POWER ELECTRONICS Device

1. POWER ELECTRONICS
It deals with control and conversion of high power applications with high efficiency.
ASHUTOSH
Wednesday, 02 November 2022
DEPARTMENT OF INSTRUMENTATION AND CONTROL
Bhartiya Vidyapeeth
Delhi-1100663

2. Content
• Introduction
Characteristics and switching behaviour of Power Diode, SCR, UJT, TRIAC, DIAC, GTO, MOSFET, IGBT, MCT and
power BJT, two-transistor analogy of SCR, firing circuits of SCR and TRIAC, SCR gate characteristics, SCR ratings.
Protection of SCR against over current, over voltage, high dV/dt, high dI/dt, thermal protection, Snubber circuits,
Methods of commutation, series and parallel operation of SCR, Driver circuits for BJT/MOSFET.
• AC to DC Converter
Classification of rectifiers, phase controlled rectifiers, fully controlled and half controlled rectifiers and their performance
parameters, .three phase half wave, full wave and half controlled rectifiers and their performance parameters, effect of
source impedance on the performance of single phase and three phase controlled rectifiers, single-phase and three
phase dual converter.
• DC to DC Converter
Classification of choppers as type A, B, C, D and E, principle of operation, switching mode regulators: Buck, Boost,
Buck-Boost, Cuk regulators.
• AC to AC Converter
AC voltage Controllers, Cyclo-converters : single phase to single phase, three phase to single phase, three phase to
three phase Cyclo-converter circuit and their operation, Matrix converter.
• DC to AC Converter
single phase single pulse inverter: Square wave, quasi square. Three phase single pulse inverters (120̊ and 180 ̊
conduction) Modulation Techniques and reduction of harmonics, PWM techniques, SPWM techniques, SVM, Carrier
less modulation. , PWM Inverter, Bidirectional PWM converters, voltage source inverters and current source inverter,
Multi level Inverter: cascaded and NPC Inverters.

3. Objective of the Switch
• Power electronics involves the study of
electronic circuits intended to control the flow of
electrical energy. These circuits handle power
flow at levels much higher than the individual
device ratings.
• Ideally, when a switch is on, it has zero voltage
drop and will carry any current imposed on it.
When a switch is off, it blocks the flow of current
regardless of the voltage across it.
• Device power, the product of the switch voltage
and current, is identically zero at all times. The
switch controls energy flow with no loss.
• Switching devices are selected based on their
power handling rating— the product of their
voltage and currents ratings.
A basic power electronic system.

4. Classification of the Power Semiconductors

5. Device Type Characteristics Of Power Devices
• Diode Current ratings from <1 to >5000 A. Voltage ratings from 10V to 10 kV or more. The fastest
power devices switch in <20 ns, while the slowest require 100 ms or more. The function of a diode
applies in rectifiers and dc-dc circuits.
• BJT (Bipolar Junction Transistor) Conducts collector current (in one direction) when sufficient base
current is applied. Power device current ratings from 0.5 to 500 A or more; voltages from 30 to 1200V.
Switching times from 0.5 to 100 ms. The function applies to dc-dc circuits; combinations with diodes are
used in inverters. Power BJTs are being supplanted by FETs and IGBTs.
• FET (Field Effect Transistor) Conducts drain current when sufficient gate voltage is applied. Power
FETs (nearly always enhancement mode MOSFETs) have a parallel connected reverse diode by virtue
of their construction. Ratings from 1 to 100 A and 30 up to 1000V. Switching times are fast, from 50 or
less up to 200 ns. The function applies to dc-dc conversion, where the FET is in wide use, and to
inverters.
• IGBT (Insulated Gate Bipolar Transistor) A special type of power FET that has the function of a BJT
with its base driven by a FET. Faster than a BJT of similar ratings, and easy to use. Ratings from 10 to
>600 A, with voltages of 600 to 1700V. The IGBT is popular in inverters from 1 to 100kW or more. It is
found almost exclusively in power electronics applications.
• SCR (Silicon Controlled Rectifier) A thyristor that conducts like a diode after a gate pulse is applied.
Turns off only when current becomes zero. Prevents current flow until a pulse appears. Ratings from 10
up to more than 5000 A, and from 200V up to 6 kV. Switching requires 1 to 200 ms. Widely used for
controlled rectifiers. The SCR is found almost exclusively in power electronics applications, and is the
most common member of the thyristor family.
• GTO (Gate Turn-Off Thyristor) An SCR that can be turned off by sending a negative pulse to its gate
terminal. Can substitute for BJTs in applications where power ratings must be very high. The ratings
approach those of SCRs, and the speeds are similar as well. Used in inverters rated >100 kW.
• TRIAC A semiconductor constructed to resemble two SCRs connected in reverse parallel. Ratings from
2 to 50 A and 200 to 800V. Used in lamp dimmers, home appliances, and hand tools. Not as rugged as
many other device types, but very convenient for many ac applications.
• MCT (MOSFET Controlled Thyristor) A special type of SCR that has the function of a GTO with its gate
driven from a FET. Much faster than conventional GTOs, and easier to use. These devices are
supplanting GTOs in some application areas.
MOSFET Symbol
Power Diode
BJT
Thyristor
GTO
P-Type MCT

