Tweet

Posted on 28 January 2019

Overload and Short Circuit Behavior of IGBTs and MOSFETs

SC II characteristics of an IGBT with external dynamic gate voltage limitation

 

 

 

 

 

 

 

 

 

 

Overload

Essentially, the switching and on-state behavior of IGBTs and MOSFETs under overload does not differ from "standard operation" under rated conditions. In order not to exceed the maximum junction temperature and to ensure safe operation, the overload range has to be restricted since increased load current may cause increased power dissipation in the device or destruction of components such as diodes due to dynamic failure mode effects.

Here, limits are set by the absolute value of the junction temperature as well as by overload temperature cycles. These limits are specified in the datasheet SOA diagrams (Safe Operating Area).

Figure 1. shows the example of an IGBT.

SOA diagram for an IGBT

Figure 1. SOA diagram for an IGBT

Short circuit

Essentially, IGBT and MOSFET are short circuit proof, i.e. they may be subjected to short circuits under certain given conditions and actively turn these off without damaging the power semiconductors.

Two different types of short circuits must be distinguished between (on the the example of an IGBT).

Short circuit I (SC I)

With SC I, the transistor is turned on when a load or bridge short circuit already exists, i.e. the full DC link voltage is applied to the transistor before the short circuit even occurs. The di/dt of the short circuit current is determined by the driver parameters (driver voltage, gate resistor) and the transfer characteristic of the transistor. This transistor current increase will induce a voltage drop over the parasitic inductance of the short circuit, which is manifested as a decrease in the collector-emitter voltage characteristic (Figure 2).

SC I characteristics of an IGBT

Figure 2. SC I characteristics of an IGBT

The static short circuit current adjusts itself to a value determined by the output characteristic of the transistor. Typical values for IGBTs of different technologies are as much as 6...10 fold the rated current.

Short circuit II (SC II)

In the case of SC II, the transistor is already turned on before the short circuit occurs. Compared to SC I, this case is much more critical with respect to transistor stress.

Figure 3 shows an equivalent circuit and principle characteristic to explain the SC II procedures.

Equivalent circuit and principle characteristics of SC II

Figure 3. Equivalent circuit and principle characteristics of SC II

As soon as the short circuit has occurred, the collector current will increase very steeply. The di/dt is determined mainly by the DC link voltage VDC and the inductance of the short circuit loop.

During time interval 1, the IGBT is desaturated. The resultant high dv/dt of the collector-emitter voltage will cause a displacement current to flow through the gate-collector capacitance which will increase the gate-emitter voltage. This in turn will cause a dynamic short circuit peak current IC/SCM .

Once the desaturation phase is complete, the short circuit current will drop to its static value IC/SC (time interval 2). During this procedure, a voltage will be induced over the parasitic inductances which becomes effective as overvoltage in the IGBT.

The stationary short circuit phase (time interval 3) is followed by turn-off of the short circuit current towards the commutation circuit inductance LK, which will in turn induce an overvoltage in the IGBT (time interval 4).

The transistor overvoltages induced during a short circuit may be several times higher than the normal operating values. The processes that occur during the stationary short circuit and turn-off phase are identical for SC I and II.

SC II characteristics of an IGBT with external dynamic gate voltage limitation

Figure 4. SC II characteristics of an IGBT with external dynamic gate voltage limitation

The SCSOA diagram (SC = short circuit) for short circuit, as shown in the IGBT datasheets or technical explanations, displays the limits for safe control of a short circuit (Figure 5a).

Short circuit specifi cations of an IGBT

Figure 5. Short circuit specifications of an IGBT a) SCSOA; non-periodic, parameters: tsc_max, inductance in the commutation circuit, VGE , Tjmax b) short circuit current normalized to rated current level as a function of the gate-emitter voltage

The following important conditions must be complied with in order to guarantee safe operation under short circuit conditions:

  • The short circuit has to be detected and turned off within a defined maximum period of time (typically tsc_max = 10 μs for many technologies).
  • The time between two short circuits must be within a defined range (typical value for many technologies: 1 s).
  • The IGBT must not be subjected to more than a specified maximum number of short circuits during its entire operating time (typical value for many technologies: 1,000).

Figure 5b shows the influence of gate-emitter voltage and junction temperature on the static short circuit current. Short circuits I and II cause high losses in the transistor which increase the junction temperature. In such case, the negative temperature coefficient of the static short circuit current, also depicted in the IGBT and MOSFET output characteristic, proves its benefits (Figure 5b).

With respect to the turn-off overvoltages involved, a static short circuit is often not the most critical consequence, especially in applications that use modern IGBT technologies. Tests have shown that maximum overvoltages are induced by turning the IGBT off exactly at its desaturation limit. This case must always be considered when verifying the selected drive and protection concept.

IGBT desaturation limit

Figure 6. IGBT desaturation limit

Reliable methods of limiting resultant overvoltages are given in "Overvoltage Limitation for Power Transistors".

 

For more information please read:

Overvoltage Limitation for Power Transistors

Reliability Engineering in Power Electronics

Basic Considerations for Semiconductor Protection with Fuses

 

VN:F [1.9.17_1161]
Rating: 4.5/6 (2 votes cast)
Overload and Short Circuit Behavior of IGBTs and MOSFETs, 4.5 out of 6 based on 2 ratings

This post was written by:

- who has written 197 posts on PowerGuru - Power Electronics Information Portal.


Contact the author

Leave a Response

You must be logged in to post a comment.