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

[Klaus Vestergaard Kragelund]

Hi

So there\'s been some nice discussion about Triacs.

I have an application where an opto diac is controlling a Triac that can
powers an ac load.

During commutation, when the current in the Triac is below holding
current, the Triac turns off. It is turned on again when the diac turns
on in the zero crossing, but delayed a bit by the voltage across the
Triac needing to develop to a certain value at which the gate current is
enough to turn the Triac on.

This dropb of voltage before the zero crossing and the delay of turn on
creates conducted emission problems below 5-10MHz and is a well known
problem.

For example:

https://www.st.com/resource/en/application_note/dm00451014-controlling-a-triac-with-a-phototriac-stmicroelectronics.pdf

https://www.vishay.com/docs/84630/84630.pdf

A number of solutions are presented, but none of them talks about just
forcing the Triac on during the zero crossing. So adding a separate
power supply, and turning the Triac on some time before the zero
crossing, and some time after also. It seems it would be a good idea.

Any one tried this, before I go and simulate/build it?

Cheers

Klaus

 

[boB]

On Mon, 12 Jun 2023 01:00:01 +0200, Klaus Vestergaard Kragelund
<klauskvik@hotmail.com> wrote:

Hi

So there\'s been some nice discussion about Triacs.

I have an application where an opto diac is controlling a Triac that can
powers an ac load.

During commutation, when the current in the Triac is below holding
current, the Triac turns off. It is turned on again when the diac turns
on in the zero crossing, but delayed a bit by the voltage across the
Triac needing to develop to a certain value at which the gate current is
enough to turn the Triac on.

This dropb of voltage before the zero crossing and the delay of turn on
creates conducted emission problems below 5-10MHz and is a well known
problem.

For example:

https://www.st.com/resource/en/application_note/dm00451014-controlling-a-triac-with-a-phototriac-stmicroelectronics.pdf

https://www.vishay.com/docs/84630/84630.pdf

A number of solutions are presented, but none of them talks about just
forcing the Triac on during the zero crossing. So adding a separate
power supply, and turning the Triac on some time before the zero
crossing, and some time after also. It seems it would be a good idea.

Any one tried this, before I go and simulate/build it?

Cheers

Klaus

Haven\'t tried that before, no, but it just might work OK.

One of the big reasons I like triacs is that with a somewhat inductive
load, usually for me it is an inductive power resistor.... is that it
turns off naturally at zero crossing at least up to a few hundred Hz.

So, maybe if you add a bit of indcuctance in series, it could slow
that turn on at a few volts above/below zero crossing and still turn
off OK ?

Nice app notes. thanks.

boB

 

[Phil Hobbs]

Klaus Vestergaard Kragelund <klauskvik@hotmail.com> wrote:

Hi

So there\'s been some nice discussion about Triacs.

I have an application where an opto diac is controlling a Triac that can
powers an ac load.

During commutation, when the current in the Triac is below holding
current, the Triac turns off. It is turned on again when the diac turns
on in the zero crossing, but delayed a bit by the voltage across the
Triac needing to develop to a certain value at which the gate current is
enough to turn the Triac on.

This dropb of voltage before the zero crossing and the delay of turn on
creates conducted emission problems below 5-10MHz and is a well known
problem.

For example:

https://www.st.com/resource/en/application_note/dm00451014-controlling-a-triac-with-a-phototriac-stmicroelectronics.pdf

https://www.vishay.com/docs/84630/84630.pdf

A number of solutions are presented, but none of them talks about just
forcing the Triac on during the zero crossing. So adding a separate
power supply, and turning the Triac on some time before the zero
crossing, and some time after also. It seems it would be a good idea.

Any one tried this, before I go and simulate/build it?

Cheers

Klaus

Zero-voltage switching is used in most SSRs, I think. It’s been around for
ages.

Cheers

Phil Hobbs

--
Dr Philip C D Hobbs Principal Consultant ElectroOptical Innovations LLC /
Hobbs ElectroOptics Optics, Electro-optics, Photonics, Analog Electronics

 

[Klaus Vestergaard Kragelund]

On 12-06-2023 03:11, boB wrote:

On Mon, 12 Jun 2023 01:00:01 +0200, Klaus Vestergaard Kragelund
klauskvik@hotmail.com> wrote:

Hi

So there\'s been some nice discussion about Triacs.

I have an application where an opto diac is controlling a Triac that can
powers an ac load.

During commutation, when the current in the Triac is below holding
current, the Triac turns off. It is turned on again when the diac turns
on in the zero crossing, but delayed a bit by the voltage across the
Triac needing to develop to a certain value at which the gate current is
enough to turn the Triac on.

This dropb of voltage before the zero crossing and the delay of turn on
creates conducted emission problems below 5-10MHz and is a well known
problem.

For example:

https://www.st.com/resource/en/application_note/dm00451014-controlling-a-triac-with-a-phototriac-stmicroelectronics.pdf

https://www.vishay.com/docs/84630/84630.pdf

A number of solutions are presented, but none of them talks about just
forcing the Triac on during the zero crossing. So adding a separate
power supply, and turning the Triac on some time before the zero
crossing, and some time after also. It seems it would be a good idea.