6. Four Modes of Ideal Switch
• Forward Blocking
• Forward Conduction
• Reverse Blocking
• Reverse Conduction

7. The types of restricted switches

8. • A generic switching function can be characterized completely with three parameters:
1. The duty ratio D is the fraction of time during which the switch is on. For control purposes the pulse width can be adjusted to achieve a desired
result. We can term this adjustment process pulse-width modulation (PWM), perhaps the most important process for implementing control in power
converters.
2. The frequency fswitch 1/T(with radian frequency ω = 2πfswitch) is most often constant, although not in all applications. For control purposes, frequency
can be adjusted. This is unusual in power converters because the operating frequencies are often dictated by the application.
3. The time delay t0 or phase φ0=ωt0. Rectifiers often make use of phase control to provide a range of adjustment. A few specialized ac-ac converter
applications use phase modulation.
• With just three parameters to vary, there are relatively few possible ways to control any power electronic circuit. The dc-dc converters
usually rely on duty ratio adjustment (PWM) to alter their behavior. Phase control is common in controlled rectifier applications. Pulse-
width modulation is used formally for many types of inverters.
A generic switching function with period T; duty ratio D; and time reference t0

9. Requirement of a Switch
• AC-AC Conversion
[FB,FC,RB,RC] Eg. TRIAC
• AC-DC Conversion
[FB,FC,RB] Eg. SCR
• DC-DC Conversion
[FB,FC] Eg. BJT, IGBT
• DC-AC Conversion
INVERTER
VSI CSI
[FB,FC,RC] Eg. RCT [FB,FC,RB] Eg. SCR

10. Control Characteristics of Power Devices
1. Uncontrolled turn-on and turn-off (e.g., diode);
2. Controlled turn-on and uncontrolled turn-off (e.g., SCR);
3. Controlled turn-on and -off characteristics (e.g., BJT, MOSFET, GTO, SITH,
IGBT, SIT, MCT);
4. Continuous gate signal requirement (BJT, MOSFET, IGBT, SIT);
5. Pulse gate requirement (e.g., SCR, GTO, MCT);
6. Bipolar voltage-withstanding capability (SCR, GTO);
7. Unipolar voltage-withstanding capability (BJT, MOSFET, GTO, IGBT, MCT);
8. Bidirectional current capability (TRIAC, RCT);
9. Unidirectional current capability (SCR, GTO, BJT, MOSFET, MCT, IGBT,
SITH, SIT, diode).

