Simple isolated mosfet driver

[Jon Slaughter]

Is it possible to drive the gate of a high side n-ch mosfet using an
optocoupler to isolate and float the gate w.r.t to the source and still be
effective?

I want to continuously control the gate of a high side n-ch mosfet to prove
a variable resistance for a high voltage load.


G = Gate, S = Source, D = Drain
OE = Opto Emitter, OC = Opto Collector

Vcc--D


OC--R2--D
OE--G

G--R1--S
|
Load
|
Gnd


The idea is simple, a resistor(R1) connects the gate to the source. When no
current is flowing the gate is then held at the same voltage as the source
and the mosfet is turned off.

An optocoupler is added to control current through that gate/source resistor
which will "bias" the gate relative to the source and allow turning on the
mosfet. A simple simulation shows this works but I'm not use how useful it
is. Some protection mechanism for the opto would be needed as well as
driving the gate too high.

The isolation is necessary because of the high voltage used. I'm not worried
about the "speed" as this isn't used for switching.

 

[Jamie]

Is it possible to drive the gate of a high side n-ch mosfet using an
optocoupler to isolate and float the gate w.r.t to the source and still
be effective?

I want to continuously control the gate of a high side n-ch mosfet to
prove a variable resistance for a high voltage load.


G = Gate, S = Source, D = Drain
OE = Opto Emitter, OC = Opto Collector

Vcc--D


OC--R2--D
OE--G

G--R1--S
|
Load
|
Gnd


The idea is simple, a resistor(R1) connects the gate to the source. When
no current is flowing the gate is then held at the same voltage as the
source and the mosfet is turned off.

An optocoupler is added to control current through that gate/source
resistor which will "bias" the gate relative to the source and allow
turning on the mosfet. A simple simulation shows this works but I'm not
use how useful it is. Some protection mechanism for the opto would be
needed as well as driving the gate too high.

The isolation is necessary because of the high voltage used. I'm not
worried about the "speed" as this isn't used for switching.

It will work as long as you have some head room to bias the gate.

Also, you should use a limiting zener diode from the Source to the
gate to prevent over voltage for Vgs..

For example, if your fet has a full turn on of 10v's above the source,
you need to have at least that much head room from the supply that is on
the DRAIN side.

In all, it means your drain supply must have 10 or more volts then
you'll ever expect on the source. You also need to factor in loss via
other circuits like the opto-isolator. etc..

You really should look at a charged pumped isolated high side
driver.. It would save you a lot of work..

You could also consider a P channel on the high side

 

[Hammy]

Is it possible to drive the gate of a high side n-ch mosfet using an
optocoupler to isolate and float the gate w.r.t to the source and still be
effective?

I want to continuously control the gate of a high side n-ch mosfet to prove
a variable resistance for a high voltage load.


G = Gate, S = Source, D = Drain
OE = Opto Emitter, OC = Opto Collector

Vcc--D


OC--R2--D
OE--G

G--R1--S
|
Load
|
Gnd


The idea is simple, a resistor(R1) connects the gate to the source. When no
current is flowing the gate is then held at the same voltage as the source
and the mosfet is turned off.

An optocoupler is added to control current through that gate/source resistor
which will "bias" the gate relative to the source and allow turning on the
mosfet. A simple simulation shows this works but I'm not use how useful it
is. Some protection mechanism for the opto would be needed as well as
driving the gate too high.

The isolation is necessary because of the high voltage used. I'm not worried
about the "speed" as this isn't used for switching.

Simplest soloution if you dont need high speed switching is something
like this.

VO1263

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

http://canada.newark.com/vishay-semiconductor/vo1263ab/solid-state-relay/dp/93K1109

 

[Jon Slaughter]

Simplest soloution if you dont need high speed switching is something
like this.

VO1263

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

http://canada.newark.com/vishay-semiconductor/vo1263ab/solid-state-relay/dp/93K1109

Thanks, That might do the trick. A bit expensive though for something that
seems relatively simple. The method they use with the fet for discharging
the gate might be all I need with any old optocoupler method.

