Digital dimmer: Triac vs MOSFET in a circuit?

[Jean]

Digital dimmer: Triac vs MOSFET in a circuit?

I want to dim a 120Volt-60Watt light with a digital controller.
I already have a circuit that use a Triac but I would prefer to use a
power MOSFET.

*******************************************************
This is not the purpose of this post.

For the ones who want to know why I would prefer MOSFET, some reasons
are summerised at
http://www.epanorama.net/documents/lights/lightdimmer.htm
like these:
Reverse phase controlling has some advantages over traditional dimmers
in many dimmer applications. The manufactuers of inverse phase dimmers
adverstise their products to be more efficent and less noisy. Using
proper controlling electronics it is possible to build a reverse phase
dimmer without any magnetics or vibrations caused by them.
Because turning on point is always exact at the zero phase there are
no huge current spikes and EMI caused by turn on. Using power MOSFETs
it is possible to make the turn-off rate relatively slot to achieve
quite operations in terms of EMI and acoustical or incandescent lamp
filament noise.

For the ones who want to know the basic functionning of a light dimmer
with Triac:
http://www.interq.or.jp/japan/se-inoue/e_ckt24_2.htm
*******************************************************

The Triac is turned on with the impulse current on the gate and shuts
off when the AC is zero.
The MOSFET turns on when the AC is zero and shuts off with the impulse
current on the gate.

Besides this basic functionning, my question is:
Is there any other considerations I should take into account by
replacing a Triac by a MOSFET in a circuit?

Do you have a dimmer circuit with MOSFET to suggest?
Where in the net I could have more information about light dimmer
circuit with MOSFET, IGBT...?

 

[Andrew Gabriel]

Digital dimmer: Triac vs MOSFET in a circuit?

I want to dim a 120Volt-60Watt light with a digital controller.
I already have a circuit that use a Triac but I would prefer to use a
power MOSFET.

*******************************************************
This is not the purpose of this post.

For the ones who want to know why I would prefer MOSFET, some reasons
are summerised at
http://www.epanorama.net/documents/lights/lightdimmer.htm

"We are sorry...
....but the URL you are trying to access could not be found or is
not available any more."

Interesting site though -- I notice it links to an article I
must have posted 10-15 years ago on dimming fluorescent lamps.

like these:
Reverse phase controlling has some advantages over traditional dimmers
in many dimmer applications. The manufactuers of inverse phase dimmers
adverstise their products to be more efficent and less noisy. Using
proper controlling electronics it is possible to build a reverse phase
dimmer without any magnetics or vibrations caused by them.
Because turning on point is always exact at the zero phase there are
no huge current spikes and EMI caused by turn on. Using power MOSFETs
it is possible to make the turn-off rate relatively slot to achieve
quite operations in terms of EMI and acoustical or incandescent lamp
filament noise.

I built a MOSFET power controller -- effectively a dimmer except
used for a heating load. My circuit actually chopped the middle
out of each half-cycle by switching off when the full wave rectified
sine wave voltage exceeded a threshold, which was adjusted so
the resulting output voltage was 120V RMS.

The reason was that this was to drop the effective voltage so I
could run a 120V heating appliance (electric frying pan) from
a 240V source. I wanted to minimize the peak voltage in the
element, to reduce the risk of insulation breakdown. Another
side effect is that although the power factor was only 0.5, I
was only drawing current in the portions of the sine wave where
there's an excess available, and not at the peaks where there's
an overload on the supply network due to switched mode PSU
appliances, so power generating plant probably though I was
the best type of load it could possibly have;-).

I used two MOSFETs opposite way round in series, utilising the
reverse polarity diode in each to conduct when polarity was
wrong way round for that FET. An alternative would be to use one
FET and a full-wave rectifier which would be fine for 60W, but
in my case it was a 10A load and the bridge rectifier would have
been one extra PN junction heat load to dissipate which I wanted
to avoid in a unit which was only a little bigger than a large
wall-wart.

