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Innoswitch3-EP SR drive capability

Posted by: Oly_willnick on 01/30/2023


I am using the above Inn3678 for a 60W power supply, from a PFC which will supply 385V nominal. The main output is 12V

I did not initially realise that the SR gate drive was only 4.5V so had a MOSTFET which did not turn on well enough at this drive. The gate drive however was nice and square and fast rising. Today I tried a larger FET which also is not well specified at 4.5V drive, but should have worked to some degree. This device has an input capacitance of 6.3nF. When I look at the drive to this, the waveform is slow rising and distorted.  The on time is also much reduced. The power supply performance is worse. The BPS rail is stable at 4.5V.

I have another device on order but I was wondering if there is a limit to to the drive capability and the input capacitance/charge of the SR FET has to be kept below a certain figure?




Submitted by PI-Jedidiah on 02/06/2023

Hi Nick,

You should use logic FET that can be turn-on properly with 4.5V gate drive voltage. SR pin pull-up and pull-down current are all specified in the datasheet which was tested with 2nF load. The higher your load capacitance (CGS), the slower dv/dt will be....

How much was the reduction of on-time you are seeing? Can you share some waveforms?



Submitted by Oly_willnick on 02/07/2023

HI Jedidiah

I have some 4.5V RDS rated FETS on order so I will see what happens with them. The Alpha Omega ones in the app notes do not seem to be widely available so I dont want to use them. It wasnt that the edges were slow; the whole waveforrm was disrupted.. Unfortunately did not record them - I will be sure to if the problem persists with the neww MOSFET.