DHANALAKSHMI COLLEGE OF ENGINEERING DEPARTMENT OF ELECTRICAL AND ELECTRONICS ENGINEERING

EE6503 POWER ELECTRONICS UNIT I- POWER SEMI-CONDUCTOR DEVICES

PART - A 1. What is a SCR?

A silicon-controlled rectifier (SCR) is a three terminal, three-junction semiconductor device that acts as a true electronic switch. It is a unidirectional device. It control the amount of power fed to the load.

2. Define break over voltage of SCR. Break over voltage is defined as the minimum forward voltage (gate being open) at which the SCR starts conducting heavily.

3. Draw the two transistor model of a SCR and mention its applications. (MAY 2016)

4. List the applications of SCR. (i) It can be used as a speed controller in DC and AC motors (ii)It can be used as inverter. (iii) It can be used as converter. (iv)It is used in battery charges. (v) It is used for phase control and heater control. (vi)It is used in light dimming control circuits.

5. What is meant by latching current & holding current? (Nov-Dec 2012)(Jun 2014) (MAY 2016)

Latching current is the minimum anode current required to maintain the thyristor in the on State immediately after a thyristor has been turned on and gate signal has been removed.Holding current is the minimum anode current to maintain the thyristor in the on state.

6. Draw the VI characteristics of a SCR and mark important points.

7. What is meant by switching losses in devices.?(DEC 2015)

The losses that occur during turning on and turning off of the devices is known as switching losses

8. What is a TRIAC? Triac is a three terminal bi-directional semiconductor-switching device. It can conduct in both the directions for any desired period. In operation it is equivalent to two SCR’s connected in ant parallel. Next to SCR it is the widely used device for power control.

9. Why Triac not popular compared to SCR?Justify. Commutation of Triac in inductive load is difficult when compared to SCR. Hence it is not popular.

10. What is power MOSFET?

A power MOSFET is a voltage-controlled device and requires only a small input current. The switching speed is very high and the switching times are of the order of nanoseconds.

11.Explain the importance of threshold voltage in gate circuit When the voltage Vgs is increased beyond threshold voltage, the device starts to conduct and the current will be increased from zero.

12.Draw the volt-ampere characteristic of a Triac.(MAY 2015)

13.Distinguish between SCR and TRIAC.(Dec 2014)

14.Draw the construction, equivalent circuit and symbol of Triac.

15.What is the common method used for di / dt protection?

The value of the di / dt can be maintained below acceptable limit by using a small inductor called di / dt inductor in series with the anode circuit.

16.What are the advantages of IGBTs? (Nov-Dec 2016) The main advantages of using the Insulated Gate Bipolar Transistor over other types of transistor devices are its high voltage capability, low ON-resistance, ease of drive, relatively fast switching speeds and combined with zero gate drive current makes it a good choice for moderate speed, high voltage applications

S.No

SCR TRIAC

1 It is unidirectional device It is a bidirectional device

2 It has fast turn off time It has comparatively longer turn off time

3 It can be used to switch AC supply frequencies upto few KHz

It can be used to switch AC supply frequencies upto 40Hz only

4 It is triggered by positive voltage applied to the gate

It is triggered by either positive or negative voltage applied to the gate.

17. Define pinch off voltage of MOSFET (May-June 2012)

Maximum drain source voltage beyond which the drain current becomes constant is called pinch-off voltage of MOSFET.

18.What are the drawbacks of GTO? (Nov-Dec 2012) *Mechanical Stress is high *On & off pulse required *very high switching frequency

19. Why are IGBT becoming popular in their application to controlled converters?(May-June 2012)

(i)They have high input gate impedance. (ii).They have low conduction loss.(iii).They have fast switching characteristics.(iv).They have very high operating frequency.

20. What is the limitation of high frequency operation of a power electronic device?( June 2013)

(i) More switching losses (ii) Electro magnetic interference

21. What is the use of snubber circuit? (June 2013) (Nov 2013)(DEC 2015) (Nov-Dec 2016)

Snubber circuit is used to prevent failure due to dv/dt. Snubber uses a small resistor (R) in series with a small capacitor (C). This combination can be used to suppress the rapid rise in voltage across a thyristor , preventing the erroneous turn-on of the thyristor .

22. List the various forced commutation techniques used to turn off SCR (Nov 2013)

Self commutation (ii) Resonant pulse commutation (iii) Complementary commutation(iv)Impulse commutation (v)External commutation (vi)Load commutation (vii) Line commutation

23. What is meant by current commutation of SCR?(Dec 2014) To turn OFF a thyristor, the forward anode current should be brought to zero for sufficient time to allow the removal of charged carriers. In case of DC circuits the forward current should be forced to zero by means of some external circuits. This process is called as Current Commutation.