 

[Jon Slaughter]

It will work as long as you have some head room to bias the gate.

Also, you should use a limiting zener diode from the Source to the
gate to prevent over voltage for Vgs..

For example, if your fet has a full turn on of 10v's above the
source, you need to have at least that much head room from the supply
that is on the DRAIN side.

In all, it means your drain supply must have 10 or more volts then
you'll ever expect on the source. You also need to factor in loss via
other circuits like the opto-isolator. etc..

You really should look at a charged pumped isolated high side
driver.. It would save you a lot of work..

You have any recomendations? Only ones I have seen are for low voltage or
for switching only(rather than linear).

 

[Jon Slaughter]

Is it possible to drive the gate of a high side n-ch mosfet using an
optocoupler to isolate and float the gate w.r.t to the source and
still be effective?

I want to continuously control the gate of a high side n-ch mosfet
to prove a variable resistance for a high voltage load.


G = Gate, S = Source, D = Drain
OE = Opto Emitter, OC = Opto Collector

Vcc--D


OC--R2--D
OE--G

G--R1--S
|
Load
|
Gnd


The idea is simple, a resistor(R1) connects the gate to the source.
When no current is flowing the gate is then held at the same voltage
as the source and the mosfet is turned off.

An optocoupler is added to control current through that gate/source
resistor which will "bias" the gate relative to the source and allow
turning on the mosfet. A simple simulation shows this works but I'm
not use how useful it is. Some protection mechanism for the opto
would be needed as well as driving the gate too high.

The isolation is necessary because of the high voltage used. I'm not
worried about the "speed" as this isn't used for switching.

If I understand your circuit, it puts a lot of voltage across the opto
output. A shunt circuit wouldn't. But then there *are* optos with 400
volt phototransistors. [1]

A photovoltaic coupler would be nice, as it can drive the gate
directly and doesn't need a high-side supply. They tend to be wimpy,
10s of uA output, so they would be pretty slow. An advantage is that
there's no sneak current path to the load.


Or do this maybe:


V+-------+---------+
| |
| |
Q |
| |
| d
+--------g
| s
c |
b |
e |
| |
| |
+---------+
|
|
|
load

where Q is a Supertex depletion fet current limiter, cbe is the opto
phototransistor, and there should be a zener g-s which I'm too lazy to
draw. It does leak a litle current into the load.

My original idea was somewhat similar. The problem is that the initial state
is with the load recieving the full voltage. Hammy gave a datasheet where
they use a jfet to discharge the gate and keep it at 0V relative to the
source when no current. It should work well and also removes any need for
protection.

 

[Tim Williams]

http://www.voltagemultipliers.com/pdf/Opto-Diode 01_12_09.pdf

http://www.voltagemultipliers.com/pdf/OC100HG_11_05_09.pdf

It looks like a surface-conduction silicon tube thing maybe, which can
be a little leaky. This looks like fun for, well, *something*

Not quite, it says it's a diode... Vf = 12V or so.

How do they make those, anyway? Are they monolithic? Even with a
wide-assed intrinsic region (PIN structure), I don't know of 10kV being held
off by a single junction. Besides, such a junction would have to be so
thick that diffusion effects within would not be negligible.

I can still imagine them being made monolithically, but laterally, so you
basically make a stack of PN's, shorting every other so it makes an
always-on SCR.

Tim

 

[Hammy]

Thanks, That might do the trick. A bit expensive though for something that
seems relatively simple. The method they use with the fet for discharging
the gate might be all I need with any old optocoupler method.

Yes but it saves you from having to make an isolated supply.

Page 18 of this pdf shows how to use a HS driver for continuous on
time. It also shows some of IR's HS drivers.

http://www.irf.com/technical-info/appnotes/an-978.pdf

Some of Fairchild's HS drivers.

FAN7371 4A, FAN7382 HI/LOW , FAN7361