Once a thyristor is triggered into conduction by its gate, it can be turned off by

reducing its anode current below the holding current, via commutation

increasing the gate current further

An IGBT is preferred over a plain power MOSFET for a medium-to-high power inverter chiefly because

it combines a voltage-controlled gate with a low conduction drop at high current, like a bipolar device

it switches faster than any MOSFET

it needs no gate drive circuit at all

it combines a voltage-controlled gate with a low conduction drop at high current, like a bipolar device

it can be turned off only by commutation, like a thyristor

Which of the following are correctly true of buck, boost and buck-boost DC-DC converters?

A boost converter's output is always greater than or equal to its input.

A buck-boost converter can produce an output either greater than or less than the input, from the same topology.

Every DC-DC converter formula ultimately derives from the inductor's volt-second balance in steady state.

A boost converter's output is always greater than or equal to its input.

A buck-boost converter can produce an output either greater than or less than the input, from the same topology.

Every DC-DC converter formula ultimately derives from the inductor's volt-second balance in steady state.

A buck converter can produce an output voltage greater than its input.

A phase-controlled thyristor converter's firing angle alpha exceeding 90 degrees (with current still flowing in the same direction) puts the converter into

inverting mode, feeding power from the DC side back to the AC side

rectifying mode with higher output

inverting mode, feeding power from the DC side back to the AC side

A Current Source Inverter (CSI), unlike a Voltage Source Inverter (VSI), must always

have a closed current path at the load, and cannot drive an open circuit

have a large filter capacitor across its DC link

have a closed current path at the load, and cannot drive an open circuit

operate only at unity power factor

use only thyristors, never MOSFETs or IGBTs

Power Devices, DC-DC Converters, Rectifiers and Inverters

Section 10 of the GATE Electrical Engineering (EE) paper, and the last of this syllabus: the semiconductor switches and converter topologies that connect AC and DC power at every scale, from a laptop charger to an HVDC link. Start with the static characteristics and gating requirements of the thyristor, MOSFET and IGBT, the three switches every converter in this section is built from; then DC-DC converters that step a DC voltage up or down; then rectifiers that convert AC to DC, controlled or not, and the harmonics and power factor penalty they impose on the supply; and finally inverters that convert DC back to AC, and the pulse-width modulation that shapes their output.

1. Thyristor, MOSFET and IGBT: static characteristics and gating

A thyristor (SCR) is a latching device: once triggered into conduction by a gate pulse (while forward-biased), it continues conducting even after the gate signal is removed, and turns off only when its anode current falls below a small holding current — this is exactly why thyristor circuits need a specific commutation strategy (natural, from the AC supply itself, or forced, using an auxiliary circuit) to turn the device off, unlike a fully controllable switch. A MOSFET is a voltage-controlled device (gate voltage alone decides its state, drawing negligible steady gate current) with fast switching but limited voltage/current handling per device, while an IGBT combines a MOSFET's easy voltage-controlled gate with a bipolar-like low conduction drop at high current, making it the standard choice for medium-to-high power inverters where a plain MOSFET's on-resistance would dissipate too much and a plain BJT's base drive would be too complex.

Three switches, at a glance
Device	Control	Turn-off
Thyristor (SCR)	gate pulse triggers ON only; latches	needs commutation (natural or forced), not gate control
MOSFET	gate voltage, fully controllable both ways	gate voltage removed — turns off directly
IGBT	gate voltage, fully controllable both ways	gate voltage removed — turns off directly

⚠️ A thyristor's gate cannot turn it off

Once a thyristor latches into conduction, removing or reversing the gate signal has NO effect on its state at all — the gate's only job is to initiate turn-on. This is the single most tested conceptual trap in this topic: a candidate who treats a thyristor like a MOSFET (assuming gate control works both ways) will get every commutation-circuit question wrong, since those circuits exist specifically to force the anode current below the holding current by external means, which a gate signal alone cannot do.

2. DC-DC converters: buck, boost and buck-boost

A buck converter steps DC voltage down: the output-to-input voltage ratio in continuous conduction is Vo/Vin = D (the switch duty cycle, 0 to 1), so the output is always less than or equal to the input. A boost converter steps DC voltage up: Vo/Vin = 1/(1-D), always greater than or equal to the input, and rising sharply (and in practice becoming poorly regulated) as D approaches 1. A buck-boost converter can step the voltage either up or down (with an inverted output polarity relative to the input in the classic topology): Vo/Vin = -D/(1-D), less than 1 in magnitude for D < 0.5 and greater than 1 for D > 0.5 — the sign is the standard clue that distinguishes this topology's output from a buck or boost's.

