POWER ELECTRONICS Instructor: Eng.Moayed N. EL MobaiedThe Islamic University of GazaFaculty of EngineeringElectrical Engineering Department EELE 5450 Fall 2009-2010Lecture 29

Power switches

Power Semiconductor Devices Power devices are the key elements of a power converter. The commonly used devices are: (1) Power Diode(2) Silicon-Controlled Rectifier (SCR) or Thyristor(3) Gate Turn-off Thyristor (GTO)(4) Power Bipolar Junction Transistor (Power BJT)(5) Power Metal-Oxide Field-Effect Transistor (Power MOSFET)(6) Insulated-Gate Bipolar Transistor (IGBT) (7) Gate controlled thyristors (IGCT).

Bipolar Junction Transistor (BJT)Ratings: Voltage: VCE

MosfetRatings: Voltage VDS

Insulated Gate Bipolar Transistor (IGBT)Combination of BJT and MOSFET characteristics. Gate behaviour similar to MOSFET - easy to turn on and off.Low losses like BJT due to low on-state Collector-Emitter voltage (2-3V).

Ratings: Voltage: VCE

Gate turn-off thyristor (GTO)Behave like normal thyristor, but can be turned off using gate signal

However 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

Insulated Gate-Commutated Thyristor (IGCT)Among the latest Power Switches.

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

Power Switches: Power Ratings

Switches comparisons (2003)

Thy

BJT

FET

GTO

IGBT

IGCT

Avail-

abilty

Early 60s

Late 70s

Early 80s

Mid 80s

Late 80s

Mid 90s

State of

Tech.

Mature

Mature

Mature/

improve

Mature

Rapid improve

Rapid improvement

Voltage

ratings

5kV

1kV

500V

5kV

3.3kV

6.5kV

Current

ratings

4kA

400A

200A

5kA

1.2kA

4kA

Switch

Freq.

na

5kHz

1MHz

2kHz

100kHz

1kHz

On-state Voltage

2V

1-2V

I* Rds (on)

2-3V

2-3V

3V

Drive

Circuit

Simple

Difficult

Very simple

Very difficult

Very simple

Simple

Comm-ents

Cannot turn off using gate signals

Phasing out in new product

Good performance in high freq.

King in very high power

Best overall performance.

Replacing GTO

Drivers and Snubbers

Interface between control (low power electronics) and (high power) switch.

Functions: amplifies control signal to a level required to drive power switch

provides electrical isolation between power switch and logic level

Complexity of driver varies markedly among switches. MOSFET/IGBT drivers are simple but GTO drivers are very complicated and expensive.Driver circuit (Base / gate)

ELECTRICAL ISOLATION FOR DRIVERSIsolation is required to prevent damages on the high power switch to propagate back to low power electronics.

Normally opto-coupler (shown below) or high frequency magnetic materials (as shown in the thyristor case) are used.

Many standard driver chips have built-in isolation. For example TLP 250 from Toshiba, HP 3150 from Hewlett-Packard uses opto-coupling isolation.

ELECTRICAL ISOLATION FOR DRIVERSPower semiconductor devices can be categorized into 3 types based on their control input requirements:

Current-driven devices BJTs, MDs, GTOsVoltage-driven devices MOSFETs, IGBTs, MCTsPulse-driven devices SCRs, TRIACs

CURRENT DRIVEN DEVICES (BJT)Power BJT devices have low current gain due to constructional consideration, leading current than would normally be expected for a given load or collector current.The main problem with this circuit is the slow turn-off time.

ELECTRICALLY ISOLATED DRIVE CIRCUITS

EXAMPLE : GATE DRIVE FOR THYRISTORSPulse transformer is used for isolation. R1 is to limit the gate current

Normally a pulse with length 10us with amplitude of 50mA is sufficient to turn-on the thyristors. It is quite common to fire the thyristors with successive pulses to ensure proper turn-on.

It is not possible to turn-off a thyristor with the above circuit

SIMPLE MOSFET GATE DRIVERNote: MOSFET requires VGS =+15V for turn on and 0V to turn off. LM311 is a simple amp with open collector output Q1.

When B1 is high, Q1 conducts. VGS is pulled to ground. MOSFET is off.

When B1 is low, Q1 will be off. VGS is pulled to VGG. If VGG is set to +15V, the MOSFET turns on.

RCD SNUBBERSIn general, snubbers are used for:

turn-on: to minimize large overcurrents through the device at turn-on

turn-off: to minimize large overvoltages across the device during turn- off.

Stress reduction: to shape the device switching waveform such that the voltage and current associated with the device are not high simultaneously.

Heatsink

Heat Removal MechanismSCR (stud-type) on air-cooled kitsFin-type Heat SinkSCR (hokey-puck-type) on power pak kitsAssembly of power converters

Heat Removal MechanismSCR (stud-type) on air-cooled kitsAssembly of power convertersSCR (stud-type) on air-cooled kitsAssembly of power converters

Applications

Static Application: DC Power Supply

Drive Application: Air-Conditioning System

Uninterruptible Power Supply (UPS)

Powering Automotive System

Motor Drive System

Motor Drive System End of Lecture

*****************************