High Side MOSFET Driver

[kyujin]

I'm trying to drive a 10ohm load with 20Volts using a MOSFET.
I want the load to be grounded, so My Vs would be around 20volts, so I
need my gate voltage to be at least 25Volts or more.
So I got a ISL6801 High side mosfet driver, hooked up everything as
depicted in the datasheet.
http://www.intersil.com/data/fn/fn9087.pdf
But I'm not getting a output voltage that is 25Volts, instead below
20Volts.
What am I doing wrong?
I hooked up a function generator with 5Vpp with frequency changing from
1Hz to 1kHz.
I'm using Vcc of 5Volts, which is connected to VB via diode. 22uF
capacitor between VB and VS.

i'm not sure if I'm not understanding the circuit very well, or if
there is something that I didn't do.
Help!

 

[Tom Bruhns]

It's a "bootstrap" part. That means that when the output is low, a
capacitor is charged, and when it switches high, the "bottom" of the
capacitor is carried up...

If you try to hold the output high for a long time, the capacitor will
discharge (e.g., through leakage of the diode used to charge the
capacitor. The part is intended for use in something like a
half-bridge circuit that switches frequently--fast enough to keep the
cap charged.

Does that help?

Cheers,
Tom

 

[Johnny Boy]

It's a "bootstrap" part. That means that when the output is low, a
capacitor is charged, and when it switches high, the "bottom" of the
capacitor is carried up...

If you try to hold the output high for a long time, the capacitor will
discharge (e.g., through leakage of the diode used to charge the
capacitor. The part is intended for use in something like a
half-bridge circuit that switches frequently--fast enough to keep the
cap charged.

Does that help?

Cheers,
Tom

Tom is right.
I haven't checked out the linked datasheet, but if you want to keep the
output high for any length of time, you'll need to use a combination of a
bootstrap capacitor and something along the lines of a charge-pump or other
supply that goes about 10V above the source voltage. The bootstrap is needed
for quickly charging the MOSFET's gate capacitance and the charge-pump will
then supply enough current to keep the output high.
As a reference/learning tool, check out the datasheet for the
Intersil/Harris HIP4080/HIP4081 or similar. Even if you don't need these
chips, the datasheets are very informative on this subject.

.... Johnny

 

[kell]

I'm trying to drive a 10ohm load with 20Volts using a MOSFET.
I want the load to be grounded, so My Vs would be around 20volts, so I
need my gate voltage to be at least 25Volts or more.
So I got a ISL6801 High side mosfet driver, hooked up everything as
depicted in the datasheet.
http://www.intersil.com/data/fn/fn9087.pdf
But I'm not getting a output voltage that is 25Volts, instead below
20Volts.
What am I doing wrong?
I hooked up a function generator with 5Vpp with frequency changing from
1Hz to 1kHz.
I'm using Vcc of 5Volts, which is connected to VB via diode. 22uF
capacitor between VB and VS.

i'm not sure if I'm not understanding the circuit very well, or if
there is something that I didn't do.
Help!

Look at a couple of datasheets for simple charge pumps like the 8-pin
TC7662. Somewhere you'll see a diagram of a voltage doubler circuit.

 

[John B]

I'm trying to drive a 10ohm load with 20Volts using a MOSFET.
I want the load to be grounded, so My Vs would be around 20volts, so I
need my gate voltage to be at least 25Volts or more.
So I got a ISL6801 High side mosfet driver, hooked up everything as
depicted in the datasheet.
http://www.intersil.com/data/fn/fn9087.pdf
But I'm not getting a output voltage that is 25Volts, instead below
20Volts.
What am I doing wrong?
I hooked up a function generator with 5Vpp with frequency changing
from 1Hz to 1kHz.
I'm using Vcc of 5Volts, which is connected to VB via diode. 22uF
capacitor between VB and VS.

i'm not sure if I'm not understanding the circuit very well, or if
there is something that I didn't do.
Help!

Why not use a P-channel FET, such as ISTH50P085 which you can find here:

http://www.ixys.com/

 

[Eeyore]

I'm trying to drive a 10ohm load with 20Volts using a MOSFET.
I want the load to be grounded, so My Vs would be around 20volts, so I
need my gate voltage to be at least 25Volts or more.
So I got a ISL6801 High side mosfet driver, hooked up everything as
depicted in the datasheet.
http://www.intersil.com/data/fn/fn9087.pdf
But I'm not getting a output voltage that is 25Volts, instead below
20Volts.

That's what you *will* get with a 20V supply !

The 25V is just for the gate.

Graham

 

[Eeyore]

sorry, that component is design to be pulsed or gated for a short
time. what your seeing is the effects of Fet biasing after the
cap internally has charged/discharged.
have you thought of about a Pmos or reconfiging your app ?

The application note he refers to shows an optional high-side supply.

The OP needs to be more explicit about what he's doing.

