Position of output current sensor in DCDC module schematic? - Page 1
Forum section: electronics / general

author: ocset
author_rank: Super Contributor
timestamp: March 16, 2020, 09:46:22 pm
content: Hi,We have a DCDC module generally giving a 48V, 6.5A output. We wish to clamp the output current to approximately 7A so have a current sensor in the output, which feeds an external error amplifier. (as in the attached schematics)Our contractor wants to put the current sensor upstream of the bulk of the output capacitance, as inschematic A….but we believe that we should have it the other way round, as inschematic B(ie downstream of the bulk of the output capacitance) …do you agree with us?

author: nctnico
author_rank: Super Contributor
timestamp: March 16, 2020, 11:22:49 pm
content: It doesn't really matter. Either way you'll need to include some delay to make sure the current limit doesn't trip due to rush-in currents.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: March 17, 2020, 12:52:22 am
content: Why would the ESC not include a hard current limit?Tim

author: ocset
author_rank: Super Contributor
timestamp: March 17, 2020, 06:13:09 am
content: QuoteThanks, on the bench, "A" is worse than "B".....ie, there is more disturbance in the sensed current in the "A" setup....................*.............................. ........*......................................*.. ....................................*............. .........QuoteThanks, I have implemented the external current clamp, because there will be forty of these DCDC modules all operating together….kind of all in parallel with each other….the modules have a droop sharing algorithm (vout reduces by 5% from no load to full load)….however, due to the sheer size of our system, the volt drops from each module to the load will be widely different, and so droop sharing alone won’t be enough…and we believe we need the current clamp as shown…(to stop any individual module over-hogging current).Also, the module's internal current clamp can be at up to 144% of nominal max current (nom=6.67A)......and thats too high for us...so we limit it to 7A as shown....................*.............................. ........*......................................*.. ....................................*............. .........Taking circuit A, there is an LC output filter there with a resonant frequency of 10273Hz (150nH and 1.6mF). The crossover frequency of the DCDC module is said to be 20kHz…….so isn’t this a recipe for instability?.....having said that….i would expect the added external output capacitance to reduce the crossover frequency of the overall power supply……so this stability fear could be wide of the mark…….with Vicor modules, there is no way of measuring the crossover of the overall thing, so we’re stuffed?The Hall sensor has to be downstream of the 150nH inductor ("L6" or "L2" respectively) otherwise the current seen in the sensor has a 80kHz oscillation in it.....I am just wondering if it would be even better to have just a 10uF film capacitor at the DCDC module output, then the 150nH inductor, and then the Hall Sensor, and then all the electrolytic capacitance (approx 2mF's worth)?I initially declined to do it like this because the DCDC module needs 220uF at its output for stability......and so i put this 220uF right at the module's output.

author: ocset
author_rank: Super Contributor
timestamp: March 17, 2020, 11:40:06 am
content: These 4 scope shots (two from circuit A of top post, and two from circuit B of top post), show how circuit B appears more stable when on constant load.There is a scope shot of current_sensor_output and SMPS_output_on_AC_coupled for A and B cases.I made the external current clamp fix the current to 2.53A in both cases, A and B.The load is 15R.Vin is 31VAs you can see, in the “A” case, there is a disturbance in the VIOUT signal of the ACS722 current sensor IC that is not there with the B case.This is not expected.Do you know why “A” appears better than “B”?

author: duak
author_rank: Super Contributor
timestamp: March 18, 2020, 04:10:25 am
content: treeze, what is the load on your test unit? I see it is 15R so I assume it is a resistor. Is there something that may cause a variation in current draw?When you say the Vicor modules have a crossover frequency of 20 KHz, do you mean the frequency at which their AC gain is unity or do you mean the corner frequency or dominant pole?

author: ocset
author_rank: Super Contributor
timestamp: March 19, 2020, 06:03:04 am
content: thanks, it was a 15R res load.I mean the fequency at which AC gain crosses zero dB.

author: Ccandrews
author_rank: Contributor
timestamp: March 19, 2020, 06:26:58 am
content: To me the answer seems to depend on what you are wanting to protect. If you want to limit the current from the power supply, put the current limiter before the bulk capacitors. If you want to limit the current available to the system (load), put after the bulk capacitors.