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Unit 7

Unit 7. Linear Power Supplies, Switched Mode Power Supply. Learning Objectives:. Linear Power Supplies: 14.1 Constituents of Linear Power Supply 14.2 Designing Mains Transformer 14.6 Linear IC Voltage Regulators 14.7 Regulated Power Supply Parameters Switched Mode Power Supplies:

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Unit 7

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  1. Unit 7 Linear Power Supplies, Switched Mode Power Supply

  2. Learning Objectives: Linear Power Supplies: 14.1 Constituents of Linear Power Supply 14.2 Designing Mains Transformer 14.6 Linear IC Voltage Regulators 14.7 Regulated Power Supply Parameters Switched Mode Power Supplies: 15.1 Switched Mode Power Supplies 15.6 Connecting Power Converters in Series 15.7 Connecting Power Converters in Parallel

  3. 14.1 Constituents Linear Power Supply: • Stepdown transformer • Rectifier • Filter (C, LC) • Regulator (ICs LM78XX, LM79XX, LM321)

  4. 14.2 Designing Mains Transformer: • To avoid magnetic saturation – core cross section area, Ac Ac = √P / 5.6 where P=power required in Watts • t / W ratio = stack thickness (t) -------------------------------- = 1.1 to 1.5 Width of central limb (W)

  5. … • Turns per volt = 108 ----------------------- 4.44 x f x Ac x B Where f=frequency=50 or 60 Hz B=flux density=50000 lines / square inch (gauss)

  6. Turns ratio (Ns / Np), n =Vp / Vs = Ns / Np • Primary current = P ------------------------------------- Efficiency x Primary Voltage

  7. … • Secondary current = Primary Current ----------------------- n where n is turns ratio

  8. 14.1 Numerical Example:

  9. 14.6 Linear IC Regulators: Features: • PCB mountable • Precision • Best voltage & current regulation • Wide range of output voltages • Protection inbuilt: current limit & thermal shutdown

  10. Three terminal regulators:

  11. Boosting current delivery capability:

  12. 14.7 Regulated Power Supply Para meters: Load regulation: Change in regulated output voltage as the load current varies. VNL – VFL % load regulation = --------------- X 100 VFL

  13. Line regulation: is defined in terms of variation of regulated output voltage for a specified change in line (input AC) voltage. Eg. For input voltage variation, say 170 V to 220V, output voltage 10V, varies ±1% has line-regulation (1+1) = ---------- x 100 10 = 0.2 %

  14. Output Impedance: In Thevenin’s equivalent circuit of RPS, output impedance comes in series with load resistance. It determines load regulation of the power supply.

  15. Ripple rejection factor:

  16. 15.1 Switched Mode Power Supplies: power supply Linear Switched • Stepped down, • Rectified, • Filtered, • Regulated • Loss • Costly • The unregulated DC is chopped at a • high frequency (using transistor / • MOSFET / IGBT) • The chopped waveform is then rectified • and filtered to get the desired DC voltage.

  17. 15.5 Switching Regulator: • Buck regulator – step down • Boost regulator – step up • Buck-boost regulator – both step down / up

  18. 15.6 Connecting Power Converters in Series: • Can be connected in series, provided specifications allow for the series connections. • It is possible that the output of one converter affects the output loop of another. • Output voltage should not exceed breakdown voltage of the devices.

  19. 15.6 Connecting Power Converters in parallel: • Again, if specification allow the connection, then they can be connected in parallel.

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