Wrong mosfet choice for h-bridge

[BC547]

Hi Folks,
This is my first post here, so don't be to hard on me. :thumbsup:

I need an very precise ac 64Khz square wave current source of 200µA.
To accomplish this i'm using a **broken link removed** in combination with a h-bridge.
This works not that bad, except that during the switching there's a small delay before the mosfet switches on and the other switching off. This causes that the current source sees the R-load not for that short moment and overshoots for a short moment.
I think this has to do with the mosfet's i have chosen (irlml6302,2803). What mosfet do you advice or at what parameters i have to look to get it working correct.

 

[FvM]

I would use CMOS analog switches like CD4053 for this rather small current.

 

[chuckey]

Are you driving the FET gates from a very low impedance source, because of their high input capacitance?
Frank

 

[ftsolutions]

Both the input capacitance, and the 470 ohm gate resistor, will delay turn on/off somewhat. You didn't indicate how much a delay you are having, but I'd first experiment with reducing the gate resistance value to 100-200 ohms, if your driving device can handle the current. What sort of method are you using to avoid shoot-through?

 

[BC547]

Thanks for the reply.

@FvM, i had already had an look at the CD4053, but the on resistance (>80 ohm x2) is to high. R-load will change and by adding to much R-load the measuring range will decrease to much.
@Chuckey & ftsolutions, I tried already different gate resistors values, even 0 ohm but the delay of 1,7µS is still there. I looks indeed that the driving device can't handle the current.
I didn't do anything about shoot trough, never heard of it before. I'm gone read what it is and how to something about it.

 

[FvM]

CD4053 was just an example. CMOS analog switches are available with Rdson downto a few ohms.

Generally CMOS switches have (almost) balanced gate charge, so the switch signal to output feedthrough is considerably smaller. And the required level converters are integrated in the IC.