MOSFET drain-source voltage drop

B

Bob Engelhardt

Guest
I am using a MOSFET driver in a PWM circuit. I am switching 12v
(battery) with a Schmitt trigger oscillator on the gate. It's running
about 7kHz. The MOSFET is a IRF1404
www.irf.com/product-info/datasheets/data/irf1404.pdf

The oscillator works nicely and puts 12v on the MOSFET gate. But the
MOSFET drops 2v and I don't get full speed from the motor that I'm
driving. The MOSFET was salvaged from the original motor controller
which did run the motor full speed (I think).

Am I missing something?

Thanks,
Bob
 
On Tuesday, January 26, 2016 at 7:51:46 PM UTC-8, Bob Engelhardt wrote:
I am using a MOSFET driver in a PWM circuit. I am switching 12v
(battery) with a Schmitt trigger oscillator on the gate. It's running
about 7kHz. The MOSFET is a IRF1404
www.irf.com/product-info/datasheets/data/irf1404.pdf

The oscillator works nicely and puts 12v on the MOSFET gate. But the
MOSFET drops 2v and I don't get full speed from the motor
Click to expand...

Look at the gate risetime under load; there's a lot of gate
capacitance and some Miller effect.
DC motors' commutators are a source of much noise and consternation.
Could switching transients be depressing your gate voltage at switch edges?
 
On Tue, 26 Jan 2016 22:51:06 -0500, Bob Engelhardt wrote:

I am using a MOSFET driver in a PWM circuit. I am switching 12v
(battery) with a Schmitt trigger oscillator on the gate. It's running
about 7kHz. The MOSFET is a IRF1404
www.irf.com/product-info/datasheets/data/irf1404.pdf

The oscillator works nicely and puts 12v on the MOSFET gate. But the
MOSFET drops 2v and I don't get full speed from the motor that I'm
driving. The MOSFET was salvaged from the original motor controller
which did run the motor full speed (I think).
Click to expand...

What chip for the oscillator, and what's it's supply? That looks like an
older chip, and it may not turn on all the way with just 5 volts.

--
www.wescottdesign.com
 
On Tuesday, January 26, 2016 at 10:51:46 PM UTC-5, Bob Engelhardt wrote:
I am using a MOSFET driver in a PWM circuit. I am switching 12v
(battery) with a Schmitt trigger oscillator on the gate. It's running
about 7kHz. The MOSFET is a IRF1404
www.irf.com/product-info/datasheets/data/irf1404.pdf

The oscillator works nicely and puts 12v on the MOSFET gate. But the
MOSFET drops 2v and I don't get full speed from the motor that I'm
driving. The MOSFET was salvaged from the original motor controller
which did run the motor full speed (I think).

Am I missing something?

Thanks,
Bob
Click to expand...

Like whit3rd said, I'd like to see a 'scope shot of the gate voltage.
How much current is it drawing?

George H.
 
On Wednesday, January 27, 2016 at 12:24:22 AM UTC-5, Tim Wescott wrote:
On Tue, 26 Jan 2016 22:51:06 -0500, Bob Engelhardt wrote:

I am using a MOSFET driver in a PWM circuit. I am switching 12v
(battery) with a Schmitt trigger oscillator on the gate. It's running
about 7kHz. The MOSFET is a IRF1404
www.irf.com/product-info/datasheets/data/irf1404.pdf

The oscillator works nicely and puts 12v on the MOSFET gate. But the
MOSFET drops 2v and I don't get full speed from the motor that I'm
driving. The MOSFET was salvaged from the original motor controller
which did run the motor full speed (I think).

What chip for the oscillator, and what's it's supply? That looks like an
older chip, and it may not turn on all the way with just 5 volts.

--
www.wescottdesign.com
Click to expand...

He said he's putting 12V on the gate.. so that should be OK.
(Assuming it is getting to 12V.)

George H.
 
On 1/27/2016 12:10 AM, whit3rd wrote:
On Tuesday, January 26, 2016 at 7:51:46 PM UTC-8, Bob Engelhardt wrote:
I am using a MOSFET driver in a PWM circuit. I am switching 12v
(battery) with a Schmitt trigger oscillator on the gate. It's running
about 7kHz. The MOSFET is a IRF1404
www.irf.com/product-info/datasheets/data/irf1404.pdf

The oscillator works nicely and puts 12v on the MOSFET gate. But the
MOSFET drops 2v and I don't get full speed from the motor


Look at the gate risetime under load; there's a lot of gate
capacitance and some Miller effect.
DC motors' commutators are a source of much noise and consternation.
Could switching transients be depressing your gate voltage at switch edges?
Click to expand...

The gate rises to 3 or 4v in about 1us then to 12v too fast for me to
see (7kHz is 140us period).

I've tried it unloaded and it still only outputs 10v. (This is a
brushless 2-wire motor - the electronic commutation is inside the motor.)

Thanks,
Bob
 
On Tue, 26 Jan 2016 22:51:06 -0500, Bob Engelhardt
<BobEngelhardt@comcast.net> wrote:

I am using a MOSFET driver in a PWM circuit. I am switching 12v
(battery) with a Schmitt trigger oscillator on the gate. It's running
about 7kHz. The MOSFET is a IRF1404
www.irf.com/product-info/datasheets/data/irf1404.pdf

The oscillator works nicely and puts 12v on the MOSFET gate. But the
MOSFET drops 2v and I don't get full speed from the motor that I'm
driving. The MOSFET was salvaged from the original motor controller
which did run the motor full speed (I think).