Any one tried this, before I go and simulate/build it?

Cheers

Klaus


Haven\'t tried that before, no, but it just might work OK.

One of the big reasons I like triacs is that with a somewhat inductive
load, usually for me it is an inductive power resistor.... is that it
turns off naturally at zero crossing at least up to a few hundred Hz.

So, maybe if you add a bit of indcuctance in series, it could slow
that turn on at a few volts above/below zero crossing and still turn
off OK ?

Nice app notes. thanks.

I tried a simulation, and looks very good. No cross over distortion at
all. Now, to build it

 

[Klaus Vestergaard Kragelund]

On 12-06-2023 04:50, Phil Hobbs wrote:

Klaus Vestergaard Kragelund <klauskvik@hotmail.com> wrote:
Hi

So there\'s been some nice discussion about Triacs.

I have an application where an opto diac is controlling a Triac that can
powers an ac load.

During commutation, when the current in the Triac is below holding
current, the Triac turns off. It is turned on again when the diac turns
on in the zero crossing, but delayed a bit by the voltage across the
Triac needing to develop to a certain value at which the gate current is
enough to turn the Triac on.

This dropb of voltage before the zero crossing and the delay of turn on
creates conducted emission problems below 5-10MHz and is a well known
problem.

For example:

https://www.st.com/resource/en/application_note/dm00451014-controlling-a-triac-with-a-phototriac-stmicroelectronics.pdf

https://www.vishay.com/docs/84630/84630.pdf

A number of solutions are presented, but none of them talks about just
forcing the Triac on during the zero crossing. So adding a separate
power supply, and turning the Triac on some time before the zero
crossing, and some time after also. It seems it would be a good idea.

Any one tried this, before I go and simulate/build it?

Cheers

Klaus


Zero-voltage switching is used in most SSRs, I think. It’s been around for
ages.

Yes, SSRs use zero crossing detectors, but they don\'t seem to turn on
during the zero crossing. None that I have found anyway.

Reason is obvious IMHO, the SSR is a controlled device, they don\'t know
the future, so if the Triac is meant to be turned on for the next
period. In my system I know that.

 

[Phil Hobbs]

Klaus Vestergaard Kragelund <klauskvik@hotmail.com> wrote:

On 12-06-2023 04:50, Phil Hobbs wrote:
Klaus Vestergaard Kragelund <klauskvik@hotmail.com> wrote:
Hi

So there\'s been some nice discussion about Triacs.

I have an application where an opto diac is controlling a Triac that can
powers an ac load.

During commutation, when the current in the Triac is below holding
current, the Triac turns off. It is turned on again when the diac turns
on in the zero crossing, but delayed a bit by the voltage across the
Triac needing to develop to a certain value at which the gate current is
enough to turn the Triac on.

This dropb of voltage before the zero crossing and the delay of turn on
creates conducted emission problems below 5-10MHz and is a well known
problem.

For example:

https://www.st.com/resource/en/application_note/dm00451014-controlling-a-triac-with-a-phototriac-stmicroelectronics.pdf

https://www.vishay.com/docs/84630/84630.pdf

A number of solutions are presented, but none of them talks about just
forcing the Triac on during the zero crossing. So adding a separate
power supply, and turning the Triac on some time before the zero
crossing, and some time after also. It seems it would be a good idea.

Any one tried this, before I go and simulate/build it?

Cheers

Klaus


Zero-voltage switching is used in most SSRs, I think. It’s been around for
ages.

Yes, SSRs use zero crossing detectors, but they don\'t seem to turn on
during the zero crossing. None that I have found anyway.

Reason is obvious IMHO, the SSR is a controlled device, they don\'t know
the future, so if the Triac is meant to be turned on for the next
period. In my system I know that.

Hmm, I don’t think that’s the case. The SSR basically just resynchronizes
the control signal at the next zero crossing.

Been done for ages.

Cheers

Phil Hobbs

--
Dr Philip C D Hobbs Principal Consultant ElectroOptical Innovations LLC /
Hobbs ElectroOptics Optics, Electro-optics, Photonics, Analog Electronics

 

[legg]

On Mon, 12 Jun 2023 01:00:01 +0200, Klaus Vestergaard Kragelund
<klauskvik@hotmail.com> wrote:

Hi

So there\'s been some nice discussion about Triacs.

I have an application where an opto diac is controlling a Triac that can
powers an ac load.

During commutation, when the current in the Triac is below holding
current, the Triac turns off. It is turned on again when the diac turns
on in the zero crossing, but delayed a bit by the voltage across the
Triac needing to develop to a certain value at which the gate current is
enough to turn the Triac on.

This dropb of voltage before the zero crossing and the delay of turn on
creates conducted emission problems below 5-10MHz and is a well known
problem.

