How should a PoE flyback transformer look on the oscilloscope

Measure at the pin not connected to the 48VDC line i.e. the pin of the transformer connected to the MOSFET / integrated flyback regulator IC (If unsure, try it out).

Depending on how exactly the IC works and the load (and the circuitry), these waveforms look quite differently. So the following pictures are mostly for basic reference.

Posted by Uli Köhler in Electronics, EMI, PoE

What 1500pF 1kV capacitor should you use for Ethernet termination?

Nowadays you can use cheap ceramic capacitors for Ethernet termination. While they are not self-healing like foil capacitors, they work fine for all but the most demanding applications. Also, it’s typically much cheaper to assemble SMD capacitors as opposed to through-hole foil types since the SMD types can be more easily picked & placed by machines.

I recommend to use the Yageo CC1206KKX7RCBB152 because it’s cheap (0,10€ @100pcs) and readily available at every major distributor.

Furthermore, it has a X7R ceramic, meaning that its capacitance doesn’t change too much with temperatur (not as much as, for example, Y5V ceramics). Note that X5R and X7R types have a capacity that depends on the voltage being applied, so if you have an application that is really sensitive to RF noise, keep in mind that if a significant DC voltage is applied to the capacitor, its capacitance will drop by up to tens of percents – leading to impromper Ethernet termination and hence more EMI.

Additionally, keep in mind that ceramic capacitors are somewhat susceptiple to mechanical flexing of the PCB. This is especially the case if you have a very thin (or even rigid-flex) PCB, or if high mechanical loads (either static loads or vibrations) are applied directly to your PCB. In that case, consider buying e.g. a slightly more expensive, “flexible termination” type capacitor.

Posted by Uli Köhler in Compliance, Electronics, EMI

What is performance criterion A/B/C/D in EMI tests?

When doing EMI immunity tests, you will often encounter specifications that your device should e.g. “meet Performance Criterion B during the ESD test”.

The performance criteria, as defined in the IEC 61000 series of standards, e.g. IEC 61000-4-5:2014 (surge test standard) are:

  • Performance Criterion A: Your device continues to function normally during the test (i.e. no interruption)
  • Performance Criterion B: Your device shows a malfunction but recovers automatically  (without user interaction) and then continues to operate normally.
  • Performance Criterion C: Your devices shows a malfunction, but continues to operate normally only after user interaction (e.g. the user resets the device)
  • Performance Criterion D: Your device has a permanent malfunction (e.g. some part of its hardware has been destroyed, or data being lost) which cannot be recovered from even after user interaction.

Additionally note that your device must not become dangerous under any circumstances, even if destroyed as per Performance Criterion D.

What defines a malfunction is up to the manufacturer of the device, but usually it’s related to whatever function it is advertised to perform. The manufacturer should define

Example: 230V LED bulb

  • Performance Criterion A: LED Bulb continues to operate normally after the test
  • Performance Criterion B: Bulb flickers during the test, but operates normally after the test
  • Performance Criterion B: Bulb is less bright during the test but then operates normally.
  • Performance Criterion B: Bulb blinks 10 times slowly but then operates normally
  • Performance Criterion B: Bulb stays off for 20 seconds after the test but then goes back to normal.
  • Performance Criterion C: Bulb goes out, but works normally after you switch off and on the power
  • Performance Criterion C: Bulb continues to blink, but works normally after you switch off and on the power
  • Performance Criterion D: Bulb goes out and does not turn on after you switch off and on the power
  • Performance Criterion D: Bulb blinks after the test and continues to blink even if you switch off and on the power.

As a manufacturer you could define e.g.

  • that the light intensity should be judged by eye (i.e. it should not be significantly dimmer after the test than a new bulb)
  • that a luxmeter needs to be used to judge the light intensity after tests
  • that the bulb may stay off for at most 15 seconds

and so on. In all but the most demanding applications, manufacturers define the criteria conservatively in order to increase the likelihood of passing the test. Since EMI laboratory tests are expensive, it’s in your best interest as a manufacturer not to over-define the criteria.

Posted by Uli Köhler in Electronics, EMI