DC Power Supplies and Rectifiers: An Introduction to Power Electronics

Introduction to Power Electronics

ENEL270: Lecture 32Outline

• Introduction
• Rectifiers
• Linear Regulators

. Introduction to Power Electronics / Switching Regulators 05/24 2Introduction . DC supplies are an extremely important aspect of almost all electronic systems. · DC supplies provide stable and constant voltages that circuits use for all electric power needs and setting of bias levels. · DC supplies are expected to be stable (handle fluctuating conditions at the input and at the output, yet maintain an acceptably constant voltage) and have very little noise at the output. · Other application-specific considerations will influence the type of dc supply that is implemented (especially aspects such as cost, efficiency, and sustainability). 05/24

Conventional Grid-Sourced DC Power Supply System

Intro Continued

3Intro Continued · A conventional grid-sourced dc power supply system (material covered in Power Supplies (Voltage Regulators) Chapter in Boylestad and Nashelsky).

1. Rectifiers

Transformer Rectifier Filter IC regulator Load 230 V rms 05/24 4Intro Continued · A conventional grid-sourced dc power supply system Transformer Rectifier Filter IC regulator Load 230 V rms · The mains (grid) ac voltage (typically 230 Vrms, 50 Hz in NZ) is conventionally connected to a mains-frequency step-down transformer where the secondary winding (output) voltage is appropriate for the desired dc supply voltage. 05/24 5Intro Continued · A conventional grid-sourced dc power supply system Transformer Rectifier Filter IC regulator Load 230 V rms · The stepped-down ac voltage is rectified (usually a diode-rectifier) to provide a basic dc voltage. Here, full- wave rectification is shown (the usual case). 05/24 6Intro Continued · A conventional grid-sourced dc power supply system 7 Transformer Rectifier Filter IC regulator Load 230 V rms . The rectified voltage is then made to be more "constant" with respect to time by using a filter (often just a simple parallel-connected capacitor filter). . The output as shown still has an appreciable amount of ripple voltage that is too large for most applications. 05/24 7Intro Continued · A conventional grid-sourced dc power supply system 7 Transformer Rectifier Filter IC regulator Load 230 V rms · To create the required level of constant dc voltage, a voltage regulator IC (or electronic system) is usually employed. · A voltage regulator will output a dc voltage with very little ripple voltage, and remains constant (stable), even for large changes in input voltage (known as line regulation or ripple rejection) and output load (known as load or voltage regulation). 05/24

Transformer Function

8Transformer . The transformer is simply used to step-down the mains voltage to the required voltage for the electronics system. N1: N2 11 12 VI V 2 RL V2 _N2 - V1 N 1 12 _ N1 = 11 N2 05/24

Rectifier Circuits

Half-Wave Rectifier

9Rectifier . Almost always want full-wave rectification. · Start off considering half-wave rectifier Vi Vm 0 T T t 1 2 1 1 cycle - vị = Vm sin œt Input waveforms - 0- 0 + + Vi R Vo - Half wave rectifier 1 A Vo 1 Vm 1 Vdc = 0.318V, 0 t - T - Output waveform 05/24 10 - +Rectifier · Conducts for positive voltage, blocks for negative voltage. + - 0 0 O O + + + + 1 Vo m - Vm 0 T T t 2 1 1 0 T - - - vi = Vm sin ot - + O o 0 0 O O 4 Vi + A Vo Vm 0 T Tt 2 0 T £ T t 1 cycle" + - + - 2 Vi = Vm Sin @t - + - Vi Vo + Vị R W Vo = 0V Vo = 0 V R T Vi R Vo = Vi Vi R W Vo T - 1 cycle t - 2 Vi 0 05/24 11Rectifier · Half-wave rectifier output now has an average net dc value. AVi Vdc = 0 V 0 t A Vo I 1 I V, m I 1 I Vdc = 0.318V, m 0 t T Output waveform Vm m Vac = - π 05/24

Full-Wave Rectifier with Diode Bridge

12Rectifier · Now consider a full-wave rectifier · Use a diode bridge A Vi Vm m 0 T 2 T t 0 + D1 D2 Vo + Vi R D3 D4 - 0 05/24 13Rectifier · Diode bridge enables use of both halves of input voltage. A Vi 0 + D2 - Vo + - Vị R + + D3 DA - "on" "off" - - o "on" "off" + X + Va + Vị + Vi 0 TI T 1 2 + "off"" + "on" o + O + 0 4 Vo 0 R Vị 1 V - + A 0 T t 2 0 - 0 Vi 0+ D1 D2 a Vo + Vi 1 0 T T 0 2 a + - O + + "off" + "on" + Va Vi 0 T 2 T A Vo - Vm 1 1 R R D3 DA o 05/24 14 Vm R 0- - - RRectifier · Diode bridge enables use of both halves of input voltage. 2Vm m Vdc = A Vi TU - Vm 0 T 2 T t A Vo m - - Vdc = 0.636V, 0 T Tt 2 05/24

Rectifier Filter Design

Capacitor Filter

15Rectifier Filter · Now consider a full-wave rectifier with simple capacitor filter. 000000000 DC Load + = C GND I DC Load 230 V rms + C GND V. m -1 0 MM A VC Vm 0 05/24 16 230 V rmsRectifier Filter · Full-wave rectifier with capacitor filter. • Time T1 is the time during which the associated diode pairs of the full-wave rectifier bridge conduct, charging the capacitor up to the voltage Vm. • Time T2 is the time interval during which the rectifier voltage drops below the peak voltage and the capacitor starts discharging through the load. • The output wavefrom has a dc level V dc and a ripple voltage V, as the capacitor charges and discharges. I 000 DC Load 230 V rms + C GND (Vr) 1 VC p-p 1 V V dc 1 1 1 t T1 - T2 T 2 05/24 17Rectifier Filter · Full-wave rectifier with capacitor filter. · If we approximate the ripple voltage with a triangular wave, then the following expression can be used to help size a capacitor for a required ripple level given a particular de load current. V,(rms) = V,(p-p) 2V3 Idc 4V3 fc Vo V, (p-p) V, (rms) = 2 13 Approximate triangular waveform V, (p-p) Vm Vắc + 1 T1 -T2 T T 4 2 05/24

Diode Peak Current Considerations

18Rectifier Filter · Full-wave rectifier with capacitor filter. · Increasing the capacitance reduces the ripple voltage amplitude, but also increases the peak current through the conducting diodes. • The diode peak current rating must therefore be considered when designing the rectifier bridge and choosing the capacitor size. 4 VC VC 1 1 Aip ID 1 T1 T T (a) (b) Output voltage and diode current waveforms: (a) small C; (b) large C. 1 peak = T TI 05/24