For example:

https://www.st.com/resource/en/application_note/dm00451014-controlling-a-triac-with-a-phototriac-stmicroelectronics.pdf

https://www.vishay.com/docs/84630/84630.pdf

A number of solutions are presented, but none of them talks about just
forcing the Triac on during the zero crossing. So adding a separate
power supply, and turning the Triac on some time before the zero
crossing, and some time after also. It seems it would be a good idea.

Any one tried this, before I go and simulate/build it?

Cheers

Klaus

If there is a downstream converter, you can always use an extra
winding to get triac or thyristor conduction.

The usual question of \'who\'s on first\' apply.

This is also another possible route for EMI, at the converter\'s
frequency.

If you capacitively couple the RCdiac circuit, you can advance
it\'s firing angle by whatever degrees the RCdiac feed permits.
You can also create a low power capacitive reservoir, with the
RCdiac circuit downstream. Shorting the C optically will
disable it. Triacs will fire with both polarity of gate pulse,
though not as reliably in both quadrants.

To ensure triacs or thyristors don\'t turn off to high
harmonic/reversing current flow, you need to use a pulse
train, anyways.

RL

 

[Ricky]

On Monday, June 12, 2023 at 3:01:30 AM UTC-4, Klaus Vestergaard Kragelund wrote:

On 12-06-2023 04:50, Phil Hobbs wrote:
Klaus Vestergaard Kragelund <klau...@hotmail.com> wrote:
Hi

So there\'s been some nice discussion about Triacs.

I have an application where an opto diac is controlling a Triac that can
powers an ac load.

During commutation, when the current in the Triac is below holding
current, the Triac turns off. It is turned on again when the diac turns
on in the zero crossing, but delayed a bit by the voltage across the
Triac needing to develop to a certain value at which the gate current is
enough to turn the Triac on.

This dropb of voltage before the zero crossing and the delay of turn on
creates conducted emission problems below 5-10MHz and is a well known
problem.

For example:

https://www.st.com/resource/en/application_note/dm00451014-controlling-a-triac-with-a-phototriac-stmicroelectronics.pdf

https://www.vishay.com/docs/84630/84630.pdf

A number of solutions are presented, but none of them talks about just
forcing the Triac on during the zero crossing. So adding a separate
power supply, and turning the Triac on some time before the zero
crossing, and some time after also. It seems it would be a good idea.

Any one tried this, before I go and simulate/build it?

Cheers

Klaus


Zero-voltage switching is used in most SSRs, I think. It’s been around for
ages.

Yes, SSRs use zero crossing detectors, but they don\'t seem to turn on
during the zero crossing. None that I have found anyway.

Reason is obvious IMHO, the SSR is a controlled device, they don\'t know
the future, so if the Triac is meant to be turned on for the next
period. In my system I know that.

??? That makes no sense. It is relatively easy to create a pattern of on and off half cycles to approximate virtually any setting of the average duty cycle. You don\'t need to predict the control input. The duty cycle responds to the control input.

--

Rick C.

- Get 1,000 miles of free Supercharging
- Tesla referral code - https://ts.la/richard11209

 

[John Larkin]

On Mon, 12 Jun 2023 09:01:23 +0200, Klaus Vestergaard Kragelund
<klauskvik@hotmail.com> wrote:

On 12-06-2023 04:50, Phil Hobbs wrote:
Klaus Vestergaard Kragelund <klauskvik@hotmail.com> wrote:
Hi

So there\'s been some nice discussion about Triacs.

I have an application where an opto diac is controlling a Triac that can
powers an ac load.

During commutation, when the current in the Triac is below holding
current, the Triac turns off. It is turned on again when the diac turns
on in the zero crossing, but delayed a bit by the voltage across the
Triac needing to develop to a certain value at which the gate current is
enough to turn the Triac on.

This dropb of voltage before the zero crossing and the delay of turn on
creates conducted emission problems below 5-10MHz and is a well known
problem.

For example:

https://www.st.com/resource/en/application_note/dm00451014-controlling-a-triac-with-a-phototriac-stmicroelectronics.pdf

https://www.vishay.com/docs/84630/84630.pdf

A number of solutions are presented, but none of them talks about just
forcing the Triac on during the zero crossing. So adding a separate
power supply, and turning the Triac on some time before the zero
crossing, and some time after also. It seems it would be a good idea.

Any one tried this, before I go and simulate/build it?

Cheers

Klaus


Zero-voltage switching is used in most SSRs, I think. It’s been around for
ages.

Yes, SSRs use zero crossing detectors, but they don\'t seem to turn on
during the zero crossing. None that I have found anyway.

Reason is obvious IMHO, the SSR is a controlled device, they don\'t know
the future, so if the Triac is meant to be turned on for the next
period. In my system I know that.

If the gate driver is powered by the switched voltage, it needs that
voltage to be up some before it can fire the gate. 5, 10 volts maybe?

The gate driver won\'t blow up at the AC peak because the triac shorts
its power supply! Dimmers similarly protect themselves by crowbarring
their own power supply.

The alternate way would use a battery or supercap and some logic to
fire exactly at the zero cross.

