Purpose of resistance between SENSE/FORCE line of power supply
I am reading through a multi-channel power supply manual and at some point it discusses the ability to chain channels in parallel to increase current output.
In the schematic, there is one resistor between sense and force line of each channel which I can't explain why it is there and what purpose it serves.
Furthermore, an amplifier symbol called "gm" is labeled as transconductor. What does it do? Transconductors are voltage-controlled current sources, but what are they doing here?
3 answers
You are accessing this answer with a direct link, so it's being shown above all other answers regardless of its score. You can return to the normal view.
Furthermore, an amplifier symbol called "gm" is labeled as transconductor. What does it do? Transconductors are voltage-controlled current sources, but what are they doing here?
The diagram looks like a voltage output power supply. I'll make a working assumption that it's voltage output power supply.
[Access to the manual might shed some light on this. Please post a link to the manual.]
In the schematic, there is one resistor between sense and force line of each channel which I can't explain why it is there and what purpose it serves.
The resistor in question is R5 in the diagram. Its value isn't given, I'll make a working assumption that the resistance is sufficiently large.
Its purpose is to tie weakly the Sense feedback input to the Force output when Sense terminal is disconnected. That allows the output to maintain a sane value if the Sense input gets disconnected. Otherwise the output could go crazy without any feedback connection, and potentially damage the DUT. It also allows to omit the sense input intentionally at the expense of a larger output error.
When the sense terminal is connected externally, it's a low impedance connection, it overpowers the R5, and the current through R5 is negligible.
0 comment threads
I see that Nick has already provided a good answer to R5, so I'll only address this:
Furthermore, an amplifier symbol called "gm" is labeled as transconductor. What does it do?
What you show is a rough diagram only intended to give enough insight into the internal workings to use the power supply effectively. A lot of details are left out.
You say:
ability to chain channels in parallel to increase current output
You can't just connect ordinary constant voltage power supplies together and expect them to play nice and share current. This is a capability that must be designed in, and requires something different from typical topologies.
My guess is that the output stages are indeed current sources. The feedback loop controls the current source to maintain the desired output voltage. When multiple supplies are connected in parallel, their feedback signals are also effectively shorted together. That way, each supply will produce roughly the same output current as the others. The overall feedback still adjusts the control inputs to these current sources to cause the desired output voltage. If you assume that the output voltage is held constant and that each supply puts out about the same current, then all the supplies share the load current nicely.
0 comment threads
This looks like the block diagram was done by a technical writer, not the designer. It is someone's conceptual misunderstanding of a linear servo with feedback relays and optional FET drivers with error amplifiers (and R5) between Vo and Sense (not accurate). It has no power supply.
Some old HP power supplies use resistance-controlled voltage for remote control. Perhaps this supply permits a single control input with ganged +/- Vout or scaled outputs.
I would ignore this diagram and find a better way to understand the text. due to non-std. symbolic block diagram used. The feedback select has only one input(!). not well done.
FETs use gm to represent the inverse of RdsOn or the inverse of Beta with a gain in Amps/V as a small signal conductance gain. But you don't need to know this.
An error voltage controls a transistor resistance that feeds voltage from input to output and varies depending on the load and voltage-controlled conductance. A high gain error amplifier uses the difference between Vout and Vsense to power and regulate the output.
0 comment threads
1 comment thread