Scoping the post rectifier DC bus in a 240VAC , 100W PFC'd flyback

treez · Nov 9, 2019

Hello
I was scoping the Primary , Post rectifier DC Bus in a 100W, 240VAC, offline PFC’d Flyback to see what the voltage ripple was like on this rail.
As you can see, at the mains peak, there is a lot of noise at the peak of the waveform that makes the measurement impossible.
The blue waveforms were done with a x10 scope probe (with a dangling ground clip) with its lead wound in three turns round a ferrite torroid. The yellow waveforms are done with a diff probe.
I obviously need a ten times coaxial probe without the dangling ground clip.
Do you know of one?...or must I do the old trick of wrapping wire round the ground barrel of the probe?

wwfeldman · Nov 9, 2019

input frequency is 50Hz as seen by the 10 ms for 1/2 cycle

what is the frequency of the PFC stage? i think you're seeing the switching
stretch out the time scale and see if the "noise" looks like a constant frequency

Easy peasy · Nov 9, 2019

it appears the PFC is somehow going to a lower sw freq once it passes a certain voltage ( or current ) and this is causing the increase in Vripple observed - can you post the PFC schematic? this could also happen if the choke is saturating or iron powder, kool mu going lower in L as the current climbs up forcing more current ripple for a given sw freq - the fact that it ( the ripple) looks to be progressive in the last screen shot ( yellow ) would seem to suggest iron powder / kool mu toroid going lower in L as I climbs ...

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the increased current ripple - due to variable L choke is causing increase V ripple on the filter caps ...

please send internet procured beer if correct ...

c_mitra · Nov 9, 2019

Looks tell a lot. This is not regular noise. All the noise is coming from the down side.

the increased current ripple - due to variable L choke is causing increase V ripple on the filter caps ...

And that may be the cause of the untimely passing away of the filter caps (sooner than later)

treez · Nov 9, 2019

Thanks, its Constant off time flyback (off time = 8.2us). I scoped the primary sense resistors and theres no saturation anywhere in the 10ms half cycle.
That ripple shown is on a small capacitor bank comprising
1 x 100n, 1kV, 2220, X7R
1 x 33n, 1812, x7r, 1kV
1 x 10n, 1206, x7r, 1kv
There is a filter inductor just upstream of this small capacitor bank.

Easy peasy · Nov 9, 2019

What is the boost choke --- ?

the PFC flyback is obviously doing more work at higher loads / higher mains, the associated volt ripple on the nearest filter cap will likely see its demise after not too long in the field ... unless it is a very high quality type ...

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what are the temps on the caps you mention when it is going flat out at 230Vac?

treez · Nov 9, 2019

There is no boost choke. The flyback does pfc and outputs to the leds.

wwfeldman · Nov 10, 2019

is there anything different in the variuos scope traces other than the voltage scale and the x10 scope probe and diff probe?

you said its a 100W converter. what is the actual design level and what is the actual power drawn by the load?

i am thinking the converter is overdesigned - the load is too low and it is skipping cycles
is it continuous or discontinuous?

Easy peasy · Nov 10, 2019

sorry - brain fade - of course the ferrite Tx is the boost choke - actually flyback Tx performing same function...

your constant off control is part of the issue - the waveforms are normal for the cap sizes listed - you'll have to filter it

p.s. for 100W, (200W peak off the mains for UPF) the peak current in the pri will be 1.4 amps ( average peak ) for DCM at the peak, I/C = dv/dt

thus for the 143nF this gives 80V for the dv for 8.2uS which is about what you are seeing ... voila ...!

Easy peasy · Nov 10, 2019

The first picture is a bit odd, it is almost as if you are going from DCM to CCM for the middle (noisy) part - CCM makes the o/p diodes reverse recover and causes noise - which might be what is being seen in No.1 pic - if so if will be part of the mix in the high Vripple observed in other pics too ...

treez · Nov 10, 2019

The first picture is a bit odd, it is almost as if you are going from DCM to CCM for the middle (noisy) part - CCM makes the o/p diodes reverse recover and causes noise - which might be what is being seen in No.1 pic - if so if will be part of the mix in the high Vripple observed in other pics too ...

Thanks yes, I believe you are right, the ripple seen is part real and part due to the noise of it edging into CCM at the peak. This flyback was designed for 60W, but we gave it a run at 100W.
On the simulator, there is 0.88ARMS of ripple current in the 143nF ceramic capacitor bank (at 100W operation), therefore, with its low ESR, we believe this to be OK even for 100W. The high ripple voltage on the ceramic capacitror bank seems disconcerting, but since it works fine, we don’t feel inclined to increase the capacitance here for 100W operation.

So it seems the dangling ground clip probe means we get common mode effects in the scope probe lead. Even wrapping it round a torroid three times did not help.

And of course, the diff probe, is terrible here, we cannot even thread the two probes through the torroid, as their shrouds are is too wide.

Easy peasy · Nov 10, 2019

running CCM is a bad idea for RFI noise, as the o/p diode gets quite the hit - this explains the band of noise in pic #1 ...

treez · Nov 10, 2019

Thanks, its only just creeping into CCM...so the reverse recovery of the output diode wont be as bad as it could be in hard CCM.
But i guess you are pointing out that passing EMC will be much harder?

Easy peasy · Nov 10, 2019

Yup - emc is always measured at worst case - so min AC line max current & max AC line - max load both cases ....

c_mitra · Nov 11, 2019

Thanks, its only just creeping into CCM...so the reverse recovery of the output diode wont be as bad as it could be in hard CCM.

If you do not mind a little heat, you can perhaps turn on (and off) the drive FET a little softly. Most of the noise (I am afraid) is coming from the hard turn on of the pass transistor. Perhaps if you increase the gate drive resistor a bit...

You have not shown us how the noise looks; same traces but with a faster time base so that we can see the noise more clearly...

Some compromise between efficiency and noise.

Easy peasy · Nov 11, 2019

X7R has a worst case dissipation factor ( tan delta ) of 2.5% for HV caps

The rms voltage during the noisy periods is about 42V rms at sw freq, the current at sw freq is 730mA rms thus the losses are 0.8 watts worst case in the 3 capacitors during CCM operation in the middle part of the mains - the average losses will be lower.

This is perhaps a bit high for these caps for long term operation - but for a commercial / domestic product not operated at full power very often - you may well get away with it - esp for larger pcb lands on the caps to wick the heat away ...

wwfeldman · Nov 11, 2019

Easy peasy said:
Yup - emc is always measured at worst case - so min AC line max current & max AC line - max load both cases ....

i thought worst conditions were max line and minimum load, and minimum line and maximum load?

Easy peasy · Nov 11, 2019

it is not a constant thing - certainly for a flyback entering CCM - high mains and full load will be an issue - the most stress on the o/p diode ...

Scoping the post rectifier DC bus in a 240VAC , 100W PFC'd flyback

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