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

[Fred Bloggs]

Now, Michael, that doesn't look too painful, does it?

The OP should just use a relay...

 

[Winfield]

The OP should just use a relay...

Spoil sport!

 

[Michael]

The OP should just use a relay...

I don't think that would work for this application - it'll be subject
to 10s of gs of acceleration in all directions. I don't think relays
can handle that, but I guess there could be some special purpose ones
out there. Also, I'm vertically constrained - I have about 2x8x8cm for
this guy - and most relays (of course not all, but most) are pretty
tall.

-Michael

 

[Hammy]

I don't think that would work for this application - it'll be subject
to 10s of gs of acceleration in all directions. I don't think relays
can handle that, but I guess there could be some special purpose ones
out there. Also, I'm vertically constrained - I have about 2x8x8cm for
this guy - and most relays (of course not all, but most) are pretty
tall.

-Michael

An alternative to IR's photovoltaic driver is Vishays LH1262CAC .

http://www.vishay.com/docs/83802/83802.pdf

They also make a new one with higher Isc but I cant find it anywhere.
This one vo1263aa .

http://www.vishay.com/docs/84639/vo1263aa.pdf

The LH1262 is available at Newark in through hole, they did have over
900 SMD versions two days ago but somebody snapped them all up. I got
four just in time.

IR has some good literature here (you have to send away for it though)
https://ec.irf.com/v6/en/US/adirect/ir?cmd=eRegMotorDrive

Dioncs also makes them with pretty good drive current but again none
of the regular distributors seem to stock it.

 

[John Larkin]

Michael wrote:
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. [...]

There are many cheap, ready-made integrated solutions to exactly this
problem. It gets a bit more complicated if the high-side switch is "on" for
extended periods of time, but if it reliably goes "off" every now and then
there's no problem.

Since you're worried about the FET overheating I guess you have frequent
switching in mind.

Nope, not frequent switching! The application is that there are a
couple power sources, and each power source will be switched with a
FET. A logic circuit chooses which power source to use and turns on
that FET. The circuit will use that FET for probably 10-30 minutes and
then once it is drained it'll switch to a difference source. That was
one of the main issues I ran into when initially looking for a driver
for the FET. Now I'm alot more interested in seeing if it's possible
to roll my own driver.

John posted a hand drawing of a simple scheme using
two optoisolators (John likes ordinary optoisolators).

I like all sorts of optocouplers, dc to GHz, LED, laser, silicon,
GaAs. I like SSR's too, which could be used here for beefier drive.

Here's an ASCII drawing of John's drawing:

floating FET driver, JL Dec07
,---+----------,
| | | in
_____ | | A |/
---| +|--' | -> | |
| | _|_ |\v |--'
---|____-|--, --- C | |<-,
| | +---+----||--+
dc-dc | | | | |
converter | | B |/ / Rs |
| | -> | \ 1M |
| | |\v / |
| | | | |
'---+----------+---+--------+---- out


+5 --+-/\/\-+--------+
| _|_ A _|_ B
| \_/ -> \_/ ->
TTL | | |
-----|>o-----+---|>o--' dual
inverters optoisolator

A 160% CTR optoisolator like Fairchild's FOD617D can
provide 7.5mA (min) to charge or discharge a MOSFET's
gate, if the opto-s LEDs are driven by 5mA. A large-
die MOSFET like an IRF1407, rated at 75V and 92A,
http://www.irf.com/product-info/datasheets/data/irf1407pbf.pdf
with Rds(on) = 0.0078 ohms (min), has Ciss = 5600pF of
gate capacitance. In particular, it takes Qgd = 54nC
of charge to move the gate through the "Miller" region.
That's the potentially-dangerous period during which
the MOSFET is neither fully ON nor OFF, as it swings
the drain voltage from one extreme to the other. The
Miller capacitance is Crss = 190pF at 25V, which has
to be charged by our 7.5mA of gate current.

Using t = Q/i we get 54nC/7.5mA = 7.2us. During this
time the MOSFET can be dissipating what, Michael, up
to 30A * 55V = 1650 watts? Not to worry, this is a
big MOSFET, and it can handle massive heat for a short
time by absorbing it in its thermal mass. Looking at
fig 11, Effective Transient Thermal Impedance plots,
we read ZthJC = 0.008 C/W for 10us. This means that
we'll see only 1650 watts * 0.008 = 13C of junction-
temperature rise during this "dangerous" transition.

Michael later noted that the load is a motor, occasionally switched,
so the fet will be turning on into inductance. Piece of cake,
dissipation-wise.

So that issue, which you were worrying about, is fine.

