CHAPTER 18 Power Supplies

Objectives Describe and Analyze: Power Supply Systems Regulation Buck & Boost Regulators Flyback Regulators Off-Line Power Supplies Troubleshooting

Introduction Electronic equipment requires DC power. But electricity is distributed as AC. Power supplies convert AC to a steady DC. They must work with minimum AC voltage as well as maximum AC voltage. Regulator circuits keep DC voltage constant. Some power supplies convert one DC voltage into another DC voltage.

Block Diagram

Regulation Regulation is a measure of how well a power supply can hold its DC output steady as its operating point changes. Two things make up the operating point: –The AC input voltage. –The current drawn by the load on the DC output. Line regulation measures the effect of the AC input. Load regulation measures the effect of the DC load. A value of 0% means perfect regulation.

Load Regulation A perfect power supply would have a constant DC output voltage as the DC load current varied from 0 to the maximum level. The output of real power supplies changes slightly with the load current. V NL = DC output voltage with no load current. V FL = DC output voltage with maximum load current. Load Regulation = ([V NL – V FL ] / V FL )  100%

Line Regulation A perfect power supply would have a constant DC output voltage as the AC input voltage varied between specified minimum and maximum levels. The output of real power supplies changes slightly with the AC input voltage. Line Regulation can be calculated as a percentage of rated DC output (%R) or as a percentage per volt (%R/  V AC ) of AC change : %R = [  V out / V out(rated) ]  100% %R / V AC = %R /  V AC

Linear vs. Switching Low efficiency limits linear to low-power applications.

Linear vs. Switching Switchers are more efficient, but also more complicated. Switching control circuitry available in an IC. Switchers require high-speed transistors. Switching speeds from 50 kHz to 500 kHz or higher are common. Can generate electrical noise (EMI). Switcher efficiency due to transistor being either ON or OFF. Linears are simple, and can be inexpensive.

Linear Supplies A typical linear supply design.

Linear Supplies Linears require a large, heavy, 60 Hz transformer. Require large filter capacitors. Dissipate heat in the series pass transistor. Requires a heat sink, and maybe a fan. Easier to have an adjustable DC output voltage than it is with switchers. Often used for “bench” supplies for powering circuits under test. Linears often have better regulation and less ripple and noise than switchers.

Linear Supplies Typical linear regulator circuit.

3-Terminal Regulators A typical circuit, good for about an Amp or less.

3-Terminal Regulators Fixed-voltage 3-terminal regulator ICs allow simple linear supplies at 1 Amp DC or less. 78XX are positive voltage regulators (7805 = 5 Volts, 7812 = 12 Volts, etc.). 79XX are negative voltage regulators (7905 = –5 Volts, 7912 = –12 Volts, etc.). Typically housed in a TO-220 case, but available in a TO-92 case for currents under 100 mA. LM317 is an adjustable 3-terminal regulator.

Switching Regulators

Switching Regulators Typical switching waveforms.

Switching Regulators The previous slide showed the basic components of a switching regulator: –A Switch: typically an E-MOSFET. –An Inductor: often a few turns of wire on a ferrite core. –A Switching Diode: must be fast; it carries the inductor discharge current when the switch opens. –A Filter: typically a Tantalum electrolytic; a few  F. –The Load: unlike linears, switchers don’t like to be run without a load. Typically, switchers achieve higher efficiency with higher load current.

Switching Regulators There are many types of switchers. Here are a few common ones: –Buck: V out is lower than V in –Boost: V out is higher than V in –Flyback: V out polarity opposite V in The inductor in a Flyback can be made as a transformer, allowing V out to be higher or lower, same or opposite polarity.

Boost Regulator

Flyback Regulator

Off-Line Switching Supply

Switching Regulator IC One of many.

Troubleshooting Be careful! If possible, use an isolation transformer when testing off-line supplies. Don’t touch a transistor to see if it is hot. Replace a bad fuse only once. If it blows again, there is a reason. First check the components that are under stress from high voltage, high current, high temperature. That includes filter capacitors, power transistors, rectifiers, and switching diodes. Look for components that are discolored, swollen, cracked, or show other show signs of damage.