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EUT Monitoring Hardware

For Equipment Under Test (EUT) monitoring during EMC tests we’ve adapted a Digilent Analog Discovery 2, added an input filter board, and enclosed in a nice case from Lincoln Binns.

This connects to the (rapidly evolving) Monitor-o-Matic 8000 software mentioned in a previous post.

This adaptor and software is going to be used this week during testing of a piece of industrial equipment. This test adaptor will be monitoring both 4-20mA and relay outputs (using the in built power supplies to generate the voltage on one pin of the relay contacts and the digital inputs to monitor the other pin).

 

Analog Discovery 2

The AD2 is a very versatile piece of kit with a good balance of analogue and digital input and output for a reasonable price. It includes

  • 2 x 14-bit, 30MHz differential scope channels
  • 2 x 14-bit, 10MHz waveform generators
  • 16 x digital logic I/O with pattern generation and logic analyser
  • 2 x programmable power supplies

 

Future Plans

We’re going to be monitoring how well this device performs for monitoring during testing. We may need to add extra filtering beyond that already fitted such as optical isolation for the digital inputs.

We may also look at fitting a battery and a USB to fibre optic converter for fully isolated measurements in a variety of EMC environments.

 

 

ESD Latch Up Behaviour in Diodes Inc. Power Switch Parts

A new customer came to me with their product that was having problems during testing at another laboratory. There were radiated emissions problems (mostly solved with improvements to the ground plane scheme on the PCB) and a very interesting (and challenging) ESD problem which I’ll cover in this blog.

Here was the device exhibiting the problem, a Diodes Inc AP22802AW5-7 “power distribution load switch”. Input VBAT from a stick of AA batteries, SW_PWR from a rotary switch, and output to the rest of the circuit.

Problem outline

The ESD problem was described by the customer:

The EUT stopped working when 4kV contact discharges were applied on discharge point shown. I removed the batteries and I put them [in] again and there was not any response from the sample (no otuput and the green LED remained OFF).

[A second sample] was then tested with the same result, although this time not on the first discharge

Upon inspection both devices had failed due to the load switch (AP22802AW5-7Diodes), with one failing open and one failing short and both becoming very warm.

ESD diode placed on input and output of load switch (with no effect)

ESD diodes placed on all [discharge points] (with no effect)

ESD diode places on VCC close to pullup resistors for [discharge points] with no effect

First thing first was to get the product set up on the ESD table (with a bit of added blur to protect the innocent).

It was very easy to re-create the problem observed at the original test lab with the second contact discharge to the EUT exposed contact point causing the unit to shut down.

In each case, the power switch was failing low resistance from IN to GND. The initial theory was that the device was being damaged by the high voltage punching through the silicon layers leaving a conductive path.

 

Eliminate the possible

I made a series of experiments to determine the coupling path into the problematic device. Working on the principle that, because of the 15cm distance between discharge point and problem device, that conduction might have been the problem.

  • Capacitors on Vin and EN
  • plus disconnect EN line
  • plus ferrite beads and capacitors on Vin, Vout and EN
  • plus local TVS diodes on pins of device
  • plus ferrite beads in series with [EUT input] lines

Whilst none of these experiments were successful they certainly helped eliminate conduction as the coupling path.

Because of the very high frequency content of the ESD pulse, capacitive coupling is likely going to be the dominant coupling method. Whilst it could couple into the device directly, there was more opportunity for the pulse to couple into the traces connected to the device first. Filtering the inputs eliminates two coupling possibilities

 

Change of sample

The PCB was starting to get a bit tired from the repeated hot air SMT de-soldering and re-soldering so I swapped to another supplied sample. To be able to operate the unit out of the casing I swapped to a linear DC bench supply instead of the AA batteries.

This proved to be an interesting mode as it allowed me to kill the power quickly. The next set of experiments were in an attempt to reduce the effect of capacitive coupling to the problem device.

