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

Dec 12, 2007 141 Replies
M

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


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J

View in Courier: .10-50DC>--------------+-------+------+ . FWB | | | . +----+ | | | .120AC>-+ +--|~ -|---+ | | . P||S | | | | | . R||E | | [BFC] [R] | . I||C | | |+ | | .120AC>-+ +--|~ +|---+->12V | | . +----+ | | | . E | | . PNP B-----+ | . C | | . | | D . +-------|----G NCH . | | S . | [R] | . [R] | | . | C | .TTL>---[R]------------|-----B NPN [LOAD] . | E | . | | | .GND-------------------+-------+------+
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W

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)

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J

Please define "fast"

John

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J

[snip]

As opposed to half-fast?:-)

...Jim Thompson

-- | James E.Thompson, P.E. | mens | | Analog Innovations, Inc. | et | | Analog/Mixed-Signal ASIC's and Discrete Systems | manus | | Phoenix, Arizona Voice:(480)460-2350 | | | E-mail Address at Website Fax:(480)460-2142 | Brass Rat | |

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| 1962 | America: Land of the Free, Because of the Brave

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J

milli/micro/nano would be a start.

John

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J

He said: "The OPA445\'s 15V/us typical slew rate should probably be fast enough." Here\'s something just a little faster than that: ;) Version 4 SHEET 1 1220 1156 WIRE 400 304 32 304 WIRE 736 304 400 304 WIRE 32 384 32 304 WIRE 400 384 400 304 WIRE 736 384 736 304 WIRE 560 432 512 432 WIRE 672 432 640 432 WIRE 32 528 32 464 WIRE 880 528 32 528 WIRE 736 560 736 480 WIRE 400 608 400 464 WIRE 512 608 512 432 WIRE 512 608 400 608 WIRE 880 608 880 528 WIRE 736 688 736 640 WIRE 832 688 736 688 WIRE 400 752 400 608 WIRE 736 752 736 688 WIRE 880 752 880 704 WIRE 224 800 160 800 WIRE 336 800 304 800 WIRE 512 800 512 608 WIRE 560 800 512 800 WIRE 672 800 640 800 WIRE 32 832 32 528 WIRE 160 832 160 800 WIRE 32 960 32 912 WIRE 160 960 160 912 WIRE 160 960 32 960 WIRE 400 960 400 848 WIRE 400 960 160 960 WIRE 736 960 736 848 WIRE 736 960 400 960 WIRE 880 960 880 832 WIRE 880 960 736 960 WIRE 32 1008 32 960 FLAG 32 1008 0 SYMBOL pnp 672 480 M180 WINDOW 0 75 83 Left 0 WINDOW 3 53 48 Left 0 SYMATTR InstName Q1 SYMATTR Value 2N5401 SYMBOL npn 336 752 R0 WINDOW 0 71 15 Left 0 WINDOW 3 45 48 Left 0 SYMATTR InstName Q2 SYMATTR Value 2N5550 SYMBOL voltage 32 816 R0 WINDOW 3 40 81 Left 0 WINDOW 123 0 0 Left 0 WINDOW 39 0 0 Left 0 WINDOW 0 40 53 Left 0 SYMATTR Value 55 SYMATTR InstName V2 SYMBOL res 864 736 R0 SYMATTR InstName R4 SYMATTR Value 1.8 SYMBOL voltage 160 816 R0 WINDOW 3 24 104 Invisible 0 WINDOW 123 0 0 Left 0 WINDOW 39 0 0 Left 0 WINDOW 0 39 57 Left 0 SYMATTR Value PULSE(0 5 0 1e-6 1e-6 0.001 0.002) SYMATTR InstName V3 SYMBOL res 320 784 R90 WINDOW 0 -46 56 VBottom 0 WINDOW 3 -38 58 VTop 0 SYMATTR InstName R9 SYMATTR Value 1000 SYMBOL nmos 832 608 R0 SYMATTR InstName M2 SYMATTR Value Si4470DY SYMBOL voltage 32 368 R0 WINDOW 3 36 73 Left 0 WINDOW 123 0 0 Left 0 WINDOW 39 0 0 Left 0 WINDOW 0 36 44 Left 0 SYMATTR Value 10 SYMATTR InstName V1 SYMBOL res 384 368 R0 SYMATTR InstName R2 SYMATTR Value 10k SYMBOL res 656 416 R90 WINDOW 0 -46 56 VBottom 0 WINDOW 3 -38 58 VTop 0 SYMATTR InstName R1 SYMATTR Value 50k SYMBOL npn 672 752 R0 WINDOW 0 72 17 Left 0 WINDOW 3 47 50 Left 0 SYMATTR InstName Q3 SYMATTR Value 2N5550 SYMBOL res 656 784 R90 WINDOW 0 65 58 VBottom 0 WINDOW 3 73 54 VTop 0 SYMATTR InstName R3 SYMATTR Value 50k SYMBOL res 720 544 R0 WINDOW 0 41 43 Left 0 WINDOW 3 34 74 Left 0 SYMATTR InstName R5 SYMATTR Value 1000 TEXT 54 984 Left 0 !.tran .01
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J

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

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J

So what, you smoke-blowing ass? The point is, he defined 15V/µs as "probably fast enough" and if the '445 won't get him there then it'll still be 15V/µs, as you very well know.
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J

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

That's actually not a serious threat here.

John

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M

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

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J

Nope, but what makes you think someone has to be drunk to flame you?
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M

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

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J

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

John

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W

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.

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J

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

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J

That was when you were nice; before Usenet took its toll on you.
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J

If the BTS660P\'s control pin isn\'t pulled down to 50V RTN how will it know to turn on? That is, shouldn\'t your drawing (BTW, I admire the doubly dashed lines for the heavy power. Very nice!) look like: . ___ BTS660P . 10-58 dc >============|___|======o===(O) . | | . D | . TTL >--+-----G _|_ LOAD . | S /_\\ . 10k | | . | | | . 50V RTN >======o=======o========o===(O) Nice chip, but with worst case Ton of 400µs, Toff of 110µs, slew rate ON of 2.2V/µs and slew rate OFF of 2.6V/µs it won\'t meet the OP\'s stated need for a slew rate of 15V/µs.
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