Transformer coupling into the gate would work, but the driver would
need to know what the AC line waveform was doing.

 

[Klaus Vestergaard Kragelund]

On 12-06-2023 18:31, Ricky wrote:

On Monday, June 12, 2023 at 3:01:30 AM UTC-4, Klaus Vestergaard Kragelund wrote:
On 12-06-2023 04:50, Phil Hobbs wrote:
Klaus Vestergaard Kragelund <klau...@hotmail.com> wrote:
Hi

So there\'s been some nice discussion about Triacs.

I have an application where an opto diac is controlling a Triac that can
powers an ac load.

During commutation, when the current in the Triac is below holding
current, the Triac turns off. It is turned on again when the diac turns
on in the zero crossing, but delayed a bit by the voltage across the
Triac needing to develop to a certain value at which the gate current is
enough to turn the Triac on.

This dropb of voltage before the zero crossing and the delay of turn on
creates conducted emission problems below 5-10MHz and is a well known
problem.

For example:

https://www.st.com/resource/en/application_note/dm00451014-controlling-a-triac-with-a-phototriac-stmicroelectronics.pdf

https://www.vishay.com/docs/84630/84630.pdf

A number of solutions are presented, but none of them talks about just
forcing the Triac on during the zero crossing. So adding a separate
power supply, and turning the Triac on some time before the zero
crossing, and some time after also. It seems it would be a good idea.

Any one tried this, before I go and simulate/build it?

Cheers

Klaus


Zero-voltage switching is used in most SSRs, I think. It’s been around for
ages.

Yes, SSRs use zero crossing detectors, but they don\'t seem to turn on
during the zero crossing. None that I have found anyway.

Reason is obvious IMHO, the SSR is a controlled device, they don\'t know
the future, so if the Triac is meant to be turned on for the next
period. In my system I know that.

??? That makes no sense. It is relatively easy to create a pattern of on and off half cycles to approximate virtually any setting of the average duty cycle. You don\'t need to predict the control input. The duty cycle responds to the control input.

If a certain percentage of cycles on are needed, then I certainly don\'t
want the SSR to take the decision to delay turn on to the next cycle.

 

[Klaus Vestergaard Kragelund]

On 12-06-2023 20:26, John Larkin wrote:

On Mon, 12 Jun 2023 09:01:23 +0200, Klaus Vestergaard Kragelund
klauskvik@hotmail.com> wrote:

On 12-06-2023 04:50, Phil Hobbs wrote:
Klaus Vestergaard Kragelund <klauskvik@hotmail.com> wrote:
Hi

So there\'s been some nice discussion about Triacs.

I have an application where an opto diac is controlling a Triac that can
powers an ac load.

During commutation, when the current in the Triac is below holding
current, the Triac turns off. It is turned on again when the diac turns
on in the zero crossing, but delayed a bit by the voltage across the
Triac needing to develop to a certain value at which the gate current is
enough to turn the Triac on.

This dropb of voltage before the zero crossing and the delay of turn on
creates conducted emission problems below 5-10MHz and is a well known
problem.

For example:

https://www.st.com/resource/en/application_note/dm00451014-controlling-a-triac-with-a-phototriac-stmicroelectronics.pdf

https://www.vishay.com/docs/84630/84630.pdf

A number of solutions are presented, but none of them talks about just
forcing the Triac on during the zero crossing. So adding a separate
power supply, and turning the Triac on some time before the zero
crossing, and some time after also. It seems it would be a good idea.

Any one tried this, before I go and simulate/build it?

Cheers

Klaus


Zero-voltage switching is used in most SSRs, I think. It’s been around for
ages.

Yes, SSRs use zero crossing detectors, but they don\'t seem to turn on
during the zero crossing. None that I have found anyway.

Reason is obvious IMHO, the SSR is a controlled device, they don\'t know
the future, so if the Triac is meant to be turned on for the next
period. In my system I know that.

If the gate driver is powered by the switched voltage, it needs that
voltage to be up some before it can fire the gate. 5, 10 volts maybe?

The power supply demand is probably quite small, so it will be up in
about a millisecond.

 

[Ricky]

On Monday, June 12, 2023 at 7:51:44 PM UTC-4, Klaus Vestergaard Kragelund wrote:

On 12-06-2023 18:31, Ricky wrote:
On Monday, June 12, 2023 at 3:01:30 AM UTC-4, Klaus Vestergaard Kragelund wrote:
On 12-06-2023 04:50, Phil Hobbs wrote:
Klaus Vestergaard Kragelund <klau...@hotmail.com> wrote:
Hi

So there\'s been some nice discussion about Triacs.

I have an application where an opto diac is controlling a Triac that can
powers an ac load.

During commutation, when the current in the Triac is below holding
current, the Triac turns off. It is turned on again when the diac turns
on in the zero crossing, but delayed a bit by the voltage across the
Triac needing to develop to a certain value at which the gate current is
enough to turn the Triac on.

This dropb of voltage before the zero crossing and the delay of turn on
creates conducted emission problems below 5-10MHz and is a well known
problem.