24. Compare MOSFET and BJT?(Jun 2014)

S.No MOSFET BJT

1 Output current is controlled by input gate voltage

Output current is controlled by input base current

2 More expensive Lower cost

3 Very high current gain and is nearly constant

Lower current gain and is not constant

4 Input resistance is very high

Input resistance is low.

PART-B

1. Explain the switching model, equivalent circuit and switching characteristics of

power MOSFET.?(MAY 2015) (Nov-Dec 2016)

BJT is a current controlled device and MOSFET is a Voltage controlled device

Its flow depends upon Majority carriers only, MOSFET is a

unipolar device Base current or control signal required in

MOSFET is lesser than BJT

This is because of high gate circuit impedance of MOSFET when compared to BJT.

So we can directly connect MOSFET to microelectronic circuits

Low power high frequency converter

Types: n-channel enhancement

MOSFET

p-channel enhancement MOSFET

n-channel enhancement MOSFET is more common because of higher mobility of

electrons.

Without a gate-to-source voltage applied, no current can flow between the source and

drain regions.

Above a certain gate-to-source voltage (threshold voltage VT), a conducting layer of

mobile electrons is formed at the Si surface beneath the oxide. These electrons can

carry current between the source and drain.

Turn on process: Turn on time is defined as the sum of turn-on delay time and rise

time of the device Turn on delay time Tdn :During turn-on delay time Tdn period the

input capacitance charges to the gate threshold voltage Vgst and the drain current is

zero

Rise Time Tr :During rise time period, gate voltage rise to Vgsp-it is the gate source

peak voltage, this voltage is sufficient to drive the MOSFET into On state, then drain

current increases from 0 to full value of current Id.Thus the total turn on time is Ton =

Tdn+ Tr.The turn on time can be reduced by using low impedance gate drive circuit

Turn-off process : It is initiated by the removal of gate source voltage Vgs at time

t1,because MOSFET is a majority carrier device. The Turn off time is the sum of Turn

off delay time Tdf and fall timeTf

Turn off delay time Tdf::During this period , the input capacitance discharges from

overdrive gate voltage v1<= V gsp but drain current Id does not change

Fall time Tf : During this time period , the input capacitance discharges from Vgsp to

threshold voltage Vgst, then the drain current fall from Id to zero, so when Vgs<=Vgst

power MOSFET Turn off is completed.Toff=Tdf+Tf

2. Explain the static and switching characteristics of IGBT and MOSFET?(DEC-

2012)(DEC 2014)(JUN 2014)

SWITCHING CHARACTERISTICS OF IGBT

The switching characteristics of an IGBT are very much similar to that of a Power

MOSFET. The major difference from Power MOSFET is that it has a tailing collector

current due to the stored charge in the N--drift region. The tail current increases the

turn-off loss and requires an increase in the dead time between the conduction of two

devices in a half-bridge circuit. The Figure 8 shows a test circuit for switching

characteristics and the Figure 9 shows the corresponding current and voltage turn-on

and turn-off waveforms. IXYS IGBTs are tested with a gate voltage switched from +15V

to 0V. To reduce switching losses, it is recommended to switch off the gate with a

negative voltage (-15V)

.

The turn-off speed of an IGBT is limited by the lifetime of the stored charge or minority

carriers in the N--drift region which is the base of the parasitic PNP transistor. The

base is not accessible physically thus the external means can not be applied to sweep

out the stored charge from the N--drift region to improve the switching time. The only

way the stored charge can be removed is by recombination within the IGBT. Traditional

lifetime killing techniques or an N+ buffer layer to collect the minority charges at turn-

off are commonly used to speed-up recombination time.

Switching characteristics of MOSFET

Turn on process: Turn on time is defined as the sum of turn-on delay time and risetime

of the device Turn on delay time Tdn :During turn-on delay time Tdn period the input

capacitance charges to the gate threshold voltage Vgst and the drain current is zero

Rise Time Tr :During rise time period, gate voltage rise to Vgsp-it is the gate soure

peak voltage, this voltage is sufficient to drive the MOSFET into On state,then drain

current increases from 0 to full value of current Id.Thus the total turn on time is Ton =