11. POWER DIODE
• A power diode has a P-I-N structure as compared to the signal diode having a P-N structure.
Here, I (in P-I-N) stands for intrinsic semiconductor layer to bear the high-level reverse voltage as
compared to the signal diode (n- , drift region layer). However, the drawback of this intrinsic layer
is that it adds noticeable resistance during forward-biased condition. Thus, power diode requires a
proper cooling arrangement for handling large power dissipation.
DC Diode parameters: The most important are the following:
• Forward voltage VF is the voltage drop of a diode across A and K at a defined current level when
it is forward biased.
• Breakdown voltage VB is the voltage drop across the diode at a defined current level when it is
beyond reverse-biased level. This is known as avalanche.
• Reverse current IR is the current at a particular voltage, and which is below the breakdown
voltage.
Structure of Power Diode
Power Diode
Amp-Volt Characteristics of Power Diode
Power diode: (a) symbol; (b) and (c) types of packaging.

12. Reverse Recovery Characteristics
• It is switching behavior of the diode from ON state to OFF state.
• After the forward diode comes to null, the diode continues to conduct in the
opposite direction because of the presence of stored charges in the depletion
layer and the p or n-layer. The diode current flows for a reverse-recovery time trr.
It is the time between the instant forward diode current becomes zero and the
instant reverse-recovery current decays to 25 % of its reverse maximum value.
• Time Ta : Charges stored in the depletion layer removed.
• Time Tb : Charges from the semiconductor layer is removed.
• Shaded area in Fig. (a) represents stored charges QR which must be removed
during reverse-recovery time trr.
• Power loss across diode = vf * if (shown in Fig. c)
• As shown, major power loss in the diode occurs during the period tb.
• Recovery can be abrupt or smooth as shown in Fig. To know it quantitatively, we
can use the S – factor.
• Ratio Tb/Ta : Softness factor or S-factor.
• S-factor: measure of the voltage transient that occurs during the time the diode
recovers.
• S-factor = 1 ⇒ low oscillatory reverse-recovery process. (Soft –recovery diode)
• S-factor <1 ⇒ large oscillatory over voltage (snappy-recovery diode or fast-
recovery diode).
Turn-Off Characteristics of Power Diode: a) Variation of
Forward Current if ; b) Variation of Forward Voltage Drop vf ; c)
Variation of Power Loss

13. Reverse Recovery Characteristics
As a design engineer frequently needs to
calculate reverse recovery time in order to
evaluate the possibility of high-frequency
switching. As a rule of thumb, the lower trr is, the
faster the diode can be switched.
If tb is negligible compared to ta (which
commonly occurs), then the following expression
is valid:
from which the reverse recovery current
Q. The manufacturer of a selected diode gives the rate of fall
of the diode current di/dt = 20 A/μs, and a reverse recovery
time of trr = 5 μs. What value of peak reverse current do you
expect?
Note: The reverse recovery time decides the maximum switching frequency of a diode.

16. Thyristor
• Thyristors are the family of solid-state devices extensively used in
power electronics circuitry such as SCR (silicon-controlled rectifier),
DIAC (diode on AC), TRIAC (triode on AC), GTO (gate turn-off
thyristors), MCT (MOS-controlled thyristor), RCT, UJT, and etc.
• Thyristors are usually three-terminal devices with four layers of
alternating p- and n-type material (i.e. three p-n junctions) in their
main power handling section.
• The control terminal of the thyristor, called the gate (G) electrode,
may be connected to an integrated and complex structure as part of
the device. The other two terminals, anode (A) and cathode (K),
handle the large applied potentials (often of both polarities) and
conduct the major current through the thyristor.
Typical thyristor and the
associated electrical schematic
symbols.
Static Characteristics Curve of SCR

17. Thyristor

18. Thyristor

19. Thyristor

20. Thyristor

21. Thyristor

22. Thyristor
• The forward anode current of a thyristor must be more than its latching
current to latch into the conduction state; otherwise, the device reverts to the
blocking condition as the anode-to-cathode voltage falls.
• If the forward anode current of a thyristor is reduced below its holding
current, the device becomes unlatched and remains in the blocking state.
• Once a thyristor conducts, it behaves like a conducting diode and there is no
control over the device. That is, the device cannot be turned off by another
positive or negative gate pulse.

