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> How To Make Bipolar Totem-pole Mosfet Driver?
Ironicster
Posted: May 20, 2008 04:35 pm
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Like it says in the topic, i am trying to build a bipolar totem-pole Mosfet Driver.

My problem is that i havenīt found a bipolar pair with the characteristics i need:

Fast switching (less then 1us)
High Pulse Current: to get the switching speed i want i need at least 10A pulsed.

But i suspect, that since that spike time is so low, maybe even a transistor with "normal" pulse current off 5A will do the trick. What do you think?

So can anyone recommend a npn/pnp complementary pair that will work for me? Tks

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GPG
Posted: May 21, 2008 04:22 am
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What is(are) the MOSFET(S) to be driven. What is the source of the drive signal. What supply voltage is available for the driver??
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Sch3mat1c
Posted: May 21, 2008 08:43 am
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Must it be discrete? There are MOSFET drivers rated up to (I believe) 14A peak. Needless to say you will need *excellent* layout, well bypassed, to use the entire capacity in the <100ns it can be delivered in!

Zetex makes small, fast, absurdly high current bipolar transistors. Offhand, the ZTX849 does Ic(max) = 7A, in a package ("E-line") smaller than a TO-92. It is also available surface mount, if you swing that way.

10A is an awful lot. I suggest you reconsider your driving strategy. You could use one driver per gate, or if your problem is one big honkin' gate, you could break it up into multiple parallel devices. Would you care to tell a bit more about your application?

Tim


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Ironicster
Posted: May 27, 2008 02:06 am
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srry, but havent been around lately.

i want to drive a irf1405 at around 30khz, in a car electric system.

i am planing on generating a negative voltage so that the swing is faster. maybe from 12 to -5V.

i am trying to build a pwm circuit at around 20A. if i turn the mosfet full on, it barely gets warm to touch, but if i try to pulse it it will heat too much, so i know the problem is not fast enough switching.

This post has been edited by Ironicster on May 27, 2008 02:20 am
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telomere
Posted: May 27, 2008 07:45 am
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There are a ton of pre-built drivers that will go to 1MHz or more - 30 KHz is nuttin' to them. Why not use one?


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GPG
Posted: May 27, 2008 11:59 am
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QUOTE
What is the source of the drive signal.
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Ironicster
Posted: May 27, 2008 02:13 pm
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using a 555 connect to bipolar totem pole

Can they go that fast with a 250nC gate? give me a reference then please.
tks
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Sch3mat1c
Posted: May 27, 2008 03:30 pm
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250nC at what voltage?
- Nevermind, I saw IRF1405;
http://pdf.elenota.pl/pdf/International_Re...ier/irf1405.pdf 260nC @ 10V.

To charge 260nC in under 1us needs a current I = dQ/dt more than 260nC/1us = 260mA = 0.26A. Considerably less than you estimated! In fact, a 555 will drive that, having about 200mA output current. A 27 ohm gate resistor would be appropriate for this driver.

Drive current isn't as big an issue as you may think. I have 600V, 50A highspeed IGBTs -- pairs of two -- driven by 2N4401/4403 emitter followers, moving the gates from +10V to -5V in about 200ns. The collector waveform shows a 40ns rise time when unconstrained, which essentially corresponds to the collector capacitance -- I = C * dV/dt so C = I * dt / dV = 10A * 40ns / 100V = 4nF, pretty damn close to the typical collector output capacitance. Evidently, actual turn-off time is nearly inconsequential!

With the output loaded down properly (a snubber capacitor), the transistors are off long before the waveform moves very far (rise time circa 1us), as you no doubt will also have to manage. Turning off 100A in a fraction of a microsecond is a bad plan; turning it off over a microsecond or three is much wiser. With a sufficient snubber, you shouldn't have much problem with gate drive.

Tim


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