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Is our 30W offline LED driver suffering from noise issues, or software bug?

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Flyback

Well-Known Member
Hello,
We have an offline (240VAC) 30W offline LED driver with a high power factor. It is controlled by a microcontroller which dims it depending on light levels etc. The problem is that we are getting lots of noise issues with the operation. (eg sometimes it doesn’t dim when it should etc etc).

We are not sure if the problem is with the code, or whether it is noise corrupting the circuit, or noise corrupting the microcontroller. Please help us to know this.

The designer was very “economic” with the components, partly because he was given a tiny space on the PCB in which to squeeze the components on to….there are no local decoupling capacitors immediately local to the opamp, the photodiode, the temperature sensor IC etc…….these are all powered from the 2V5 bias rail but the 2v5 rail does have 20uF of ceramic capacitance on it, so I suppose its not completely without de-coupling.

Another point is that the PIC18F26K20 microcontroller that is also supplied from the 2V5 bias rail has no 4k7 pullup from its MCLR pin to the 2V5 rail, and no 10nF capacitor from the MCLR pin to ground. The MCLR pin has no connections to anything, so it may be getting noise corrupted(?), even though the MCLR function has been disabled in the microcontroller.

Also, there is no mains filter, and no capacitance on the DC Bus just downstream of the Mains rectifier bridge. I requested an AC input filter be added, but this was rejected, saying it would be too costly.

Another point is that the 2V5 bias rail is supplied by an LR8N8 high voltage linear regulator. The designer put no input capacitance at this regulator’s input. I can’t see this being a problem, but I recognise that it is often classed as ‘standard procedure’ to put a capacitor at the input to a linear regulator (but the LR8N8 datasheet doesn’t even mention an input capacitor.)

LR8N8 datasheet:
http://ww1.microchip.com/downloads/en/DeviceDoc/20005399A.pdf

Do you think by having no AC filter that we are leaving ourselves wide open to mains bourn noise coming into the product and corrupting it? I have never seen an offline product without an AC filter before, so I don’t have experience with this.

I have a funny feeling that without a Mains AC filter with the product, we are going to get endless issues with noise. Do you agree?

A representative schematic of the Bias section is shown here…also a representative LTspice simulation of it.
 

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alec_t

Well-Known Member
Most Helpful Member
If the bean counters won't allow sensible suppression components to be added then faulty operation due to interference is extremely likely. Management should be made aware of that so that the appropriate head (not yours :) ) can roll when things go pear shape.
 

Flyback

Well-Known Member
Thanks but they would say that the 2V5 bias rail has 20uF of ceramic capacitance on it so the control circuitry should be OK.

The LED current regulator chips however, connect directly to the DC Bus which has no capacitance whatsoever.

I mean, I personally would never attempt to do an offline LED driver (linear or switch mode) without an AC Mains filter…but I couldn’t absolutely proove that without it there would be noise susceptibility issues….and unless one can give absolute proof that the filter is needed, then there’ll be no money for it.
 

alec_t

Well-Known Member
Most Helpful Member
You'll know soon enough if it's susceptible to interference when it goes for EMC testing!
 

Flyback

Well-Known Member
Thanks,
We'd like to do that susceptibility test to it first,but do you know what equipment is used for it?
 

ci139

Active Member
etc etc).
as it causes nukes in russia to launch at random times :) etc. etc. ?
those etc.-s may have their relevance . . .
interesting.png
why your simulation shows 339V pk-pk next to label 240V mains (what spice does not simulate - is some sort of rect. bridge effect of rising the input voltage name value to 1.2x up to 1.7x at output capacitor ... in your case 1.41... x ) . . . also i suggest stating your signal(or reference) ground in between L & N by 2 symmetrical , in series capacitor 1n resistor 10meg pairs and re-naming your rectified signal ground (such becomes more relevant when inductors / filters follow the rectifier)
 
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Flyback

Well-Known Member
Thanks, ......though our vout is 4mA and only 2V5...I presume you mean our LR8N8 regulator.

Seriously, i do wonder what are the things that could happen from so called noisy power supplies with no filters.
Half the time you find yourself wondering if most emc regs are just there to keep smaller rivals out of the market...more moolah for the big corporations.
 

alec_t

Well-Known Member
Most Helpful Member

Cicero

Active Member
Not sure if I'm missing something, but you've already hinted on multiple possible issues. Without speculating more, I'd dive right in and tackle them learning more by process of elimination.

On the micro side, it should be simple to add extra code to see if anything dodgy is going on internally due to noise. If its resetting/getting stuck somewhere, and you have no debug UART out or similar, toggle an extra pin or something.

As for the hardware issues, simply add in an off-the-shelf mains filter and see if it helps. Add in more capacitance and see if it helps.

Personally during a design, I usually add in a decent amopunt of 'what-if' components and footprints for possible EMC issues that arise. Then during testing, I get rid of whatever I can to make it smaller/cheaper if need be. These EMC delays cost a hell of a lot more money in the long run than an extra board spin. I'm very sceptical of not having a mains filter when there's any type of switching involved, and also decent decoupling/pullups etc.
 

Cicero

Active Member
Thanks,
We'd like to do that susceptibility test to it first,but do you know what equipment is used for it?
I don't know specifically for lighting equipment, but it can't be that different to the commercial/industrial testing that I'm used to.

For radiated immunity they use an RF signal source, a broadband power amplifier and a set of antennas, and blast your equipment from all angles with 3V/m 80MHz - 1GHz.

For conducted immunity you'd need a couple of CDN's (coupling-decoupling networks), also an RF signal and amplifier etc, I think 150Khz-80MHz.

The problem with trying to DIY this is that there's a lot they test for compliance, so what about everything else? Surges, fast transients (EFT), voltage dips & interruptions, the dreaded ESD testing? Once you factor in everything the cost of one or two days at a lab doesn't seem that bad. Also labs can do unsupervised testing if the equipment is well documented. This can work out much cheaper than a whole day and your time.
 

JimB

Super Moderator
Most Helpful Member
We'd like to do that susceptibility test to it first,but do you know what equipment is used for it?
There are fast pulse generators for this, used in conjunction with a LISN.
Have you tried to find any info for yourself?

Half the time you find yourself wondering if most emc regs are just there to keep smaller rivals out of the market.
No, the EMC regs are there to stop cheap crappy equipment from vomiting their spurious emissions all over other users of the electromagnetic spectrum.

...more moolah for the big corporations
Your poor little me, persecuted by all and sundry is wearing a bit thin.
Your recent (in another thread) defamatory comments regarding the ethics of Accredited EMC Test Houses, are to be quite honest rather galling.

Go get some big boy trousers, man up and grow a pair and stop your continual bleating.

JimB
 

Cicero

Active Member
This may help a bit, yeah its dated but still looks good to me:

http://www.compliance-club.com/archive/keitharmstrong/emc_testing4.html

http://www.compliance-club.com/archive/old_archive/011021.htm

Keep this in mind as well:
"Because of the possibilities for causing interference over a wide area, on-site (or in-situ) radiated RF immunity testing using a transmitting antenna without a shielded room (anechoic or not) is illegal in the UK unless a special license has been received from the Radiocommunication Agency (www.radio.gov.uk)."
 
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