives traditionally traditionally use a heavy-duty four-wire connector connector to provide 1

P8

P8 1 Orange 2 Red 3 Yellow 4 Blue 5 Black 6 Black

PwrGood

5Vdc 12Vdc 12Vdc G nd G nd

6 1

P9

6


P9 1 Black 2 Black 3 White 4 Red 5 Red 6 Red

G nd G nd 5 Vdc 5Vdc 5Vdc 5Vdc

AT-style motherboard power connections.


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+12 Vdc and +5 Vdc to each drive. The +12-Vdc signal powers the drive’s motor(s), motor(s), while the +5-Vdc signal operates the drive’s logic circuits. The wire colors are identified as follows: s s s s

Yellow Black Black Red

+12 Vdc Ground Ground +5 Vdc

As a rule, one drive power connector should be available for each drive in the system. Higher-capacity Higher-capacity power supplies typically typically offer more drive power connectors. If you do not have enough drive power connectors to power all of the drives in your system, you might be able to use a Y splitter to transform one power connector into two. However, you should be extremely judicious in the use of Y splitters. splitters. The use of inadequate power connectors might indicate that you’re pushing the power supply beyond its capacity, and erratic system behavior can result result (if the system boots at all). Also, never split the power connector operating a hard drive—the power diverted from a hard drive might result in erratic HDD performance and data corruption.

ATX/NLX-STYLE POWER CONNECTIONS Although ATX and NLX form-factor systems now constitute the majority of new systems entering service, their power requirements are remarkably similar. The ATX/NLX power supply provides five voltages to the motherboard (+5 Vdc, –5 Vdc, +12 Vdc, –12 Vdc, and +3.3 Vdc) through a 20-pin connector (Fig. 36-5). The +3.3-Vdc supply is added to support the growing base of “low-voltage logic” appearing in the PC. Older AT-style motherboards also incorporate low-voltage logic, but require an on-board voltage regulator to supply the +3.3 Vdc, rather than the power supply. The signals can be identified by their colors: s s s s s s s s s s

Black Blue Brown Gray Green Orange Purple Red White Yellow

Gnd –12 Vdc 3.3 V sense Power OK PS-ON +3.3 Vdc 5 VSB +5 Vdc –5 Vdc +12 Vdc

In addition to the actual voltages feeding the motherboard, several logic signals are used to control power: PS-ON PS-ON is an active-low signal that turns on all of the main power outputs (+3.3

Vdc, +5 Vdc, –5 Vdc, +12 Vdc, and –12 Vdc). When this signal is held high (logic 1) or left open-circuited, the power-supply power-supply outputs should be off. In effect, this is the signal that allows “soft control” of the system power.

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3.3Vdc

11

1

3.3Vdc

12Vdc

12

2

3.3Vdc

Gnd

13

3

Gnd

PS ON

14

4

5Vdc

15

5

Gnd

16

6

5Vdc

17

7

Gnd

18

8

Power OK

19

9

5VSB

20

10

12Vdc

Gnd

Gnd

Gnd

5Vdc 5Vdc

5Vdc


ATX/NLX-style motherboard power connector.

5VSB 5VSB is a “standby voltage” source that can be used to power circuits that require

power input during the powered-down state. state. The 5VSB pin should deliver 5 Vdc (+/- 5%) at a minimum of 10 mA for PC board circuits to operate. PW-OK PW-OK (Power (Power OK) is a “power good” signal and should be set at logic 1 by the

power supply to indicate that the +5-Vdc and +3.3-Vdc outputs are above the undervoltage thresholds of the power supply.

OPTIONAL ATX/NLX POWER CONNECTOR The ATX and NLX form-factor specifications also provide for an optional 6-pin power connector (Fig. 36-6). Each signal adds a certain amount amount of versatility to the ATX/NLX ATX/NLX system. You can identify the optional optional power connector signals by their their wire colors: s s s s s

White White/blue st stripe Whit White/ e/br brow own n stri stripe pe White/red stripe Whit hite/bl e/blaack str stripe ipe

FanM FanC 3.33.3-V V sens sensee 1394V 1394 1394R R


1111 1111

FanM signal The FanM signal is an open collector, 2-pulse per revolution tachometer

signal from the power-supply power-supply fan. This signal allows the system to monitor monitor the power sup ply for fan speed or failures. failures. If this signal is not implemented implemented on the motherboard, motherboard, it should not impact the power-supply function. FanC signal The FanC signal is an optional fan-speed and shutdown-control signal.