Am I missing something?

Thanks,
Bob
Click to expand...

Probably gate capacitance. At high PWM rates, the gate capacitance is
significant and slows the rise time until the device is working in the
linear region.

Your options are to drive the gate with enough current (a low
impendence driver amplifier) to switch it quickly, or lower the
frequency you are driving it with.

Here's an explanation for some ready-made chips designed specifically
to drive power mosfets and IGBTs:

https://en.wikipedia.org/wiki/Gate_driver
A gate driver is a power amplifier that accepts a low-power input from
a controller IC and produces a high-current drive input for the gate
of a high-power transistor such as an IGBT or power MOSFET. Gate
drivers can be provided either on-chip or as a discrete module. In
essence, a gate driver consists of a level shifter in combination with
an amplifier.
 
On 1/27/2016 12:24 AM, Tim Wescott wrote:
On Tue, 26 Jan 2016 22:51:06 -0500, Bob Engelhardt wrote:

I am using a MOSFET driver in a PWM circuit. I am switching 12v
(battery) with a Schmitt trigger oscillator on the gate. It's running
about 7kHz. The MOSFET is a IRF1404
www.irf.com/product-info/datasheets/data/irf1404.pdf

The oscillator works nicely and puts 12v on the MOSFET gate. But the
MOSFET drops 2v and I don't get full speed from the motor that I'm
driving. The MOSFET was salvaged from the original motor controller
which did run the motor full speed (I think).


What chip for the oscillator, and what's it's supply? That looks like an
older chip, and it may not turn on all the way with just 5 volts.
Click to expand...

The oscillator is a CD40106
https://www.fairchildsemi.com/datasheets/CD/CD40106BC.pdf

It is using a 12v battery as its supply.

Thanks,
Bob
 
On Wednesday, January 27, 2016 at 12:39:21 PM UTC-5, Bob Engelhardt wrote:
I tried a couple of things:

- I was way off on the gate rise time; here's a picture:
https://www.dropbox.com/l/s/SC14PWZep6ri4BIsLVPVlo
sweep is 0.1us/ & signal is 5v/

- I paralleled 3 of Schmitt triggers for extra drive & that had no
effect (driven by oscillator & driving gate)

- I swapped the MOSFET with another that I had also taken from the
original controller. No change.

- I put a 10R on the MOSFET & the voltage dropped to 7-1/2.

Bob
Click to expand...

Hmmm OK maybe they are bad FET's... or you have source and drain reversed?
(I hooked up a IRF520 to a power supply and measured a D-S voltage of
25mV with 200 mA flowing. An easy check of the FET.

George H.
 
I tried a couple of things:

- I was way off on the gate rise time; here's a picture:
https://www.dropbox.com/l/s/SC14PWZep6ri4BIsLVPVlo
sweep is 0.1us/ & signal is 5v/

- I paralleled 3 of Schmitt triggers for extra drive & that had no
effect (driven by oscillator & driving gate)

- I swapped the MOSFET with another that I had also taken from the
original controller. No change.

- I put a 10R on the MOSFET & the voltage dropped to 7-1/2.

Bob
 
On 1/27/2016 11:38 AM, Bob Engelhardt wrote:
I tried a couple of things:

- I was way off on the gate rise time; here's a picture:
https://www.dropbox.com/l/s/SC14PWZep6ri4BIsLVPVlo
sweep is 0.1us/ & signal is 5v/

- I paralleled 3 of Schmitt triggers for extra drive & that had no
effect (driven by oscillator & driving gate)

- I swapped the MOSFET with another that I had also taken from the
original controller. No change.

- I put a 10R on the MOSFET & the voltage dropped to 7-1/2.

Bob
Click to expand...

You could always use a Fet Driver
http://www.microchip.com/wwwproducts/Devices.aspx?product=TC4422
Or build one from discretes. As below.
http://www.eetimes.com/author.asp?doc_id=1279277

https://www.google.com/search?q=fet+driver+circuit&ie=utf-8&oe=utf-8
Click to expand...

Mikek
 
Are you measuring the Vgs at the mosfet, or assuming that the source is
the same as some other ground in your circuit?

Are you measuring the Vds drop at the mosfet, or calculating it based on
some other measurement?

IMHO you should connect the scope's ground to the mosfet's source pin,
and two input channels to gate and drain, and see what's actually
happening at the mosfet. You might find, for example, that you have a
dead battery ;-)
 
On Wednesday, January 27, 2016 at 8:10:25 PM UTC-5, Bob Engelhardt wrote:
More:

To totally remove effects of motor & oscillator, I connected the MOSFET
alone: 12v to D & G, S to gnd thru 1K:
Click to expand...

That's what I did, but make the R smaller (50 or 100 ohms) for more current.
Then measure Vds at DC.

George H.
V(DS) = 2-4v pp, about 3MHz sine (kinda) (S to gnd "output" 8-10v pp)

The MOSFET is clearly marked IRF1404Z

Double (triple, ...) checked pin order (G-D-S)

battery: 12v drops to 11v with 10R load

At this point I am abandoning the idea of making a PWM controller, but
the MOSFET REALLY bugs me.

Thanks to all,
Bob
Click to expand...
 
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