Alex Lidow Answers Your Questions

Last month Power Electronics received a question about how GaN transistors compare to power MOSFETs in power switching applications in terms of efficiency . We asked our expert in GaN/power MOSFETs, Alex Lidow to weigh in on the subject (view his response here) and also encourged readers to submit other questions they had about the space.

Below are the responses to the questions that Alex had time to answer. Several of the questions that are not included required long responses and additional research from Alex and his team. If your question was not answered this time, please keep an eye out for up and coming Q&A's with our future experts.

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Have experienced failures of MOSFETs from two different manufacturers when used in a phase shifted ZVS full bridge topology. The failures occurred under load at elevated ambient with an 85C case temp measured at the MOSFETs. Extensive investigation showed the circuitry to be working properly and that the FETs were clearly the issue. When the offending FETs were replaced with IR devices (IRFPS40N50L) the failures ceased. Further investigation showed hard commutaion of the body diode at elevated ambient produced easily repeated failures in the offending FETs (5A reverse current at 80C resulted in failure) while the IR devices were much more robust allowing 10A of reverse current at 125C without failure. My question is: What made the IR device so much more rugged as far as hard commutation of the body diode? -Lynn

Dear Lynn, I can’t comment on how another company makes their devices however, I do know that the ruggedness of a body diode in a MOSFET is closely related to how well the channel region (defined by a p-type diffusion) is shorted to the source region. This is particularly important around the device perimeter and the bonding pads. Under light load conditions in ZVS fill bridge circuits, it is possible for the high side FETs to turn on when the bottom side FET is still in the process of reverse recovering. When this happens, a shoot through condition is occurring causing the device to blow up. Using a fast body diode device, this issue can be resolved, Almost all MOSFET vendors are selling fast body diode devices today.

Hi Alex,
Is there any inherent overvoltage withstand capability for an eGAN FET (like the avalanche capability of a conventional power MOSFET)?
Regards, SL.

Dear SL, eGaN FETs are rated on the data sheet for a 20% overvoltage in pulsed mode. There is no intrinsic reason why eGaN FETs cannot have an avalanche capability similar to a lateral silicon device. The second generation eGaN FETs now on the market are not yet designed for avalanche. Look to the third generation for that.

Continue reading Alex's responses pg. 2

Hello Dr. Lidow,
Thank you so much for this Q & A session. These type of sessions are very beneficial to new entrants like us into Power Electronics. We are a small start-up and would like to offer products and services in small to medium power (< 1000W) applications such SMPS, Solar Inverters etc. Could you recommend a list of equipment that our lab must have in order to develop and test such equipment. Thank you very much for your valuable reply. Regards, Anand

Dear Anand, A lot depends on your budget. You will need some power supplies in various voltages and currents, a good oscilloscope, function generator, multimeters, soldering iron, a variety of resistors, capacitors, and inductors. A lot of this equipment can be purchased used at a steep discount. In general I have been able to put together a basic lab for our applications engineers for about $20K.

Can you explain the fundamental design issues and usage of 'linear mode' mosfets - mosfets designed for slow-switching/DC operation with 'square' SOA curves that extend to full voltage/current. Is the basic issue with 'non-linear-mode' mosfets in slow/DC switching applications a basic die hotspot issue where the individual mosfet cells aren't sharing the power evenly? thanks - Kevin

Dear Kevin, Thermal is the most serious issue when MOSFETs are used in a linear mode. The RDS of the power MOSFETs rise 60 to 80% as the operating temperature goes up from 25C to Tjmax also. Further silicon FETs have a negative temperature coefficient below the “zero tempco” point,. This causes the RDS(ON) to go lower pulling higher currents through one of the FETs in a parallel connected situation, causing even more uneven distribution of current between FETs.. This is the reason why several manufacturers do not recommend parallel FETs for hotswap applications. However, enhancement mode GaN FETs do not exhibit “zero tempco” characteristics and are ideal for parallel operation in the linear mode.

Continue reading Alex's responses pg. 3

Dear Alex,
Especially for MOSFETs, what are the effects of parasitic capacitance such as wire bond capacitance on the MOSFET capacitance such as drain-source capacitance during switch operation. - Ahmed

Dear Ahmed, There are several articles published by different manufacturers discussing the parasitic capacitances. Parasitic capacitance has to be charged and discharged during every switching operation, it impacts the switching time and switching losses.

We have a project where we are creating a printed circuit board that will interact with a commercial washing machine's mechanical timer. The circuitry has to be able to read when 115VAC is present or in the case of a 230VAC machine when 230VAC is present. Is there more than one approach when it comes to picking off that part of the logic within the circuit board. Since I am the person underwriting the project ... I am hoping the very young electronic engineer is not in over his head when it comes to the most reliable & durable way of reading the presence of the proper voltage. Thanks! - Mike

Dear Mike, Not sure how to answer this question. We don’t know what is in this PWB. (Post in the comment section below to provide more information regarding your question. This may allow Alex to provide a more detailed response)

Why are power MOSFs limited to -55C storage temperature while most power BJTs are rated for -65C storage temperature. - Jim

Dear Jim, There isn’t a one line answer to your question. Can we get in touch with you to discuss your question in depth? (Post in the comment section below to provide more information regarding your question. This may allow Alex to provide a more detailed response)