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I always wandered how cheap pnp-npn totem poles would do compared to a dedicated gate driver IC, like the TC4420CPA. So, I finally got around to try it, and this is what I got so far
The mosfets are not powered,
First, the TC4420CPA, no gate resistance used:
Driving a irfp250:
Driving a irf510:
Now, a BD681 + BD682 totem pole, the base resistance of the totem pole is 270ohm, no gate resistance used:
Driving a irfp250:
Driving a irf510:
So, what do you think?
Edit: My bad, the big mosfet is irfp250, not irfp260
Registered Member #3888
Joined: Sun May 15 2011, 09:50PM
Location: Erie, PA
Posts: 649
is the totem pole doing a better job on the irf510 than the ic is? I don't imagine that ringing from the ic being too good for the gate (but you could probably remedy that with a capacitor)
Registered Member #2099
Joined: Wed Apr 29 2009, 12:22AM
Location: Los Altos, California
Posts: 1716
Great experiment; thanks for sharing the scope pictures.
By the way, as one of those annoyingly wonderful complexities of the English language: Discreet and discrete are two different words with the same pronunciation. The latter is the right one for this thread's title.
Also, newbies should note that the term "totem pole" correctly refers to both the pair of bipolar transistors, and to the TC4420CPA's output stage.
Registered Member #816
Joined: Sun Jun 03 2007, 07:29PM
Location:
Posts: 156
Are you using your transistors connected emitter to emitter? The only reason I say is these are Darlington’s so you will have 2x base emitter voltage drops, probably not a problem for the +ve rise, but for turn off if your only using a 0v rail it can only pull down to 1.2v till the gate capacitance has discharged.
The gate drive Ic’s also accept logic level inputs which is convenient as you don’t have to add a voltage level shifter.
To be fair if you can build discrete circuits that work equally well, I think that makes you a better designer, as with a lot IC’s made for a specific purpose, so many people just automatically choose to use them, rather than try and understand alternative ways it can be done.
the IC is one part vs multiple. it really just comes down to choosing what best suits your application. for us tinkerers if you blow up a discrete driver you will probably just need to replace the totem pole rather than what can be an expensive IC. a cool thing we could do as a community is come up with some open hardware driver designs. I'm thinking of trying out a hybrid, driver IC with external totem pole
I will try different pnp + npn transistors later and post the waveforms. And if I can get some small signals mosfets I will also try them, since 4A 30V mosfets have minuscule gate capacitance, but they can source sink a lot of current in pulses.
For me the problem with dedicated ICs is that, I have about 10 TC4420 but 100 of the bd681/682, and they costed less.
Switching from signal levels (0-5v) to 0-15v should not be too hard, two R + npn inverters, or just one if you don't mind the inverted signal.
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Discrete gate driver
