Thyristor Characteristics and Triggering Questions and Answers

Answer: A

The UJT is used because of its excellent triggering properties.

A UJT switches on sharply when its emitter voltage reaches the peak point, discharging a capacitor in a fast pulse. This makes the UJT relaxation oscillator a simple, stable source of sharp trigger pulses for firing SCRs and triacs in power control circuits.

The UJT is a low-power device, so high power dissipation and high current supply are not its virtues; it is used to trigger the power devices, not to handle power itself. A large breakdown voltage is not the reason it is chosen.

Answer: B

di/dt protection is given by connecting an inductor in series with the load (in series with the thyristor).

When a thyristor turns on, conduction starts in a small area near the gate and spreads across the junction. If the anode current rises too fast, the current crowds into that small area and creates local hot spots that can destroy the device. A series inductor limits the rate of rise to about V/L.

An inductor across the load does nothing to limit the current through the thyristor, and an inductor in the gate circuit affects the gate pulse, not the anode current that must be limited.

Answer: B

Hot spots are avoided by applying gate current nearer to the maximum gate current.

A hot spot forms when the SCR turns on over only a small area near the gate and the anode current crowds into it. A strong gate drive, close to the rated maximum gate current, turns on a larger area of the cathode at once and makes conduction spread faster across the whole cross-section, so the current density stays low.

Reducing the junction temperature and mounting on a heat sink deal with the average heating of the whole device, not with local current crowding at turn-on. Using only resistive loads does not limit the rate of rise of current.

Answer: C

dv/dt protection is given by connecting a capacitor and resistor in parallel with the device.

The junctions of an SCR have capacitance, and a rapidly rising anode voltage drives a charging current \(i = C\, dv/dt\) through them. That current can act like gate current and turn the SCR on falsely. An RC snubber across the SCR absorbs the fast voltage change, slowing the rise of voltage across the device; the resistor limits the capacitor's discharge current when the SCR turns on.

A series inductor protects against di/dt, not dv/dt, and a capacitor across the load or an inductor across the device does not slow the voltage across the SCR.

Answer: B

The maximum frequency is about 4.9 kHz.

In each cycle the thyristor must turn on and then turn off completely before the next cycle, so the shortest possible period is the sum of the two times:

\[ \begin{aligned} T_{min} &= 3 + 200 = 203\ \mu\text{s} \\ f_{max} &= \frac{1}{203 \times 10^{-6}} \approx 4.93\ \text{kHz} \end{aligned} \]

5.00 kHz comes from ignoring the turn-on time and using 200 μs alone, which slightly overstates the limit. 10 kHz and 20 kHz would require periods of 100 μs and 50 μs, shorter than the turn-off time itself.

Answer: D

An SCR has 3 junctions.

The silicon controlled rectifier is a four-layer PNPN device. Four alternating layers create three P-N junctions, usually labelled J1, J2 and J3, between anode and cathode, with the gate connected to the inner P layer next to the cathode.

A single junction is a diode, and two junctions make a bipolar transistor. Four junctions would need five layers, which is not the structure of an SCR.

Answer: D

At high frequency, switching losses become significant.

Every time an SCR turns on or off, there is a short interval during which both the voltage across it and the current through it are appreciable, and energy is dissipated. The average switching loss is that energy multiplied by the switching frequency:

\[ P_{sw} = (E_{on} + E_{off}) \times f \]

so it grows in proportion to frequency and limits SCRs to relatively low frequencies. di/dt and dv/dt ratings must always be respected, and reverse blocking voltage is set by the circuit voltage; none of these grows with operating frequency in the way switching loss does.

Answer: B

A triac is a pair of converter grade SCRs connected in anti-parallel.

A triac behaves like two SCRs connected in inverse parallel with a common gate, built as one device. It can therefore conduct in both directions and be triggered in either half cycle, which makes it suitable for AC control such as light dimmers and fan regulators.

A pair of UJTs would not give controlled conduction of load current. Two SCRs in plain parallel both conduct in the same direction, and two in series also conduct in one direction only.

Answer: C

Fibre optic triggering is preferred because it provides a very high level of insulation/isolation.

In high voltage applications such as HVDC valves, SCRs sit at potentials of many kilovolts above ground, while the gate control electronics are near ground. An optical fibre carries the trigger pulse as light, and since glass does not conduct, it isolates the control circuit completely from the high voltage. It is also immune to electromagnetic interference.

An optical fibre carries a signal, not power, so it does not provide low impedance, gain or high current for the power circuit.

Answer: C

The key gives 60 m sec; the paper writes 'm sec' where microseconds are meant, since thyristor turn-off times are measured in microseconds.

Turn-off time is the time the anode current must stay at zero, with reverse bias, before forward voltage can be reapplied without the thyristor turning on again. It depends on how quickly the excess carriers stored in the inner junction recombine. Carrier lifetime increases with temperature, so the stored charge takes longer to disappear, and turn-off time rises roughly in proportion to junction temperature in the working range. The standard textbook statement used for this question is that doubling the temperature from 50°C to 100°C doubles the turn-off time, from 30 to 60.

The turn-off time therefore does not stay the same or fall to 15, and 100 is not supported by that proportionality.