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Chapter 4. Switch Realization 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 Fundamentals of Power Electronics1 Chapter 4: Switch realization SPST (single-pole single-throw) switches Buck converter SPST switch, with voltage and current with SPDT switch: L polarities defined 1 iL(t) + 1 2 + i Vg CRV + – v – – with two SPST switches: 0 i A L A iL(t) + + vA – – v All power semiconductor V + B B CRV g – devices function as SPST + iB switches. – Fundamentals of Power Electronics2 Chapter 4: Switch realization Realization of SPDT switch using two SPST switches G A nontrivial step: two SPST switches are not exactly equivalent to one SPDT switch G It is possible for both SPST switches to be simultaneously ON or OFF G Behavior of converter is then significantly modified —discontinuous conduction modes (chapter 5) G Conducting state of SPST switch may depend on applied voltage or current —for example: diode Fundamentals of Power Electronics3 Chapter 4: Switch realization Quadrants of SPST switch operation 1 Switch i on state A single-quadrant + current switch example: v ON-state: i > 0 – OFF-state: v > 0 0 Switch off state voltage Fundamentals of Power Electronics4 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 off-state voltage switch off-state voltage switch switch on-state on-state current current Voltage- Four- bidirectional quadrant two-quadrant switch switch off-state switch off-state switch voltage voltage Fundamentals of Power Electronics5 Chapter 4: Switch realization 4.1.1. Single-quadrant switches 1 Active switch: Switch state is controlled exclusively i + by a third terminal (control terminal). v Passive switch: Switch state is controlled by the applied current and/or voltage at terminals 1 and 2. – SCR: A special case — turn-on transition is active, 0 while turn-off transition is passive. Single-quadrant switch: on-state i(t) and off-state v(t) are unipolar. Fundamentals of Power Electronics6 Chapter 4: Switch realization The diode • A passive switch i • Single-quadrant switch: 1 on • can conduct positive on- + i state current off v v • can block negative off- state voltage – • provided that the intended 0 on-state and off-state operating points lie on the diode i-v characteristic, Symbol instantaneous i-v characteristic then switch can be realized using a diode Fundamentals of Power Electronics7 Chapter 4: Switch realization The Bipolar Junction Transistor (BJT) and the Insulated Gate Bipolar Transistor (IGBT) 1 • An active switch, controlled BJT i by terminal C i + C • Single-quadrant switch: v on – • can conduct positive on- off v 0 state current • can block positive off-state 1 IGBT voltage i + C • provided that the intended v on-state and off-state – operating points lie on the 0 instantaneous i-v characteristic transistor i-v characteristic, then switch can be realized using a BJT or IGBT Fundamentals of Power Electronics8 Chapter 4: Switch realization The Metal-Oxide Semiconductor Field Effect Transistor (MOSFET) • An active switch, controlled by i terminal C • Normally operated as single- 1 on quadrant switch: i + C off v • can conduct positive on-state v current (can also conduct – negative current in some on circumstances) 0 (reverse conduction) • can block positive off-state voltage Symbol instantaneous i-v characteristic • provided that the intended on- state and off-state operating points lie on the MOSFET i-v characteristic, then switch can be realized using a MOSFET Fundamentals of Power Electronics9 Chapter 4: Switch realization Realization of switch using transistors and diodes Buck converter example i A L A iL(t) + + vA – – v V + B B CRV g – + Switch A: transistor iB – Switch B: diode iA iB Switch A Switch B i i SPST switch on L on L operating points Switch A Switch B off off Vg vA –Vg vB Switch A Switch B Fundamentals of Power Electronics10 Chapter 4: Switch realization Realization of buck converter using single-quadrant switches i vA L A +– iL(t) +– vL(t) – V + v g – B + iB iA iB Switch A Switch B on iL on iL Switch A Switch B off off Vg vA –Vg vB Fundamentals of Power Electronics11 Chapter 4: Switch realization 4.1.2. Current-bidirectional two-quadrant switches • Usually an active switch, controlled by terminal C i 1 on • Normally operated as two- i (transistor conducts) quadrant switch: + C off v • can conduct positive or v negative on-state current – on • can block positive off-state 0 (diode conducts) voltage • provided that the intended on- state and off-state operating i-v BJT / anti-parallel instantaneous i-v points lie on the composite diode realization characteristic