PCB creepage/clearance etc. for offline SMPS

[jmbw]

This has probably been done to death but I got nowhere searching the archives...

I'm trying to build my first off-line SMPS (+24@4A, +5@2A, -8@.2A, universal input) and I want to do it as safely as possible. If this sees the light of day it'll be super low volume (like a dozen total sold) so getting UL certification is out of the question, but I'd like to design it *as if* it were going to pass.

I'm too stupid to fully understand the actual design (my brain is digital-only) so I'm relying on the MC34262 data sheet examples for the PFC part and PowerInt's software-generated schematics for the actual circuit and design of the transformers (separate PWM for +24 and +5 so they're both regulated). But the safety/RFI aspects are all greek to me, and what I've seen of the UL language is very hard to understand. So I'm trying to match it up with what I've noticed in actual commercial SMPSes that I've opened up:

- The PCBs seem to normally be single-sided (with wire jumpers -- 1970s all over again!) and made out of something other than our old friend FR4. Is this for safety/reliability reasons (dielectric strength of .062" board too low for tracks to cross?), or is it just to cut costs? I got my design to route with just one jumper but some of the nets had to get a bit long, which makes me uncomfortable (it would have been nice to send them over the top, but it seems as if it can't be a coincidence that commercial SMPSes avoid that).

- Somewhere I read that you need a min of 4mm spacing between the primary side tracks and AC earth, and 8mm spacing between pri and sec. Sounds reasonable and of course I have I have a nice obvious 8mm stripe of nothingness across the board under the xfmrs/Y-caps/optoisolators (although I can't believe that simply bending the leads on a DIP-4 optoisolator out to .4" spacing really counts, when the package is still the same size so its own creepage is unchanged), but what about spacing between tracks within the primary-side circuit? If I understand the online PCB trace calculators correctly, 50 mils is enough spacing between tracks with a 380VDC difference on a single-sided PCB with solder mask (right???) and I guess that's just about believable but I'd like to be sure. It sort of seems like no big deal -- if the PFC stuff shorts out it'll just fry components and/or blow the fuse but that's much less of a big deal than zapping the user.

- What about grounding? Since the pri and sec are totally isolated (except for Y caps) it's obviously not cool to tie *both* the AC earth and the DC ground to the case (or in this case to the mounting holes -- it's an open-air PCB that will mount in a user-supplied case), but it seems like *one* of them should be grounded to the case.

- Not relevant to this project but I'm curious for the future: does an offline SMPS always have to be on a separate PCB from the rest of the device? This seems to be true in stuff that I've disassembled, but I don't see why it has to be, as long as proper clearance/creepage from the primary side is observed everywhere. In that case is it OK for the non-SMPS part of the PCB to be double-sided? (If it wasn't already OK for the SMPS.)

Thanks!

John Wilson
D Bit

 

[goldsmith]

Dear John
Hi
The design of an off line SMPS is very simple . it is not clear to me that what is your clear accurate problem . is with it's transformer design ? or with it's PCB ( PCB at high frequencies need more care to design ) . So can you tell me that waht is your clear problem , please?
Best Wishes
Goldsmith

 

[jmbw]

My questions are:

- double-sided PCB: OK or not? (from UL point of view)

- 4mm spacing: between pri side nets and external earth (center AC wire), or between pri side nets and each other?

- grounding: none at all, caps in RFI filter (pri side) only, return legs of sec side only, or (I strongly doubt it) pri side caps *and* DC returns?

- combining off-line SMPS and the circuit it powers on one PCB: OK or not?

Thanks!

John Wilson
D Bit

 

[mtwieg]

Double sided PCB is okay of course. FR4 is fine too. Trace spacing depends on the applied voltage, and also how dirty/humid your environment is. If your PCB has soldermask, then usually a few mm is fine for line level voltages, but exposed pads require caution because current may conduct through dirt and moisture on the PCB surface. All the guidelines should be laid out in the UL specs.

Combining the offline SMPS with another system shouldn't be an issue, so long as the SMPS still has all the necessary safety features.

Also one thing to keep in mind is that transformers for offline supplies usually include at least one screen between the windings. Often their will be two: one for carrying ground fault currents, and another one to shield RFI from being coupled into the secondary circuitry.

edit: I didn't post twice...

 

[goldsmith]

Dear John
Again Hi
AS Mtweig said , the double PCB's are ok . and the distance between pins and circuits depends on your voltage and frequency .
and can you give me more explanation about :"- grounding: none at all, caps in RFI filter (pri side) only, return legs of sec side only, or (I strongly doubt it) pri side caps *and* DC returns? " please?

It is better that you have 2 board . one of them will be control board ( PWM and the other low power circuits ) and the other board will be power board.
Good luck
Goldsmith

 

[jmbw]

What I mean by grounding is, which nets on the PCB should be grounded to the case (through the mounting holes)?

Obviously NOT the high-voltage negative line.

The AC earth (third wire -- green in US) is the logical ground for the caps in the RFI filter, and *probably* also the case?

But in an all-DC circuit (powered by a wall wart and maybe some DC-DC converters) I normally connect the DC return to the case. So I feel like I want to do it here, but I probably shouldn't since the whole point of AC coupling (transformers and optoisolators) is so the secondary side can float. But the primary side is really what's floating? (Since it's what we *really* don't want the user to touch.)

Thanks!

John Wilson
D Bit

 

[goldsmith]

Hello Dear John
At first step , about grounding :
At these circuits we will have to type of ground ! 1st , is Digital ground (logic) , and the other is analogue ground . so those grounds should connect together , with star arrangement .
and you'de better , try to use polygon as your ground . ( to prevent from some problems)
And for next step , about AC line , i think your mean is EMI filter. , so it will use to avoid from interference between switching harmonics and power line . it's impedance instead of AC is very negligible , and instead of switching component will be about infinite ( it is like a high order filter ) .
and , you should prevent to connect your AC input ground to the case , because , it can become the cause of some injures and it is dangerous.
And the reason of using opto couplers at these circuits is that , we dont want any electrical connection between AC ground and out put , because it will be dangerous for your health , my mean is that , if your power supply isn't isolated , and suppose it's out put voltage is about just 5 volt . but if you touch it's ground , and if the place of phase wire and null wire is changed , thus creator should help you !!!!!!!!!!!! :wink:
Good luck
Goldsmith