How to fix PS conducted interference triggers RCD?

Hi there,

A friend bought a Taiwanese made switchmode power supply that delivers 0 - 24 V at several amps. Problem is that it triggers his RCD (Residual Current Device, Core Balance Relay) in normal use. He asked me to take a look, since the nuisance tripping is major annoyance.

Seems the RCD is rectifying the 40kHz ripple from the PS to produce what it thinks is a fault current.

Unlike PC power supplies I've opened up, this one solidly grounds the output and does not have the usual large clearance between mains side and load side on the PCD, that I'd expect to see in switching power supplies. Other PS units I've seen usually have a large gap with an obvious break between mains side and load side.

I made up a power lead with a 1.0 Ohm, 3W resistor (6 x 1R5,

1/2W, 1%, with some bypass diodes for fault current) in the ground lead to measure earth current from the PS. It's bad.

The RCD triggers at around 50mA p-p of 40kHz noise from the PS.

I've been asked if I can stop the nuisance RCD tripping.

Because the DC side is grounded, isolating the DC side from mains earth is non-trivial as I can't just cut the earth trace and isolate the DC, as that trace is needed for safety. Need to isolate the DC side then add tiny caps to ground to stop RF interference.

Got me beat why this PS doesn't have the usual clearances?

I put photos of the waveform, measurement setup, RCD and the offending PS PCB up on

formatting link
for the curious. With summary of this problem.

What I'm seeking is an easier way to stop the PS dumping that

40kHz switching noise current down the ground lead? I have no circuit for the things.

This is second one I've looked at, the first one triggers the RCD too, so it's not a unit fault, it appears to be a design fault.

In the worst case, we may need to dump this brand PS and get a better ones for the task, I don't like chucking stuff if it can be made to work.

The PS claims a 'C tick' approval mark, I didn't think lack of trace clearance and mains separation like I see here was allowed any more? But I haven't worked (professionally) with consumer gear since the '70s.

Thanks, Grant.

Reply to
Grant
Loading thread data ...

Use a line filter

Reply to
cbarn24050

Board top view?

I'm guessing it has neither X nor Y-type capacitors on the input? The = way it's supposed to be laid out, Y-types RF-ground the circuit side, = and a common mode choke limits ground loop current. X type caps provide = differential filtering, bypassing line and circuit sides; leakage = inductance in the CM choke provides a crude lowpass, but this is = sometimes enhanced with a series choke of 10-100uH to provide more = differential filtering.

The maximum value for Y-type caps is somewhere around 4700pF; more and = the 60Hz leakage can cause problems (564kohm at 60Hz, or 0.4mA at 240V, = enough for a minor shock hazard I guess). However, this sounds = substantially better than the ~18mA you measured.

Tim

--=20 Deep Friar: a very philosophical monk. Website:

formatting link

Reply to
Tim Williams

Yup, there now, with commentary :)

" Those three resistors standing up behind the fuse are part of the output current shunt resistor, another three resistors are further back.

Next trace to left is mains neutral, next trace to the left of that is mains active, going to the fuse's right hand terminal. Next trace goes under the fuse is mains earth!

There's none of the usual large separation of secondary side and mains side traces.

Left side of fuse goes offboard to power switch, back on board to the right of the yellow X2 cap onto the tab also marked switch.

Behind the green core common-mode filter there's bridge rectifier just out of sight. It feeds the two 330/200 electrolytics connected in series in foreground with the classic voltage doubler option tabs east and northeast of the left electrolytic cap. A couple of

150k 1/2W resistors balance the caps.

On the left side just behind the mains entry, and to the left of the right hand side electrolytic capacitor you see the two small blue Y caps.

In particular, note how the Y cap on left goes from neutral to earth, which is also the DC output negative line.

The blue cap to the left rear of the green cored CM filter is centre-tap series capacitor for half bridge drive to the power transformer. Half bridge power transistors are K3469, controlled by a signal transformer. No sign of opto, but there's a second signal transformer likely used for feedback.

Output is rectified by TO247 U30D20C with snubbers, feeding

2200/35 with 200 Ohm 2W across it, through a filter toroid to another 2200/35. Secondary power centre tap goes to the right hand side of the vertical current sense resistors mentioned above.

There's more control circuitry in a tinplate shielded box not in frame, control signal in front panel is a triple(!) gang pot, two sections control voltage while a third section in opposite sense controls max current.

Output adjustable 0 - 24V and output current depends on voltage and loading. " Hope that helps?

Got them, just doesn't follow the common layouts I'm used to seeing :(

common mode choke limits ground loop current. X type caps provide differential filtering, bypassing line and circuit sides; leakage inductance in the CM choke provides a crude lowpass, but this is sometimes enhanced with a series choke of

10-100uH to provide more differential filtering.

Yeah, that I know :) X2 cap is 100n, Y2 rated caps are 2200pF.

No extra series chokes, there's a couple more Y caps on other side of CM choke, snubber cap 4700pF plus 5W resistor across power transformer primary, that's about it for the mains side.

leakage can cause problems (564kohm at 60Hz, or 0.4mA at 240V, enough for a minor shock hazard I guess). However, this sounds substantially better than the ~18mA you measured.

