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CHAPTER 18

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.

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CHAPTER 18

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  1. CHAPTER 18 Power Supplies

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

  3. 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.

  4. Block Diagram <insert figure 18-2 here>

  5. 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.

  6. 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. VNL = DC output voltage with no load current. VFL = DC output voltage with maximum load current. Load Regulation = ([VNL – VFL] / VFL)  100%

  7. 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/VAC) of AC change: %R = [Vout/Vout(rated)]  100% %R /VAC = %R/VAC

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

  9. 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.

  10. Linear Supplies A typical linear supply design.

  11. 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.

  12. Linear Supplies Typical linear regulator circuit.

  13. 3-Terminal Regulators A typical circuit, good for about an Amp or less.

  14. 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.

  15. Switching Regulators <insert figure 18-22 here>

  16. Switching Regulators Typical switching waveforms.

  17. 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.

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

  19. Boost Regulator

  20. Flyback Regulator

  21. Off-Line Switching Supply

  22. Switching Regulator IC One of many.

  23. 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.

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