Chapter 4. Switch Realization 4.1. Switch applications Single-, - PowerPoint PPT Presentation

Chapter 4. Switch Realization 4.1. Switch applications Single-, two-, and four-quadrant switches. Synchronous rectifiers 4.2. A brief survey of power semiconductor devices Power diodes, MOSFETs, BJTs, IGBTs, and thyristors 4.3. Switching loss Transistor switching with clamped inductive load. Diode recovered charge. Stray capacitances and inductances, and ringing. Efficiency vs. switching frequency. 4.4. Summary of key points 1 Fundamentals of Power Electronics Chapter 4: Switch realization

SPST (single-pole single-throw) switches Buck converter SPST switch, with voltage and current with SPDT switch: L 1 i L (t) polarities defined + 1 2 + i V g C R V – + – v – with two SPST switches: A L i A i L (t) 0 + + v A – – v B B All power semiconductor + V g C R V – + devices function as SPST i B switches. – 2 Fundamentals of Power Electronics Chapter 4: Switch realization

Realization of SPDT switch using two SPST switches A nontrivial step: two SPST switches are not exactly equivalent to one ● SPDT switch It is possible for both SPST switches to be simultaneously ON or OFF ● Behavior of converter is then significantly modified ● —discontinuous conduction modes (ch. 5) Conducting state of SPST switch may depend on applied voltage or ● current —for example: diode 3 Fundamentals of Power Electronics Chapter 4: Switch realization

Quadrants of SPST switch operation 1 switch A single-quadrant on-state i current + switch example: ON-state: i > 0 v OFF-state: v > 0 – switch 0 off-state voltage 4 Fundamentals of Power Electronics Chapter 4: Switch realization

Some basic switch applications switch switch on-state on-state Single- Current- current current quadrant bidirectional switch two-quadrant switch switch switch off-state voltage off-state voltage switch switch on-state on-state current current Voltage- Four- bidirectional quadrant two-quadrant switch switch switch off-state off-state switch voltage voltage 5 Fundamentals of Power Electronics Chapter 4: Switch realization

4.1.1. Single-quadrant switches Active switch: Switch state is controlled exclusively 1 by a third terminal (control terminal). i + Passive switch: Switch state is controlled by the applied current and/or voltage at terminals 1 and 2. v SCR: A special case — turn-on transition is active, – while turn-off transition is passive. 0 Single-quadrant switch: on-state i(t) and off-state v(t) are unipolar. 6 Fundamentals of Power Electronics Chapter 4: Switch realization

The diode • A passive switch i • Single-quadrant switch: 1 • can conduct positive on- i on + state current v • can block negative off- off v state voltage – • provided that the intended on-state and off-state 0 operating points lie on the diode i-v characteristic, then switch can be Symbol instantaneous i-v characteristic realized using a diode 7 Fundamentals of Power Electronics Chapter 4: Switch realization

The Bipolar Junction Transistor (BJT) and the Insulated Gate Bipolar Transistor (IGBT) • An active switch, controlled 1 BJT by terminal C i + i • Single-quadrant switch: C v • can conduct positive on- on – state current 0 v off • can block positive off-state voltage IGBT 1 • provided that the intended i + on-state and off-state C v operating points lie on the transistor i-v characteristic, instantaneous i-v characteristic – then switch can be realized 0 using a BJT or IGBT 8 Fundamentals of Power Electronics Chapter 4: Switch realization

The Metal-Oxide Semiconductor Field Effect Transistor (MOSFET) • An active switch, controlled by terminal C i • Normally operated as single- 1 quadrant switch: i on + • can conduct positive on-state C v v current (can also conduct off negative current in some – circumstances) on 0 (reverse conduction) • can block positive off-state voltage • provided that the intended on- Symbol instantaneous i-v characteristic state and off-state operating points lie on the MOSFET i-v characteristic, then switch can be realized using a MOSFET 9 Fundamentals of Power Electronics Chapter 4: Switch realization

Realization of switch using transistors and diodes Buck converter example A L i A i L (t) + + v A – – v B B + V g C R V – + Switch A: transistor i B – Switch B: diode i A i B switch A SPST switch switch B i L i L on on operating points switch A switch B off off V g v A –V g v B Switch A Switch B 10 Fundamentals of Power Electronics Chapter 4: Switch realization

