Why is IGBT used in VFD?

What is IGBT?

IGBT is a type of transistor. Basically, the family of transistors consists of three main types – BJT (Bipolar Junction Transistor), FET (Field Effect Transistor), and IGBT (Insulated Gate Bipolar Transistor).

IGBT is a more advanced type of transistor. As a standard construction of a transistor, it too has three terminals – gatecollector, and emitter.

When voltage is applied to the gate, it opens and allows the current to flow between the collector and emitter. When voltage is removed from the gate, it closes and blocks the current to flow between the collector and emitter.

In this way, an IGBT behaves like a switch; on when the gate is open and flowing current and off when it is closed.

IGBT is a combination of BJT and FET. It has the gate function of FET and the low saturation voltage/output characteristics of a BJT. That is why you can see it in its name too; it is a combination of both FET (IG) and BJT (BT). It is capable of handling large collector-emitter currents with virtually zero gate current drive.

Why is IGBT used in VFD?

Coming to the main topic, it must be confusing for you because each type of transistor has its separate characteristics. But, it is important to know that the highest level of advantages are offered in an IGBT.

Let us understand step by step why an IGBT is used in VFD.

  1. VFD is used to provide variations in voltage and frequency to control the motor speed. As the IGBT can act as a switch, it will act so rapidly that it can instantly create pulsating (on and off) waveforms. This is called a PWM (pulse width modulation) waveform. Depending on the output you require, you can control the switching rate of the IGBT. If the gate is opened and closed slowly, then you will get a slow frequency and if the gate is opened and closed quickly, then you will get a fast frequency. This is the first basic requirement of a VFD. And you get this easily from an IGBT.
  2. Now, let us focus on the main advantages of an IGBT over other semiconductors. BJT can be used at higher voltages, but it has a slow switching frequency. MOSFET can be used at higher switching frequencies, but it has a lower voltage rating. But, IGBT can be used at both higher voltages and higher frequencies. Continuing this, IGBT can thus be used for higher collector current exceeding 100A. And if this is inadequate, two or more IGBTs may be paralleled quite easily.
  3.  The voltage drop and conduction loss are lesser in IGBT as compared to other semiconductors.
  4. The IGBT has a much lower “on-state” resistance, RON than an equivalent MOSFET. This means that the I2R drop across the bipolar output structure for a given switching current is much lower. This means that it has a much lower on-state channel resistance than a standard MOSFET; which automatically increases the current ratings.
  5. The IGBT has a fast switching speed. This minimizes switching losses and allows for high switching frequencies which are good for motor harmonic and noise reduction.
  6. A VFD IGBT can turn on in less than 400 nanoseconds and off in less than 500 nanoseconds. This itself is enough for you to imagine how fast it works.

In this way, we understand the use of IGBT in a VFD.

Source: <https://instrumentationtools.com/why-is-igbt-used-in-vfd/>