Besides this basic functionning, my question is:
Is there any other considerations I should take into account by
replacing a Triac by a MOSFET in a circuit?

I don't know how well a triac would handle end of life lamp failure
where an arc manages to establish in the bulb. OTOH, triacs are
not known for surviving such events too well either -- they usually
blow quite a bit faster than any fuse protecting them;-).
In the case of a heating element, I didn't care about end of life
failure, since if that happened, this circuit would become redunant
anyway. Also, MOSFETs are static sensitive so you need some
protection built into the circuit so it doesn't get zapped whilst
being handled, whereas triacs are nowhere near as static sensitive
and don't need any protection unless going into a particularly
harsh environment.

 

[Tomi Holger Engdahl]

"We are sorry...
...but the URL you are trying to access could not be found or is
not available any more."

The correct URL to this document is
http://www.epanorama.net/documents/lights/lightdimmer.html

This document has never been on the address you gave.
You mistyped (or maybe just a cut&past error) the URL here.

Interesting site though -- I notice it links to an article I
must have posted 10-15 years ago on dimming fluorescent lamps.

Thank you for your feedback.

 

[Tomi Holger Engdahl]

Digital dimmer: Triac vs MOSFET in a circuit?

I want to dim a 120Volt-60Watt light with a digital controller.
I already have a circuit that use a Triac but I would prefer to use a
power MOSFET.

*******************************************************
This is not the purpose of this post.

For the ones who want to know why I would prefer MOSFET, some reasons
are summerised at
http://www.epanorama.net/documents/lights/lightdimmer.htm

The correct URL for this document is
http://www.epanorama.net/documents/lights/lightdimmer.html

like these:
Reverse phase controlling has some advantages over traditional dimmers
in many dimmer applications. The manufactuers of inverse phase dimmers
adverstise their products to be more efficent and less noisy. Using
proper controlling electronics it is possible to build a reverse phase
dimmer without any magnetics or vibrations caused by them.
Because turning on point is always exact at the zero phase there are
no huge current spikes and EMI caused by turn on. Using power MOSFETs
it is possible to make the turn-off rate relatively slot to achieve
quite operations in terms of EMI and acoustical or incandescent lamp
filament noise.

For the ones who want to know the basic functionning of a light dimmer
with Triac:
http://www.interq.or.jp/japan/se-inoue/e_ckt24_2.htm
*******************************************************

The Triac is turned on with the impulse current on the gate and shuts
off when the AC is zero.
The MOSFET turns on when the AC is zero and shuts off with the impulse
current on the gate.

Besides this basic functionning, my question is:
Is there any other considerations I should take into account by
replacing a Triac by a MOSFET in a circuit?

The MOSFET should be suitable for the given application
(vortage and current ratings). Triacs can pass
AC power to both directions.
MOSFETs work only on one direction, so you need to either
use two MOSFETs (one for each AC current direction) or use
one MOSFET plus a rectifier.
On the control circuitry you need to take care of the following
things:
1. Triacs are triggered with current usually at low voltage,
while MOSFET is a voltage driven (make sure control circuit gives
high enough voltage)
2. MOSFET needs drive signal all the time it needs to conduct,
for a triac only pulse is enough. Some TRIAC circuits
send contant drive from the start of conduction to the end
of phase to triac (those suit for FET as well) while some
other designs just send s brief pulse at one point of
phase where triac is needed to start conducting (does not
suit well for FET application).

Those are the first points that come to my mind on this.

Do you have a dimmer circuit with MOSFET to suggest?

CMOS gate implements reverse phase control - this circuit implements a
"reverse" phase control, using only a single CMOS 4001 quad NOR gate,
conduction begins at the zero crossing of the ac sine wave and the
turn-off timing is adjusted based on dimmer setting
http://www.e-insite.net/ednmag/archives/1998/052198/11DI.htm#CMOS gate implements

Where in the net I could have more information about light dimmer
circuit with MOSFET, IGBT...?