characteristic, then switch can be realized as shown Fundamentals of Power Electronics12 Chapter 4: Switch realization Two quadrant switches switch i on-state on current 1 (transistor conducts) i + v v off switch – off-state voltage 0 on (diode conducts) Fundamentals of Power Electronics13 Chapter 4: Switch realization MOSFET body diode i 1 on (transistor conducts) i + v C 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 Fundamentals of Power Electronics14 Chapter 4: Switch realization A simple inverter iA + Q1 V + D vA v0(t)=(2D –1)Vg g – 1 – L iL + + V + D CRv g – 2 vB 0 Q2 – – iB Fundamentals of Power Electronics15 Chapter 4: Switch realization Inverter: sinusoidal modulation of D v0(t)=(2D –1)Vg Sinusoidal modulation to v0 produce ac output: V g D(t)=0.5 + Dm sin (ωt) D The resulting inductor 0 0.5 1 current variation is also sinusoidal: –V v (t) Vg g i (t)= 0 =(2D –1) L R R Hence, current-bidirectional two-quadrant switches are required. Fundamentals of Power Electronics16 Chapter 4: Switch realization The dc-3øac voltage source inverter (VSI) ia + Vg – ib ic Switches must block dc input voltage, and conduct ac load current. Fundamentals of Power Electronics17 Chapter 4: Switch realization Bidirectional battery charger/discharger D1 L + + Q vbus 1 D vbatt spacecraft 2 main power bus Q2 – – vbus > vbatt A dc-dc converter with bidirectional power flow. Fundamentals of Power Electronics18 Chapter 4: Switch realization 4.1.3. Voltage-bidirectional two-quadrant switches • Usually an active switch, controlled by terminal C 1 i • Normally operated as two- i + on quadrant switch: • can conduct positive on-state v v current C off off (diode (transistor • can block positive or negative 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 Fundamentals of Power Electronics19 Chapter 4: Switch realization Two-quadrant switches 1 i switch + i on-state current v on – 0 v switch 1 off off off-state (diode (transistor voltage i + blocks voltage) blocks voltage) v C – 0 Fundamentals of Power Electronics20 Chapter 4: Switch realization A dc-3øac buck-boost inverter φ iL a + vab(t) – φ – b + + V g vbc(t) – φc Requires voltage-bidirectional two-quadrant switches. Another example: boost-type inverter, or current-source inverter (CSI). Fundamentals of Power Electronics21 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 off-state • can block positive or negative voltage off-state voltage Fundamentals of Power Electronics22 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 Fundamentals of Power Electronics23 Chapter 4: Switch realization A 3øac-3øac matrix converter 3øac input 3øac output ia + van(t) – vbn(t) i + – b – vcn(t) + ic •All voltages and currents are ac; hence, four-quadrant switches are required. • Requires nine four-quadrant switches Fundamentals of Power Electronics24 Chapter 4: Switch realization 4.1.5. Synchronous rectifiers Replacement of diode with a backwards-connected MOSFET, to obtain reduced conduction loss i 1 1 1 on i i (reverse conduction) + + i + C off v v v v – – – on 0 0 0 ideal switch conventional MOSFET as instantaneous i-v diode rectifier synchronous characteristic rectifier Fundamentals of Power Electronics25 Chapter 4: Switch realization Buck converter with synchronous rectifier •MOSFET Q2 is controlled to turn on i vA L A +– iL(t) when diode would normally conduct Q1 – • Semiconductor + C Vg vB conduction loss can – C + be made arbitrarily Q 2 iB small, by reduction of MOSFET on- resistances •Useful in low-voltage high-current applications Fundamentals of Power Electronics26 Chapter 4: Switch realization 4.2. A brief survey of power semiconductor devices G Power diodes G Power MOSFETs G Bipolar Junction Transistors (BJTs) G Insulated Gate Bipolar Transistors (IGBTs) G Thyristors (SCR, GTO, MCT) G On resistance vs. breakdown voltage vs. switching times G Minority carrier and majority carrier devices Fundamentals of Power Electronics27 Chapter 4: Switch realization 4.3.1. Transistor switching with clamped inductive load transistor waveforms v L V i A i (t) g A +– L vA(t) iL physical i (t) MOSFET – A + ideal Vg vB – 00 – + diode gate + t i DT T driver i L s s B diode waveforms i (t) Buck converter example 00B t vB(t) vB(t)=vA(t)–Vg transistor turn-off transition iA(t)+iB(t)=iL –Vg p (t) A Vg iL = vA iA area W 1 off Woff = 2 VgiL (t2 – t0) t t0 t1 t2 Fundamentals of Power Electronics28 Chapter 4: Switch realization Switching loss induced by transistor turn-off transition Energy lost during transistor turn-off transition: 1 Woff = 2 VgiL (t2 – t0) Similar result during transistor turn-on transition.
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