Case Study: AC Mains Input EMC and Safety Troubleshooting

Many of the customers I deal with are technically savvy and extremely good at designing innovative and clever devices. I’m always learning something new every time I get a different product through the door. Unfortunately it isn’t practical or possible to be good at everything and EMC expertise, especially when it comes to fault finding and problem solving, can be hard to come by. This is where I come in.

I’ve been helping a good customer on a product that they’ve been working with that had some EMC troubles on a prototype design. It had originally been taken to a different test lab where they had performed a mains conducted emissions measurement showing a clear failure at low frequencies. There were a couple of other hard copy scans supplied where a capacitor value had been adjusted to try and improve the emissions but with no effect.

In need of some expertise, they got in touch.

Mains Conducted Emissions Testing

I received the product and quickly set it up in our screened room to perform some EN 55014-1 conducted emissions measurements. Below you can see the first scan result, showing a failure of up to 10dB on the Quasi Peak detector. There’s clearly some room for improvement so let’s analyse the problem and see what we can do.

mains conducted emissions - before

Our starting point for the improvement work

Lower frequency mains conducted emissions are not uncommon and are usually caused by differential mode voltage noise. This is generated by current flowing through the impedance presented by the primary side bulk decoupling and switching circuit. The switching frequencies of the power supply controller are usually in the 30 kHz to 250 kHz range putting it (and it’s harmonics) right in this lower frequency (sub 1MHz) range for this test.

Improving differential mode noise can be done in a number of ways. Removing the noise at source is the approach I advocate, in this case this can be achieved by reducing the impedance of the rectified mains bulk decoupling capacitor. A review of the BOM showed that the units had been built with some general purpose electrolytic capacitors with a relatively high impedance. So the first thing that I did was to swap out these parts for ones from the Nichicon PW series of low impedance electrolytic capacitors.

after fitting low impedance bulk decoupling

Changing the electrolytics to a low impedance variety

The result: a big improvement on the QP measurements, bringing some of them down by around 10dB. The improvement on the Average detector readings was less pronounced, especially around 550 kHz where only a 3dB improvement was registered. It is likely that the HF impedance of the decoupling capacitor is still a problem. One option is to apply a suitably rated high frequency decoupling capacitor in parallel with the bulk decoupling capacitor. The other option is to improve the filtering on the AC mains input to prevent the noise from escaping back down the line.

Filtering for differential mode noise can be provided in several ways. The most common method is to make an LC filter from the leakage inductance of a common mode choke paired with a Class X safety capacitor between Live and Neutral. The leakage inductance is in the tens of micro-Henries whereas the common mode inductance is often a couple of magnitudes larger up in the tens of milli-Henries. Simplistically (there are other effects to consider) a 10uH leakage inductance paired with a 470nF capacitor will roll off frequencies above 100 kHz. Well, let’s try that!

now with added class X cap

Now with an additional 470nF Class X capacitor soldered across the mains input terminals

Performance is improved by around 5dB across a wide range of frequencies; indeed the improvement can be seen up to 15 MHz. This leaves a margin of around 2dB to the average limit line which is perhaps a bit close for comfort and I would generally recommend looking at a little more filtering to bring this down a bit further to allow for variations in production and tolerance of components. Options for further improvements could include a second Class X capacitor to form a pi filter but because of the low impedance of the differential mode noise this approach might not be as effective. Adding some inductance to form an LC filter with the bulk decoupling capacitor is another approach.

However this proved the case to the customer for a PCB redesign to make space for the larger bulk decoupling capacitors and at least one Class X capacitor.

Surge and Safety

Following on from this work, at the customers request, I carried out a full suite of EMC tests on the product to EN 55014-1 (emissions) and 55014-2 (immunity). One thing that I noticed was the sound of an electrical breakdown during the application of a differential mode surge test. Taking off the outer casing, I managed to catch the below arc on camera during a 1kV surge event.

Arcing caught on camera

Snap, crackle and pop.

The arc appeared around the resistor; desoldering and removing it from the PCB showed a couple of points where there was arcing between the resistor body and the trace running underneath it.

Arcind damage to the PCb to surface

Arcing evidence on the PCB

This problem has occurred because the resistor R1 is in series with the Live phase and the trace underneath is connected to the Neutral phase. When mounted flush to the PCB normally, the resistor has only its outer insulation between live and neutral. Reviewing the relevant electrical safety standard for the product requires a minimum clearance (air gap) for basic and functional insulation is 1.5mm. This can be achieved by standing the resistor up on spacers to keep it away from the PCB but then it starts to approach VDR1 and Q4 meaning a considered manufacturing approach is required. This was another incentive for redesigning the PCB.

The take-away lesson from this finding is to consider the Z axis / third dimension when reviewing a PCB as it can be easy to see things purely in two dimensions!