Take a look at the links at
http://www.epanorama.net/links/lights.html#dimmer

 

[Adam Aglionby]

So why are reverse phase dimmers/mosfet/igbt so rare?

Avolites had a prototype on their stand at PLASA a while back but the
production models went back to triacs.

Seem to remember there being a lot of wrangling about a major London
theatre installation of reverse phase dimmers that didn`t work as
intended, has the dust settled enough for anyone to say what the
problem was?

Thanks
Adam

 

[Clive Mitchell]

So why are reverse phase dimmers/mosfet/igbt so rare?

The circuitry is a bit more complex, particularly given the ease with
which a MOSFET/IGBT can be blootered. (Technical term for extreme
failure.)

Fundamentally I think the issue is with the sheer expense of developing
new RPC dimmers and the simple fact that most manufacturers are waiting
for their rivals to bring one out so that they can copy it's circuitry.

Meeeeeyow!

 

[Winfield Hill]

Tomi Holger Engdahl wrote...

The correct URL to this document is
http://www.epanorama.net/documents/lights/lightdimmer.html

It's also available without the epanorama.net header at
http://home2.planetinternet.be/gronsijn/ham/Schemas/Light dimmer circuits.htm

Tomi, I'm curious, who owns and operates the epanorama website, with
all your material on it? Who owns ELH Communications Ltd? Are you
personally involved? Who receives the [email protected] email?

Much of the epanorama.net material has the phrase, "Published by ELH
Communications Ltd., all rights reserved." How is material selected
for this designation, is it mostly your own stuff?

 

[Winfield Hill]

Andrew Gabriel wrote...

I built a MOSFET power controller -- effectively a dimmer except
used for a heating load. My circuit actually chopped the middle
out of each half-cycle by switching off when the full wave rectified
sine wave voltage exceeded a threshold, which was adjusted so
the resulting output voltage was 120V RMS.

The reason was that this was to drop the effective voltage so I
could run a 120V heating appliance (electric frying pan) from
a 240V source. I wanted to minimize the peak voltage in the
element, to reduce the risk of insulation breakdown. Another
side effect is that although the power factor was only 0.5, I
was only drawing current in the portions of the sine wave where
there's an excess available, and not at the peaks where there's
an overload on the supply network due to switched mode PSU
appliances, so power generating plant probably though I was
the best type of load it could possibly have;-).

If your MOSFET failed, it likely would have failed shorted, which
would then force up to 2x more current = 4x more power through the
frying pan, creating a new meaning to the word fry. :>)

I used two MOSFETs opposite way round in series, utilising the
reverse polarity diode in each to conduct when polarity was
wrong way round for that FET. ...in my case it was a 10A load...

What kind of FET did you use? Did you consider IGBTs instead?

 

[Tomi Holger Engdahl]

Tomi Holger Engdahl wrote...

It's also available without the epanorama.net header at
http://home2.planetinternet.be/gronsijn/ham/Schemas/Light dimmer circuits.htm

Somebody seems to have copied that document there...
Lots of my material seems to been copied by different people around...
Most copies without any permission from me. :-(

Tomi, I'm curious, who owns and operates the epanorama website, with
all your material on it? Who owns ELH Communications Ltd? Are you
personally involved? Who receives the [email protected] email?

I am personally heavily involved.
I am one of the founders and one of the owners of ELH Communications Ltd.
I used to receive all the [email protected] email.
Now there is another person who handles those mails
(mostly requests to add links). The relevant mails
get forwarded to me if there are things I should handle in them.

Much of the epanorama.net material has the phrase, "Published by ELH
Communications Ltd., all rights reserved." How is material selected
for this designation, is it mostly your own stuff?

It is mostly my own stuff.

 

[Andrew Gabriel]

Andrew Gabriel wrote...

