Test: Power Electronics - 1

The reverse recovery characteristic of a power diode is shown below. reverse recovery time t_rr composed of t_a and t_b

 

t_a is the time between zero crossings and peak reverse current I_RR and t_b is measured from reverse peak I_RR value to 0.25 I_RR.

If the characteristics are assumed to be triangular (i.e. abrupt recovery) then from the figure charge stored is

Q_RR = area of the triangle

The short circuiting fingers, also called anode-shorts, leads to short-circuit of the emitter p⁺(Anode A) with base n of Q₁ transistor as shown in Fig. For anode or emitter current I_a, effective emitter current I_E1 is reduced because of anode short. This further decreases collector current I_C1. Therefore, effective current gain α₁ of Q₁, now given by I_C1/I_a gets reduced. So by anode-short structure, α₁ is reduced but α₂ remains unchanged as desired.

Anode-short, however, reduces reverse voltage blocking capability. With reverse biased GTO, junction that blocks reverse voltage is J₃ only. Junction J₁ blocks no voltage because of n⁺fingers in between p⁺ anode. As J₃ junction has large doped layers p⁺, n⁺ on its two sides, J₃ has lower breakdown voltage, of the order of about 20 to 30 V.

• Thyristor is unidirectional and bipolar switch. It carry current Anode to cathod and block the current cathode to anode
• Triac conduct in both the direction A triac is thus a bidirectional thyristor with three terminal
• GTO is just like SR it is unidirectional and bipolar It conducts only Anode to cathode
• MOSFET is bidirectional but unipolar device
• LASER is conduct only one direction.

Concept:

Change in V and L are shown below

Energy loss during turn on process =
Calculations:

= 20 × 10³ × 3 × 10^-6

= 0.06 J

Energy loss during turn – on

= 0.2 watt – sec.

Energy loss during turn – off

= 0.27 watt – sec

Total energy loss in one cycle = 0.2 + 0.27

= 0.47 watt – sec.

Average power loss in transistor

= switching frequency × energy loss in one cycle

⇒ Switching frequency =470/0.47

= 1000 Hz.

Concept:

Average power loss during delay time with o < t < t_d

Average power loss during rise time is

Calculations:

P_d = 5 × 10³ × 2 × 10⁻³ × 220 × 4 × 10⁻⁶ = 88mW

Pr = 5 × 10³ × 80 × 1 × 10⁻⁶ = 14.933W
Total power loss during turn on time = 14.933 + 0.00088 = 14.9339 W

Concept:


Calculations:

P = I_rms² × 50 × 10^-3 = 24 × 50 × 10^-3 = 1.2 W

Correct Answer :- 3.14

Explanation : For half-wave diode rectifier

R.M.S value of the output voltage

Vor = [1/2π ∫(0 to π) Vm² sin²ωt d(ωt)]^1/2

= Vm/(2π)^1/2[∫(0 to π)(1-cos²ωt)/2 d(ωt)]^1/2

= Vm/2

Ior = Vm/2R

Average value of load current I₀ = Vm/πR

I_or = Vm/2E

I_o × time = 400 Ah

∴ Time required = (400*π)/400

= 3.14 hrs

• The Gate turn off thyristor (GTO) is a four-layer PNPN power semiconductor switching device that can be turned on by a short pulse of gate current and
• can be turned off by a reverse gate pulse.
• The magnitude of latching, holding currents is more. The latching current of the GTO is several times more as compared to conventional thyristors of the
• same rating.
• On state voltage drop and the associated loss is more.
• Due to the multi-cathode structure of GTO, triggering gate current is higher than that required for normal SCR.
• Gate drive circuit losses are more. Its reverse voltage blocking capability is less than the forward voltage blocking capability.
• GTO has the capability of being turned off by a negative gate – current pulse

Concept:

Apply KVL

The roots of the characteristic equation are

Ringing Frequency in rod/sec = ω_r = 
 

Calculations:

Conduction time of diode = π/ω_r

Conduction time =