I hope you found this case study useful and that it has given you some tools with which you can improve your designs.

If you need some EMC fault finding expertise then get in touch: I’d be happy to help and I love a good challenge!

Not just EMC – we get our hands dirty

More than just a test lab, we are your product design partner.

A busy week of EMC testing this week on a variety of products but, as the picture suggests, our usefulness doesn’t just stop at testing.

brass gear in vise

One customer posted a sample of their product for testing with the latest sample of DC motor in. However the mechanism was jamming part way through the operating cycle meaning radiated emissions testing couldn’t be completed (no motor brush noise, stall current doesn’t count). Rather than just sit on my hands and wait for them to sort it out, I stripped the mechanics down and found some rough edges on two of the parts. A bit of work with a needle file and some light grease and it was back together and working just fine.

Also this week we’ve helped a customer with designing a regulator circuit to control some LEDs and provided a little bit of training and explanation time too, receiving some good feedback in the process. Explaining something and seeing the understanding take hold is a great experience, we love sharing our knowledge and helping de-mystify EMC.

Here at Unit 3 Compliance we don’t just test, we solve your problems proactively – providing a versatile and friendly engineering service.

Unit 3 Compliance anechoic chamber

Recent Work: Medical Laboratory Equipment EMC Testing and more

It has been an good couple of weeks here at Unit 3 Compliance with some very interesting products to work on.

Firstly, I received confirmation from a previous customer that their product has passed the required EMC certification testing at their accredited laboratory with the modifications that we incorporated during our problem resolution work. This is excellent news for all concerned!

Then things started off with some radiated emissions EMC testing and fault finding on quite a complex and clever piece of medical laboratory equipment with multiple interconnected boards, display, motors and servos. A few problems were identified and feedback given to the customer about potential improvements.

Noisy DC motors were again the theme in some more radiated emissions testing, requiring additional suppression on the motor terminals, made all the more challenging by the tight mechanical constraints of the product. Differential mode suppression on the terminals using ferrite beads to reduce the brush noise is the most effective solution but without a well defined RF return path to the brushes any noise reduction will ultimately be a compromise. More testing is being performed with some small filter PCBs mounted right on the motor terminals.

Lastly, our screened room test facility is almost completed and is being used for some mains and DC port conducted emissions testing with buck converter switching noise providing a challenge.

“Problems worthy of attack prove their worth by fighting back!” 

Unit 3 Compliance anechoic chamber

Radiated emissions fault finding and pre-compliance in the Unit 3 Compliance fully anechoic chamber

 

radiated emissions plot showing improvements made to customers product

Brushed DC Motor – EMC Radiated Emissions Problems and Improvements

I’ve been working with a customer whose product had significant broadband radiated emissions throughout the spectrum, helping them with fault finding and improving their EMC performance. Partial information from their certifying test lab appeared to show a regular harmonic series suggesting noise from a digital clock of some kind. However, near field probing with a spectrum analyser on the bench disproved this theory and identified the brushed DC motor as the cause. It is believed that a combination of the certifying test lab only performing one frequency sweep, the long cycle time of the EUT and the random arcing noise from the DC motor brushes caused a series of regular peaks to appear distributed throughout the spectrum.

In reality, both spectrum analyser measurements and the radiated emissions plot shown below show a wide range of broadband noise once the graph had multiple sweeps through the spectrum to enable it to become fully populated.

Working closely with the customer, we selected an alternate motor from the same manufacturer part with suppression capacitors built in to decouple the brush noise at the source; always the most efficient way of solving an EMC problem is to knock it on the head where is is being generated. Putting the EUT in the anechoic chamber showed that this provided a significant improvement, especially at higher frequencies but there remained an hump at around 150MHz to 300MHz.

We followed up with testing various ferrite cores around the DC motor power leads to further reduce the noise being conducted down the lead, leading to the selection of a small ferrite core with two turns around the motor DC supply leads.

Below can be seen the scan data showing before any modifications (blue) and after changing the motor and adding the ferrite core (green). Testing also revealed what appeared to be a satisfied customer 😉

radiated emissions plot showing improvements made to customers product

NFC Antenna Optimisation

We’ve recently been working on optimising an Near Field Communications (NFC) card reader antenna using our skills in wireless integration and antenna design. The customer had an existing system and a new requirement to mount the read antenna at the end of a cable away from the NFC reader mainboard.

This involved us working on the original system by modifying the existing antenna to the required inductance, implementing and tuning an impedance matching network and trialling different cable types. Remember, the rules for tuning the read antenna are different to those for the receive antenna.

Despite the design challenges we were able to achieve a card read distance up to 30mm away from the antenna and provide a robust solution for mounting on different surface materials. The customer was happy and we look forward to seeing the product out in the field!