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

[Michael]

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 (http://www.linear.com/pc/productDetail.jsp?
navId=H0,C1,C1003,C1042,C1031,C1061,P1316) 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 (http://focus.ti.com/
docs/prod/folders/print/opa445.html), 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

 

[John Fields]

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 (http://www.linear.com/pc/productDetail.jsp?
navId=H0,C1,C1003,C1042,C1031,C1061,P1316) 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 (http://focus.ti.com/
docs/prod/folders/print/opa445.html), 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?

---
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-------------------+-------+------+

 

[John Fields]

---
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-------------------+-------+------+

 

[Winfield]

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 (http://www.linear.com/pc/productDetail.jsp?
navId=H0,C1,C1003,C1042,C1031,C1061,P1316) 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 (http://focus.ti.com/
docs/prod/folders/print/opa445.html), 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?

---
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-------------------+-------+------+

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 |
.. | +---|<|---o=o===(O)
.. R1 | |
.. | R3 _|_ LOAD
.. | | /_\
.. GND >----------+-----' |
.. 50V RTN >=====================================o===(O)

 

[John Fields]

John Fields wrote:

---
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-------------------+-------+------+

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 |
. | +---|<|---o=o===(O)
. R1 | |
. | R3 _|_ LOAD
. | | /_\
. GND >----------+-----' |
. 50V RTN >=====================================o===(O)

 

[John Larkin]

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 (http://www.linear.com/pc/productDetail.jsp?
navId=H0,C1,C1003,C1042,C1031,C1061,P1316) 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 (http://focus.ti.com/
docs/prod/folders/print/opa445.html), 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

Please define "fast"

John

 

[Jim Thompson]

[snip]

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

Please define "fast"

John

As opposed to half-fast?

...Jim Thompson

 

[John Larkin]

[snip]

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

Please define "fast"

John

As opposed to half-fast?

...Jim Thompson

milli/micro/nano would be a start.

John

 

[John Fields]

Please define "fast"

---
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

 

[John Larkin]

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

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

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

John

 

[John Fields]

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

---
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.

 

[John Larkin]

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

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.

That's actually not a serious threat here.


John

 

[Michael]

.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-------------------+-------+------+

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 |
. | +---|<|---o=o===(O)
. R1 | |
. | R3 _|_ LOAD
. | | /_\
. GND >----------+-----' |
. 50V RTN >=====================================o===(O)

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

 

[John Fields]

On Wed, 12 Dec 2007 14:38:33 -0600, John Fields


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

 

[Michael]

Please define "fast"

John

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

 

[John Larkin]

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

John

 

[Winfield Hill]

John Fields wrote:

---
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-------------------+-------+------+

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 |
. | +---|<|---o=o===(O)
. R1 | |
. | R3 _|_ LOAD
. | | /_\
. GND >----------+-----' |
. 50V RTN >=====================================o===(O)

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.

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.

 

[John Larkin]

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

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:

http://img105.imageshack.us/my.php?image=fetdriverwc6.jpg

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

John

 

[John Fields]

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

 

[John Fields]

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)

---
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.