can turn on 1 FET but not another 3

[GoKid]

I have two FETs I am trying to turn on - IR620 and IR 3215, both N-channel FETs. VDD = 9v, gate signal is 40KHz +5V. The International Rectifier IR3415 N-channel was turned on with a TC428 FET driver no problem outputting a nice sqaure wave. It was also turned on with 6 parallel 74A0C inverters. Ok, no problem. Then I tried turning on the IR620 - it did not turn on first with either the inverter or FET driver scenario. I made a voltage doubler from one TC428 (in spec sheet) and used its output to feed VDD of another TC428 and hence, doubled my gate voltage. The IR620 still did not turn on. I increased VDD to the first TC428 to 9v doubling to 18v, and it gave a partial, sloppy, non-sqaure 40KHz signal.

Is the RDS(On) the culprit? =.018 for the 3415 and .8 for the 620.

Spec sheets are here:

http://www.irf.com/product-info/datasheets/data/irf3415.pdf

http://www.irf.com/product-info/datasheets/data/irf620.pdf

 

[VEBill]

This may be overly simplistic, but have you tried reducing the 40kHz frequency to maybe 1Hz ? This would obviously distinguish "AC" problems (gate capacitance) from "DC" problems.

 

[GoKid]

I haven't because I am trying to switch Vdd at 40Khz, e.g. 40KHz square wave driver for transducer. I don't know what to do - I have increased both current and voltage and I my returns aren't much.

 

[IRstuff]

The transistor has a relatively normal threshold, so either it's dead or your circuit isn't driving it the way you think.

TTFN

 

[Skogsgurra]

I would have expected the opposite. The 620 has much lower gate capacitance (unless there's a typo in the data sheet) and should be easier to turn on. Gate voltages are the same.

The only situation where channel resistance would have a say is if you load the output. You haven't said if you run lightly or heavily loaded. And if you have capacitors in the load. Do you?

Gunnar Englund

http://www.gke.org

 

[itsmoked]

You either have a bad part or you have a board/solder/trace defect. You don't need voltage doublers, etc. to run that device. Unless you have it as a high side switch.

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[melone]

Is 5V VGS going to turn on the device hard enough?

 

[GoKid]

I've posted the schematic here. Please tell me whether this would change the response(s). Thank you for the replies.

http://216.77.188.54/coDataImages/p/Groups/292/292938/folders/243015/1909634Push-PullDriver.JPG

 

[benta]

Several points:
1: With this kind of setup, you have absolutely no control over turn-on and turn-off times. I strongly suggest that you have a problem with "shoot-through" (but FETs on at the same time.)
2: 1N4001 at 40 kHz? Think again.
I'd strongly advise you to use a deidcated driver like ON NCP5181 or IR IRC2114, there is a reason they've been made.

Benta.

 

[zeitghost]

The IRF9530 is P channel.

According to your schematic, the source of the IRF9530 is connected to the drain of the N channel, which will turn on the body diode of the P channel thus connecting the drain of the N channel directly to the power rail...

 

[GoKid]

I corrected my drawing per Zeitghosts comment, it wasn't actually connected that way. Benta I will look into those dedicated drivers to strengthen my design.

Replacing the P channel 9630 with an IRF9640 and the circuit above began working (bad FET?). I used a 9v battery test source for the push-pull network and the output is a 9Vpp 40.00KHz square wave. FYI, once the FET was changed, the voltage doubler was unnecessary and was driven soley with the TC428 at +5V.

My new problem is when I change the 9v source for the network to 100V DC HV converter (EMCO Q0-05) no square output. Is this more likely a problem with my converter or shoot through.

http://www.emcohighvoltage.com/

go to "Q series"

Thanks
corrected:

http://216.77.188.54/coDataImages/p/Groups/292/292938/folders/243015/1909692Push-PullDriver.JPG

 

[IRstuff]

Well, what are you trying to drive with this thing?

Your power supply only puts out maybe 2.5 mA? That means you can't drive a load larger than about 150 pF. Your transistors' drain capacitances alone probably exceed that value.

TTFN

 

[GoKid]

I am driving a transducer with a 200ohm load resistor connected to the push-pull output. Are you suggesting that getting a converter with a higher output amperage may solve the problem? That would make sense as other lower voltage higher amp sources have worked while this one doesn't.