Switching Realization

Switch and Circuit Realization

• Switches– Transistor, BJT, MOSFET, IGBT, Diodes,

Thyristors

• How are switches arranged?– To realize circuit requirements and to satisfy

KVL and KCL of circuits– For quadrant operations

Realization of Switch

• Realization of switch: Depends on– Polarity of voltage that the devices must block

in the OFF state– Polarity of the current the device must

conduct during the conduction state

Realization of Switch contd..

• Refer the figure above– Switch A must block positive voltage Vg when

in OFF state

– Must conduct positive current IL in positive direction in the ON-state

• If for all operating conditions the current and blocking voltage lie in a single quadrant

Realization of Switch contd..

• The operation can be realized using a transistor and/or a diode

SPST-Single Pole, Single Throw

Passive Switches

• Single-Quadrant Switches SPST Passive and Active Switches– Passive switches have terminals 1 and 0

while active switches have additional terminal C for control action

– Passive switches operate in ON and OFF positions depending on the voltage and current waveforms [V,I]

Passive Switches : Diode

• Passive Switch- Diode

• Ideal Diode: Requires V(t), I(t) waveforms to turn ON/OFF – Diode OFF, when– Diode ON, when

0)( tV0I0I 0)( tV

Active Switches: BJT

• Active Switch- With Control C– Thyristor, BJT, MOSFET, IGBT

• BJT: Bipolar Junction Transistor

• MOSFET: Metal Oxide Semiconductor Field Effect Transistor

Active Switches: MOSFET

Active switches contd..

• When control terminal C causes device to be OFF and the device is capable of blocking positive voltage

• When control terminal C causes device to be in ON-state V = 0 and is capable of conducting positive current

• The reverse block characteristics of BJT and IGBT are poor or non-existent

0V

0I

0I

Active switches contd..

• MOSFET is able to conduct current in reverse direction but is rarely used– Example:

Active Switches contd..

• Switch A can be IGBT, BJT or MOSFET

• Switch B can be Diode i.e. OFF when

0BV

Current Bidirectional 2-Quadrant Switches

• DC-AC Inverters require switches to conduct both negative and positive currents but block only positive voltages

• A current Bidirectional two Quadrant SPST switch can be realized using a transistor and an anti-parallel diode

Current Bidirectional 2 Quadrant Switches contd..

• A SPDT-single pole double throw current bidirectional two quadrant switch can be derived using two SPST switches

• Two quadrant SPDT switch

• A current Bidirectional SPDT switch– Conducts both positive and negative currents– Blocks positive voltage

Current Bidirectional 2 Quadrant Switches contd..

Three phase DC-AC Inverter• Has 3 two quadrant SPDT switches one

per phase• Each switch block input voltage Vg,

conduct output currents

Three phase DC-AC inverter contd..

Three phase Inverter

Battery Charger• Battery charger

• Vbus > 0, Vbatt > 0

• Q1, Q2, D1, D2 block positive voltage Vbatt

Battery Charger contd..

• Charging mode:– IL > 0, Q1 and D2 alternately conduct current

• Discharging mode:– IL < 0, Q2, D1 alternately conduct current

Voltage Bidirectional Two Quadrant Switches

• I > 0, V < 0• V > 0

Voltage Bidirectional two quadrant switches contd..

• Switch must block both positive and negative voltages and conduct positive current

• Requires an SPST switch using a series connected transistor and diode

Voltage Bidirectional 2- Quadrant Switches contd..

• In OFF state, controller turns OFF the transistor. Diode blocks negative voltage and transistor blocks positive voltage

• Blocks negative voltage up to diode voltage rating

• Blocks positive voltage up to transistor voltage rating

• Example: DC-AC Buck-Boost Inverter

DC-AC Buck Boost Inverter

• Inverter mode IL > 0

• All switches conduct only positive current

• Switches must block output AC line-line voltage

Four Quadrant SPST switch

• Four quadrant switch capable of conducting currents of either polarity and blocking voltages of either polarity

Four Quadrant SPST switch contd..

Three ways of implementing a four-quadrant SPST switch