  • Improved ground stitching / connection
  • Changing supply voltage
  • Indirect HCP discharge – not to EUT but to the Horizontal Coupling Plane albeit with a vertical ESD gun to increase capacitive coupling to EUT.
  • Reduction of coupling into Vin terminal by removing components and copper
  • Addition of copper foil shield over the top of the device

 

Failure mode discovery

Setting the current limit on the DC supply to a fairly low value (about 20% higher than nominal current draw) was a good idea.

When applying the ESD strikes the supply went into foldback as the EUT power input went low resistance. I discovered that quickly turning off the power and then turning it back on effectively reset the failure mode of the device. This proved to be repeatable over several discharges: zap – foldback – power cycle – EUT OK.

What silicon component behaves like this? A thyristor.

This is a phenomena known as “latch up” where the parasitic thyristor structure present in the CMOS process fires due to over voltage… such as an ESD strike for instance!

Because the device is only small the power dissipation caused by the battery short circuit current is enough to “pop” the device through overheating.

 

Out of circuit testing

Whilst it doesn’t get used very often, my Sony Tektronix 370 curve tracer is perfect for testing components like this.

(not mine, picture From CAE Online)

Here’s the VI curve of an undamaged device. It’s a bipolar voltage between VIN and GND. On the left of centre is the standard forward biased body diode. On the right is the reverse biased breakdown of around 8V.

Now for a damaged device. In this case the current changes quickly for a small applied voltage and there is no non-linear characteristic. Essentially, a short circuit.

Turning up the maximum voltage that the curve tracer can apply and dialling down the series impedance allowed me to simulate the over voltage fault condition and create a latch up condition. This latch up wasn’t permanent due to the bipolar sine wave nature of the curve tracer applied voltage.

However turning up the voltage enough to cause excess power dissipation inside the device did result in the same failure mode using the curve tracer.

 

Summary

I have never encountered a device that is this unusually sensitive to ESD events before. A nearby 2kV discharge on the PCB top layer ground plane was enough to cause the latch up condition.

I noted in the report to the customer that this device had been changed to “not recommend for new designs” by Diodes Inc. I wonder if they identified this condition in the device and withdrew it for that reason.

The customer resolved the issue by replacing the device with a different part and we all lived happily ever after.

The end.

 

 

 

Schaffner/Teseq NSG 5500 test system

New Automotive Test Capabilities ISO 7637-2

The best day is new equipment day 🙂

We are continuing to invest in our test capabilities. As such, the Unit 3 Compliance EMC test laboratory has just acquired a Schaffner (Teseq) NSG 5500 automotive surge/EFT test generator.

Schaffner NSG 5500 test systemWith this, we now have the capability to test your equipment to the ISO 7637-2 standard for automotive conducted transients.

The NSG 5500 will generate the ISO pulses 1, 2a, 3a and 3b, along with the Load Dump and Clamped Load Dump pulses 5a and 5b.

This gives us the capability to support your automotive product development to these standards:

  • EN 50498:2010 – Aftermarket electronics for vehicles – full testing for CE marking
  • CISPR 25 for non Immunity Related Function EUTs
  • UNECE R10.06 (pre-compliance)
  • ISO 13766-1:2018 Earth Moving Machinery (pre-compliance)
  • ISO 7637-2:2011 automotive conducted transients
  • ISO 16750-2:2012 automotive electrical loads (part)

 

Footnote:

Timing is a curious thing. Like two buses arriving simultaneously after a long wait I find things tend to cluster up. This acquisition occurred not long after publishing this blog post on how to test to the automotive standards without an automotive surge generator.

Compromise EFT Test Setup

When the customer supplied cable isn’t long enough to fit inside the standard EFT/B capacitive clamp what do you do?

One answer, for pre-compliance testing, is to make your own clamp from aluminium foil cut to length and separated from the GRP by expanded foam blocks.

The capacitive clamp is not a sophisticated piece of test equipment and a close compromise can be achieved quickly with commonly available lab materials.

