power electronics - philadelphia university...some power electronic circuits where control of switch...

Power Electronics Power semiconductor devices

1

Dr. Firas Obeidat

2

Table of contents

1 • Introduction

2 • Classifications of Power Switches

3 • Power Diodes

4 • Thyristors (SCRs)

5 • The Triac

6

• The Gate Turn-Off Thyristor (GTO)

7 • Insulated Gate-Commutated Thyristor (IGCT)

8 • The MOS-Controlled Thyristor

9 • Bipolar Junction Transistor (BJT)

10 • MOSFET

11 • IGBT

Dr. Firas Obeidat Faculty of Engineering Philadelphia University

3 Dr. Firas Obeidat Faculty of Engineering Philadelphia University

Introduction

Electronic switches capable of handling high voltage and current operations at

high frequency (HF) are the most important devices needed in the design of

energy conversion systems that use power electronic.

An ideal power electronic switch can be represented as a three terminals

device as shown in the figure; The input, the output, and a control terminal

that imposes ON/OFF conditions on the switch.

• When the switch is open, it has zero-current through it and can handle infinite voltage.

• When the switch is closed it has zero-voltage across it and can carry infinite current.

• An ideal switch changes condition instantly, which means that it takes zero-time to switch from ON-to-OFF or OFF-to-ON.

• exhibits zero-power dissipation, carries bidirectional current, and can support bidirectional voltage.

• By definition, an ideal switch can operate in all four quadrants.

A switch is considered ‘‘ideal’’


Introduction

Practical or real switches do have their limitations in all of the characteristics explained in an ideal switch.

When a switch is on, it has some voltage across it, known as the on-voltage and it carries a finite current.

During the off stage, it may carry a small current known as the leakage current while supporting a finite voltage.

The switching from ON-to-OFF and vice versa does not happen instantaneously.

• The first loss occurs during the on and off-states and is defined as the ‘‘conduction loss’’.

• The second loss is defined as the ‘‘switching loss’’ which occurs just as the switch changes state as either opening or closing. The switch losses result in raising the overall switch temperature.

There is voltage and current across the non-ideal switch at all times, which will result in two types of losses


Classifications of Power Switches

There are three classes of power switches

Uncontrolled switch: The switch has no control terminal. The state of the switch is determined by the external voltage or current conditions of the circuit in which the switch is connected. A diode is an example of such switch.

Semi-controlled switch: In this case the circuit designer has limited control over the switch. For example, the switch can be turned-on from the control terminal. However, once ON, it cannot be turned-off from the control signal. The switch can be switched off by the operation of the circuit or by an auxiliary circuit that is added to force the switch to turn-off. A thyristor or a SCR is an example of this switch type.

Fully controlled switch: The switch can be turned ON and OFF via the control terminal. Examples of this switch are the BJT, the MOSFET, the IGBT, the IGCT, the GTO thyristor, and the MOS-controlled thyristor (MCT).


Power Diodes A diode is the simplest electronic switch. It is uncontrollable in that the on and off conditions are determined by voltages and currents in the circuit.

Diode terminals are known as Anode (A) and cathode (K) as shown in fig. a.

The diode is forward-biased (ON) when the current iD is positive while supporting a small voltage (0.2V to 3V) - (quadrant I in fig. b). The diode current varies exponentially with the diode voltage.

When the diode is reversed-biased (OFF), it supports a negative voltage and carries a negligible current (leakage current from μA to mA) - (quadrant III in fig. b).

When the negative voltage exceeds a certain limit, known as the breakdown voltage, the leakage current increases rapidly while the voltage remains at the breaking value, which potentially damages the device.

The ideal diode characteristics are shown in fig. c. During the ON-state, the diode has zero-voltage across it and carries a positive current. During the OFF state, the diode carries zero-current and supports a negative voltage.


Types of Power Diodes

• On state voltage: very low (below 1V).

• Large trr (about 25us) (very slow response).

• Very high current ratings (up to 6kA).

• Very high voltage ratings(8kV).

• Used in line-frequency (50/60Hz) applications such as rectifiers.

Line frequency (general purpose)

• Very low trr (<1μs).

• Power levels at several hundred volts and several hundred amps.

• Normally used in high frequency circuits.

Fast recovery

• Very low forward voltage drop (typical 0.3V).

• Limited blocking voltage (50-100V).

• Used in low voltage, high current application such as switched mode power supplies.

Schottky


Thyristors


Thyristors ( Silicon Controlled Rectifiers “SCRs”)

Thyristors are electronic switches used in some power electronic circuits where control of switch turn-on is required. But once it turns ON, the control terminal becomes ineffective and the thyristor behaves similar to a diode.

Thyristors are capable of large currents and large blocking voltages for use in high-power applications, but switching frequencies cannot be as high as when using other devices such as MOSFETs.

The thyristor current, IA, flows from the anode (A) to the cathode (K) and the voltage VAk across the thyristor is positive when the anode is at higher voltage than the cathode.