Clearly one painful little problem in John's design is
the dc-dc converter.

Mouser has them starting around $6 or so. Given the fets, motors,
power supplies, all that, another 6 or 8 bucks is invisible in the
noise.

It's not a painful little problem. It's not a problem at all. Why
don't you be positive about suggestions, rather than combative and
defensive? Nobody posts fully-engineered designs here, well, except JF
maybe.


But, since you're only turning

the MOSFET on once every now and then, if we change
the standby pulldown resistor Rs to 10M, it's clear
that only a modest amount of power is required to turn
on the MOSFET, enough to charge the gate capacitance,
and run the 10M resistor thereafter. Well, capacitor
C can be a 0.1uF, etc., and provide that gate charge,
if we can find a way to charge C. My suggestion is to
use another kind of opto-isolator, an extra-ordinary
type, made by IR. John suggested a 9-volt battery,
which isn't a bad idea, although a bit brute-force.
Also, that's not a semiconductor, and is frowned upon.

Frowned upon by who?

John

 

[Michael]

Looking at the response-time specs of the FOD617D,
http://www.fairchildsemi.com/ds/FO/FOD617A.pdf
I do see one possible issue, namely the rail-rail
current through transistors A and B, as one turns
off and the other turns on. This can last several
us, and partially discharge C. If C = 0.1uF, and
we allow a 2-volt drop, then we can handle a 20mA
shoot-through current for 0.1*2/0.02 = 10us. OK,
that isn't too bad, but let's increase C to 0.22uF
to give us more safety margin. That's easier than
providing 20us turn-on delays, etc., in the TTL-
logic cmos-inverter network, which is a commonly-
employed solution to this issue.

Here's the modified new circuit.

floating MOSFET driver, JL, WH Dec07

,-----+----------,
_____|_ | | in, 75V max
+5 | + | | A |/ (o)
---| | | -> | |
| PVI | _|_ |\v |--'
,--| 1050 | --- C | |<-,
| | | | 0.22uF +---+----||--+
'--| | | | | | IRF1407
| | | B |/ / Rs |
,--| | | -> | \ 10M |
| |_____-_| | |\v / |
240 | | | | |
| '-----+----------+---+--------+---(o) out
gnd
470
+5 --+-/\/\-+--------+
| _|_ A _|_ B
| \_/ -> \_/ ->
TTL | | |
-----|>o-----+---|>o--' FOD617D (2)
inverters optoisolator

Yep, that doesn't look too painful.

Hi Winfield - I really like that IRF photovoltaic isolator chip. I saw
those a while back, but had completely forgotten about them.

Can you tell me, would this circuit be faster (or better in some way?)
than the circuit John Fields posted? For some odd reason it sits
better than me, I guess because using isolated parts always seems like
the easy/lazy way out . I guess I'm a bit of a masochist in that
way...

-Michael

 

[John Larkin]

The OP should just use a relay...

OK, keep the dc/dc converter, but drive the fet gate with a tiny spdt
relay. That ain't bad. Gate rise/fall times will be insane, so add a
series gate resistor.

John

 

[James Arthur]

Michael later noted that the load is a motor, occasionally switched,
so the fet will be turning on into inductance. Piece of cake,
dissipation-wise.





Mouser has them starting around $6 or so. Given the fets, motors,
power supplies, all that, another 6 or 8 bucks is invisible in the
noise.

It's not a painful little problem. It's not a problem at all. Why
don't you be positive about suggestions,

I didn't take "painful" as derogatory; I just understood Win's comment
to mean the DC-DC itself still needed designing, which would involve
some additional effort.

I like the photo ideas and dc-dc both. All photo & semi is sexy
somehow.

For more frequent switching rates than our OP needs, I was going to
suggest directly driving the big FET with a rectified fly-back pulse &
turn-off with an opto or some such might be fun.

+50v / 30A
---
o D1 |
+12V >--, ,--|>|--+----. |--'
)||( | | |<-. Q2
)||( ZD | |--+
| T1 |o 15v |
|/ | | |
--| Q1 '-------+---------+
|>. |
| .-.
--- | | Load
| |
'-'
|

(Pulse Q1 to load up T1, flyback pulse charges Q2 gate capacitance &
leaves it on. Discharge via opto--possibly darlington'd for extra
drive--not shown. Isolated, simple.)

Cheers,
James Arthur

 

[John Larkin]

Oops, piece'o'cake on the rising edge. Payback on the falling edge.

I didn't take "painful" as derogatory; I just understood Win's comment
to mean the DC-DC itself still needed designing, which would involve
some additional effort.

How would you like being criticized by the author of The Art Of
Electronics?

