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Common-mode current sensing in the NG 304 power supply
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profdc9:
In another thread a really cool resource was mentioned, the manual to the NG 304 power supply:
https://www.mikrocontroller.net/attachment/412953/DBL_BEHA_NG304_MANUAL_DEUTSCH.PDF
So I looked at the schematic (which I have included a copy of), and there is an interesting feature which I have drawn red boxes around. Normally the current sense resistor is on the low side of the power supply, and so as the current increases the negative output of the supply is lifted off of ground. However, in the NG 304, the sense resistor is on the high side.
Now I was thinking of using this approach, but there are two potential problems I thought I identified:
1. The common-mode rejection of the op-amp must be really good to ensure that the current sensing threshold doesn't depend on the output voltage level.
2. The circuit is not really a difference amplifier, only a comparator. But if used to actually measure the quantitative current in the circuit (for example putting in a meter), the ability of the circuit to reject the common mode would likely be limited by how closely the divider resistor networks to the + and - terminals are matched to each other. This problem occurs in something like an instrumentation amplifier I suppose.
So why is high-side sensing like is done in the NG 304 not done more often? Is it because of the difficultly of the common-mode rejection when the output voltage changes?
I'm just curious but it's not something I've seen in many power supply designs.
Jay_Diddy_B:
Hi,
This isn't really high side current sensing.
The common for +/-12V supply is connected to the positive output terminal. So from a signal point of view the positive output is signal ground.
I have marked the ground connections in green
They have a 5V6 Zener and diode in series for temperature compensation.
This is fairly common in power supplies. A lot of the HP power supplies were designed like this.
You can make high voltage power supplies without using high voltage op-amps or level shifters.
Regards,
Jay_Diddy_B
profdc9:
Thanks for the information. I was a bit confused about that from the schematic and so your help saved me from going down the wrong rabbit hole. I suppose then for a high voltage supply the two sets of windings on the transformer must be well insulated from each other so that the current return is only through the box and upside-down-T shaped terminals.
That is still an interesting idea however but it requires a special transformer or a pair of well-insulated transformers to do it.
Jay_Diddy_B:
Hi,
By high voltage I mean 60 or 100V maximum output (in this context).
Regards,
Jay_Diddy_B
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