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 <insert figure 18-2 here> 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... 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,... 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 . 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. • Some power supplies convert one DC voltage into another DC voltage. Block Diagram <insert figure 18-2 here> Regulation • Regulation is a measure of how well a power supply