Graham

 

[Jamie]

I'm trying to drive a 10ohm load with 20Volts using a MOSFET.
I want the load to be grounded, so My Vs would be around 20volts, so I
need my gate voltage to be at least 25Volts or more.
So I got a ISL6801 High side mosfet driver, hooked up everything as
depicted in the datasheet.
http://www.intersil.com/data/fn/fn9087.pdf
But I'm not getting a output voltage that is 25Volts, instead below
20Volts.
What am I doing wrong?
I hooked up a function generator with 5Vpp with frequency changing from
1Hz to 1kHz.
I'm using Vcc of 5Volts, which is connected to VB via diode. 22uF
capacitor between VB and VS.

i'm not sure if I'm not understanding the circuit very well, or if
there is something that I didn't do.
Help!

sorry, that component is design to be pulsed or gated for a short
time. what your seeing is the effects of Fet biasing after the
cap internally has charged/discharged.
have you thought of about a Pmos or reconfiging your app ?

 

[Tom Bruhns]

Why not use a P-channel FET, such as ISTH50P085 which you can find here:

http://www.ixys.com/

Maybe that should be IXTH50P085?

I'd be cautious about exceeding the GS max voltage, with a 20V supply;
and though it's not difficult to do a level shift (e.g. with an NPN
with base tied to the logic supply and emitter driven from the logic
source through a resistor), it's not quite as straightforward as "high
side MOSFET gate driver with logic-level input," at least if you're not
accustomed to thinking in terms of transistor amplifier circuits.
Depends too on how fast it must act.

Another alternative is an optoisolator of the sort that has
photovoltaic output and can drive a low current to a few volts. By
itself, that's not going to drive a FET gate very fast, but in
conjunction with a storage cap and something like the "bootstrap high
side driver" (now fed high-side gate drive power from the opto output),
it should work. It's a takeoff on the theme mentioned by other posters
of a simple switching supply to get the gate voltage.

Cheers,
Tom

 

[[email protected]]

I'm trying to drive a 10ohm load with 20Volts using a MOSFET.
I want the load to be grounded, so My Vs would be around 20volts, so I
need my gate voltage to be at least 25Volts or more.
So I got a ISL6801 High side mosfet driver, hooked up everything as
depicted in the datasheet.
http://www.intersil.com/data/fn/fn9087.pdf
But I'm not getting a output voltage that is 25Volts, instead below
20Volts.
What am I doing wrong?
I hooked up a function generator with 5Vpp with frequency changing from
1Hz to 1kHz.
I'm using Vcc of 5Volts, which is connected to VB via diode. 22uF
capacitor between VB and VS.

i'm not sure if I'm not understanding the circuit very well, or if
there is something that I didn't do.
Help!

A pnp transistor is the simple way.

 

[Johnny Boy]

A pnp transistor is the simple way.

In this application, since the current is fairly low, a P-channel MOSFET
or PNP transistor isn't too bad an idea.
Usually, as high-side drivers, P-channel MOSFETs are avoided because the
on resistance, (Rds on) is higher than in an N-channel device, resulting in
a higher power dissipation due to the higher voltage drop across the device.
Also, in high current apps, transistors tend to have higher dissipation than
MOSFETs and so are usually avoided. In this case, even if you used a
P-channel MOSFET with an 'Rds on' of 0.1 ohm, you'd only drop about 100mV,
resulting in a device power dissipation of 200mW. You shouldn't even need a
heatsink for this. (I'm assuming that your load is resistive and not
inductive) What type of load are you driving?

.... Johnny

 

[Johnny Boy]

In this application, since the current is fairly low, a P-channel MOSFET
or PNP transistor isn't too bad an idea.
Usually, as high-side drivers, P-channel MOSFETs are avoided because the
on resistance, (Rds on) is higher than in an N-channel device, resulting in
a higher power dissipation due to the higher voltage drop across the device.
Also, in high current apps, transistors tend to have higher dissipation than
MOSFETs and so are usually avoided. In this case, even if you used a
P-channel MOSFET with an 'Rds on' of 0.1 ohm, you'd only drop about 100mV,

*** The above should be 200mV voltage drop, 2A * 0.1 ohms.

resulting in a device power dissipation of 200mW. You shouldn't even need

a

*** This should have read 400mW dissipation, 200mV * 2A - sorry about that.

 

[kyujin]

Thanks for all the replies.

I need to use N-channel mosfet, because actual application uses 9Amps
of current, and 60volts.
I was using 20Volts just to test to see how the high side mosfet driver
works.

I have 8 loads, which I will be multiplexing with 80ms period, meaning
each load gets turned on for 10ms, every 80ms. (about 12Hz) I'm using
PIC to do this.

My Question:

1. Where do I connect the charge pump? Do I connect it to the bootstrap
power supply, which is connects to the diode?
All the diagrams I saw seemed like I should connect the diode to the
Vcc of the gate driver, which is 5Volts.

2. What do I use for the charge pump? Does the charge pump need the
voltage that I will be driving the gate with? So does this mean I need
an extra power supply with 70volts?

So I basically need to drive the gate to 70volts using a 5 volt output
from PIC.