For example:

https://www.st.com/resource/en/application_note/dm00451014-controlling-a-triac-with-a-phototriac-stmicroelectronics.pdf

https://www.vishay.com/docs/84630/84630.pdf

A number of solutions are presented, but none of them talks about just
forcing the Triac on during the zero crossing. So adding a separate
power supply, and turning the Triac on some time before the zero
crossing, and some time after also. It seems it would be a good idea..

Any one tried this, before I go and simulate/build it?

Cheers

Klaus


Zero-voltage switching is used in most SSRs, I think. It’s been around for
ages.

Yes, SSRs use zero crossing detectors, but they don\'t seem to turn on
during the zero crossing. None that I have found anyway.

Reason is obvious IMHO, the SSR is a controlled device, they don\'t know
the future, so if the Triac is meant to be turned on for the next
period. In my system I know that.

??? That makes no sense. It is relatively easy to create a pattern of on and off half cycles to approximate virtually any setting of the average duty cycle. You don\'t need to predict the control input. The duty cycle responds to the control input.

If a certain percentage of cycles on are needed, then I certainly don\'t
want the SSR to take the decision to delay turn on to the next cycle.

Sorry, I don\'t understand your concern. What are you talking about? Are you not talking about a dimmer? If not, then it won\'t delay, it will just turn on. If you are talking about something like a dimmer, why do you care which cycles are on and which are off?

--

Rick C.

+ Get 1,000 miles of free Supercharging
+ Tesla referral code - https://ts.la/richard11209

 

[Klaus Vestergaard Kragelund]

On 13-06-2023 03:17, Ricky wrote:

On Monday, June 12, 2023 at 7:51:44 PM UTC-4, Klaus Vestergaard Kragelund wrote:
On 12-06-2023 18:31, Ricky wrote:
On Monday, June 12, 2023 at 3:01:30 AM UTC-4, Klaus Vestergaard Kragelund wrote:
On 12-06-2023 04:50, Phil Hobbs wrote:
Klaus Vestergaard Kragelund <klau...@hotmail.com> wrote:
Hi

So there\'s been some nice discussion about Triacs.

I have an application where an opto diac is controlling a Triac that can
powers an ac load.

During commutation, when the current in the Triac is below holding
current, the Triac turns off. It is turned on again when the diac turns
on in the zero crossing, but delayed a bit by the voltage across the
Triac needing to develop to a certain value at which the gate current is
enough to turn the Triac on.

This dropb of voltage before the zero crossing and the delay of turn on
creates conducted emission problems below 5-10MHz and is a well known
problem.

For example:

https://www.st.com/resource/en/application_note/dm00451014-controlling-a-triac-with-a-phototriac-stmicroelectronics.pdf

https://www.vishay.com/docs/84630/84630.pdf

A number of solutions are presented, but none of them talks about just
forcing the Triac on during the zero crossing. So adding a separate
power supply, and turning the Triac on some time before the zero
crossing, and some time after also. It seems it would be a good idea.

Any one tried this, before I go and simulate/build it?

Cheers

Klaus


Zero-voltage switching is used in most SSRs, I think. It’s been around for
ages.

Yes, SSRs use zero crossing detectors, but they don\'t seem to turn on
during the zero crossing. None that I have found anyway.

Reason is obvious IMHO, the SSR is a controlled device, they don\'t know
the future, so if the Triac is meant to be turned on for the next
period. In my system I know that.

??? That makes no sense. It is relatively easy to create a pattern of on and off half cycles to approximate virtually any setting of the average duty cycle. You don\'t need to predict the control input. The duty cycle responds to the control input.

If a certain percentage of cycles on are needed, then I certainly don\'t
want the SSR to take the decision to delay turn on to the next cycle.

Sorry, I don\'t understand your concern. What are you talking about? Are you not talking about a dimmer? If not, then it won\'t delay, it will just turn on. If you are talking about something like a dimmer, why do you care which cycles are on and which are off?

it\'s a SSR function, to turn on a motor, so ideally it is fully on all
the time, but when commanded on from the off state it needs to turn on
at the zero crossing, and during each period it also needs to turn on a
the zero crossing.

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.

 

[piglet]

On 14/06/2023 01:01, Klaus Vestergaard Kragelund wrote:

On 13-06-2023 03:17, Ricky wrote:
On Monday, June 12, 2023 at 7:51:44 PM UTC-4, Klaus Vestergaard
Kragelund wrote:
On 12-06-2023 18:31, Ricky wrote:
On Monday, June 12, 2023 at 3:01:30 AM UTC-4, Klaus Vestergaard
Kragelund wrote:
On 12-06-2023 04:50, Phil Hobbs wrote:
Klaus Vestergaard Kragelund <klau...@hotmail.com> wrote:
Hi

So there\'s been some nice discussion about Triacs.

I have an application where an opto diac is controlling a Triac
that can
powers an ac load.