If your MOSFET failed, it likely would have failed shorted, which
would then force up to 2x more current = 4x more power through the
frying pan, creating a new meaning to the word fry. :>)

Yes, which is one reason why the circuit includes an FF10A fuse.
The other reason being that if the control circuit failed to switch
the MOSFETs off, the fuse should operate before the pan or MOSFETs
overheat (the MOSFET being designed to handle double the current
for long enough to blow the fuse).

What kind of FET did you use?

Don't seem to be able to locate my design notes at the moment, but
looking back at my purchase orders for 1998 when I designed and
built this, it looks like I chose Motorola MTW24N40E TMOS power
FETs (400V, 24A). (ISTR the problem choosing a high voltage/current
MOSFET was finding one that anyone stocked and would supply in
small numbers.)

Did you consider IGBTs instead?

No, mainly because they weren't around when I learned electronics
so I don't know much about their characteristics or have any real
life experience using them.

 

[Winfield Hill]

Tomi Holger Engdahl wrote...

I am personally heavily involved.
I am one of the founders and one of the owners of ELH Communications Ltd.
I used to receive all the [email protected] email. Now there is
another person who handles those mails (mostly requests to add links).
The relevant mails get forwarded to me if there are things I should
handle in them.


It is mostly my own stuff.

Well, your name, Tomi Engdahl, has a respected and extensive reputation,
so why not highlight it, or at least reveal it where appropriate, on the
epanorama site? Take advantage of the brand, so to speak, rather than
hiding it while relying on creating a new brand. Or at a minimum, make
it clear on the epanorama.net site that the official site for the classic
Tomi Engdahl material is now epanorama.

What else does ELH Communications do? What does ELH and epanorama mean?

 

[Winfield Hill]

Andrew Gabriel wrote...

Yes, which is one reason why the circuit includes an FF10A fuse.
The other reason being that if the control circuit failed to switch
the MOSFETs off, the fuse should operate before the pan or MOSFETs
overheat (the MOSFET being designed to handle double the current
for long enough to blow the fuse).

It's risky to rely on a fuse conducting without incident at current
I, while reliably blowing at curent 2I.

Don't seem to be able to locate my design notes at the moment, but
looking back at my purchase orders for 1998 when I designed and
built this, it looks like I choose Motorola MTW24N40E TMOS power
FETs (400V, 24A). (ISTR the problem choosing a high voltage/current
MOSFET was finding one that anyone stocked and would supply in
small numbers.)

Pretty good. With a 160m-ohm Rdss(on), at 10A that would only be
16W dissipation. What was your peak load current for 1kW average
power?

No, mainly because they weren't around when I learned electronics
so I don't know much about their characteristics or have any real
life experience using them.

I'll find a good one for comparison.

 

[Jim Thompson]

On 7 Feb 2005 18:57:17 -0800, Winfield Hill

[snip]

I'll find a good one for comparison.

Win, Did you ever get anywhere with comparing the MOSFET models?

...Jim Thompson

 

[Tomi Holger Engdahl]

Tomi Holger Engdahl wrote...

Well, your name, Tomi Engdahl, has a respected and extensive reputation,
so why not highlight it, or at least reveal it where appropriate, on the
epanorama site? Take advantage of the brand, so to speak, rather than
hiding it while relying on creating a new brand. Or at a minimum, make
it clear on the epanorama.net site that the official site for the classic
Tomi Engdahl material is now epanorama.

There is some material on this at
http://www.epanorama.net/index.php?index=info

Maybe I should highlight that more.

What else does ELH Communications do?

Running the http://www.epanorama.net/ site is the main activity.
There has been some small consulting/design projects..

What does ELH and epanorama mean?

ELH = Engdahl Lindeman Hyötyläinen
(from the names of the founders)

 

[Andrew Gabriel]

Andrew Gabriel wrote...

