The driver circuits known under various designations, such as "two-stage turn-off", "soft switch-off", "slow turn-off" uses a low-ohmic gate resistor in normal operation to turn the IGBT off in order to minimise the switching losses, and a high-ohmic resistor (or lower gate current) whenever a short-circuit or over-current is detected. However the problem lies in the reliable detection of these conditions: VCE monitoring always involves a delay (known in this case as the response time) that must elapse before an error is detected. This time is as a rule up to 10us. If a short circuit is in fact present and the IGBTs are driven with a pulse which is shorter than the response time, the error is not detected and the circuit switches off too quickly. The IGBT is then destroyed via an over-voltage. Moreover the coverage of limit cases (between over-current / not over-current) poses a problem.
As a rule, such circuits must be regarded as dangerous and are therefore not used in CONCEPT products.
CONCEPT recommends mounted parts with minimum inductance values and worst-case dimensioning of the power parts, i.e. the gate resistance values should be selected so that over-currents and short circuits can be safely controlled at every turn-off and at maximum intermediate link voltages.
For high-power applications, CONCEPT has developed the SCALE plug-and-play driver series of drivers with an active clamping function. This represents a more complex but a better and more reliable solution than the "slow turn-off" approach already described.
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