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I am looking to incorporate a 45V supply into a design. These PFE1000-48V supplies have a trimming ability to get +/- 20%, based on +/- 20% variable resistor. I am trying to confirm what this resistor value should be. If 45V is 93.75% of 48V. So the resistor value should be 93.75% of the base, correct?

Per the manual, you have this:

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This suggests that R2 should be 18k and that VR should be 46.875k. However, I am unsure how that works based on the internal circuits here.

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I'm assuming the resistor to S+ is the line resistance to the output of the supply. Given that I do not see how the Trim voltage changes at all based on VR value. Shouldn't it change based on the R2 Value?

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2 Answers 2

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It changes the R2/R1 voltage divider and therefore the voltage at the non-inverting input of the error amp. TDK actually will forward requests to their engineers if you contact customer service; here's some additional info I got from them. Emphasis theirs for my specific question about raising output voltage; you'll need Ra. FYI, they prefer applying an external voltage to the TRM pin to adjust output voltage.

PFE1000 voltage control

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V+ changes with both values of Rb on S+ to V and Ra to S-, resulting in matched voltages to error amplifier.

Rb adds to the string that derives the feedback ratio to result in 1V without Ra.

Ra lowers the 1.0002 V reference by shunting the bandgap reference voltage even more thus multiplying the output voltage more.

You have some choices for a fixed V+=45V.
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All choices result Vfb = Vref=TRIM or in other words the R ratio of 45V out to match the S- voltage reference input which is normally 1.0002 V with a Thevenin resistance of 5.971 kohms.

  1. You can use Rb = 500 Ohms an use Ra = 77k then TRM voltage = 927.4 mV or use a 1K pot and have 0.5k/48.5k *100% = +/-1% tuning range.
  2. You can double Rb to 2k pot and get +/-2% tuning range. then use Ra ~= 67K
  3. or use your suggestion of 48.875k for nearest pot of 50k and Ra=10k for > =/-20% range
  4. Use Rb=1k fixed between S+<>V+ and Ra= 67k fixed between S-<>V-

Your choice depending on what tuning range you desired for cable loss and load regulation PSU specs.

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The key to low noise performance is to have area wire loops to all external sense resistors. This is why they prefer a low impedance voltage offset to TRIM instead, but that still needs to be shielded from any high pulse current dI/dt. However you can remove cable losses using remote sensing for reasonable lengths by connecting the senses at the end of the cable and adding ferrite +RF Cap filter near supply, but not too much as that can add to step overshoot from delays.

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