Buck, boost and buck-boost, side by side
Converter	Vo/Vin (continuous conduction)	Range
Buck	D	0 to 1 (step-down only)
Boost	1/(1-D)	greater than or equal to 1 (step-up only)
Buck-boost	-D/(1-D)	either side of 1 in magnitude, inverted polarity

🎯 Volt-second balance is the one idea behind every formula here

In steady-state periodic operation, an inductor's average voltage over one switching cycle must be exactly zero (otherwise its current, and hence its stored energy, would ramp away without bound) — this single volt-second balance condition, applied to each converter's specific switch states, is what derives every one of the Vo/Vin formulas above; there is no need to memorise them independently once this one principle is understood.

3. Rectifiers, thyristor converters and harmonics

An uncontrolled rectifier (diode bridge) gives a fixed DC output for a given AC input, single-phase or three-phase, and its output ripple frequency is 2 times supply frequency for single-phase full-wave and 6 times supply frequency for three-phase full-wave — the higher pulse number directly means less filtering effort for the same output smoothness. A phase-controlled (thyristor) converter adds a firing-angle-alpha delay to each thyristor's turn-on relative to where a diode would naturally conduct, and this single angle continuously varies the average DC output from maximum (alpha near 0) down through zero (alpha = 90 degrees for a purely resistive load) to negative values (alpha beyond 90 degrees) — a negative average output voltage with current still flowing in the same direction is exactly the inverting mode that lets the same hardware feed power back from the DC side to the AC side.

ℹ️ Line current harmonics and the power factor/distortion factor split

A rectifier's AC-side line current is inherently non-sinusoidal (it is drawn only during each thyristor or diode's conduction interval, not continuously), so it carries harmonics beyond the fundamental. The overall power factor of such a load splits into two independent factors: the DISPLACEMENT factor cos(phi1), which is the cosine of the phase angle between the FUNDAMENTAL component of line current and the supply voltage (this is what a firing-angle delay directly worsens), and the DISTORTION factor (the ratio of fundamental RMS current to total RMS current), which captures the harmonic content alone — overall power factor is the PRODUCT of the two, so a converter can have perfect displacement (phi1 = 0) and still have a poor overall power factor purely from harmonic distortion.

4. Voltage/current source inverters and sinusoidal PWM

A Voltage Source Inverter (VSI) is fed from a stiff DC voltage (a large filter capacitor holds the DC link voltage nearly constant) and its switches directly impose that voltage's polarity onto the load, so it can drive any load impedance including an open circuit; a Current Source Inverter (CSI) is fed from a stiff DC current (a large series inductor holds the DC link current nearly constant) and its switches direct that fixed current into the load, so it must always have a closed current path and cannot drive an open circuit. VSIs are far more common in practice, from small drives to grid-tied solar inverters, precisely because most loads (including a motor's stator winding) are naturally more compatible with a voltage source than a forced current.

🧠 Sinusoidal PWM: the modulation index shortcut

Sinusoidal PWM compares a low-frequency sinusoidal reference (the desired output waveform) against a high-frequency triangular carrier, switching the inverter on whenever the reference exceeds the carrier — the resulting pulse pattern's average value over each carrier period traces the sinusoid, pushing the switching-frequency harmonics far away from the fundamental where they are easily filtered. The modulation index ma = (peak of reference) / (peak of carrier) controls output amplitude directly: ma <= 1 stays in the linear range where fundamental output voltage is proportional to ma, while ma > 1 (overmodulation) trades linearity for a higher achievable fundamental, eventually reaching square-wave (six-step) operation at the maximum possible fundamental output.

Key takeaways

A thyristor's gate can only turn it ON; turning it off needs commutation (natural or forced) since the device latches — unlike a MOSFET or IGBT, which turn off directly by removing gate voltage.
Every buck/boost/buck-boost formula derives from one idea: an inductor's average voltage over one switching cycle is zero in steady state (volt-second balance).
A phase-controlled converter's firing angle alpha continuously varies output from maximum through zero to negative (inverting) — negative average output with unchanged current direction is what feeds power back to the AC side.
Overall power factor is displacement factor (fundamental phase) times distortion factor (harmonic content) — perfect displacement can still coexist with a poor overall power factor from harmonics alone.
A VSI (stiff DC voltage) can drive any load including an open circuit; a CSI (stiff DC current) always needs a closed current path — which is why VSIs dominate practical inverter applications.

Practice questions (9)

Attempt each one before opening the answer. Every explanation names the tempting wrong option as well as the right one, because that is where marks are lost.