The fan speed and shutdown are controlled by a variable voltage voltage on this pin. This signal allows the system to request control control of the power supply fan from full speed to off. The control circuit on the motherboard should supply voltage to this pin from +12 Vdc to 0 Vdc for the fan-control request. 3.3-V sense line A remote 3.3-V sense line can be added to the optional connector to

allow for accurate control of the 3.3-Vdc line directly at motherboard loads. 1394V pin This pin on the optional connector allows for implementation of a segregated

voltage supply rail for use with unpowered IEEE-1394 IEEE-1394 (“fire wire”) wire”) solutions. The power derived from this pin should be used to power only 1394 connectors (unregulated anywhere from 8 to 40 V). 1394R pin The 1394R pin provides an isolated ground path for unpowered IEEE-1394 (“fire wire”) implementations. implementations. This ground should be used only for 1394 connections, and should be fully isolated from other ground planes in the system.

VOLTAGE TOLERANCES If you pursue power-supply testing or troubleshooting at any level, you’re going to need to test the output voltages. voltages. One important aspect aspect of voltage measurements measurements that are often often overlooked by novice technicians is the idea idea of “voltage tolerance.” Voltage outputs are rarely exact, and might vary from their rated value by as much as 5% (often 3 to 4% for the +3.3-Vdc output). For example, a +5-Vdc output might actually read between between +4.75 Vdc and +5.25 Vdc, and a +12-Vdc output output might read from +11.4 Vdc Vdc to +12.6 Vdc. As long as the measured voltage is within a reasonable tolerance, the output should be considered good. If the measured voltage voltage strays outside of this reasonable reasonable tolerance (usually (usually to the

1394R

1394V

Reserved

4

1

5

2

6

3

Fan M

Fan C

3.3V sense


Optional ATX/NLX motherboard power connector.

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low side), chances are that the output is being overloaded by excessive devices. If the out put measures extremely low (or is absent) chances are that the output (and the power sup ply) is defective. You can then choose to repair or replace replace the power supply.

Troubleshooting Troubleshooting Switching Power Power Supplies Su pplies Troubleshooting a switching power supply can be a complex and time-consuming task. Although the operation of rectifier and filter sections are reasonably straightforward, sensing/switching circuits can be complex oscillators that are difficult to follow without a schematic. Sub-assembly replacement replacement of dc switching supplies is quite common.

TIPS FOR POWER-SUPPLY POWER-SUPPLY SERVICE Power and power-supply problems can manifest themselves in a stunning variety of ways, but the following tips should help you to stay out of trouble: s s s s s s s s

Power-supply cooling is important—keep important—keep the vent openings and fan blades clean. Be sure that the line-voltage switch (120/220 Vac) is set correctly for your region. Verify that the power-supply connectors are attached to the motherboard and drives securely. Remember that for AT-style power connections, the “black wires go together.” Do not use a Y splitter to split power from a HDD (avoid Y splitters entirely, if possible). Some Y splitters are wired improperly. If you have trouble with a device after installing a Y splitter, check the splitter or try powering the device directly. Voltage tolerances are usually ±5% (±4% for 3.3 Vdc), so be sure that each output is within tolerance. If you experience erratic system behavior after adding a new device, this can be the result of an overload. overload. Try removing the device.

AN EXAMPLE POW ER SUPPLY For the purposes of this troubleshooting section, consider the IC-based switching supply of Fig. 36-7. The STK7554 is a switching regulator IC manufactured manufactured as a 16-pin SIP (single in-line package). package). It offers a dual output output of 24 Vdc and 5 Vdc. Vdc. Notice that both both output waveforms from the STK7554 are 38-V square waves, but the duty cycle of those square waves sets the desired output levels. The square wave’s amplitude simply simply provides energy to the filter circuits. Filters made from coils coils (“chokes”) and high-value polarized polarized capacitors smooth the square-wave input (actually a form of pulsating dc) into a steady source of dc. Some small amount amount of high-frequency high-frequency ripple will be on each dc output. Smaller, non polarized capacitors on each output act to filter out high-frequency components of the dc out put. Finally, notice the resistor-capacitor-diode combinations on each output. These form a surge and flyback protector, which prevents energy stored in the choke from re-entering the IC and damaging it. Refer to Fig. 36-7 for the following following symptoms.

