Linear Power Supplies, Switched Mode Power Supply 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 LM78XX, LM79XX, LM321)

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)

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

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

… 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 Para meters: 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 220V, output voltage 10V, varies ±1% has line-regulation (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 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.

15.5 Switching Regulator: Buck regulator – step down Boost regulator – step up 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.