IGBT- Insulated gate bipolar transistor

Power Electronics

Chapter-01

(IGBT- insulated-gate bipolar transistor)

    -Asiqur Rahman Milon,EETE-DIU,EEE-IEB(Con)

#What is IGBT?

Insulated Gate Bipolar Transistor

The IGBT is a power switching transistor which combines the advantages of MOSFETs and BJTs for use in power supply and motor control circuits

#What is the operation of IGBT?

-Operation of IGBT is

Turn on-Forward blocking state

From picture we can write-

VCE>OV   and VGE=0V

From above picture we can easily know this device is now Forward bias.Cause J1 positive point connect with positive charge and J3 Negative point connect with negative charge.But in this device only junction J2 is not forward bias this junction is reverse bias.For this reverse bias carrier can’t go other layer due to depletion layer.For this reverse junction it’s called Forward blocking state in IGBT.

Forward condition state-

From conduction state figure we can know by changing gate voltage we can make a channel to make device forward or reverse. Means here by creation of inverse layer we can easily flow carrier from one layer to another layer.

IGBT working for minority charge carrier.

Turn off-Gate voltage reverse-

Generally IGBT takes some time after turn off switch that’s why IGBT has some limitations. If we remove gate voltage source or other source then this will turn off but slowly.

If we consider temperature then we should use MOSFET,if we consider time we should use BJT,if we consider high power device then we must use IGBT.

Applications for IGBTs:

IGBTs are used in high power applications such as:

·       Appliance motor drives

·       Electric vehicle motor drives

·       Power factor correction converters

·       Uninterruptible power supplies

·       Solar inverters

·       High frequency welders

·       Inductive heating cookers

Advantages and Disadvantages of IGBT

Advantages:- Advantages of IGBT are showing below

·       Lower gate drive requirements

·       Low switching losses

·       Small snubber circuitry requirements

·       High input impedance

·       Voltage controlled device

·       Temperature coefficient of ON state resistance is positive and less than PMOSFET, hence less On-state voltage drop and power loss.

·       Enhanced conduction due to bipolar nature

·       Better Safe Operating Area

Disadvantages:- Disadvantages of IGBT are showing below

·       Cost

·       Latching-up problem

·       High turn off time compared to PMOSFET

IGBT is a relatively new device in power electronics and before the advent of IGBT, Power MOSFETs and Power BJT were common in use in power electronic applications. Both of these devices possessed some advantages and simultaneously some disadvantages. On one hand we had bad switching performance, low input impedance, secondary breakdown and current controlled Power BJT and on the other we had excellent conduction characteristics of it. Similarly we had excellent switching characteristics, high input impedance, voltage controlled PMOSFETs, which also had bad conduction characteristics and problematic parasitic diode at higher ratings. Though unipolar nature of PMOSFETs leads to low switching times, it also leads to high ON-state resistance as the voltage rating increases.

Thus the need was for such a device which had the goodness of both PMOSFETs and Power BJT and this was when IGBT was introduced in around early 1980s and became very popular among power electronic engineers because of its superior characteristics. IGBT has PMOSFET like input characteristics and Power BJT like output characteristics and hence its symbol is also an amalgamation of the symbols of the two parent devices. The three terminals of IGBT are Gate, Collector and Emitter. Figure below shows the symbol of IGBT.

IGBT is known by various other names also, such as- Metal Oxide Insulated Gate Transistor (MOSIGT), Gain Modulated Field Effect Transistor (GEMFET), Conductively Modulated Field Effect Transistor (COMFET), Insulated Gate Transistor (IGT).

Structure of IGBT

The structure of IGBT is very much similar to that of PMOSFET, except one layer known as injection layer which is p⁺ unlike n⁺ substrate in PMOSFET. This injection layer is the key to the superior characteristics of IGBT. Other layers are called the drift and the body region. The two junctions are labeled J₁ and J₂. Figure below show the structure of n-channel IGBT.

Upon careful observation of the structure, we’ll find that there exists an n-channel MOSFET and two BJTs- Q₁ and Q₂ as shown in the figure. Q₁ is p⁺n^-p BJT and Q₂ is n^-pn ⁺ BJT. R_d is the resistance offered by the drift region and R_b is the resistance offered by p body region. We can observe that the collector of Q₁ is same as base of Q₂ and collector of Q₂ is same as base of Q₁. Hence we can arrive at an equivalent circuit model of IGBT as shown in the figure below. 

The two transistor back to back connection forms a parasitic thyristor as shown in the above figure. N-channel IGBT turns ON when the collector is at positive potential with respect to emitter and gate also at sufficient positive potential (>V_GET) with respect to emitted. This condition leads to formation of an inversion layer just below the gate, leading to a channel formation and a current begins to flow from collector to emitter. The collector current I_c in IGBT constitutes of two components- I_e and I_h. I_e is the current due to injected electrons flowing from collector to emitter through injection layer, drift layer and finally the channel formed. I_h is the hole current flowing from collector to emitter through Q₁ and body resistance R_b. Hence Although I_h is almost negligible and hence I_c ≈ I_e. A peculiar phenomenon is observed in IGBT known as Latching up of IGBT. This occurs when collector current exceeds a certain threshold value (I_CE). In this the parasitic thyristor gets latched up and the gate terminal loses control over collector current and IGBT fails to turn off even when gate potential is reduced below V_GET. For turning OFF of IGBT now, we need typical commutation circuitry as in the case of forced commutation of thyristors. If the device is not turned off as soon as possible, it may get damaged.

Characteristics of IGBT

Static I-V Characteristics of IGBT

The figure below shows static i-v characteristics of an n-channel IGBT along with a circuit diagram with the parameters marked. The graph is similar to that of a BJT except that the parameter which is kept constant for a plot is V_GE because IGBT is a voltage controlled device unlike BJT which is a current controlled device. When the device is in OFF mode (V_CE is positive and V_GE < V_GET) the reverse voltage is blocked by J₂ and when it is reverse biased, i.e. V_CE is negative, J₁ blocks the voltage.

Transfer Characteristics of IGBT

Figure below shows the transfer characteristic of IGBT, which is exactly same as PMOSFET. The IGBT is in ON-state only after V_GE is greater than a threshold value V_GET.

Switching Characteristics of IGBT

The figure below shows the typical switching characteristic of IGBT. Turn on time t_on is composed of two components as usual, delay time (t_dn) and rise time (t_r). Delay time is defined as the time in which collector current rises from leakage current I_CE to 0.1 I_C (final collector current) and collector emitter voltage falls from V_CE to 0.9V_CE. Rise time is defined as the time in which collector current rises from 0.1 I_C to I_C and collector emitter voltage falls from 0.9V_CE to 0.1 V_CE. The turn off time t_off consists of three components, delay time (t_df), initial fall time (t_f1) and final fall time (t_f2). Delay time is defined as time when collector current falls from I_C to 0.9 I_C and V_CE begins to rise. Initial fall time is the time during which collector current falls from 0.9 I_C to 0.2 I_C and collector emitter voltage rises to 0.1 V_CE. The final fall time is defined as time during which collector current falls from 0.2 I_C to 0.1 I_C and 0.1V_CE rises to final value V_CE.