But in a situation like this, big-buck power parts and probably
one-off, it's easier to buy a dc/dc for $6 or so.

The dc/dc could *be* the gate driver, with a dirty trick for speedup
maybe.

I like the photo ideas and dc-dc both. All photo & semi is sexy
somehow.

For more frequent switching rates than our OP needs, I was going to
suggest directly driving the big FET with a rectified fly-back pulse &
turn-off with an opto or some such might be fun.

+50v / 30A
---
o D1 |
+12V >--, ,--|>|--+----. |--'
)||( | | |<-. Q2
)||( ZD | |--+
| T1 |o 15v |
|/ | | |
--| Q1 '-------+---------+
|>. |
| .-.
--- | | Load
| |
'-'
|

(Pulse Q1 to load up T1, flyback pulse charges Q2 gate capacitance &
leaves it on. Discharge via opto--possibly darlington'd for extra
drive--not shown. Isolated, simple.)

After the first flyback, you could just bang it now and then to keep
the gate charged.

A second transformer could turn on a smaller npn or fet to discharge
the gate, plus a resistor across the zener.

I've combined transformers (for high current) with a wimpy pv isolator
(for dc).

John

 

[Jim Thompson]

Oops, piece'o'cake on the rising edge. Payback on the falling edge.



How would you like being criticized by the author of The Art Of
Electronics?

But in a situation like this, big-buck power parts and probably
one-off, it's easier to buy a dc/dc for $6 or so.

The dc/dc could *be* the gate driver, with a dirty trick for speedup
maybe.



After the first flyback, you could just bang it now and then to keep
the gate charged.

A second transformer could turn on a smaller npn or fet to discharge
the gate, plus a resistor across the zener.

I've combined transformers (for high current) with a wimpy pv isolator
(for dc).

John

On the Hubble Telescope platform support thingy I did years ago, I
used transformers with a DC-restore circuit to drive the HexFETs yet
allow PWMing.

Likewise on the satellite launch spinner for Honeywell Satellite
Systems Division.

...Jim Thompson

 

[John Larkin]

On the Hubble Telescope platform support thingy I did years ago, I
used transformers with a DC-restore circuit to drive the HexFETs yet
allow PWMing.

Likewise on the satellite launch spinner for Honeywell Satellite
Systems Division.

...Jim Thompson

I have a great book, The Hubble Wars by Chaisson. Whan they couldn't
get it to focus, they had excuses then meetings and then
investigations and then screaming. At one meeting, when they finally
figured out that the mirror was ground wrong, one of the optics
designers stepped out into the corridor and vomited.

An amateur telescope maker, with a candle and a straight-edge, could
have spotted it.


John

 

[Winfield Hill]

I like all sorts of optocouplers, dc to GHz, LED, laser, silicon,
GaAs. I like SSR's too, which could be used here for beefier drive.

I stand corrected, I should have said "John likes
optoisolators" I was thining of the many sketches
you've posted using them for various unusual things,
like HV amplifiers. Do you remember those?

Michael later noted that the load is a motor, occasionally
switched, so the fet will be turning on into inductance.
Piece of cake, dissipation-wise.

Yes, although turn-off protection may be in order.

Mouser has them starting around $6 or so. Given the fets,
motors, power supplies, all that, another 6 or 8 bucks is
invisible in the noise.
Yes.

It's not a painful little problem. It's not a problem at all.
Why don't you be positive about suggestions, rather than
combative and defensive?

Hah, now just who's being combative and defensive?

Nobody posts fully-engineered designs here, well,
except JF maybe.

I often do, because it's fun to work things out, and
more often than not something changes as one goes from
a nice-sounding concept to a circuit with specific part
values. You know that experience well, I'm sure.

I'm also more likely to work out the details and post
them if it's something I'm interested in for my own
purposes, and then the usenet archive becomes something
of a notebook. I'm continually surprised when I do a
Google search on some exotic topic, seeking information,
and right away up pops a post I made 7 years ago, or
whatever, with all kinds of detail worked out. I'm
surprised first of all because I'd forgotten working on
the topic, and second sometimes because the post looks
to be much better than anything I could have written.
The implications of that are a little bit disturbing.

Frowned upon by who?

That, clearly I hope, was a joke. If a subtle, wry
joke is made, isn't it wrecked by adding a smily?

However, the IR and Vishay opto-voltage parts are
truly cool and useful, especially for switching huge
MOSFETs that aren't available in the small SSR IC
packages, but they do suffer from excessively-slow
turn on and turn off, e.g., 5uA into 2000pF, etc.
Combining one of those with your opto-isolator idea
was appealing to me, so I pursued it in detail. I
would have thought this was paying you a compliment.
But if you don't see it that way, I take it back,
by all means, and ask for your forgiveness.

