Derating methodology

We are considering using a pair of GaN FETs in our new active clamp forward dc-dc converter. We need to derate the transistors average power dissipation in relation to their average maximum dc power dissipation. This parameter is not listed on your data sheets. Also the SOA curve only goes to 1 ms, so it is not possible to get this parameter from the SOA curve either.

Can you please help us get this data? We need to be able to derate this part.

An active-clamp forward converter sounds like a good use for GaN FETs. Please note we are always available to recommend the optimized controller and/or gate drivers for your design. You can find example layouts, Altium board layout models, an online thermal calculator, and other tools on to help in your evaluation.

We don’t publish maximum power dissipation on our data sheets. The real limitation/requirement is to keep the junction of the FET at 150 deg. C maximum and that depends on the thermal design. One way to approach this is to take the thermal resistance, junction-to-case (i.e. top of FET) from the data sheet, (as an example 1.4 deg. C/W).
o Assume heat sink is at 25 deg. C. and Max. junction for GaN FET is 150° C (from data sheet)
o Max power for this ideal (not real!) situation: P = (delta T, junction to case) / (thermal impedance, C/W)  P = (150-25) / 1.4 = 89 W.
o These ideal and not real calculations imply an infinite heat sink, assumes perfect coupling to heat sink, and running at max. junction. And ambient is 25 C max.
o Therefore a more real situation would be to use : Ambient 85 C max, junction 125 C max (design point, for margin), thermal impedance (junction to ambient) 5 C/W. So: P = (125-85) / 5 = 8 W.
Another requirement is to stay below the continuous current max. values.

To better understand the SOA Curve (figure 11 in the data sheet), please note: to the left of the dark green line is not limited by time.