During commutation, when the current in the Triac is below holding
current, the Triac turns off. It is turned on again when the diac
turns
on in the zero crossing, but delayed a bit by the voltage across the
Triac needing to develop to a certain value at which the gate
current is
enough to turn the Triac on.

This dropb of voltage before the zero crossing and the delay of
turn on
creates conducted emission problems below 5-10MHz and is a well
known
problem.

For example:

https://www.st.com/resource/en/application_note/dm00451014-controlling-a-triac-with-a-phototriac-stmicroelectronics.pdf

https://www.vishay.com/docs/84630/84630.pdf

A number of solutions are presented, but none of them talks about
just
forcing the Triac on during the zero crossing. So adding a separate
power supply, and turning the Triac on some time before the zero
crossing, and some time after also. It seems it would be a good
idea.

Any one tried this, before I go and simulate/build it?

Cheers

Klaus


Zero-voltage switching is used in most SSRs, I think. It’s been
around for
ages.

Yes, SSRs use zero crossing detectors, but they don\'t seem to turn on
during the zero crossing. None that I have found anyway.

Reason is obvious IMHO, the SSR is a controlled device, they don\'t
know
the future, so if the Triac is meant to be turned on for the next
period. In my system I know that.

??? That makes no sense. It is relatively easy to create a pattern
of on and off half cycles to approximate virtually any setting of
the average duty cycle. You don\'t need to predict the control input.
The duty cycle responds to the control input.

If a certain percentage of cycles on are needed, then I certainly don\'t
want the SSR to take the decision to delay turn on to the next cycle.

Sorry, I don\'t understand your concern.  What are you talking about?
Are you not talking about a dimmer?   If not, then it won\'t delay, it
will just turn on.  If you are talking about something like a dimmer,
why do you care which cycles are on and which are off?


it\'s a SSR function, to turn on a motor, so ideally it is fully on all
the time, but when commanded on from the off state it needs to turn on
at the zero crossing, and during each period it also needs to turn on a
the zero crossing.

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.

Rather than use an opto triac with internal zero cross logic to drive
the bigger power triac could you use a photo-mos type ssr to drive the
big power triac and do the zero cross timing yourself?

piglet

 

[whit3rd]

On Tuesday, June 13, 2023 at 8:01:13 PM UTC-4, Klaus Vestergaard Kragelund wrote:

On 13-06-2023 03:17, Ricky wrote:

Sorry, I don\'t understand your concern. What are you talking about? Are you not talking about a dimmer? If not, then it won\'t delay, it will just turn on. If you are talking about something like a dimmer, why do you care which cycles are on and which are off?

it\'s a SSR function, to turn on a motor, so ideally it is fully on all
the time, but when commanded on from the off state it needs to turn on
at the zero crossing, and during each period it also needs to turn on a
the zero crossing.

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.

So, the zero-voltage turnon is using a diac? That\'s usually used for
phase control (turnon at arbitrary times). If you want near-zero-volts
switching, the diac has to have a boost capacitor (with enough charge to
reliably trigger) and gate resistor (enough resistance to stretch the
start current time out), and should trigger off a shifted sinewave,
not the AC directly. You want a phase-shift network to fill the boost
capacitor if you want a 30V diac to breakover when MT1 vs MT2
voltage is low (3V?).

 

[legg]

On Wed, 14 Jun 2023 02:01:06 +0200, Klaus Vestergaard Kragelund
<klauskvik@hotmail.com> wrote:

On 13-06-2023 03:17, Ricky wrote:
On Monday, June 12, 2023 at 7:51:44?PM UTC-4, Klaus Vestergaard Kragelund wrote:
On 12-06-2023 18:31, Ricky wrote:
On Monday, June 12, 2023 at 3:01:30?AM UTC-4, Klaus Vestergaard Kragelund wrote:
On 12-06-2023 04:50, Phil Hobbs wrote:
Klaus Vestergaard Kragelund <klau...@hotmail.com> wrote:
Hi

So there\'s been some nice discussion about Triacs.

I have an application where an opto diac is controlling a Triac that can
powers an ac load.

During commutation, when the current in the Triac is below holding
current, the Triac turns off. It is turned on again when the diac turns
on in the zero crossing, but delayed a bit by the voltage across the
Triac needing to develop to a certain value at which the gate current is
enough to turn the Triac on.

This dropb of voltage before the zero crossing and the delay of turn on
creates conducted emission problems below 5-10MHz and is a well known
problem.

For example:

https://www.st.com/resource/en/application_note/dm00451014-controlling-a-triac-with-a-phototriac-stmicroelectronics.pdf

https://www.vishay.com/docs/84630/84630.pdf

A number of solutions are presented, but none of them talks about just
forcing the Triac on during the zero crossing. So adding a separate
power supply, and turning the Triac on some time before the zero
crossing, and some time after also. It seems it would be a good idea.

Any one tried this, before I go and simulate/build it?

Cheers

Klaus


Zero-voltage switching is used in most SSRs, I think. It’s been around for
ages.

Yes, SSRs use zero crossing detectors, but they don\'t seem to turn on
during the zero crossing. None that I have found anyway.