 

[Michael]

Looking at the response-time specs of the FOD617D,
http://www.fairchildsemi.com/ds/FO/FOD617A.pdf
I do see one possible issue, namely the rail-rail
current through transistors A and B, as one turns
off and the other turns on. This can last several
us, and partially discharge C. If C = 0.1uF, and
we allow a 2-volt drop, then we can handle a 20mA
shoot-through current for 0.1*2/0.02 = 10us. OK,
that isn't too bad, but let's increase C to 0.22uF
to give us more safety margin. That's easier than
providing 20us turn-on delays, etc., in the TTL-
logic cmos-inverter network, which is a commonly-
employed solution to this issue.

Here's the modified new circuit.

floating MOSFET driver, JL, WH Dec07

,-----+----------,
_____|_ | | in, 75V max
+5 | + | | A |/ (o)
---| | | -> | |
| PVI | _|_ |\v |--'
,--| 1050 | --- C | |<-,
| | | | 0.22uF +---+----||--+
'--| | | | | | IRF1407
| | | B |/ / Rs |
,--| | | -> | \ 10M |
| |_____-_| | |\v / |
240 | | | | |
| '-----+----------+---+--------+---(o) out
gnd
470
+5 --+-/\/\-+--------+
| _|_ A _|_ B
| \_/ -> \_/ ->
TTL | | |
-----|>o-----+---|>o--' FOD617D (2)
inverters optoisolator

Yep, that doesn't look too painful.

Hi Winfield - a couple more questions for you: What is the purpose of
the resistor across the bottom optoisolator? Also, you mentioned that
the IRF1407 is a large die mosfet. How can you tell this? The reason I
ask is that I am interested in using a FET with a smaller on
resistance, but I want to make sure I know what to be looking for.
Also, why not only use one inverter and drive the B optoisolator
directly? I'm assuming it's because you want the A optoisolator to
switch before the B optoisolator, but why?

By the way - wonderful weather today, right? I'm very tempted to just
pull up a couch and sleep at work tonight.

Thanks so much for all your (and everybody else's) help, I really
appreciate it.

-Michael

 

[Winfield Hill]

Hi Winfield - a couple more questions for you: What is the purpose of
the resistor across the bottom optoisolator? Also, you mentioned that
the IRF1407 is a large die mosfet. How can you tell this? The reason I
ask is that I am interested in using a FET with a smaller on
resistance, but I want to make sure I know what to be looking for.

Well, that's John's part, but I can answer for him.
That's just to insure the MOSFET is off, and stays
off, when the 5V logic supply is off.

Also, why not only use one inverter and drive the B optoisolator
directly? I'm assuming it's because you want the A optoisolator to
switch before the B optoisolator, but why?

It's necessary to supply a reasonably-high current
both to turn ON and to turn OFF the MOSFET's gate.
So you have to pull both directions. In John's
idea, either A or B is always on.

By the way - wonderful weather today, right? I'm very tempted to
just pull up a couch and sleep at work tonight.

I may not have a choice. The cars are stuck on
Land Blvd outside, and I wouldn't be able to even
get out onto the street. People are out of their
cars, cleaning snow off their windows, etc., and
it's coming down hard. I was going to leave over
an hour ago, but it's been stalled like that the
whole time.

Thanks so much for all your (and everybody else's) help, I really
appreciate it.

Well, we're enjoying ourselves. You're getting
a half dozen good ways to do it, so have fun.
But if this is for a serious real-world high-
production project, and not just a one-off, the
maybe you'd better get some real help involved
in the decision making, instead of all of us
usenet playing-around wannabies!

Oops, the cars are moving again outside, so I'd
better get going.

 

[John Larkin]

I stand corrected, I should have said "John likes
optoisolators" I was thining of the many sketches
you've posted using them for various unusual things,
like HV amplifiers. Do you remember those?


Yes, although turn-off protection may be in order.


Hah, now just who's being combative and defensive?


I often do, because it's fun to work things out, and
more often than not something changes as one goes from
a nice-sounding concept to a circuit with specific part
values. You know that experience well, I'm sure.

I'm also more likely to work out the details and post
them if it's something I'm interested in for my own
purposes, and then the usenet archive becomes something
of a notebook. I'm continually surprised when I do a
Google search on some exotic topic, seeking information,
and right away up pops a post I made 7 years ago, or
whatever, with all kinds of detail worked out. I'm
surprised first of all because I'd forgotten working on
the topic, and second sometimes because the post looks
to be much better than anything I could have written.
The implications of that are a little bit disturbing.


