IGBT｜Basic

Short Circuit Withstand Time (SCWT) of IGBTs

2023.08.30

Points of this article

・SCWT (Short Circuit Withstand Time) is the time a power device can withstand a short circuit without destruction.

・Short-circuit withstand is the time required to ensure the protection function of the system.

・The longer a short circuit withstand time, the more reliable and safer the system will be.

table of contents

Short Circuit Withstand Time (SCWT) of IGBTs

IGBTs and other power devices have an electrical characteristic (parameter) called the short circuit withstand time (SCWT). When a power device is in a short-circuited state, a large current flows, normally resulting in destruction in a short time; the SCWT is the time for which the device can withstand the short-circuit without destruction, and is also called the short circuit tolerance time.

Short-circuiting of a power device means, for the case of an IGB (and the following explanation is for IGBTs), that the IGBT is turned on with a high voltage (VCC) applied across the collector and emitter, so that a large collector current IC flows in the IGBT in the conducting state. Such a state may occur due to a control circuit malfunction or some other erroneous operation.

In order to better understand this short-circuiting, below are shown a basic circuit example when measuring the short circuit withstand time, as well as the resulting waveforms. When VCC is applied to the IGBT in the off state and the gate driving circuit causes the IGBT to be turned on, the charge accumulated in the capacitor suddenly flows into the IGBT, and after some time elapses the IGBT reaches destruction. The time elapsed until destruction differs depending on the VCC voltage, temperature, package type, and other parameters, but general ranges from several microseconds to several tens of microseconds. In tests, the gate driving voltage is controlled to increase the on time slightly and determine whether destruction occurs; by repeating this procedure, the time until destruction is measured. Alternatively, there are cases in which a pass/fail judgment is rendered by confirming that a device does not fail within a stipulated on time.

The waveform diagram is for a product in the ROHM RGS series of IGBTs. A minimum short circuit withstand time of 8 µs is guaranteed. When a gate signal caused the IGBT to enter the on (short circuit) state, a collector current flowed, but when after 13.5 µs the device was turned off by a gate signal, the collector current was shut off and the IGBT did not undergo destruction, indicating that under these test conditions, the IGB withstood a short circuit for 13.5 µs. Of course, the guaranteed value of 8 µs is cleared, with room to spare. The rise and fall of the collector voltage upon short circuit and shutoff was due to the charging and discharging of the parasitic inductance from the capacitor to the IGBT collector pin; thereafter the collector voltage returned to VCC. The decrease in the collector current with the passage of time is due to heat generation.

In the event that an IGBT should be destroyed during short circuiting, in essence a short circuit malfunction initially occurs, so that current continues to flow to the IGBT more or less without limit, so that the collector voltage = VCC falls to near ground level. Of course even if an off signal is sent to the gate, the IGBT does not turn off, and the collector current is not shut off. If an IGBT is destroyed during tests or evaluations, the current must be turned off rapidly, to avoid smoke due to overcurrent-induced heating and even the possibility of fire. Thorough safety measures, such as providing VCC (the power supply) with means for appropriate current limiting, are essential.

The Importance of the Short Circuit Withstand Time

The short circuit withstand time is an important parameter for protection of power devices and peripheral circuitry as well as connected components. Circuits that use power devices are generally equipped with circuits for protection against overcurrents and other phenomena. Should a power device enter a short-circuited state, a protection circuit will detect the short circuit and execute a protection operation; but time is required from detection of the problem until functioning of the protection action, typically for MCU system processing and the like. If there is sufficient time, this processing can be performed reliably. That is, the short circuit withstand time is time needed to ensure operation of system protection functions; the longer this time is, the greater the safety margin for system processing, resulting in enhanced system reliability and safety.

Thus the short circuit withstand time is an important characteristic, but a value for the SCWT and a guarantee of same are not provided for all power devices. Depending on the grade of the product and its application areas, a value may not be provided, or a provided value may be only representative (“Typ.”) with no guarantee, or a guaranteed value may be explicitly stated. In all cases, the data sheet must be checked.

Moreover, while a longer short circuit withstand time is better, guaranteed values vary greatly depending on the manufacturer and the model series. For example, the previously mentioned RGS series of IGBTs has a guaranteed value of 8 µs (minimum), but the value for products in another series, the RGT series, is 5 µs (minimum). Moreover, the VCC and temperature conditions are also different, so the conditions as well as the values themselves must be checked. As actual examples, please have a look at these IGBT data sheets. The conditions and guaranteed values are stated for parameter name: Short Circuit Withstand Time, symbol: tSC.

RGS series of IGBTs: https://www.rohm.com/products/igbt/field-stop-trench-igbt?page=1&SearchWord=rgs

RGT series of IGBTs: https://www.rohm.com/products/igbt/field-stop-trench-igbt?page=1&SearchWord=rgt

【Download Documents】 Basics of IGBTs

IGBTs are one of the typical power devices and are used in a wide range of applications including motor drives. This handbook provides a basic understanding of IGBTs, including their application scope and application picture based on their features, their structure and principle of operation, and how they compare with and use other power devices.

Short Circuit Withstand Time (SCWT) of IGBTs
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