TROUBLESHOOTING SWITCHING POWER SUPPLIES

120 Vac input

24 Vdc output

38 Vdc

39 Vdc

1113

24-V sense F1

D1 L3

D2 L1

T1

D3



D4

C1 R1

STK 7554 IC1

R3 D6

 C8

C9

C4

5-Vdc output

Parts list

D1 to D6 L1 T1 L2, L3, L4 R1 R2, R3 C1 C2 C3, C4, C7 C5, C6, C8 C9 F1 IC1

Rectifier diodes Surge supressor Transformer Chokes 10 k 1 / 2 W 47  1 W 6000 F 22 F 0.01 F 1000 F 1 F Fuse Switching regulator STK 7554


 C2 38 Vdc

5-V sense L2

R2

 L4

D5 C5

 C6

C7

C3

A complete IC-based switching power supply.

SYMPTOMS Symptom 36-1. The PC or peripheral is completely dead—no power indicators are lit As with linear supplies, check the ac line voltage entering the PC before

beginning any major repair repair work. Use your multimeter multimeter to measure the ac line line voltage available at the wall outlet that is powering your computer computer or peripheral. Be extremely cautious whenever measuring measuring ac line-voltage levels. Normally, you should read between 105 and 130 Vac to ensure proper-supply operation. operation. If you find either very high or low ac voltage, try the device in an outlet that provides the correct correct amount of ac voltage. Unusual line voltage levels might damage your power supply, so proceed cautiously. If ac line voltage is normal, suspect that the main power fuse in the supply has failed. Most power fuses are accessible from the rear of the computer near the ac line cord, but some fuses might only be accessible by disassembling the device and opening the supply. Unplug the device and remove the the fuse from its holder. You should find the fusible link link intact, but use your multimeter multimeter to measure continuity across the fuse. fuse. A good fuse should measure as a short circuit (0 ohms), but a failed fuse will measure as an open circuit (infinity). Replace any failed failed fuse and re-test re-test the PC. If the fuse continually continually fails, a serious defect is elsewhere within the power supply or other computer/peripheral computer/peripheral circuits. If your supply has an ac selector switch that sets the supply for 120-Vac or 240-Vac operation, be sure that switch is in the proper position for your region of the world (an improperly set ac switch can disable the entire system). Unplug the computer and disassemble it enough to expose the power supply supply clearly. Restore power to the PC and measure each dc output with your multimeter or oscilloscope (you can usually find a power connector connector at the motherboard or other main board). board). Be sure that any power cables are securely attached. attached. If each output measures correctly, correctly, then your