Reason is obvious IMHO, the SSR is a controlled device, they don\'t know
the future, so if the Triac is meant to be turned on for the next
period. In my system I know that.

??? That makes no sense. It is relatively easy to create a pattern of on and off half cycles to approximate virtually any setting of the average duty cycle. You don\'t need to predict the control input. The duty cycle responds to the control input.

If a certain percentage of cycles on are needed, then I certainly don\'t
want the SSR to take the decision to delay turn on to the next cycle.

Sorry, I don\'t understand your concern. What are you talking about? Are you not talking about a dimmer? If not, then it won\'t delay, it will just turn on. If you are talking about something like a dimmer, why do you care which cycles are on and which are off?


it\'s a SSR function, to turn on a motor, so ideally it is fully on all
the time, but when commanded on from the off state it needs to turn on
at the zero crossing, and during each period it also needs to turn on a
the zero crossing.

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.

Normal emi precautions ( >20KHz ) will employ RC snubbers, or just
C across the switch.

These allow the switch to turn on more reliably, as well.

RL

 

[legg]

On Wed, 14 Jun 2023 02:01:06 +0200, Klaus Vestergaard Kragelund
<klauskvik@hotmail.com> wrote:

On 13-06-2023 03:17, Ricky wrote:
On Monday, June 12, 2023 at 7:51:44?PM UTC-4, Klaus Vestergaard Kragelund wrote:
On 12-06-2023 18:31, Ricky wrote:
On Monday, June 12, 2023 at 3:01:30?AM UTC-4, Klaus Vestergaard Kragelund wrote:
On 12-06-2023 04:50, Phil Hobbs wrote:
Klaus Vestergaard Kragelund <klau...@hotmail.com> wrote:
Hi

So there\'s been some nice discussion about Triacs.

I have an application where an opto diac is controlling a Triac that can
powers an ac load.

During commutation, when the current in the Triac is below holding
current, the Triac turns off. It is turned on again when the diac turns
on in the zero crossing, but delayed a bit by the voltage across the
Triac needing to develop to a certain value at which the gate current is
enough to turn the Triac on.

This dropb of voltage before the zero crossing and the delay of turn on
creates conducted emission problems below 5-10MHz and is a well known
problem.

For example:

https://www.st.com/resource/en/application_note/dm00451014-controlling-a-triac-with-a-phototriac-stmicroelectronics.pdf

https://www.vishay.com/docs/84630/84630.pdf

A number of solutions are presented, but none of them talks about just
forcing the Triac on during the zero crossing. So adding a separate
power supply, and turning the Triac on some time before the zero
crossing, and some time after also. It seems it would be a good idea.

Any one tried this, before I go and simulate/build it?

Cheers

Klaus


Zero-voltage switching is used in most SSRs, I think. It’s been around for
ages.

Yes, SSRs use zero crossing detectors, but they don\'t seem to turn on
during the zero crossing. None that I have found anyway.

Reason is obvious IMHO, the SSR is a controlled device, they don\'t know
the future, so if the Triac is meant to be turned on for the next
period. In my system I know that.

??? That makes no sense. It is relatively easy to create a pattern of on and off half cycles to approximate virtually any setting of the average duty cycle. You don\'t need to predict the control input. The duty cycle responds to the control input.

If a certain percentage of cycles on are needed, then I certainly don\'t
want the SSR to take the decision to delay turn on to the next cycle.

Sorry, I don\'t understand your concern. What are you talking about? Are you not talking about a dimmer? If not, then it won\'t delay, it will just turn on. If you are talking about something like a dimmer, why do you care which cycles are on and which are off?


it\'s a SSR function, to turn on a motor, so ideally it is fully on all
the time, but when commanded on from the off state it needs to turn on
at the zero crossing, and during each period it also needs to turn on a
the zero crossing.

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 might affect harmonic distortion, if that\'s the actual issue.

RL

 

[Klaus Vestergaard Kragelund]

On 15-06-2023 14:21, legg wrote:

On Wed, 14 Jun 2023 02:01:06 +0200, Klaus Vestergaard Kragelund
klauskvik@hotmail.com> wrote:

On 13-06-2023 03:17, Ricky wrote:
On Monday, June 12, 2023 at 7:51:44?PM UTC-4, Klaus Vestergaard Kragelund wrote:
On 12-06-2023 18:31, Ricky wrote:
On Monday, June 12, 2023 at 3:01:30?AM UTC-4, Klaus Vestergaard Kragelund wrote:
On 12-06-2023 04:50, Phil Hobbs wrote:
Klaus Vestergaard Kragelund <klau...@hotmail.com> wrote:
Hi

So there\'s been some nice discussion about Triacs.

I have an application where an opto diac is controlling a Triac that can
powers an ac load.

During commutation, when the current in the Triac is below holding
current, the Triac turns off. It is turned on again when the diac turns
on in the zero crossing, but delayed a bit by the voltage across the
Triac needing to develop to a certain value at which the gate current is
enough to turn the Triac on.

