Does this EMI filter and PFC look correct? - Page 1
Forum section: electronics / projects

author: technix
author_rank: Super Contributor
timestamp: May 01, 2016, 09:05:29 pm
content: This EMI filter and PFC section is used in my M103v2 power supply since I am doing mains-side switching. Does this look correct?PFC section output specs: 400V, 250W max (so I can have 2 isolated 100W output stages along with a shared control stage)

author: johansen
author_rank: Super Contributor
timestamp: May 02, 2016, 04:32:05 am
content: Cx1 should be larger, how large I don't know. 250 watts, probably 1uf would do.the pfc chip's internal impedance is likely much higher than 1 ohm, so the gate resistor doesn't make a difference. also since dead time doesn't matter you don't need the diode.the boost diode, you can make that arbitrarly expensive to optimize losses. But much better options are available for the boost mosfet. i have not kept up with the industry so i can't point you towards any particularly good ones.. but if you really wanted to you could find a surface mount part that doesn't need a heatsink for your expected 250 watt load.

author: technix
author_rank: Super Contributor
timestamp: May 02, 2016, 07:04:05 am
content: Quote from: johansen on May 02, 2016, 04:32:05 amFrom TI's design tool CX1 have a maximum capacitance of 500nF. So...US3M is the surface-mount version of UF5408 - dirt cheap. MOSFETs get expensive fast though, that is why I am staying with an older part. I do wish I can find some good MOSFETs that is both rated for the job and comes in a surface-mount package though (preferably SO-8 which is easier to hand solder)

author: Riotpack
author_rank: Regular Contributor
timestamp: May 02, 2016, 07:35:51 am
content: For R10 I would use 2 or 3 resistors in series.

author: technix
author_rank: Super Contributor
timestamp: May 02, 2016, 08:29:00 am
content: Quote from: Riotpack on May 02, 2016, 07:35:51 amR10 is a 1M/2W resistor so I probably can get away with this.

author: dmills
author_rank: Super Contributor
timestamp: May 02, 2016, 09:56:39 am
content: Watch the voltage ratings on the resistors (Even 1206 parts are often only specified for 200V)!I would be adding a MOV across the AC side of the bridge, and maybe some resistance in series with D2 (Which plays no part once the PFC has started but will limit the inrush).A fuse in the AC line is not a bad idea.I am assuming that GNDA does not in fact ever connect to ground....Regards, Dan.

author: SeanB
author_rank: Super Contributor
timestamp: May 02, 2016, 11:19:57 am
content: As you likely are using a switcher chip after this why not use a better power supply for the PFC, using a tap off the switcher IC feedback/power supply side. That way your startup resistors are much lower dissipation, and as many SMPS drivers now have built in startup supplies built in this will obviate actually having the R6 supply at all.Inrush limiting is needed, and a NTC thermistor is both cheap and common, and will help to reduce inrush all round. 4R7 would be around the correct value. Do not forget R4, it also needs a very good high voltage rating, and if it fails the converter likely will blow up. C4 probably will need to be an electrolytic, not a ceramic, unless you use a few in parallel so the capacitance at 12V is actually 10uF.

author: technix
author_rank: Super Contributor
timestamp: May 02, 2016, 01:05:15 pm
content: Quote from: dmills on May 02, 2016, 09:56:39 amR3 is the MOV (EAGLE does not have an appropriate symbol) and I have sourced IEC connector combos that have a built-in fuse holder and switch. So the fuse do not need to be represented on the schematics explicitly (and MOV can last longer as its crowbar action will blow the fuse).GNDA is NOT the ground. My system have 4 grounds - PE, GNDA (VIN and +18V rails reference against this) for HV section and GND (+/-12V, +5V and +3.3V rails reference against this) for LV section. +18V is a goldilocks voltage that is above the starting voltage of most mains-side SMPS controller chip, but below the maximum Vgs for the MOSFET.

author: nctnico
author_rank: Super Contributor
timestamp: May 02, 2016, 01:28:10 pm
content: I'd put the MOV after the common mode choke. That way the common mode choke filters spikes before reaching the MOV.

author: technix
author_rank: Super Contributor
timestamp: May 03, 2016, 04:39:35 pm
content: Quote from: nctnico on May 02, 2016, 01:28:10 pmLike this? (the +18V comes from an auxiliary winding on the forward converter that follows this.)

author: T3sl4co1l
author_rank: Super Contributor
timestamp: May 04, 2016, 02:59:43 pm
content: Note that a MOV without a fuse is, at best, unwise...MOVs exist which have three terminals, with a thermal fuse between the two so your circuit disconnects harmlessly if the MOV starts to cook off.Tim

author: technix
author_rank: Super Contributor
timestamp: May 04, 2016, 05:41:02 pm
content: Quote from: T3sl4co1l on May 04, 2016, 02:59:43 pmAll my stock of IEC connectors comes with built-in fuse holders. The particular one I am planning to use on this project also includes a DPST switch.

author: NiHaoMike
author_rank: Super Contributor
timestamp: May 05, 2016, 02:15:55 am
content: Quote from: T3sl4co1l on May 04, 2016, 02:59:43 pmSome old HP PSUs (back when HP actually made great PSUs) I tore down put the MOV before the fuse. Not sure why but I suspect they had too many warranty returns with the MOV after the fuse as they did on some of their older PSUs.

author: Richard Head
author_rank: Frequent Contributor
timestamp: May 05, 2016, 06:40:28 am
content: Put your Y-caps on the mains side of the common mode choke. That way you actually make use of the common mode choke.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: May 06, 2016, 05:18:54 am
content: Quote from: Richard Head on May 05, 2016, 06:40:28 amDepends on the circuit.The CMC isn't useless, because the power line is assumed to be a pair of 50 ohm transmission lines. It has a 25 ohm common mode resistance to work against, which is pretty good, with peak impedances in the kohms being common.There are about four typical cases for Y caps:- Across the transformer (primary DC common to secondary DC common)- Primary DC common or AC mains (after CMC) to GND- Secondary DC common to GND (or secondary galvanically grounded*)- AC mains (input, before CMC) to GND*Grounded secondary only needs "Basic" isolation (i.e., a single failure of insulation results in a short circuit to GND, and no harmful voltages beyond). Isolated secondary requires "Reinforced" insulation.Typically, only a few permutations of these four positions are effective at filtering EMI. Example:The yellow Y-caps may not be easily visible, but they are in different places on each model of output voltage. It seems the transformers have different EMI response depending on ratio and winding design. High voltage ratios are harder to deal with.Tim

author: Richard Head
author_rank: Frequent Contributor
timestamp: May 06, 2016, 07:41:32 am
content: The CMC isn't useless, because the power line is assumed to be a pair of 50 ohm transmission lines.That's true, but you have a single pole response when you could have a double pole response. Ideally smaller value Y-caps on both sides of the common mode choke.