HELP selecting choke for AC-DC module - Page 1
Forum section: electronics / general

author: ETITsynthesizer
author_rank: Contributor
timestamp: June 27, 2020, 01:24:15 pm
content: I have here a common mode choke wired for differential mode. this AC-DC module from mornsun specs 150mA ripple max. I think putting the cap before the choke is going to provide more ripple attenuation compared to after the choke. maybe I am wrong. this feeds into multiple power distribution boards maybe two of them that have 470uF at each input. max capacitive load for the AC-DC brick is 7000uF. everything is rated for 2.6A max continuous output. 10C delta is acceptable. also, if you have suggestions for the PCB go ahead. 2oz copper no thermals 2 sided but with traces on one side.

author: duak
author_rank: Super Contributor
timestamp: June 29, 2020, 11:05:38 pm
content: Common mode chokes generally have medium to high permeability cores. When used in common mode, any DC or low frequency currents should run through in opposite directions to cancel out so there is no net flux. When used in differential mode the DC or low frequency currents add and usually cause the core to saturate and its net inductance drops to a far lower than expected value - perhaps as low as 1/100 of its specified value. If you really want some series inductance you'll have to get parts that have larger cores that are capable of operating with a DC current as high as your load current.

author: ETITsynthesizer
author_rank: Contributor
timestamp: July 02, 2020, 02:21:36 pm
content: those differential mode chokes for DC are really big and really expensive. I already ordered the PCB. the core material from kemet is proprietary low permeability. at 65KHz from my SMPS, this common mode choke is rated to have an impedance of 10K. at 1/100 that would be 100 Ohms. this is perfect for my application. probably this is a hack that no electrical engineer would attempt, but I think it may be perfect for my application when no better options exist for this price and this package size. I already ordered the PCB's. no turning back now. the choke power rating is 4A or 5A. I am only passing 2.6A max. is this ok?

author: duak
author_rank: Super Contributor
timestamp: July 03, 2020, 06:54:31 pm
content: I don't know what your load is so If the choke presents a greater impedance at lower DC currents will it cause a problem? Could it resonate with the load's capacitance? It's generally not a good idea for a power source to have an output impedance with a significant inductive component. It's a variable that I'd prefer to not have to deal with.However, when you get this thing together, observe the voltages after the choke under varying combinations of frequency and current to see what's there. If you find some sort of instability, you might be able to modify the choke by reducing the number of turns. The inductance at full load current won't change much, but it'll reduce the inductance at low current considerably as inductance is a function of the square of the number of turns.Let us know how it goes.

author: ETITsynthesizer
author_rank: Contributor
timestamp: July 04, 2020, 01:33:05 am
content: ok great. that implies that the attenuation would be really good at low currents? so in theory 100mA load on the 15v output with 150mV ripple would attenuate at >80dB? the DC component is still 0.100 Ohms in each coil. or am I wrong on the DC impedance?

author: duak
author_rank: Super Contributor
timestamp: July 05, 2020, 12:38:17 am
content: The impedance of the choke is an AC characteristic and is the vector sum of the DC resistance and the reactance of the inductance. The DC resistance doesn't change unless the conductor is heated a lot. (Copper's resistance changes ~0.4% per degree C, so for a 50 C increase the resistance will increase 20%.)Without the full magnetic characteristics of the core it's hard to say what the inductance will be for a given DC current. Only at zero net current will it have the inductance on the data sheet. You'll have to try it.