Unit 7 Linear Power Supplies Switched Mode Power

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: 15. 1 Switched Mode Power Supplies 15. 6 Connecting Power Converters in Series 15. 7 Connecting Power Converters in Parallel

14. 1 Constituents Linear Power Supply: • • Stepdown transformer Rectifier Filter (C, LC) Regulator (ICs LM 78 XX, LM 79 XX, LM 321)

14. 2 Designing Mains Transformer: 1. To avoid magnetic saturation – core cross section area, Ac Ac = √P / 5. 6 where P=power required in Watts 2. t / W ratio = stack thickness (t) ---------------- = 1. 1 to 1. 5 Width of central limb (W)

… 3. 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)

4. Turns ratio (Ns / Np), n =Vp / Vs = Ns / Np 5. Primary current = P ------------------Efficiency x Primary Voltage

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

14. 1 Numerical Example:

14. 6 Linear IC Regulators: Features: • PCB mountable • Precision • Best voltage & current regulation • Wide range of output voltages • Protection inbuilt: current limit & thermal shutdown

Three terminal regulators:

Boosting current delivery capability:

14. 7 Regulated Power Supply Parameters: Load regulation: Change in regulated output voltage as the load current varies. VNL – VFL % load regulation = -------- X 100 VFL

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 220 V, output voltage 10 V, varies ± 1% has lineregulation (1+1) = ----- x 100 10 = 0. 2 %

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.

Ripple rejection factor:

15. 1 Switched Mode Power Supplies: power supply Linear 1. 2. 3. 4. Stepped down, Rectified, Filtered, Regulated Loss Costly Switched 1. The unregulated DC is chopped at a high frequency (using transistor / MOSFET / IGBT) 2. The chopped waveform is then rectified and filtered to get the desired DC voltage.

15. 5 Switching Regulator: 1. Buck regulator – step down 2. Boost regulator – step up 3. Buck-boost regulator – both step down / up

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.

15. 6 Connecting Power Converters in parallel: • Again, if specification allow the connection, then they can be connected in parallel.