23. Two Transistors Analogy
• Basic operating principle of SCR, can easily be understood by the two transistor model of SCR,
as it is a combination of p and n layers.
This is a pnpn thyristor. If we bisect it then we will get two transistors i.e. one pnp transistor with J1 and J2
junctions and another is with J2 and J3 junctions as shown in figure.
The relation between the collector current and emitter current is shown below
Here, IC is collector current, IE is emitter current, ICBO is forward leakage current, α is common base forward
current gain and relationship between IC and IB is
Where, IB is base current and β is common emitter forward current gain.
Let’s for transistor T1 this relation holds
And that for transistor T2
Now, by the analysis of two transistors model we can get anode current,
From equation (i) and (ii), we get,
If applied gate current is Ig then cathode current will be the summation of anode current and gate current i.e.
By substituting this value of Ik in (iii) we get,
Two-transistor behavioral model of a thyristor.
J1
J2
J2
J3

24. Two Transistors Analogy
From this relation we can assure that with increasing the value of
towards unity, corresponding anode current will increase. How?
At the first stage when we apply a gate current Ig, it acts as base current of T2 transistor i.e. IB2 = Ig and
emitter current of the T2 transistor IE2 = Ik. Hence establishment of the emitter current gives rise α2 as
Presence of base current will generate collector current as
This IC2 is nothing but base current IB1 of transistor T1, which will cause the flow of collector current,
IC1 and IB1 lead to increase IC1 as
and hence, α1 increases. Now, new base current of T2 is
, which will lead to increase emitter current Two-transistor behavioral model of a thyristor.
J1
J2
J2
J3

25. Two Transistors Analogy
and as a result α2 also increases and this further increases
As
α1 again increases. This continuous positive feedback effect increases
towards unity and anode current tends to flow at a very large value. The value current then can only be
controlled by external resistance of the circuit.
Two-transistor behavioral model of a thyristor.
J1
J2
J2
J3

26. Thyristor Turn ON Method
A thyristor is turned on by increasing the anode current. This can be
accomplished in one of the following ways:
• Thermals. If the temperature of a thyristor is high, there is an increase in the
number of electron–hole pairs, which increases the leakage currents. This
increase in currents causes α1 and α2 to increase. Due to the regenerative
action, (α1 + α2) may tend to unity and the thyristor may be turned on. This
type of turn-on may cause thermal runaway and is normally avoided.
• Light. If light is allowed to strike the junctions of a thyristor, the electron–hole
pairs increase; and the thyristor may be turned on. The light-activated
thyristors are turned on by allowing light to strike the silicon wafers.
• High voltage. If the forward anode-to-cathode voltage is greater than the
forward breakdown voltage VBO, sufficient leakage current flows to initiate
regenerative turn-on. This type of turn-on may be destructive and should be
avoided.
• dv/dt. if the rate of rise of the anode–cathode voltage is high, the charging
current of the capacitive junctions may be sufficient enough to turn on the
thyristor. A high value of charging current may damage the thyristor; and the
device must be protected against high dv/dt. The manufacturers specify the
maximum allowable dv/dt of thyristors.
• Gate current. If a thyristor is forward biased, the injection of gate current by
applying positive gate voltage between the gate and cathode terminals turns
on the thyristor. As the gate current is increased, the forward blocking
voltage is decreased, as shown in Fig.
Effects of gate current on forward blocking voltage.

27. Gate Turn-off Thyristors
A GTO like an SCR can be turned on by applying a
positive gate signal. However, a GTO can be turned off
by a negative gate signal.
The GTOs have these advantages over SCRs:
(1) elimination of commutating components
in forced commutation, resulting in reduction in cost,
weight, and volume;
(2) reduction in acoustic and
electromagnetic noise due to the elimination of
commutation chokes;
(3) faster turn-off, permitting high switching
frequencies; and
(4) improved efficiency of converters

28. TRIAC
• A TRIAC can conduct in both directions and is
normally used in ac-phase control.
• It can be considered as two SCRs connected in
antiparallel with a common gate connection as
shown in Fig. a.
• Its symbol is shown in Fig. b.
• The v-i characteristics are shown in Fig. c.
TRIAC is a bidirectional device, its terminals cannot
be designated as anode and cathode. If terminal
MT2 is positive with respect to terminal MT1, the
TRIAC can be turned on by applying a positive
gate signal between gate G and terminal MT1. If
terminal MT2 is negative with respect to terminal
MT1, it is turned on by applying a negative gate
signal between gate G and terminal MT1. Characteristics of a TRIAC.