Tdn+ Tr.The turn on time can be reduced by using low impedance gatedrive circuit

Turn-off process : It is initiated by the removal of gate source voltage Vgs at time

t1,because MOSFET is a majority carrier device.The Turn off time is the sum of Turn

off delaytime Tdf and fall timeTf

Turn off delaytime Tdf::During this period , the input capacitance descharges from

overdrive gate voltage v1<= Vgsp but drain current Id doesnot change

Fall timeTf : During this time period , the input capacitance discharges from Vgsp to

threshold voltage Vgst, then the drain current fall from Id to zero, so when Vgs<=Vgst

power MOSFET Turn off is completed.Toff=Tdf+Tf

3. Describe about any one driver circuit and snubber circuit for MOSFET &

IGBT.?(JUN 2012)(JUN 2014) (Nov-Dec 2016)

To turn the MOSFET on the logic level input to the inverting buffer is set to high state

so that transistor Q3 turns off and Q1 turns on. The top circuit of Fig 6.10 (b) shows

the equivalent circuit during turn on. Note that, during turn on Q1 remains in the

active region. The effective gate resistance is RG + R1 / (β1 + 1). Where, β1 is the

dc current gain of Q1.

To turn off the MOSFET the logic level input is set to low state. Q3 and Q2 turns on

whole Q1 turns off. The corresponding equivalent circuit is given by the bottom circuit

of Fig 6.10 (b). The switching time of the MOSFET can be adjusted by choosing a

proper value of RG. Reducing RG will incase the switching speed of the MOSFET.

However, caution should be exercised while increasing the switching speed of the

MOSFET in order not to turn on the parasitic BJT in the MOSFET structure

inadvertently. The drain-source capacitance (CDS) is actually connected to the base

of the parasitic BJT at the p type body region. The body source short has some

nonzero resistance. A very fast rising drain-source voltage will send sufficient

displacement current through CDS and RB as shown in Fig 6.10 (c). The voltage

drop across RB may become sufficient to turn on the parasitic BJT. This problem is

largely avoided in a modern MOSFET design by increasing the effectiveness of the

body-source short. The devices are now capable of dvDS/dt in excess to 10,000

V/μs. Of course, this problem can also be avoided by slowing down the

MOSFET switching speed. Since MOSFET on state resistance has positive

temperature coefficient they can be paralleled without taking any special precaution

for equal current sharing. To parallel two MOSFETs the drain and source terminals

are connected together as shown in Fig 6.10 (d). However, small resistances (R) are

connected to individual gates before joining them together. This is because the gate

inputs are highly capacitive with almost no losses. Some stray inductance of wiring

may however be present. This stray inductance and the MOSFET capacitance can

give rise to unwanted high frequency oscillation of the gate voltage that can result in

puncture of the gate qxide layer due to voltage increase during oscillations. This is

avoided by the damping resistance R.The logic level gate drive signal is first opto-

isolated and fed to a level shifting comparator. This stage converts the unipolar

(usually positive) output voltage of the opto-isolator to a bipolar (±Vgg ) signal

compatible to the IGBT gate drive levels. The output of the comparator feeds a totem

pole output amplifier stage which drives the IGBT.

4. Explain why triac is rarely used in I quadrant with negative pulse and in III

quadrant with positive pulse.(JUN 2012)

Four different possibilities of operation of triac exists. They are:

1. Terminal MT2 and gate are positive with respect to terminal MT1:

When terminal MT2 is positive with respect to terminal MT1 current flows through

path P1-N1-P2-N2. The two junctions P1-N1 and P2-N2 are forward biased whereas

junction N1 P2 is blocked. The triac is now said to be positively biased. A positive

gate with respect to terminal MT1 forward biases the junction P2-N2 and the

breakdown occurs as in a normal SCR.

2. Terminal MT2 is positive but gate is negative with respect to terminal MT1:

Though theflow path of current remains the same as in mode 1 but now junction P2-

N3 is forward biased and current carriers injected into P2 turn on the triac.

3.Terminal MT2 and gate are negative with respect to terminal MT1: When

terminal MT2isnegative with respect to terminal MT1, the current flow path is P2-

N1-P1-N4. The two junctions P2-N1 and P1 - N4 are forward biased whereas

junction N1-P1 is blocked. The triac is now said to be negatively biased. A negative

gate with respect to terminal MT1 injects current carriers by forward biasing junction

P2-N3 and thus initiates the conduction.

4. Terminal MT2 is negative but gate is positive with respect to terminal MT1:

Though theflow path of current remains the same as in mode 3 but now junction P2-

N2 is forward biased, current carriers are injected and therefore, the triac is turned

on. Generally, trigger mode 4 should be avoided especially in circuits where high

di/dt may occur. The sensitivity of triggering modes 2 and 3 is high and in case of

marginal triggering capability negative gate pulses should be used. Though the

triggering mode 1 is more sensitive compared to modes 2 and 3, it requires a positive

gate trigger. However, for bidirectional control and uniform gate trigger modes 2 and

3 are preferred.