Details of a compromise EFT test setup using aluminium foil and foam blocks.

Making sure there is good contact to the GRP from the generator is important which is partly achieved by taping the cable down with some conductive adhesive aluminium tape.

Overall area of the injection plate is reduced by 25% from the standard capacitive clamp plate area. Therefore the injection voltage was increased by 25% to compensate for the reduced capacitance.

Safety warning: don’t touch the foil when the generator is running!

Obviously not good enough for exact testing to the standard but it is within the spirit of the test and will give some useful information.

Q4/17 Updates – A good variety of work!

It feels like it has been a busy couple of months here at Unit 3 Compliance with a wide variety of projects coming through the door.

Q1/18 is already shaping up to be busy with some really interesting products booked in for pre-compliance testing and some nice meaty problems to get our teeth into. I’m looking forward to sharing some of the insights I gain from this work with you.

Here’s a quick roundup of what’s been happening…

EMC Pre-Compliance

Our key area of expertise and always the cornerstone of what we do here at Unit 3 Compliance is EMC pre-compliance testing. In the chambers recently we’ve had ticket machines, water boilers, development kits, and a light/motion sensor. Some with problems that we quickly fixed and some sailed through first time.

One particularly interesting product was an industrial lighting system that needed radiated RF immunity testing at 20V/m. This test loves to mess with products by turning on or off semiconductors that were quite happy as they were thank you very much. In this case, there was an transistor based current limiting circuit that, thanks to one of the transistors demodulating the RF carrier, decided to shut down key parts of the circuit. Replacing it with a resistor removed the problem allowing the customers development cycle to continue.

Microwave Antenna Pattern Measurement

A customer has been leasing the anechoic chamber to make some antenna pattern measurements on a complex microwave antenna system. By loading up the quiet zone of the chamber with extra microwave absorber we were able to provide a highly anechoic (low reflection) environment all the way up to 18GHz.

As part of this exercise we made some rough background noise measurements from 2GHz up to 18GHz revealing very little. This suggests that when we reassembled the chamber in its new home we didn’t leave any gaps!

Vibration Testing

The vibration shaker and amplifier have been fully commissioned after their move. They’ve been getting a good run in performing a 2g sine sweep test on a large 25kg rack mount power supply.

Jigging equipment onto the vibration table is always a challenge, especially for a large and heavy piece of equipment like this one. I like to use 1″ x 1″ x 1/8″ wall aluminium box section (really stiff and light) along with high tensile M10 threaded bar to clamp an EUT of this size. Smaller EUTs can be easily secured to lighter platforms using hot-melt glue, surprisingly effective!

I always find vibration testing fascinating, especially watching various components come in and out of resonance during a sine sweep test. It’s fun to draw parallels between mechanical and electrical resonance, stiffness, impedance and damping.

In this case we found a large resonance that caused a fracture of the base plate due to excessive motion. We suggested a few approaches to stiffening that area, one of which was implemented and successfully removed the resonance.

One piece of equipment I’m going to be designing soon is an LED strobe lamp that synchronises to the output of the vibration controller so that any flexing in resonant modes can be easily spotted. That will make analysis much easier.

Design Reviews

We’ve carried our several sets of schematic and PCB design reviews, from motion sensors to heater controllers, from pump monitors to semiconductor development kits.

Our approach is not only to look at EMC / system level but also to question and educate designers on alternative circuit choices based on our long experience in electronics design. This is part of the value that we give to our customers.

In each case we’ve addressed the circuit design, considering the EMI phenomena and levels that the ports of the design will be exposed to. This is where understanding the tests themselves is so important otherwise the circuit could be susceptible to problems.

We also look at design partitioning in some detail. This is one of the easiest ways to achieve good system level performance (and not just from an EMC perspective) by segregating the design into digital, analogue, power supply and I/O areas with the aim of keeping noise currents where they should be and away from their potential victims.