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trouble lies outside of the the supply—a key circuit has has failed elsewhere in the the device. You can try a POST board board or diagnostic to trace the specific specific problem further. further. A low output voltage is suggestive of a problem within the supply itself. itself. Check each connector and all interconnecting wiring leading to or from the supply. Remember that many switching sup plies must be attached to a load for proper proper switching to occur. If the load circuit is disconnected from its supply, the voltage signal could shutdown or oscillate wildly. If the supply outputs continue to measure incorrectly with all connectors and wiring intact, chances are that your problem is is inside the supply. With a linear supply, begin testtesting at the output, then work back toward the ac input. For a switching supply, you should begin testing at the ac input, then work toward the defective output. Measure the primary ac voltage applied applied across the transformer (T1). (T1). Use extreme caution when measuring high-voltage high-voltage ac. You should read approximately approximately 120 Vac for Fig. 36-7. If voltage has been interrupted interrupted in that primary circuit, the the meter will read 0 Vac. Check the primary circuit for any fault that might interrupt interrupt power. Measure secondary ac voltage supplying the rectifier rectifier stage. It should read higher than the highest highest output voltage that you expect. For the example of Fig. Fig. 36-7, the highest expected dc output output is 24 V, so ac secondary voltage should be several volts higher higher than this. The example shows this as 28 Vac. If primary voltage reads correctly correctly and secondary voltage does not, an open circuit might be in the primary or secondary transformer transformer winding. Try replacing the transformer. transformer. Next, check the pre-switched pre-switched dc voltage supplying supplying the switching IC. IC. Use your multimeter or oscilloscope to to measure this dc level. You should read approximately approximately the peak value of whatever secondary ac voltage voltage you just measured. For Fig. 36-7, a secondary voltage of 28 Vac should yield a dc voltage of about (28 Vac RMS ( 1.414) 39 Vdc. If this voltage is low or non-existent, unplug ac from the supply and check each rectifier diode, then inspect the filter capacitor. Use your oscilloscope to measure each chopped dc output output signal. You should find a high-frequency square wave at each output (20kHz to 40kHz) with an amplitude approximately equal to the pre-switched pre-switched dc level (38 to 39 volts in this this case). Set your oscilloscope to a time base of 5 or 10 S/DIV and start your VOLTS/DIV setting at 10 VOLTS/DIV. VOLTS/DIV. Once you have established a clear trace, adjust adjust the time base and vertical sensitivity to optimize the display. If you do not read a chopped dc output from the switching IC, either the IC is defective or one (or more) of the polarized output filter capacitors might might be shorted. Unplug the PC and inspect each questionable filter capacitor. Replace any capacitors that appear shorted. As a general rule, filter capacitors tend to fail more readily in switching supplies than in linear supplies because of high-frequency electrical stress and the smaller physical size of most switching-supply components. components. If all filter capacitors check out correctly, replace the switching IC. Use care when desoldering desoldering the old regulator. regulator. Install an IC socket socket (if possi ble) to prevent repeat soldering work, work, then just plug in the new IC. If you do not have the tools to perform this work (or the problem persists), replace the power supply outright. Symptom 36-2. Supply operation is intermittent—device operation cuts in and out with the supply Inspect the ac line volt voltage age into your printer. Be sure that

the ac line cord is secured properly properly at the wall outlet and printer. printer. Be sure that the power fuse is installed securely. If the PC/peripheral comes comes on at all, the fuse has to be intact. Unplug the device and expose your power supply. supply. Inspect every connector or interconintercon-

FURTHER FURTHER STUDY

1115

necting wire leading into or out of the supply. A loose or improperly installed installed connector can play havoc with the system’s operation. Pay particular attention attention to any output connections. In almost all cases, a switching power power supply must be connected to its load circuit in order to operate. Without a load, the supply might cut out or oscillate wildly. In many cases, intermittent intermittent operation might be the result of a PC board problem. problem. PC board problems are often the result of physical abuse or impact, but they can also be caused by accidental damage damage during a repair. Lead pull-through occurs occurs when a wire or component lead is pulled away from its solder joint, usually through its hole in the PC board. This type of defect can easily be repaired by re-inserting the pulled lead and properly re-soldering the defective defective joint. Trace breaks are hairline fractures fractures between a solder pad and its printed trace. Such breaks can usually render a circuit inoperative, and they are almost impossible to spot without without a careful visual inspection. inspection. Board cracks can sever any number of printed traces, but they are often very easy to spot. The best method for repairing trace breaks and board cracks is to solder jumper wires across the damage between two adjacent solder pads. You could also simply replace replace the power supply outright. Some forms of intermittent intermittent failures are time time or temperature related. If your system works just fine when first turned on, but fails only after a period of use, then spontaneously returns to operation later on (or after it has been off for a while), you might be faced with a thermally intermittent component—a component might work when cool, but fail later on after reaching or exceeding its working working temperature. After a system quits under such circumstances, check for any any unusually hot components. Never touch an operating circuit circuit with your fingers—injury fingers—injury is almost certain. Instead, smell around the circuit for any trace of burning semiconductor or unusually unusually heated air. If you detect an overheated component, component, spray it with a liquid liquid refrigerant. Spray in short bursts for for the best cooling. If normal operation returns, then you have isolated the defective component. Replace any components that behave intermittently. intermittently. If operation does not return, test any other other unusually warm components. If problems persist, replace the entire power supply supply outright.

Further Study This concludes the material material for Chapter 36. Be sure to review the glossary and chapter questions on the accompanying CD. CD. If you have access to the Internet, take a look at some of these power-supply resources: UL (Underwriter’s Laboratories): http://www.ul.com/ TUV (German Standards): http://www.tuv.com/ Astec: http://www.astec.com/ PC Power and Cooling: http://www.pcpowercooling.com Amtrade: http://www.amtrade.com

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Switching p Supplies

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