Guru
Joined: 21/06/2018
Location: AustraliaPosts: 1156
| Posted: 03:32am 16 May 2019 |
Thanks for the question Klaus and the answer is two fold, yes there are diodes in the FET - the added diodes were an 11th hour inclusion. Please remember this is a first prototype and I am a believer in belts and braces approach.
I have made no inclusion for snubbers in my design - it will probably require some and they will be essentially in parallel with the diodes in question so there are holes available to put them in (or legs to solder them to).
There is another benefit in not relying on the internal diodes of the FETs to conduct current internally in my opinion. The FETs are already shouldering the bulk of current switching, for every watt dissipated there is a penalty of higher RDS on generating more heat. I wanted to experiment with external diodes such as MUR420 with lower foward voltage drop to minimise heat generation inside the FET by passing current externally to the FET when it is not on & minimising the internal heat. I am also aware that in a final design the MUR420s may be under rated for full load and the TO247 U30D20C or Motorola equivalent MUR3020 will probably be required)
This is directly associated also with dead time. The diodes conduct during dead time.
Dead-time is essential to ensure no shoot through. Excess dead time I consider an enemy. Efficiency is essentially maintained with excess deadtime but the cost is extra heat in the switching FETs. The sooner the FET is turned on after deadtime the better as then the diode (inside or external to the FET is not conducting & generating heat which is wasting energy. It doesnt actually matter as much that the external diodes may get hot, they become more efficient as they get hotter as the Vf value reduces further.
Whilst discussing FETs, one application note I read recently for TO247 (same as the large HY4008) packages had some advice for reducing internal heat in the package.
They advised that the Mosfets should be inserted into the PCB up to the shoulder (wider parts of the leg close to the case) and well soldered to generous copper areas as it minimises heat build up of the internal bonding wires. Whilst I really Like the Warp terminal blocks idea and have used it myself, for a serious Inverter I chose to solder directly with short legs - every bit must help.
Happy to hear others point of view on any of the above.
If at first you dont succeed, I suggest you avoid sky diving....
Cheers Mike