Once a thyristor is triggered into conduction by its gate, it can be turned off by
1. removing the gate pulse
2. reversing the gate voltage
3. reducing its anode current below the holding current, via commutation
4. increasing the gate current further
Show answer
Answer: C — reducing its anode current below the holding current, via commutation
A thyristor latches once triggered, and the gate has no further influence on its state — only reducing the anode current below the holding current (by natural or forced commutation) turns it off, which is the standard fact this topic tests most heavily.
An IGBT is preferred over a plain power MOSFET for a medium-to-high power inverter chiefly because
1. it switches faster than any MOSFET
2. it needs no gate drive circuit at all
3. it combines a voltage-controlled gate with a low conduction drop at high current, like a bipolar device
4. it can be turned off only by commutation, like a thyristor
Show answer
Answer: C — it combines a voltage-controlled gate with a low conduction drop at high current, like a bipolar device
An IGBT's structure combines a MOSFET's easy voltage-controlled gate with a bipolar-like low forward drop at high current density, which a plain MOSFET's channel resistance cannot match at high power — it is fully gate-controllable for both turn-on and turn-off, unlike a thyristor.
A buck converter operating at duty cycle D = 0.4 with input voltage 20 V gives an output voltage (continuous conduction) of
1. 8 V
2. 20 V
3. 33.3 V
4. 50 V
Show answer
Answer: A — 8 V
Vo = D * Vin = 0.4 * 20 = 8 V — a buck converter's output is always less than or equal to its input, which immediately rules out the larger options; those larger values would instead be plausible boost-converter outputs.
Which of the following are correctly true of buck, boost and buck-boost DC-DC converters?
1. A boost converter's output is always greater than or equal to its input.
2. A buck-boost converter can produce an output either greater than or less than the input, from the same topology.
3. Every DC-DC converter formula ultimately derives from the inductor's volt-second balance in steady state.
4. A buck converter can produce an output voltage greater than its input.
Show answer
Answer: A — A boost converter's output is always greater than or equal to its input.; B — A buck-boost converter can produce an output either greater than or less than the input, from the same topology.; C — Every DC-DC converter formula ultimately derives from the inductor's volt-second balance in steady state.
The first three are all correctly true. A buck converter's D ranges only from 0 to 1, so Vo/Vin = D can never exceed 1 — a buck converter cannot step voltage up, which is exactly the boost converter's job instead.
A single-phase full-wave rectifier's output ripple frequency, for a 50 Hz supply, is
1. 50 Hz
2. 100 Hz
3. 25 Hz
4. 300 Hz
Show answer
Answer: B — 100 Hz
A single-phase full-wave rectifier produces two output pulses per input cycle (one from each half-cycle, both rectified to the same polarity), so ripple frequency is twice the supply frequency: 2 * 50 = 100 Hz; a three-phase full-wave bridge would instead give 6 times supply frequency, 300 Hz.
A phase-controlled thyristor converter's firing angle alpha exceeding 90 degrees (with current still flowing in the same direction) puts the converter into
1. rectifying mode with higher output
2. inverting mode, feeding power from the DC side back to the AC side
3. a permanently off state
4. short-circuit protection mode
Show answer
Answer: B — inverting mode, feeding power from the DC side back to the AC side
Beyond alpha = 90 degrees (for a purely resistive load reference), the average DC output voltage goes negative while current continues in the same direction — a negative voltage times a positive current means power flows OUT of the DC side and INTO the AC side, which is exactly the inverting mode of a phase-controlled converter.
A converter's overall power factor is the product of its displacement factor and its
1. efficiency
2. distortion factor
3. firing angle
4. duty cycle
Show answer
Answer: B — distortion factor
Overall power factor = displacement factor (cosine of the fundamental current's phase angle) times distortion factor (fundamental RMS current over total RMS current) — a converter can have zero displacement angle and still suffer a poor overall power factor purely from harmonic distortion.
A Current Source Inverter (CSI), unlike a Voltage Source Inverter (VSI), must always
1. have a large filter capacitor across its DC link
2. have a closed current path at the load, and cannot drive an open circuit
3. operate only at unity power factor
4. use only thyristors, never MOSFETs or IGBTs
Show answer
Answer: B — have a closed current path at the load, and cannot drive an open circuit
A CSI is fed from a stiff DC current source (a large series inductor), and its switches must always direct that fixed current somewhere — an open circuit at the load would have nowhere for the current to go, which a VSI (a voltage source, happy driving any impedance including an open circuit) does not suffer from. A large DC-link capacitor is the VSI's own characteristic feature, not the CSI's.
In sinusoidal PWM, the modulation index ma is defined as the peak of the reference signal divided by the peak of the carrier signal. If the reference peak is 8 V and the carrier peak is 10 V, ma equals ___
Numerical answer — type the value.
Show answer
Answer: 0.8
ma = 8/10 = 0.8, which is within the linear range (ma <= 1), so the fundamental output voltage stays directly proportional to ma at this operating point rather than entering overmodulation.