In quadrant I, in the absence of a gate current, the device is OFF in the forward blocking region and supports a positive voltage.

If a gate current is applied, the device switches to the ON-state region and the device has a i-v characteristic similar to that of a diode.

In quadrant III, the device is OFF and the region is known as the reverse blocking region and supports a negative voltage.

The characteristics are similar to those of a diode. Comparing the

switching characteristics of a diode and a thyristor, it appears that when

the thyristor is OFF, it can block large positive or negative voltage, which

is a fundamental feature that is important in circuit applications.

thyristor can be considered to carry an unidirectional current and

supports a bidirectional voltage.



Thyristors can only be turned on with three conditions:

• The device must be forward biased, i.e., the anode should be more positive than the cathode.

• A positive gate current (Ig) should be applied at the gate.

• The current through the thyristor should be more than the latching current.

Once conducting ,the anode current is LATCHED (continuously flowing).

• Surge Current Rating (IFM)—The surge current rating (IFM) of an SCR is the peak anode current an SCR can handle for a short duration.

• Holding Current (IL)—A minimum anode current must flow through the SCR in order for it to stay ON initially after the gate signal is removed. This current is called the latching current (IL).

• Latching Current (IH)—After the SCR is latched on, a certain minimum value of anode current is needed to maintain conduction. If the anode current is reduced below this minimum value, the SCR will turn OFF.

• Peak Repetitive Reverse Voltage (VRRM)—The maximum instantaneous voltage that an SCR can withstand, without breakdown, in the reverse direction.

• Peak Repetitive Forward Blocking Voltage (VDRM)—The maximum instantaneous voltage that the SCR can block in the forward direction. If the VDRM rating is exceeded, the SCR will conduct without a gate voltage.

• Nonrepetitive Peak Reverse Voltage (VRSM)—The maximum transient reverse voltage that the SCR can withstand.

• Maximum Gate Trigger Current (IGTM)—The maximum DC gate current allowed to turn the SCR ON.

• Minimum Gate Trigger Voltage (VGT)—The minimum DC gate-to-cathode voltage required to trigger the SCR.

• Minimum Gate Trigger Current (IGT)—The minimum DC gate current necessary to turn the SCR ON.

SCR rating



• Thyristor cannot be turned off by applying negative gate current. It can only be turned off if IA goes negative (reverse). This happens when negative portion of the of sine-wave occurs (natural commutation),

• Another method of turning off is known as “forced commutation”, The anode current is “diverted” to another circuitry.

Thyristor TURN-OFF

Types of Thyristors

Phase controlled

- Rectifying line frequency voltage and current for ac and dc motor drives.

- Large voltage (up to 7kV) and current (up to 5kA) capability.

- Low on-state voltage drop (1.5 to 3V).

Inverter grade

- Used in inverter and chopper.

- Quite fast. Can be turned-on using “force-commutation” method.

Light activated

- Similar to phase controlled, but triggered by pulse of light.

- Normally very high power ratings.

TRIAC

- Dual polarity thyristors.


The Triac (triode for alternating current)

The Triac is a member of the thyristor family which can conduct in both directions (bidirectional semi-controlled device). Thus a Triac is similar to two back to back (anti parallel) connected thyristosr but with only three terminals.

The Triac extensively used in residential lamp dimmers, heater control and for speed control of small single phase series and induction motors.

The conduction of a triac is initiated by injecting a current pulse into the gate terminal. The triac turns off only when the current through the main terminals become zero.

As the Triac can conduct in both the directions the terms “anode” and “cathode” are not used for Triacs. The three terminals are marked as MT1 (Main Terminal 1), MT2 (Main Terminal 2) and the gate by G.


The Triac (triode for alternating current)

Since a Triac is a bidirectional device and can have its terminals at various combinations of positive and negative voltages, there are four possible electrode potential combinations as given below

• MT2 positive with respect to MT1, G positive with respect to MT1.

• MT2 positive with respect to MT1, G negative with respect to MT1.

• MT2 negative with respect to MT1, G negative with respect to MT1.

• MT2 negative with respect to MT1, G positive with respect to MT1.

In trigger mode-1 the gate current flows

mainly through the P2 N2 junction like an

ordinary thyristor. When the gate current

has injected sufficient charge into P2 layer

the Triac starts conducting through the P1

N1 P2 N2 layers like an ordinary thyristor.


The Gate Turn-Off Thyristor (GTO)

The GTO thyristor is a device that operates similar to a normal thyristor except the device physics, design and manufacturing features allow it to be turned-off by a negative gate current which is accomplished through the use of a bipolar transistor.

Turning off is difficult. Need very large reverse gate current (normally 1/5 of anode current).

Gate drive design is very difficult due to very large reverse gate current at turn off.

Ratings: Highest power ratings switch: Voltage: Vak<5kV; Current: Ia<5kA. Frequency<5KHz.