My actual application is more complicated than what I've described.
For people who are interested, here it is.
------------------------------------------------------------------------------------------------
My actual application uses 9 Amps to drive multiple resistive
loads(artificial muscle actuator) that are connected in series via
multiple of IRF8901 MOSFETS, which are surface mount N-mosfets.
I have 8 of these, that I will be driving at the same time. So I use
PIC to program
(Please don't ask why I need to connect the loads with switches.
Believe me, I need to.)
The total load requires about 60Volts to get 9Amps.
So the actual problem requires different gate voltages for each MOSFETS
that drive the loads, since it cannot be over 20volts.
For this, I decided to use a zener diode at every step where I need to
get Vgs between 10Volts and 20Volts. I'm using zener diode because of
space limitations. I don't have room to use high side mosfet driver for
each MOSFETs.

60volts
|
Load
|
MOSFET gate--- zener
| |
| --------------------- |
|
Load
|
MOSFET gate--- zener
| |
| --------------------- |
Load

 

[Tom Bruhns]

If all the loads are in series as you have shown, just WHY do you need
multiple MOSFETs? If you think you are going to have isolation between
the loads when the MOSFETs are off, you are most likely fooling
yourself because of the parasitic diode between source and drain in the
MOSFETs.

If you indeed are turning each series string of loads (and there are
eight of them) on for 10 milliseconds out of each 80 milliseconds, and
if the loads pull down to "ground" when off, then the bootstrapped
driver you first posted about should work just fine. You only need to
use a storage capacitor large enough to hold its charge against
whatever leakage currents you have, for only 10 milliseconds. I gather
the supply current for the output stage is on the order of a milliamp.
Since I=C*dv/dt, to limit the voltage sag in 10 milliseconds to, say, 1
volt, and assuming 1mA, you'd need about 10uF of capacitance. That's
the capacitor between Vb and Vs in the data sheet diagram for the part
you suggested.

With that arrangement, you should not need an isolated supply; the
little SO-8 driver plus the bootstrap capacitor and the diode from the
bootstrap supply (presumably about 10 volts) should be enough, and you
can drive the input nicely from logic levels. That keeps everything
simple, no?

Just DO NOT expect that it will hold the FET ON for much longer than 10
milliseconds if you use 10uF for the bootstrap capacitor. Yeah, it may
hold it for 100 milliseconds, but not for seconds. For testing you
could use an even larger bootstrap capacitor (which should be fine with
a voltage rating twice the bootstrap supply voltage--twice is just a
safety margin), and have longer "hold" times.

Cheers,
Tom

 

[Johnny Boy]

Thanks for all the replies.

I need to use N-channel mosfet, because actual application uses 9Amps
of current, and 60volts.
I was using 20Volts just to test to see how the high side mosfet driver
works.

I have 8 loads, which I will be multiplexing with 80ms period, meaning
each load gets turned on for 10ms, every 80ms. (about 12Hz) I'm using
PIC to do this.

My Question:

1. Where do I connect the charge pump? Do I connect it to the bootstrap
power supply, which is connects to the diode?
All the diagrams I saw seemed like I should connect the diode to the
Vcc of the gate driver, which is 5Volts.

***The "Typical Application Block Diagram" that I saw in the ISL6801
datasheet does not connect the bootstrap supply to Vcc, it is a separate
supply here, connected to pin 8, and the diode is connected to that. Take
another look at the datasheet.
This is probably irrelevant anyway, since, as Tom points out, if you're
only turning the load on for 10mS, you can probably do without a charge-pump
type supply.

2. What do I use for the charge pump? Does the charge pump need the
voltage that I will be driving the gate with? So does this mean I need
an extra power supply with 70volts?

So I basically need to drive the gate to 70volts using a 5 volt output
from PIC.


My actual application is more complicated than what I've described.
For people who are interested, here it is.
-------------------------------------------------------------------------- ----------------------
My actual application uses 9 Amps to drive multiple resistive
loads(artificial muscle actuator) that are connected in series via
multiple of IRF8901 MOSFETS, which are surface mount N-mosfets.
I have 8 of these, that I will be driving at the same time. So I use
PIC to program
(Please don't ask why I need to connect the loads with switches.
Believe me, I need to.)
The total load requires about 60Volts to get 9Amps.
So the actual problem requires different gate voltages for each MOSFETS
that drive the loads, since it cannot be over 20volts.
For this, I decided to use a zener diode at every step where I need to
get Vgs between 10Volts and 20Volts. I'm using zener diode because of
space limitations. I don't have room to use high side mosfet driver for
each MOSFETs.

60volts
|
Load
|
MOSFET gate--- zener
| |
| --------------------- |
|
Load
|
MOSFET gate--- zener
| |
| --------------------- |
Load
.
.
.
.

*** I must confess that I didn't look at your diagram until now. Are these
really connected as depicted, with no ground path for each? A clearer
diagram would help.

 

[jasen]

I'm trying to drive a 10ohm load with 20Volts using a MOSFET.
I want the load to be grounded, so My Vs would be around 20volts, so I
need my gate voltage to be at least 25Volts or more.

that, or a P-channel mosfet.

 

[kyujin]

The problem was that I was not using large enough capacitor.
I started using 22uF and it's working fine now.
Thank you for all the help.