Realization of buck converter using single-quadrant switches v A i A L i L (t) + – + – v L (t) – + V g v B – + i B i B i A switch B switch A i L i L on on switch B switch A off off V g v A –V g v B 11 Fundamentals of Power Electronics Chapter 4: Switch realization

4.1.2. Current-bidirectional two-quadrant switches • Usually an active switch, controlled by terminal C i • Normally operated as two- 1 on (transistor conducts) quadrant switch: i + • can conduct positive or C v off v negative on-state current – • can block positive off-state on voltage (diode conducts) 0 • provided that the intended on- state and off-state operating points lie on the composite i-v BJT / anti-parallel instantaneous i-v characteristic, then switch can diode realization characteristic be realized as shown 12 Fundamentals of Power Electronics Chapter 4: Switch realization

Two quadrant switches switch i on-state current on 1 (transistor conducts) i + v off v switch – off-state voltage on 0 (diode conducts) 13 Fundamentals of Power Electronics Chapter 4: Switch realization

MOSFET body diode i 1 on i (transistor conducts) + C v off v – on (diode conducts) 0 Power MOSFET Power MOSFET, Use of external diodes characteristics and its integral to prevent conduction body diode of body diode 14 Fundamentals of Power Electronics Chapter 4: Switch realization

A simple inverter i A + Q 1 + v 0 ( t ) = (2 D – 1) V g v A V g D 1 – – L i L + + + V g D 2 v 0 C R v B – Q 2 – – i B 15 Fundamentals of Power Electronics Chapter 4: Switch realization

Inverter: sinusoidal modulation of D v 0 ( t ) = (2 D – 1) V g Sinusoidal modulation to v 0 produce ac output: V g D ( t ) = 0.5 + D m sin ( ω t ) D The resulting inductor 0 current variation is also 0.5 1 sinusoidal: = (2 D – 1) V g –V g i L ( t ) = v 0 ( t ) R R Hence, current-bidirectional two-quadrant switches are required. 16 Fundamentals of Power Electronics Chapter 4: Switch realization

The dc-3øac voltage source inverter (VSI) i a + V g – i b i c Switches must block dc input voltage, and conduct ac load current. 17 Fundamentals of Power Electronics Chapter 4: Switch realization

Bidirectional battery charger/discharger D 1 L + + Q 1 v bus v batt D 2 spacecraft Q 2 main power bus – – v bus > v batt A dc-dc converter with bidirectional power flow. 18 Fundamentals of Power Electronics Chapter 4: Switch realization

4.1.3. Voltage-bidirectional two-quadrant switches • Usually an active switch, controlled by terminal C • Normally operated as two- i 1 quadrant switch: i + on • can conduct positive on-state current v v C off off • can block positive or negative (diode (transistor blocks voltage) blocks voltage) off-state voltage – 0 • provided that the intended on- state and off-state operating points lie on the composite i-v BJT / series instantaneous i-v characteristic, then switch can diode realization characteristic be realized as shown • The SCR is such a device, without controlled turn-off 19 Fundamentals of Power Electronics Chapter 4: Switch realization

Two-quadrant switches 1 i switch + i on-state current v on – 0 v switch off off 1 off-state (diode (transistor voltage i + blocks voltage) blocks voltage) v C – 0 20 Fundamentals of Power Electronics Chapter 4: Switch realization

A dc-3øac buck-boost inverter φ a i L + v ab (t) – φ b + – V g v bc (t) + – φ c Requires voltage-bidirectional two-quadrant switches. Another example: boost-type inverter, or current-source inverter (CSI). 21 Fundamentals of Power Electronics Chapter 4: Switch realization

4.1.4. Four-quadrant switches switch on-state current • Usually an active switch, controlled by terminal C • can conduct positive or negative on-state current switch • can block positive or negative off-state voltage off-state voltage 22 Fundamentals of Power Electronics Chapter 4: Switch realization

Three ways to realize a four-quadrant switch 1 1 1 i i i + + + 1 i + v v v v – – – – 0 0 0 0 23 Fundamentals of Power Electronics Chapter 4: Switch realization

A 3øac-3øac matrix converter 3øac input 3øac output i a + v an (t) – v bn (t) i b + – – v cn (t) + i c • All voltages and currents are ac; hence, four-quadrant switches are required. • Requires nine four-quadrant switches 24 Fundamentals of Power Electronics Chapter 4: Switch realization