Is this considered high or lowside?

Thread Starter

shortbus

Joined Sep 30, 2009
10,044
Started working on an ongoing project again. Is the discharge mosfet in the attached circuit considered considered a high side or low side switch?
Since it goes to ground through a spark gap, that has aresistance of around 1Ω I've always thought of as a high side. But thought I should ask you guy's before spending too much more time on the circuit.
 

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BillB3857

Joined Feb 28, 2009
2,570
My personal interpretation is the Hi Side switches power while Low Side switches ground. I have no idea what you would call one between the two. :)
 

Thread Starter

shortbus

Joined Sep 30, 2009
10,044
Thanks guy's, so far high side has been my take too. Why I was asking is that I have seen some schematics with a current measuring resistor in the the low side configuration.
 

t06afre

Joined May 11, 2009
5,934
Thanks guy's, so far high side has been my take too. Why I was asking is that I have seen some schematics with a current measuring resistor in the the low side configuration.
It is more easy circuit wise to place the current sense resistor on the low side. As you do not have to use an differential amplifier. Then sensing current
 

tindel

Joined Sep 16, 2012
934
I always use a diff amp on a current sense high-side or low. I'm not sure why t06afre suggests otherwise, maybe he/she could elaborate. It seems to me that you couldn't get an accurate current measurement without a diff-amp. I also place the traces (or wires) as close to the sense resistor as possible. I don't typically like via-in-pad configurations, except in this case.

The reason for me that it is easier to place them on the low side is because I don't need a large common mode input voltage. Good luck finding a op-amp that can withstand a 100V input while you power your op-amp off of a +/-15V rail. Even a 40V common mode input is hard to find - they are out there though.
 

thatoneguy

Joined Feb 19, 2009
6,359
Thanks guy's, so far high side has been my take too. Why I was asking is that I have seen some schematics with a current measuring resistor in the the low side configuration.
What is the voltage at the source when it needs to switch? That usually answers the question, if the voltage isn't very near ground, it's a high side.
 

t06afre

Joined May 11, 2009
5,934
I always use a diff amp on a current sense high-side or low. I'm not sure why t06afre suggests otherwise, maybe he/she could elaborate. It seems to me that you couldn't get an accurate current measurement without a diff-amp. I also place the traces (or wires) as close to the sense resistor as possible. I don't typically like via-in-pad configurations, except in this case.

The reason for me that it is easier to place them on the low side is because I don't need a large common mode input voltage. Good luck finding a op-amp that can withstand a 100V input while you power your op-amp off of a +/-15V rail. Even a 40V common mode input is hard to find - they are out there though.
In some application like say a power supply. You do often not want to place the current sense resistor on the low side. But on say a constant current battery charger it may work just fine. Also by low side i mean inserted with one leg connected to the ground. Let us just use this picture for the conceptual discussion only

In this setting you could have used a differential amplifier and measured the voltage drop over Rload. And then used this as the feedback to the OPAMP. You could of course also have used a differential amplifier and measured the voltage drop over Rsense and then used the latter output as feedback. But that would be to great deal of overdesign. As the voltage over Rsense is ground referenced already. It can be used to directly as input or just amplified with a non differential amplifier.
 

tindel

Joined Sep 16, 2012
934
Ah, yes - thanks for clarifying that for me t06afre. I've used something similar to that circuit before, but I've still considered it to be a diff-amp. The circuit is just giving the non-inverting pin a higher voltage reference than ground by including the zener. The op-amp is still measuring the difference between the zener voltage and the sense voltage and driving the transistor more or less to try to make the input differential = 0V.

Just as an aside for the OP - I would personally still keep the zener as close to the ground side of the sense resistor as possible and I would 'star' the ground there. This will keep ground currents from effecting the accuracy of that circuit.

This is why I keep coming back here - I learn new ways of looking at circuits.
 

Thread Starter

shortbus

Joined Sep 30, 2009
10,044
Just as an aside for the OP - I would personally still keep the zener as close to the ground side of the sense resistor as possible and I would 'star' the ground there. This will keep ground currents from effecting the accuracy of that circuit.

This is why I keep coming back here - I learn new ways of looking at circuits.

Thanks, but was just using the current sense as an example, of seeing a small resistance in the ground path of a low side mosfet. I don't want or need current sensing in my circuit.
 

Thread Starter

shortbus

Joined Sep 30, 2009
10,044
What is the voltage at the source when it needs to switch? That usually answers the question, if the voltage isn't very near ground, it's a high side.
Thank you thatoneguy! That was how I was thinking about it, but wanted a second opinion. You solidified it for me.
 
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