As stated in the text, that signal on DPO is twice current reading for voltage shown, as I'm feeding 50R into coax with 50 Ohm termination on input (divide by two from one Ohm shunt). So you're seeing about 40mA, trip point is about 50mA (+/- 2 1/2 divs at

20mA/div).

Oh, and the PCB reference designators show component values, so those four Y caps are all marked CF222, the two 150Ks across mains filter electros: R150K, I see an R39, the colours are orange white black gold. This is one very strange layout!

I'll put the thing back together and measure the ground noise with high load (200W of lamps) connected and post results later, might need to remove the diodes -- I put the diodes in, in case of fault. After all, the idea of cutting the earth lead is not appealing, I need to check for DPO ground interference to the measurement too. That the RCD triggers on this noise is real.

Mains is 240V, 50Hz in the land downunder.

BTW: to the poster suggesting line filter, I've yet to meet a line filter that does anything for interference conducted along the ground lead ;)

Grant.

Reply to
Grant

Now added new measurements, without that confusing x2 factor :)

formatting link

DPO now shows earth 1R shunt rms and p-p voltage, as well as PS' DC output voltage.

Grant.

Reply to
Grant

(snip)

Just to humour me (note spelling - also in LDU) trying running ALL the supply (A/N/E) though a single ferrite toroid. If it makes a difference, I'll happily explain the rationale, otherwise I'll STFU.

Reply to
who where

Are you saying that safety earth ground is used for secondary refrence---> return currents?

Strange I didnt think any current other then small leakage or fault current was allowed into earth.

I know its not uncommon to have earth ground on the secondary but its for fault detection. Primary to secondary insulation failure not for secondary return currents.

If that is the case wouldnt this cause your earth ground which is ideally at 0V to bounce all over the place? Given transformer DCR and trace R and the high peak currents bounceing around the secondary. .

Reply to
Hammy

Thanks, I didn't think of running all 3 leads through toroid, will try it. I see your rationale :)

All I saw was line filter == X and Y caps and CM filter for A & N, nothing for E, therefore nothing for my problem? After all, I am asking here for suggestions, even if I need to be hit with cluebat at times.

Cheers, Grant.

Reply to
Grant

Yup, madness in there :(

Same here.

What I have seen is earthed screens (electrostatic screen?) in power transformers, single turns of foil (non-shorting) between primaries and secondaries, connected to mains (safety) earth.

That's exactly what I think is happening, seeing high injected current to earth, which is also DC output negative. There's no earth to the transformer indicating shielded windings.

The RCD is tripping at about 12mA rms, as reported by the Tek TDS3034, dunno if that is true rms, ought to be, for an instrument with AU$13k replacement value ;) But there may be some vagueness in the measurement as I don't have a diff amp input module for the DPO, and I'm wary of isolating the mains ground to it. After all, we got safety earth fitted for a reason.

In general, the product seems a new design (over-designed, too many internal connectors for a tight production design), by somebody who's not read the book on modern mains rules or layout techniques very well (example, the ref. des. -> part value, not a part reference).

Contrast this power supply with others I often see (at similar power levels) that has multiple CM chokes, several X caps, usual Y caps, TVS + PTC components, and an obvious divide between mains and output sides. Might add some examples of what I consider decent layouts for the 250W power level. Most I see are switching battery chargers, 6A to 10A into 12 cell SLA or VRLA batteries.

The problem power supply is sold by a .au & .nz nationwide electronics chain (Jaycar), you'd think they'd follow the rules?

I dunno which authority over here to ask if this does meet the C-tick mark it claims. All stuff sold here is supposed to have one, on the one hand, but is ignored by some store chains on the other, seemingly because they can claim to be simple importers and sellers? But I doubt Jaycar can claim that.

It doesn't meet the rules as I remember them from decades ago, particularly the physical isolation barrier between mains and secondary.

But then, I once return a PC PSU under warranty when its active PFC board went Bang! --> tracks too close together for 240V. Oddly enough, the warranty replacement was a different model by same manufacturer, indicating a design flaw?

Grant.

Reply to
Grant

No help :( But, there's more!

Added another filter after the toroid and now the power supply runs the 200W lamp load without tripping the RCD :o) Definitely see a reduced earth current waveform, it's an avenue worth exploring a bit more.

I could manage five turns of A/N/E through a couple of 1 inch OD by half inch joined to make 1" by 1" toroid. Did only the slightest improvement on its own, but adding the filter that has X, 2 Y caps and another CM toroid did the trick. Nothing I can fit inside the PS box though.

The ferrite used is from mains filters, not switcher toroids, I had some larger ones in a bag but cannot find them. I've got some other large toroids from switcher power supplies to try as well.

This result means I got to go through the other testing done with toroids just in the ground lead (as is quite common in this size power supply), to see if the extra filter helps there too, be a bit smaller. Five turns of just the earth lead through a small toroid helped a bit, I want to see if the extra filter helps there.

Will add this info and photos to the web page soon. Tell me your name if you'd like a credit :) (my email addr above is valid)

I'm thinking of changing the power supply DC filter by removing the first cap connected to the main diodes, a choke input filter would reduce peak current through the diodes and is a far more common configuration for these higher current converters.

Assuming, of course, that the thing wont blow up or go haywire with the different feedback dynamics.

Thanks, Grant.

Reply to
Grant

ElectronDepot website is not affiliated with any of the manufacturers or service providers discussed here. All logos and trade names are the property of their respective owners.