A (Anode)

K (Cathode)

+

Vak

_

Ia

GTO: Symbol

I

g

Ia

Vak

Vr

Ig=0 Ig>0 Ih

Ibo

Vbo

v-i characteristics


Insulated Gate-Commutated Thyristor (IGCT)

Conducts like normal thyristor (latching), but can be turned off using gate signal, similar to IGBT turn off; 20V is sufficent.

Power switch is integrated with the gate-drive unit.

Ratings:

Voltage: Vak<6.5kV; Current: Ia<4kA. Frequency<1KHz.

Currently 10kV device is being developed.

Very low on state voltage: 2.7V for 4kA device

K (Cathode)

+

Vak

_

Ia

I

g

IGCT


The MOS-Controlled Thyristor

The MCT is a hybrid or double mechanism device that was designed to have a control port of a MOSFET and a power port of a thyristor.

The characteristics of MCT are similar to the GTO, except that the gate drive circuitry for the MCT is less complicated than the design for a GTO as the control circuit of the MCT uses a MOSFET instead of a transistor.


Transistors


Bipolar Junction Transistor (BJT)

For the npn-type BJT shown in fig. a, the Base (B) is the control terminal, where the power terminals are the Collector (C) and the Emitter (E).

The real v-i characteristics of device are shown in fig. b. The device operates in quadrant I and is characterized by the plot of the collector current IC versus the collector to emitter voltage vCE as shown in fig. b. BJT is a current-controlled device.

The device has three regions two of them where the device operates as a switch and the third is where the device operates as a linear amplifier. The device is OFF in the region below iB=0 and is ON in the region where vCE is less than vCE(Sat).

Neglecting the middle region, the idealized device characteristics as a switch are shown in fig. c. During the ON state the device carries a collector current IC>0 with vCE=0. In the OFF-state, the device supports positive vCE>0 with IC = 0. Therefore, the BJT is unidirectional current and voltage device.


The Metal Oxide Semiconductor Field Effect Transistor (MOSFET)

For the n-channel MOSFET shown in fig. a, the control terminal known as the Gate (G) and the power terminals are the Drain (D) and Source (S).

The device is controlled by supplying a voltage (vGS) between the gate and the source. This makes it a voltage controlled device compared to the BJT, which is a current-controlled device.

The real v-i characteristics of device are shown in fig. b. Similar to the BJT, the MOSFET operates within three operating regions. Two of the regions are used when the device is operated as a switch, and the third is when the device is used as an amplifier.

To maintain the MOSFET in the off-state, vGS must be less than a threshold voltage known as vT, which is the region below the line marked OFF. And when the device is ON it act as resistance determined by the slope of the line marked ON.

The idealized characteristics of a MOSFET switch are shown in fig. c. When the device is ON, it has zero vDS and carriers a current ID>0 and when the device is OFF it supports a positive vDS and has zero drain current (ID = 0).


The Insulated Gate Bipolar Transistor (IGBT)

The IGBT (its symbol shown in fig. a) is a hybrid or also known as double mechanism device. Its control port resembles a MOSFET and its output or power port resembles a BJT. Therefore, an IGBT combines the fast switching of a MOSFET and the low power conduction loss of a BJT.

The control terminal is labeled as gate (G) and the power terminals are labeled as collector (C) and emitter (E).

The i-v characteristics of a real IGBT are shown in fig. b, which shows that the device operates in quadrants I and III. The ideal characteristics of the device are shown in fig. c. The device can block bidirectional voltage and conduct unidirectional current.

An IGBT can change to the ON-state very fast but is slower than a MOSFET device. Discharging the gate capacitance completes control of the IGBT to the OFF-state. IGBT’s are typically used for high power switching applications such as motor controls and for medium power PV and wind PE.

(b)


Comparison between GTO, IGCT and IGBT

Item GTO IGCT IGBT

Maximum switch power (V×I ) 36MVA 36MVA 6MVA

Active di/dt and dv/dt control No No Yes

Active short circuit protection No No Yes

Turn-off (dv/dt) snubber Required Not required No required

Turn-on (di/dt) snubber Required Required No required

Parallel connection No No Yes

Switching speed Slow Moderate Fast

Behavior after destruction Shorted Shorted Open

in most cases

On-state losses Low Low High

Switching losses High Low Low

Gate Driver Complex,

separate

Complex,

integrated

Simple,

compact

Gate Driver Power

Consumption High High Low


Device Rating

12000

10000

8000

SCR12000V/1500A

(Mitsubishi)

4500V/900A

(Mitsubishi)

6500V/1500A

(Mitsubishi)GTO/GCT

7500V/1650A

(Eupec)6500V/600A

(Eupec)

6000

4000

2000

1000 2000 3000 4000

3300V/1200A

(Eupec)

2500V/1800A

(Fuji)

1700V/3600A

(Eupec)

IGBT

SCR:

GTO/GCT:

IGBT:

27MVA

36MVA

6MVA

6000V/3000A

(ABB)

6500V/4200A

(ABB)

6000V/6000A

(Mitsubishi)

4800V

5000A

(Westcode)

5000 6000 I (A)

V (V)

00

power electronics - philadelphia university...some power electronic circuits where control of switch...

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