Power Electronics Europe April/May Issue 2022

30 WIND POWER INVERTER DESIGN www.power.com/scale-iflex-lt Issue 2 2022 Power Electronics Europe www.power-mag.com is close (within approximately 5-10 %), the module will naturally compensate and balance the collector currents due to the heating effect on RCE(ON), which has a positive temperature coefficient. Under these circumstances, current sharing will be very good once thermal equilibrium is reached in the system. To reduce common mode currents flowing between modules (Figure 5), it is important to reduce the loop area (and therefore parasitic inductance) as much as possible. The propagation delay associated with large emitter loops can be in the order of 800 ns with mismatch of as much as 80 ns between modules. Active gate drivers located on the IGBT module (MAGs), which reduce the loop area and common mode chokes, are effective in reducing common mode currents. As well as reducing current sharing, gate drive can be challenging, being susceptible to EMI which can cause false triggering – a phenomenon only partially addressed by passive filtering. Current imbalance between adjacent modules of an inverter phase means that an active MAG approach can increase output power by as much as 15 % compared to that achieved with a passive module drive stage with less accurate current sharing. Optimizing gate drive for highest switching efficiency The role of gate resistance in controlling the switching loss (especially turn-on loss) in an IGBT is well known. An actively driven gate signal located on the local MAG is able to support a low impedance gate drive for turn-on and turn-off. Turn-off losses are relatively constant compared to gate resistance, but the low gate impedance is important in preventing voltage overshoot during shutdown. The turn-on resistance of the gate-driver stage strongly influences switching loss, and there are important efficiency benefits associated with having a driver stage that can support a low on-resistance (0.7 Ω is shown in Figure 6) from a relatively high current drive stage. SCALE-iFlex LT can deliver up to 20 A of gate drive current per MAG channel due to the active drive- stage booster in the ASIC for each channel. Typical values for a discrete driver with remote driver stage will be in the order of 2-3 Ω . With discrete local gate driver stages, it is often necessary to increase gate driver resistance on individual modules to achieve better load balancing between switch modules and, in discrete circuits to increase the effectiveness of the Active Clamp circuit. Optimizing drive to enable low gate resistance and better switching performance can increase overall switch efficiency by up to 3 %. Conclusion Combining the system utilization benefits of higher DC-link voltage and better current sharing with the efficiency that can be gained from localized gate drive, we can substantially increase system utilization efficiency. There is some crossover in parameter effect, so a straight addition of the benefits is difficult, but improvements in driver utilization of more than 20% are certainly possible. This suggests that with careful design and the utilization of active MAGs supporting the IMC, it is possible to remove one in five of the IGBT modules employed in a typical wind turbine inverter application – a very significant saving in space and material. Figure 5: Large loop areas between the IMC and passively- driven gate drive modules create large parasitic inductance that can cause circulating common mode currents which limit switching frequency and can cause significant timing mismatch – often sufficient to cause jitter between modules Figure 6: A localized active gate driver (MAG) is able to support a low gate drive resistance due to the buffered drive stage capable of delivering >1 A. This reduces turn-on losses by up to 50 % compared to the higher resistance typically needed to balance a passive gate driver stage