Triac EMI during commutation and keeping it on to solve the problem...

[Klaus Kragelund]

On Friday, 16 June 2023 at 14:41:15 UTC+2, legg wrote:

On Thu, 15 Jun 2023 23:24:07 +0200, Klaus Vestergaard Kragelund
klau...@hotmail.com> wrote:

snip
The problem is that the triac turns off when the holding current is
below a certain value, and since the diac/triac is supplied from the
mains, it turns on again when some voltage has developed across the
triac. Thus crossover distortion that leads to emission problems.

Crossover distortion is not EMI.


It is. Quite clear from the measurements, that a discontinuity in the
current leads to conducted emissions. The discontinuity some times turns
the triac on several times, since it is right on the edge of the holding
current.

I did a test with an opto to keep the Triac turned on even when the
current is below the holding current, and now conducted emission is 30dB
lower.

So it\'s conducted emussions you\'re tracking down - not crossover
distortion.

One leads to the other, more or less.

In what frequency range?

The problem is seen up to 700kHz.

Measurements comparing a zero crossing controlled triac vs a opto coupler turning the Triac on during the zerocrossing:

https://www.electronicsdesign.dk/tmp/triac_emi_problem.pdf

Is your triac mounted on a chassis-grounded heatsink?
No, and the switching node of the triac is kept at low area, to avoid CM noise.

Do you have a filter, or differential/common mode caps?

There is no CM inductor, due to the high current of the triac outputs. There is both x and y caps on the triac supply lines.

It\'s not my design, I am just helping them to pass EMC. The layout is really crappy. CM inductor for a switch mode is placed next to the PSU, but other circuits on the hot side are placed close to the mains side, so it effectively shorts out the performance of the CM inductor. It is not the root cause, since shorting the triac M1/M2 removes the problem, and the implementation I did with turning on the triacs during the zero crossing had the same effect, removing the problem.

What is the physical load?

The load is a 75W Papst advanced fan, which has build in fan speed control. As seen in the curves I linked to, there is a charging of DC link capacitor for each half wave, so the speed control of the fan is done with vector control probably using a 3 phase power module on a PM motor.

 

[piglet]

On 17/06/2023 00:06, Klaus Kragelund wrote:

Measurements comparing a zero crossing controlled triac vs a opto coupler turning the Triac on during the zerocrossing:

https://www.electronicsdesign.dk/tmp/triac_emi_problem.pdf

Thanks. I don\'t get the green trace in the second experiment, is it
showing MT1-MT2 of 100V during the negative halfcycle?

piglet

 

[legg]

On Fri, 16 Jun 2023 16:06:43 -0700 (PDT), Klaus Kragelund
<klaus.kragelund@gmail.com> wrote:

On Friday, 16 June 2023 at 14:41:15 UTC+2, legg wrote:
On Thu, 15 Jun 2023 23:24:07 +0200, Klaus Vestergaard Kragelund
klau...@hotmail.com> wrote:

snip
The problem is that the triac turns off when the holding current is
below a certain value, and since the diac/triac is supplied from the
mains, it turns on again when some voltage has developed across the
triac. Thus crossover distortion that leads to emission problems.

Crossover distortion is not EMI.


It is. Quite clear from the measurements, that a discontinuity in the
current leads to conducted emissions. The discontinuity some times turns
the triac on several times, since it is right on the edge of the holding
current.

I did a test with an opto to keep the Triac turned on even when the
current is below the holding current, and now conducted emission is 30dB
lower.

So it\'s conducted emussions you\'re tracking down - not crossover
distortion.

One leads to the other, more or less.

In what frequency range?

The problem is seen up to 700kHz.

Measurements comparing a zero crossing controlled triac vs a opto coupler turning the Triac on during the zerocrossing:

https://www.electronicsdesign.dk/tmp/triac_emi_problem.pdf


Is your triac mounted on a chassis-grounded heatsink?
No, and the switching node of the triac is kept at low area, to avoid CM noise.


Do you have a filter, or differential/common mode caps?

There is no CM inductor, due to the high current of the triac outputs. There is both x and y caps on the triac supply lines.

Looks like the issue occurs at turn-off only, in one quadrant only.

Turn off, theoretically should not be affected by enhancment of
\'on\' drive.

Suggest a smaller gate resistor.

RL

 

[Klaus Vestergaard Kragelund]

On 17-06-2023 20:49, piglet wrote:

On 17/06/2023 00:06, Klaus Kragelund wrote:
Measurements comparing a zero crossing controlled triac vs a opto
coupler turning the Triac on during the zerocrossing:

https://www.electronicsdesign.dk/tmp/triac_emi_problem.pdf



Thanks. I don\'t get the green trace in the second experiment, is it
showing MT1-MT2 of 100V during the negative halfcycle?

The second plot green trace is linked to the y axis on the right. So
maximum voltage across the triac is 0.8V

 

[Klaus Vestergaard Kragelund]

On 18-06-2023 18:26, legg wrote:

On Fri, 16 Jun 2023 16:06:43 -0700 (PDT), Klaus Kragelund
klaus.kragelund@gmail.com> wrote:

On Friday, 16 June 2023 at 14:41:15 UTC+2, legg wrote:
On Thu, 15 Jun 2023 23:24:07 +0200, Klaus Vestergaard Kragelund
klau...@hotmail.com> wrote:

snip
The problem is that the triac turns off when the holding current is
below a certain value, and since the diac/triac is supplied from the
mains, it turns on again when some voltage has developed across the
triac. Thus crossover distortion that leads to emission problems.

Crossover distortion is not EMI.


It is. Quite clear from the measurements, that a discontinuity in the
current leads to conducted emissions. The discontinuity some times turns
the triac on several times, since it is right on the edge of the holding
current.

I did a test with an opto to keep the Triac turned on even when the
current is below the holding current, and now conducted emission is 30dB
lower.

So it\'s conducted emussions you\'re tracking down - not crossover
distortion.

One leads to the other, more or less.

In what frequency range?

The problem is seen up to 700kHz.

Measurements comparing a zero crossing controlled triac vs a opto coupler turning the Triac on during the zerocrossing:

https://www.electronicsdesign.dk/tmp/triac_emi_problem.pdf


Is your triac mounted on a chassis-grounded heatsink?
No, and the switching node of the triac is kept at low area, to avoid CM noise.


Do you have a filter, or differential/common mode caps?

There is no CM inductor, due to the high current of the triac outputs. There is both x and y caps on the triac supply lines.

Looks like the issue occurs at turn-off only, in one quadrant only.

Turn off, theoretically should not be affected by enhancment of
\'on\' drive.

Suggest a smaller gate resistor.

It does look funny. The load is a fan with inverter inside, so one sees
the charging of the DC link capacitor on the peak of the ac cycle.

But the charging during the negative ac cycle is very different from the
positive ac cycle. Suggests that something strange is going on inside
the fan. Maybe I should buy one of the fans and do a teardown.

Thanks for looking at the plots by the way