What is IGBT?
The Insulated Gate Bipolar Transistor or IGBT is a three-terminal power semiconductor device, noted for high efficiency and fast switching. The IGBT is a semiconductor device with four alternating layers (P-N-P-N) that are controlled by a metal-oxide-semiconductor (MOS) gate structure without regenerative action.
The IGBT combines the simple gate-drive characteristics of the MOSFETs with the high-current and low–saturation-voltage capability of bipolar transistors by combining an isolated gate FET for the control input, and a bipolar power transistor as a switch, in a single device.
Large IGBT modules typically consist of many devices in parallel and can have very high current handling capabilities in the order of hundreds of amperes with blocking voltages of 6000 V.
What are the Types of IGBTs?
Currently there are two types of IGBTs available. They are Punch-Through (PT) IGBT, and Non-Punch-Through (NPT) IGBT.
Non-Punch-Through (NPT) IGBT
The basic IGBT with P-N-P-N (P+, N-, P+, N+) structure is called as non punch-through IGBT.
To support the off-state voltage, the IGBT must support a high electric field. If the electric field reaches a critical field strength (Ecritical) determined by the concentration of the N- base region, or reaches through from the P+ at the cathode (Top) to the P+ at the anode (Bottom), the device will break down.
In NPT devices, the electric field is stopped in the N- base region. Breakdown occurs when the electric field reaches through the N- Region connecting both P+ regions shown in Figure.
Punch-Through (PT) IGBT
Punch-Through (PT) IGBT is an IGBT with additional buffer layer (N+) as shown in Figure.
In PT devices the electric field “punches through” the N- base region and is stopped by the N+ buffer layer.
PT Vs NPT IGBTs
- The NPT device has better short-circuit capability.
- The NPT device is easier to connect in parallel due to the conduction drop, positive temperature coefficient.
- The NPT has less part parameter variation than the PT.
- The PT devices offer lower ON-state voltage drop than NPT devices.
- The PT devices are made to turn-OFF with lesser turn-OFF energy than NPT devices. That means an NPT device must be capable to handle larger turn-OFF energy than a PT device for the same application.
- The PT devices are faster in switching. The NPT devices are slower but with lower amplitude current tail than the PT devices.
- The NPT has more reverse voltage capability than PT.
- The NPT has greater avalanche capability than the PT.
What are the applications of IGBT?
The IGBT is used in medium to high-power switching applications in many modern appliances: Electric vehicles including electric cars and hybrid cars, traction motor control, variable speed refrigerators, air-conditioners, and high power stereo systems with switching amplifiers, industrial SMPS. Since it is designed to rapidly turn on and off, amplifiers that use it often synthesize complex waveforms with pulse width modulation and low-pass filters.
What are the advantages of IGBT?
- The IGBT has the output switching and conduction characteristics of a bipolar transistor but is voltage-controlled like a MOSFET.
- IGBT has the advantages of high-current handling capability of a bipolar with the ease of control of a MOSFET
- IGBT has higher breakdown voltages (typically 1200V) compared to MOSFET (typically less than 250V).
- IGBT can handle high currents, say upto 120A at low switching frequencies.
- IGBT has a very low on-state voltage drop due to conductivity modulation and has superior on-state current density.
- IGBT has very good pulse handling capability
- Operation at high junction temperature is allowed (>100C)
- Very high (>5kW) output power can be achieved using IGBT
- Smaller chip size is possible and the cost can be reduced.
What are the limitations of IGBT?
- Limited to low switching frequency applications (less than 20KHz)
- Comparatively IGBT exhibits large current tail than MOSFET
- IGBT can not handle large variations in load or line power supply
- IGBT is suitable for low duty cycle applications
- Collector current is derated with switching frequency
- There is a possibility of latchup due to the internal PNPN thyristor structure.
What are the alternates to IGBT?
MOSFETs can be preferred for:
- High frequency applications (>200kHz)
- Wide line or load variations
- Long duty cycles
- Low-voltage applications (<250V)
- Less than 500W output power
How to get IGBTs?
Please check the IGBT manufacturers list.
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