I have a question about the high side gate driver and charge pump circuit. I am testing my proto making sure all the drive signals are correct. I notice that when switching my dual high side fets that the gate voltage has a nice well controlled rise time on the order that I expect (longer turn on and off then planned but I am working my way up in drive current as I verify everything) but after switching there is a long slow tail to get the gate signal to fully enhance to the final value that is 10 volts above the Vdrain voltage. It looks like it is not fully enhanced until long after the switching time. On the trailing edge it does something similar but in reverse, it slowly droops down to almost the Vdrain voltage, just before the switching event, then quickly pulls down as epxected at the switching point.
I checked my calcs and the gate charge is not too large. I have slowed the switching freq down to 5khz at 50% duty max. I am running a resistive bridge with very minimal load three 2k resistors in place of a motor. I am using the basic values called out in the data sheet and app notes - 47nf 100 V between CPL and CPH and a 1uF 16 volt between Vdrain and VCP. I am runing a split power suppy with 15 vlots at VM. FETs used are IAUT300N10S5N015. I am testing with 30 volts on the input (Vdrain)
Gate to source charge Q gs - 52, 68 nC
Gate to drain charge Q gd - 33, 50
Gate charge total Q g - 166, 216
Gate plateau voltage Vplateau - 4.4 - V
Any thoughts on what I might try to recitfy this? Changing the gate drive does not affect this effect at all my guess is that it is related to the charge pump and its capability. The lower fet does not show this, it has a nice clean frive that fully enhances the low side fets.
I do have 5 ohm resisors in between the two parallel gate drives for each set of fets. I put those in in the event we have any ringing I can use one or both to dampen that but so far it all looks good. I kept the value low to start so i don't think that would be doing it.
Should the charge pump capacitors values ever be increased or are those the specificed values good for a wide range of applicaitons - should not every need to be changed?
Ross
