How to quickly turn on/off N-FET switching high side of 55V?

Hi - I'm going to be using some N-FETs to turn on/off (from the high side) a large (10-30A), grounded load that is being powered by a ~55V power source. I would like to be able to switch them with 5V signals. Now, I am no genius, but I'm pretty sure I can't drive the gates with

5V when their drains are at 55V. So somehow I need to get a voltage say ~10-20V above the 55V power source that is being switched. I'm favoring a boost converter, just because that is the technique I'm most comfortable with. Specifically, it looks like I could use a Linear LT1171HV
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to get that supply pretty easily. I should mention that on this board I will probably also have some sort of lower voltage supply, like maybe 12V or 5V or something along those lines. Also, I should mention that on this one PCB I'll have probably 3 of these switching circuits.

But once I have this power source, how do I switch the gate, and how do I do it quickly? The hard thing is that I want to be able to switch this stuff with TTL level signals. So, right now I'm thinking about using an op-amp, say something like a TI OPA445

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though probably not that chip as it seems to have pretty limited availability right now. The OPA445's

15V/us typical slew rate should probably be fast enough. I'm trying to avoid overheating the FET while it is switching from on to off and vice versa, hence the desire for fast switching speeds.

Now, to make things a bit more interesting: The power source is going to be fairly variable. It could be anywhere between about 10 and

55VDC. That means my DC/DC converter needs to be smart enough to have an output voltage that is ~10-20V higher than the input voltage. Or the gate switching circuit needs to be smart enough to bring the gate to about 10-20V higher than the input voltage. I tried working out a circuit to handle the feedback for the DC/DC converter to give it an output voltage of 20V higher than the input voltage, but, unless I messed something up, it looked like it'd require an analog multiplier and an op-amp, which is just getting complicated. So right now I'm thinking it'd be better to take care of the variable input voltage with the gate switching circuitry. I can't seem to come up with a circuit that will do this with just a single op-amp - everything I've thought of so far has needed two op-amps - but surely there's a way to do it. If not, oh well - it's not the end of the world, but I would like to use as few of these high voltage op-amps as possible as they seem to be pretty spendy.

So - do you think this is the right direction for me to be going in? Or can anybody suggest a better method?

Thanks guys!

-Michael

Reply to
Michael
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.10-50DC>--------------+-------+------+            
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Reply to
John Fields

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Aarghhh!!! Bad design.
Reply to
John Fields

There's something wrong with your drawing, John. SFAICT, you've added a 12V dc source riding on top of the 10-50V 30A power source, let's call that 62V, but you switch it with a PNP, from a signal connected to 50V rather than 62V. That can't work. I'll fix that, and I'll change the level-shifting signal into a current source, as you know I prefer to do when the destination voltage can vary. And when it doesn't.

Also note the separate grounds and the output diode. Note the MOSFET zener, and R4 to help damp the RF oscillation possible during slow on/off switching.

. FWB . ,----, +12V . 120AC >-, ,--|~ +|---+-----+-----, . P||S | | | | | . R||E | | [BFC] 3*R1 | . I||C | | |- | E 150V pnp . 120AC >-' '--|~ -|---+ +---B . '----' | | C . | | | . 10-50DC >==============o=====================, . | | | . 150V npn | | D 100V . C +--R4-- G n-CH . TTL >------- B | 18V S . E | zener | . | +---|----------+-----' | . 50V RTN >=====================================o===(O)

Reply to
Winfield

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Yup! 

Thanks. :-)
Reply to
John Fields

Please define "fast"

John

Reply to
John Larkin

[snip]

As opposed to half-fast?:-)

...Jim Thompson

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|  James E.Thompson, P.E.                           |    mens     |
|  Analog Innovations, Inc.                         |     et      |
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Reply to
Jim Thompson

milli/micro/nano would be a start.

John

Reply to
John Larkin

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He said: "The OPA445\'s 15V/us typical slew rate should probably be
fast enough." 
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Reply to
John Fields

The '445 would be slew-rate limited, and is pretty wimpy to drive a bunch of gate capacitance.

It's a hassle to paste these things into LTspice.

John

Reply to
John Larkin

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So what, you smoke-blowing ass?

The point is, he defined 15V/µs as "probably fast enough" and if the
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Reply to
John Fields

Geeze, are you hitting the booze this early in the day?

That's actually not a serious threat here.

John

Reply to
John Larkin

You all are making an assumption that isn't correct: I don't have AC available to me. This is a battery powered application. Everything is running off of the 10-55V DC power source.

-Michael

Reply to
Michael

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Nope, but what makes you think someone has to be drunk to flame you?
Reply to
John Fields

Fast enough that nothing explodes. I don't care about the speed of switching - I just care about not damaging the electronics. I'd have to run some numbers and be smart about it (and that whole smart thing has never sat well with me) - but it seems to me that when the FET is not off and not on, but in between, that it'll have a large voltage across it and since the load is inductive, a large current as well. Thus I'm worried that if the switching is too slow it'll fry the FET. Is that realistic?

Thanks,

-Michael

Reply to
Michael

Gee, what happened to "I love you, John Larkin"?

John

Reply to
John Larkin

That's OK, Michael, John and I just like admiring our fine ASCII drawings. Aren't they nice? Actually, an attractive way to do this is to use a commercial chip with a built-in bias generator for the power n-channel switching MOSFET. Something like Infineon's BTS660P, for $6.69 at DigiKey.

. ___ BTS660P . 10-58 dc >============|___|======o===(O) . | | . D | . TTL >--+---- G _|_ LOAD . | S /_\\ . 10k | | . | | | . GND >--+-------' | . 50V RTN >=======================o===(O)

You can switch the BTS660 on and off with a 100-volt logic- level MOSFET in a TO-92 package, such as a Zetex ZVNL110A. Can you tell us about this 55 volts of yours? The BTS660 has a maximum rating of 58 volts, and it probably can go a bit beyond that, but one should know exactly what might be encountered, in case a more complex design is in order.

Reply to
Winfield Hill

So something in the few-hundred microsecond or less time frame might be safe, maybe less depending on the fets, and how many fets.

A floating power supply would be helpful, so the driver doesn't have to swing so far.

You could do something like this:

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After that, you can also add a serious driver if it's needed.

John

Reply to
John Larkin

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That was when you were nice; before Usenet took its toll on you.
Reply to
John Fields

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If the BTS660P\'s control pin isn\'t pulled down to 50V RTN how will
it know to turn on?
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Reply to
John Fields

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