29. DIAC
• A DIAC, or “diode for alternating current,” is also a member of
the thyristor family. It is just like a TRIAC without a gate
terminal. The cross section of a DIAC is shown in Fig. a. Its
equivalent circuit is a pair of inverted four-layer diodes. Either
of two symbols as shown in Fig. b and c is often used. A DIAC
is a PNPN structured four-layer, two-terminal semiconductor
device. MT2 and MT1 are the two main terminals of the
device. There is no control terminal in this device. The DIAC
structure resembles a bipolar junction transistor (BJT).
• A DIAC can be switched from off-state to the on-state for
either polarity of the applied voltage. Since it is a bilateral
device like TRIAC, the terminal designations are arbitrary. The
switching from off-state to on-state is achieved by simply
exceeding the avalanche breakover voltage in either direction.
• A typical v–i characteristic of a DIAC is shown in Fig. When
the terminal MT2 is positive enough to break the junction N2-
P2, the current can flow from terminal MT2 to terminal MT1
through the path P1-N2-P2-N3. If the polarity of terminal MT1
is positive enough to break the junction N2-P1, the current
flows through the path P2-N2-P1-N1. A DIAC may be
considered as two series-connected diodes in opposite
direction.
Cross section of DIAC and its symbols.
v–i characteristics of DIACs.

30. Series Operation of Thyristors
For high-voltage applications, two or more
thyristors can be connected in series to provide
the voltage rating.
A derating factor that is normally used to
increase the reliability of the string is defined as: Three series-connected thyristors.

31. Parallel Operation of Thyristors
When thyristors are connected in parallel, the load current is not
shared equally due to differences in their characteristics. If a
thyristor carries more current than that of others, its power
dissipation increases, thereby increasing the junction
temperature and decreasing the internal resistance. This, in turn,
increases its current sharing and may damage the thyristor. This
thermal runaway may be avoided by having a common heat sink,
so that all units operate at the same temperature.
A small resistance, as shown in Fig a, may be connected in
series with each thyristor to force equal current sharing, but there
may be considerable power loss in the series resistances. A
common approach for current sharing of thyristors is to use
magnetically coupled inductors, as shown in Fig b. If the current
through thyristor T1 increases, a voltage of opposite polarity can
be induced in the windings of thyristor T2 and the impedance
through the path of T2 can be reduced, thereby increasing the
current flow through T2.

32. Protection of Thyristor
• Overcurrent Protection – Fuse or MCB must be connected.
• Overvoltage Protection – Varistor must be connected.
• di/dt Protection
A thyristor requires a minimum time to spread the current
conduction uniformly throughout the junctions. If the rate of
rise of anode current is very fast compared with the spreading
velocity of a turn-on process, a localized “hot-spot” heating
may occur due to high current density and the device may fail,
as a result of excessive temperature.
The practical devices must be protected against high di/dt. As
an example, let us consider the circuit in Fig. Under steady-
state operation, Dm conducts when thyristor T1 is off. If T1 is
fired when Dm is still conducting, di/dt can be very high and
limited only by the stray inductance of the circuit.
In practice, the di/dt is limited by adding a series inductor Ls ,
as shown in Fig. The forward di/dt is
where Ls is the series inductance, including any stray
inductance.
Thyristor switching circuit with di/dt limiting inductors.

33. Protection of Thyristor
• dv/dt Protection
If switch S1 in Fig. is closed at t = 0, a step voltage may be
applied across thyristor T1 and dv/dt may be high enough to
turn on the device. The dv/dt can be limited by connecting
capacitor Cs. When thyristor T1 is turned on, the discharge
current of capacitor is limited by resistor Rs.
With an RC circuit known as a snubber circuit, the voltage
across the thyristor rises exponentially as shown in Fig. and
the circuit dv/dt can be found approximately from
• Thermal Protection – Heat Sink is must.
dv/dt protection circuits.