5. Describe the basic structure of IGBT and explain its working. Give its equivalent circuit

and explain the turn ON and turn OFF process.(NOV 2013)(MAY 2015)

IGBT Fundamentals: The Insulated Gate Bipolar Transistor (IGBT) is a minority-

carrier device with high input impedance and large bipolar current-carrying

capability. Many designers view IGBT as a device with MOS input characteristics

and bipolar output characteristic that is a voltage-controlled bipolar device. To make

use of the advantages of both Power MOSFET and BJT, the IGBT has been

introduced. It’s a functional integration of Power MOSFET and BJT devices in

monolithic form. It combines the best attributes of both to achieve optimal device

characteristics. The IGBT is suitable for many applications in power electronics,

especially in Pulse Width Modulated (PWM) servo and three-phase drives requiring

high dynamic range control and low noise. It also can be used in Uninterruptible

Power Supplies (UPS), Switched-Mode Power Supplies (SMPS), and other power

circuits requiring high switch epetition rates. IGBT improves dynamic performance

and efficiency and reduced the level of audible noise. It is equally suitable in

resonant-mode converter circuits. Optimized IGBT is available for both low

conduction loss and low switching loss.

SWITCHING CHARACTERISTICS OF IGBT

The switching characteristics of an IGBT are very much similar to that of a Power

MOSFET. The major difference from Power MOSFET is that it has a tailing collector

current due to the stored charge in the N--drift region. The tail current increases the

turn-off loss and requires an increase in the dead time between the conduction of two

devices in a half-bridge circuit. The Figure 8 shows a test circuit for switching

characteristics and the Figure 9 shows the corresponding current and voltage turn-on

and turn-off waveforms. IXYS IGBTs are tested with a gate voltage switched from +15V

to 0V. To reduce switching losses, it is recommended to switch off the gate with a

negative voltage (-15V).

6. Explain the operation of SCR using two transistor analogy?(JUN 2014)(DEC

2015)(May 2016)

7. Explain the structure different modes of operation and characteristics of Triac.

(May 2016)

Construction of a Triac

As mentioned above, triac is a three terminal, four layer bilateral semiconductor device.

It incorporates two SCRs connected in inverse parallel with a common gate terminal in a

single chip device. The arrangement of the triac is shown in figure. As seen, it has six

doped regions. The gate terminal G makes ohmic contacts with both the N and P

materials. This permits trigger pulse of either polarity to start conduction. Electrical

equivalent circuit and schematic symbol are shown in figure.b and figure.c respectively.

Since the triac is a bilateral device, the term “anode” and “cathode” has no meaning, and

therefore, terminals are designated as main terminal 1. (MT1), main terminal 2 (MT2) and

gate G. To avoid confusion, it has become common practice to specify all voltages and

currents using MT1 as the reference.

Triac Basic Structure

Four different possibilities of operation of triac exists. They are:

1. Terminal MT2 and gate are positive with respect to terminal MT1:

When terminal MT2 is positive with respect to terminal MT1 current flows through

path P1-N1-P2-N2. The two junctions P1-N1 and P2-N2 are forward biased whereas

junction N1 P2 is blocked. The triac is now said to be positively biased. A positive

gate with respect to terminal MT1 forward biases the junction P2-N2 and the

breakdown occurs as in a normal SCR.

2. Terminal MT2 is positive but gate is negative with respect to terminal MT1:

Though theflow path of current remains the same as in mode 1 but now junction P2-

N3 is forward biased and current carriers injected into P2 turn on the triac.

3.Terminal MT2 and gate are negative with respect to terminal MT1: When

terminal MT2isnegative with respect to terminal MT1, the current flow path is P2-

N1-P1-N4. The two junctions P2-N1 and P1 - N4 are forward biased whereas

junction N1-P1 is blocked. The triac is now said to be negatively biased. A negative

gate with respect to terminal MT1 injects current carriers by forward biasing junction

P2-N3 and thus initiates the conduction.

4. Terminal MT2 is negative but gate is positive with respect to terminal MT1:

Though theflow path of current remains the same as in mode 3 but now junction P2-

N2 is forward biased, current carriers are injected and therefore, the triac is turned

on. Generally, trigger mode 4 should be avoided especially in circuits where high

di/dt may occur. The sensitivity of triggering modes 2 and 3 is high and in case of

marginal triggering capability negative gate pulses should be used. Though the

triggering mode 1 is more sensitive compared to modes 2 and 3, it requires a positive

gate trigger. However, for bidirectional control and uniform gate trigger modes 2 and

3 are preferred.