That, clearly I hope, was a joke. If a subtle, wry
joke is made, isn't it wrecked by adding a smily?

However, the IR and Vishay opto-voltage parts are
truly cool and useful, especially for switching huge
MOSFETs that aren't available in the small SSR IC
packages, but they do suffer from excessively-slow
turn on and turn off, e.g., 5uA into 2000pF, etc.
Combining one of those with your opto-isolator idea
was appealing to me, so I pursued it in detail. I
would have thought this was paying you a compliment.
But if you don't see it that way, I take it back,
by all means, and ask for your forgiveness.

Nobody will forgive you until 3e is in the bookstores.

John

 

[Michael]

Nobody will forgive you until 3e is in the bookstores.

John

Is there one being worked on? I hadn't heard of it, though it would
seem that it is due!

-Michael

 

[Michael]

Well, that's John's part, but I can answer for him.
That's just to insure the MOSFET is off, and stays
off, when the 5V logic supply is off.

Ah - that makes good sense.

It's necessary to supply a reasonably-high current
both to turn ON and to turn OFF the MOSFET's gate.
So you have to pull both directions. In John's
idea, either A or B is always on.

I understand the necessity of needing to be able to quickly sync or
source current into our out of the gate, but it looks like the circuit
has a delay added in between A and B in the form of the inverter. I
mean, couldn't the switching circuit look like below?:

470
+5 --+-----+-/\/\-+
_|_ B | _|_ A
\_/ -> | \_/ ->
TTL | | |
------+----|>o-----+ FOD617D (2)
inverter optoisolator

With the way you have it drawn, it seems to me that there must be some
reason to switch A before B, but I'm not sure what that would be,
especially considering that the delay between switches would be so
small.

I may not have a choice. The cars are stuck on
Land Blvd outside, and I wouldn't be able to even
get out onto the street. People are out of their
cars, cleaning snow off their windows, etc., and
it's coming down hard. I was going to leave over
an hour ago, but it's been stalled like that the
whole time.


Well, we're enjoying ourselves. You're getting
a half dozen good ways to do it, so have fun.
But if this is for a serious real-world high-
production project, and not just a one-off, the
maybe you'd better get some real help involved
in the decision making, instead of all of us
usenet playing-around wannabies!

It's not for a production project - just a research project, so no
fear

Oops, the cars are moving again outside, so I'd
better get going.

Lucky! I'm still stuck.

-Michael

 

[Joel Koltner]

Is there one being worked on? I hadn't heard of it, though it would
seem that it is due!

I seem to recall I first heard that Win was already "working on it" sometime
shortly after I'd purchased the 2nd edition as an undergraduate in the
not-quite-mid-'90s...

I get the impression that Win had a lot more spare time when he and Paul were
working on the 1st and 2nd versions than he does today... victim of his own
success, I guess!

 

[Jure Newsgroups]

I stand corrected, I should have said "John likes
optoisolators" I was thining of the many sketches
you've posted using them for various unusual things,
like HV amplifiers. Do you remember those?


Yes, although turn-off protection may be in order.


Hah, now just who's being combative and defensive?


I often do, because it's fun to work things out, and
more often than not something changes as one goes from
a nice-sounding concept to a circuit with specific part
values. You know that experience well, I'm sure.

I'm also more likely to work out the details and post
them if it's something I'm interested in for my own
purposes, and then the usenet archive becomes something
of a notebook. I'm continually surprised when I do a
Google search on some exotic topic, seeking information,
and right away up pops a post I made 7 years ago, or
whatever, with all kinds of detail worked out. I'm
surprised first of all because I'd forgotten working on
the topic, and second sometimes because the post looks
to be much better than anything I could have written.
The implications of that are a little bit disturbing.


That, clearly I hope, was a joke. If a subtle, wry
joke is made, isn't it wrecked by adding a smily?

However, the IR and Vishay opto-voltage parts are
truly cool and useful, especially for switching huge
MOSFETs that aren't available in the small SSR IC
packages, but they do suffer from excessively-slow
turn on and turn off, e.g., 5uA into 2000pF, etc.
Combining one of those with your opto-isolator idea
was appealing to me, so I pursued it in detail. I
would have thought this was paying you a compliment.
But if you don't see it that way, I take it back,
by all means, and ask for your forgiveness.

and here is another idea for application of the
IR and Vishay opto-voltage parts :-

Reverse Bias supply for a detector diode !

Jure Z.

 

[Winfield]

Then you might want to look into the HIP4081 and friends.

robert

Good idea. Has anybody verified their internal
high-side charge pump?