34. Firing Circuit
1. Resistance firing Circuit
R1: is the gate current limitingresistance
R2: is used to vary the gate current and hence firing angle
R limits the voltage at Gate terminal
Diode D prevents build-up of negative voltage at Gate terminal
• The phase angle at which the SCR starts conducting is called firing angle, α
• Disadvantages:
Performance depends on temperature and SCR characteristics.
Minimum phase angle is typically 2-4 degrees only (not zero degree).
Maximum phase angle is only 90 degrees.

35. R-Trig Waveforms
The phase angle at which
the SCR starts conducting is
called firing angle, α
Vi
t
VL
VT
α

36. RC Triggering Circuit
• Capacitor charges during the negative half cycle
through D2
• When SCR is turned on, capacitor C is suddenly
discharged through D2
• D1 protects the SCR during negative half cycle
• Advantage over R-triggering Circuit:
• Controls upto 180 degrees

37. RC Trig Waveforms

38. Unijunction Transistor (UJT)
• Has a lightly doped n-type silicon layer to
which a heavily doped p-type emitter is
embedded
• The inter-base resistance is in the range of
5 – 10 kΩ
• This device cannot ‘amplify’

39. UJT Equivalent Circuit
• When Ve is more than V1+VD, then the diode is forward
biased and a current flows through RB1
• Number of carriers in RB1 increases and the resistance
reduces
• Ve decreases with increase in Ie and the therefore the device
is said to exhibit negative resistance
UJT Characteristics
• At peak point, Ve = V1+VD,
• At Valley point, RB1 is minimum

40. UJT Prameters
Maximum emitter reverse voltage
• Maximum reverse bias which the emitter – base2 junction can tolerate without breakdown. Typ: 30V
Maximum inter-base voltage
• Maximum voltage possible between base1 and base2. Decided by the power dissipation. Typ: 35 V
Interbase resistance
• Typ: 4.7 k – 9.1 k
Intrinsic stand off ratio
• Typ: 0.56 – 0.75
Maximum peak emitter current
• Typ : 2A
Emitter leakage current
• The emitter current when Ve is less than Vp and the UJT is off.
• Typ 12 μA
Typical values are of 2N2646

41. UJT Oscillator
• R1 and R2 are much less than the inter-base resistance
• The output pulses can be used to trigger an SCR

42. UJT firing circuit for Half Wave Controller

43. Full wave UJT trigger Circuit

44. Quiz 1
1. The p-region has a greater concentration of __________ as
compared to the n-region in a P-N junction.
a) holes
b) electrons
c) both holes & electrons
d) photons
2. Which of the below mentioned statements is false regarding a p-n
junction diode?
a) Diode are uncontrolled devices
b) Diodes are rectifying devices
c) Diodes are unidirectional devices
d) Diodes have three terminals
3. A power transistor is a
a) three layer, three junction device
b) three layer, two junction device
c) two layer, one junction device
d) four layer, three junction device
4. The MOSFET combines the areas of _______ & _________
a) field effect & MOS technology
b) semiconductor & TTL
c) mos technology & CMOS technology
d) none of the mentioned
5. Choose the correct statement
a) MOSFET is a uncontrolled device
b) MOSFET is a voltage controlled device
c) MOSFET is a current controlled device
d) MOSFET is a temperature controlled device
6. The three terminals of the IGBT are
a) base, emitter & collector
b) gate, source & drain
c) gate, emitter & collector
d) base, source & drain
7. The controlled parameter in IGBT is the
a) IG
b) VGE
c) IC
d) VCE
8. An SCR is a
a) four layer, four junction device
b) four layer, three junction device
c) four layer, two junction device
d) three layer, single junction device
9. Choose the false statement.
a) SCR is a bidirectional device
b) SCR is a controlled device
c) In SCR the gate is the controlling terminal
d) SCR are used for high-power applications
10. For an SCR in the forward blocking mode (practically)
a) leakage current does not flow
b) leakage current flows from anode to cathode
c) leakage current flows from cathode to anode
d) leakage current flows from gate to anode