This dropb of voltage before the zero crossing and the delay of turn on
creates conducted emission problems below 5-10MHz and is a well known
problem.

For example:

https://www.st.com/resource/en/application_note/dm00451014-controlling-a-triac-with-a-phototriac-stmicroelectronics.pdf

https://www.vishay.com/docs/84630/84630.pdf

A number of solutions are presented, but none of them talks about just
forcing the Triac on during the zero crossing. So adding a separate
power supply, and turning the Triac on some time before the zero
crossing, and some time after also. It seems it would be a good idea.

Any one tried this, before I go and simulate/build it?

Cheers

Klaus


Zero-voltage switching is used in most SSRs, I think. It’s been around for
ages.

Yes, SSRs use zero crossing detectors, but they don\'t seem to turn on
during the zero crossing. None that I have found anyway.

Reason is obvious IMHO, the SSR is a controlled device, they don\'t know
the future, so if the Triac is meant to be turned on for the next
period. In my system I know that.

??? That makes no sense. It is relatively easy to create a pattern of on and off half cycles to approximate virtually any setting of the average duty cycle. You don\'t need to predict the control input. The duty cycle responds to the control input.

If a certain percentage of cycles on are needed, then I certainly don\'t
want the SSR to take the decision to delay turn on to the next cycle.

Sorry, I don\'t understand your concern. What are you talking about? Are you not talking about a dimmer? If not, then it won\'t delay, it will just turn on. If you are talking about something like a dimmer, why do you care which cycles are on and which are off?


it\'s a SSR function, to turn on a motor, so ideally it is fully on all
the time, but when commanded on from the off state it needs to turn on
at the zero crossing, and during each period it also needs to turn on a
the zero crossing.

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.

Normal emi precautions ( >20KHz ) will employ RC snubbers, or just
C across the switch.

These allow the switch to turn on more reliably, as well.

Yes, snubbers is a given. Also avoid the triac to turn on by itself
during high dV/dt on the line.

 

[Klaus Vestergaard Kragelund]

On 15-06-2023 14:23, legg wrote:

On Wed, 14 Jun 2023 02:01:06 +0200, Klaus Vestergaard Kragelund
klauskvik@hotmail.com> wrote:

On 13-06-2023 03:17, Ricky wrote:
On Monday, June 12, 2023 at 7:51:44?PM UTC-4, Klaus Vestergaard Kragelund wrote:
On 12-06-2023 18:31, Ricky wrote:
On Monday, June 12, 2023 at 3:01:30?AM UTC-4, Klaus Vestergaard Kragelund wrote:
On 12-06-2023 04:50, Phil Hobbs wrote:
Klaus Vestergaard Kragelund <klau...@hotmail.com> wrote:
Hi

So there\'s been some nice discussion about Triacs.

I have an application where an opto diac is controlling a Triac that can
powers an ac load.

During commutation, when the current in the Triac is below holding
current, the Triac turns off. It is turned on again when the diac turns
on in the zero crossing, but delayed a bit by the voltage across the
Triac needing to develop to a certain value at which the gate current is
enough to turn the Triac on.

This dropb of voltage before the zero crossing and the delay of turn on
creates conducted emission problems below 5-10MHz and is a well known
problem.

For example:

https://www.st.com/resource/en/application_note/dm00451014-controlling-a-triac-with-a-phototriac-stmicroelectronics.pdf

https://www.vishay.com/docs/84630/84630.pdf

A number of solutions are presented, but none of them talks about just
forcing the Triac on during the zero crossing. So adding a separate
power supply, and turning the Triac on some time before the zero
crossing, and some time after also. It seems it would be a good idea.

Any one tried this, before I go and simulate/build it?

Cheers

Klaus


Zero-voltage switching is used in most SSRs, I think. It’s been around for
ages.

Yes, SSRs use zero crossing detectors, but they don\'t seem to turn on
during the zero crossing. None that I have found anyway.

Reason is obvious IMHO, the SSR is a controlled device, they don\'t know
the future, so if the Triac is meant to be turned on for the next
period. In my system I know that.

??? That makes no sense. It is relatively easy to create a pattern of on and off half cycles to approximate virtually any setting of the average duty cycle. You don\'t need to predict the control input. The duty cycle responds to the control input.

If a certain percentage of cycles on are needed, then I certainly don\'t
want the SSR to take the decision to delay turn on to the next cycle.

Sorry, I don\'t understand your concern. What are you talking about? Are you not talking about a dimmer? If not, then it won\'t delay, it will just turn on. If you are talking about something like a dimmer, why do you care which cycles are on and which are off?


it\'s a SSR function, to turn on a motor, so ideally it is fully on all
the time, but when commanded on from the off state it needs to turn on
at the zero crossing, and during each period it also needs to turn on a
the zero crossing.

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.

 

[legg]

On Thu, 15 Jun 2023 23:24:07 +0200, Klaus Vestergaard Kragelund
<klauskvik@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.

In what frequency range?

Is your triac mounted on a chassis-grounded heatsink?

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

What is the physical load?

RL