differential measurement of large swing

Thread Starter

m121212

Joined Jul 24, 2011
96
My goal is to measure the on state voltage drop across M1.

If M1 is OFF, the current through the load should be essentially zero.

Your 300K+Zener will however allow 1.7 mA to flow through the load.
 

Ron H

Joined Apr 14, 2005
7,063
My goal is to measure the on state voltage drop across M1.

If M1 is OFF, the current through the load should be essentially zero.

Your 300K+Zener will however allow 1.7 mA to flow through the load.
We are having a communication problem here.
Why do you care how much current flows through the load when the MOSFET is off?
 

Thread Starter

m121212

Joined Jul 24, 2011
96
Sorry! I'm sure I'm being unclear, and I'm not giving you all the details.

Like for example...

The load is potentially capacitive. A little bit of current can trickle charge it. The switch should really be off when the MOSFET is disabled. Even MOSFETs have some small leakage, but it is on the order of microamps in this case.
 

Ron H

Joined Apr 14, 2005
7,063
Sorry! I'm sure I'm being unclear, and I'm not giving you all the details.

Like for example...

The load is potentially capacitive. A little bit of current can trickle charge it. The switch should really be off when the MOSFET is disabled. Even MOSFETs have some small leakage, but it is on the order of microamps in this case.
How would the capacitor charging affect the Vds(on) and Id measurements of the MOSFET?
I am questioning whether you are setting an unnecessary restraint that could prevent you from being able to make the measurements you need.
 

Thread Starter

m121212

Joined Jul 24, 2011
96
It's a good question. I wish it were because I was making some ridiculous constraint.

The charge-up of a capacitive load wouldn't impact the voltage and current reading, but the danger is that the capacitor trickle charges when it's not supposed to, which can be a safety concern during maintenance.
 

Thread Starter

m121212

Joined Jul 24, 2011
96
Nope, it's part of a power control module / circuit breaker. I can see why you were confused!

One aspect that makes this all tricky is not having access to the 500V ground. I only have the two terminals - 500V and Load+.

It would be an odd test fixturing situation if I didn't have access to the 500V ground!
 
Top