Does Power Integrations offer digital IGBT drivers?
Power Integrations IGBT drivers combine both digital and analog approaches. The highly integrated SCALE and SCALE-2 chipsets in every Power Integrations driver rely on a mixed-signal architecture. The individual strengths of digital building blocks and analog cells are strategically exploited to generate maximum performance.
As an example, digital startup control can be much more precise than its analog counterpart when all operating conditions, voltage variations and device aging are considered. Also, considerable chip area, and thus costs, can be saved by implementing digital filters and digital timing management.
The situation is very different if we look at IGBT short-circuit protection. Here, the inherent speed of analog signal processing is far superior to any affordable digital emulation where it would be necessary to wait for the next few clock cycles to accommodate the request.
Another example strongly in favor of analog circuitry is management of the IGBT’s switching characteristics by means of active clamping, di/dt control and dv/dt feedback. Digital programming of these features can naturally reduce the production costs for the driver. However, such a coarse “digital” adaption to an IGBT module is no match for the excessive fine-tuning needed to generate the optimum switching performance required of these costly power modules.
It is generally a question of how much performance compromise and digital overhead you are willing to spend for a fully digital IGBT driver – and if you are getting any benefit from doing so.
The IGBT or MOSFET power switch must always be considered as an analog device to make the most of its capabilities. The optimum driver will therefore be the actual interface between the digital domain and the analog “real world”.
We at Power Integrations believe that an integrated combination of analog and digital functionality exploits the best of both domains at an optimum cost/performance ratio.
Why don't Power Integrations drivers turn off slowly in the event of a short circuit?
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 minimize 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 / no over-current) poses a problem.
As a rule, such circuits must be regarded as dangerous and are therefore not used in Power Integrations products.
Power Integrations 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 DC-link voltages.
For high-power applications, Power Integrations has developed the SCALE and SCALE-2 plug-and-play driver series with an (advanced) active clamping function. This represents a more complex, but a still better and more reliable solution than the "slow turn-off" approach already described.