Pretty good. With a 160m-ohm Rdss(on), at 10A that would only be
16W dissipation.

You have to add the forward voltage drop of the integral reverse
polarity bypass diode too, which probably takes it nearer 25W
total, but it was easily dissipated with only a small fan running
silently at low speed.

What was your peak load current for 1kW average
power?

I'd have to work it out...
Actual rating of the pan was 1050W @ 120V = 13.7 ohm.
ISTR the phase angle at cutoff was 43º (a bastard to work out;-).
240VAC * sqrt(2) = 339V peak
339 * sin(43º) = 231V at cutoff.
231 / 13.7 = 16.9A

I'll find a good one for comparison.

Interesting, cheers.

 

[Winfield Hill]

Jim Thompson wrote...

Win, Did you ever get anywhere with comparing the MOSFET models?

Thanks for the reminder.

 

[Winfield Hill]

Andrew Gabriel wrote...

You have to add the forward voltage drop of the integral reverse
polarity bypass diode too, which probably takes it nearer 25W
total, but it was easily dissipated with only a small fan running
silently at low speed.

I assume you used the standard back-to-back series connection with
both sources tied together and both gates tied together. In this
preferred configuration, both FETs are on, with one conducting
"backwards" shorting out its source-drain diode, and you end up
with twice the power dissipation of a single FET. Of course, if
the voltage drop across the FET rises to more than the diode drop,
as in your case (see below), then these mechanisms work together
sharing the load current.

I'd have to work it out... Actual rating of the pan was
1050W @ 120V = 13.7 ohm. ISTR the phase angle at cutoff was 43º
(a bastard to work out;-). 240VAC * sqrt(2) = 339V peak
339 * sin(43º) = 231V at cutoff. 231 / 13.7 = 16.9A

OK, 17A^2 * 0.16 ohms = 46 watts peak for the forward FET. It's
a bit painful to calculate the RMS power, say about 15-20 watts.
The reverse FET, with its diode helping, would be a little lower.

The reason IGBTs are attractive for applications like this is that
for a smaller part with similar or less silicon area to a FET, it
enjoys a smaller voltage drop at high currents. Your MTW24N40E FET
drops up to 2.7V at 17A, which a smaller IGBT can easily beat. For
example, a TO-220 600V 12A Fairchild HGTP12N60C3D IGBT drops 1.7V
at 17A. And Fairchild's HGTG30N60B3D, a 30A IGBT with a slightly
smaller die size than your big FET (rated at 208W compared to the
FET's 250W), drops only about 1.0 volt. That's a big improvement.

http://www.fairchildsemi.com/pf/HG/HGTP12N60C3D.html
http://www.fairchildsemi.com/pf/HG/HGTG30N60B3.html

 

[Tomi Holger Engdahl]

Hi,

In response to "Do you have a dimmer circuit with MOSFET to suggest?"
Tomi responded with "CMOS gate implement
s reverse phase control - this circuit implements a
"reverse" phase control, using only a single CMOS 4001 quad NOR gate,
conduction begins at the zero crossing of the ac sine wave and the
turn-off timing is adjusted based on dimmer setting
http://www.e-insite.net/ednmag/archives/1998/052198/11DI.htm#CMOS gate implements"

I'm interested! I can get to the article but the circuit diagram (figure
1) is supposedly stored in a .PDF file...that will not come up. Has
anybody got another source for this circuit? Or is it just me who can't
get to the .PDF file?

Downloading the PDF did not work for me either :-(
I has link to this ciruit, and posted the address whe I saw that
circuit page to work... Did not test the download of all parts..

EDN magazine seems to change their archive every now and them...
Result beuing that links tend to break down every now and them.
It is now not the particular link on the t circuit, every PDF
link on that page seems to be broken because of their new system..
One idea would be to contact the webmaster of that site
to report of the problem and ask to correct it.

I might have that circuit printed on paper somewhere, maybe
even electronic copy...