I had a similar but different problem, the triac was for a fast-cycling temperature control. My solution was to add a contactor in series with the triac. so when it was to be cycling the contactor was closed and the power was cycled on and off rapidly. When not cycling the contactor was open. No leakage, no RFI, and much less shock hazard.

 

My solution was to add a contactor in series with the triac

Do you mean, a relay?

 

Do you mean, a relay?

The device was called a "Contactor" because it has the same construction as an equally rated motor starter, but without the overload sensor section. So it provides the same switching as a relay, but at a higher current rating and a greater load circuit voltage. In addition, A "contactor" has double-break contacts , and no portion of the current carrying parts flexes or bends. The standard symbol is usually the same as for a relay, though..
So the functional difference was that it was intended to switch a 30 amp, 3-phase, 480 volt load reliably for a long life, similar to an equivalent "relay", which is probably not available for that sort of ratings.

 

I don't want to use a mosfet as the blocking device, because too large and expensive. If i use a mosfet as the blocking device, then i might as well discard the triac, and use two back-to-back mosfets. But again, too large and expensive.

Incorrect, twice. Discarding the TRIAC is a much better solution. If you want to switch high-current DC, use a part that is built from the ground up to do just that.

Two low voltage power MOSFETs rated for 10 A cost less and are way smaller than a 10 A TRIAC. Note that I'm over-rating all parts because that's the right thing to do for long-term reliability. Looking for parts rated for at least 20 V and up to 30 A, Digi-Key has over 800 line items in SMT. A lot of those are various sales options, but that still leaves over 100 part numbers.

Here's one for 55 cents in ones, so that's $1.10 for a back-to-back pair. Two of them combined are less than 1/10th the area of a TO-220 TRIAC. The off-state leakage current is 1 uA *max*. And - no heatsink required. A TRIAC operating in these conditions would dissipate around 6 W; that's a mandatory heatsink.

https://www.digikey.com/en/products/detail/alpha-omega-semiconductor-inc/AON7522E/3603566

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If you want to stick with thru-hole, a pair of these is less than $2.00, and can operate (a bit warmly) in free-air with no heatsink.

https://www.digikey.com/en/products/detail/infineon-technologies/IRFZ34NPBF/811724

Note that for any MOSFET solution, the on-stage voltage drop across the devices will be considerably less than that of a TRIAC, something like 0.4 V versus 1.2 V.

ak

 

As you continue to dribble out the system details we need for a decent solution, I don't see anything about the available circuit voltages. I've suggested only n-channel FETs because I'm free to assume that you have sufficient gate drive voltages available. If this is not the case, then a shift to P-channel parts is not a big deal.

Also, how often is this circuit being activated? Minimum on or off times?

ak

 

The device was called a "Contactor"

Is it still available? Can you share a link or part #?

Incorrect, twice.
cost less and are way smaller than a 10 A TRIAC.

Twice? I'm seeing triacs for cheaper than a mosfet, and i only need one. The larger package is fine for me.

If you want to switch high-current DC, use a part that is built from the ground up to do just that.

The mosfet isn't "built for" what i need either, because it only blocks in one direction. If it was built for my purpose, i wouldn't need two. So "built for" means different things -- every part is a tradeoff in some way.

Some triacs:

8A, 800V
$0.14 @ 1,000
https://www.utmel.com/productdetail/nxpsemiconductors-bt137800127-22730478

12 A RMS, 500V
$0.34 @ 1,000
https://www.rocelec.com/part/NXPBTA212-600B-DG127

16A RMS, 600V
$0.38 @ 500
https://www.rocelec.com/part/NXPBT139-600E127

MOSFET solution, the on-stage voltage drop across the devices will be considerably less than that of a TRIAC

For my application, that's ok. I can compensate.

My voltage requirement is 150V. Actual usage is closer to 60V, i'm over-rating.

assume that you have sufficient gate drive voltages available. If this is not the case, then a shift to P-channel parts

I didn't realize P fets require less gate current. The need for, and cost of, a gate driver is a consideration.

how often is this circuit being activated? Minimum on or off times?

The switch will be closed most of the time.
Twice a day, the switch will open for 30 minutes, then close again and remain closed.

 

My voltage requirement is 150V. Actual usage is closer to 60V, i'm over-rating.

That certainly changes things. Posts #1 and #5 say you have a 4 V source. Is this correct?

In my application, I'm passing 5A continuous DC @ 4V.

Also, that certainly changes things. 1 V of drop across a TRIAC is obviously a much bigger issue with a 4 V source than with a 60 V source. . .

I didn't realize P fets require less gate current. The need for, and cost of, a gate driver is a consideration.

Not less current, but a different voltage polarity.

For truly identical performance, n-channel FETs generally cost less than p-channel FETs. The problem in a positive-voltage, high-side switch application is that an n-channel FET's gate voltage must be greater than the signal or power rail being switched. OTOH, for a p-channel FET the gave voltage must be less, often achieved by simply pulling the gate to GND directly or through a voltage divider.

I didn't realize that this is a high-volume application. Did I miss something? Re-reading posts #5 and #7, here is what I got:

You have a system that works.
It has a characteristic that is not an actual problem for the system's performance, but bugs you.
You worked through an alternate approach with two MOSFETs, but think they are too large and expensive.

Thinking this was a one-off (or very lo volume) lab application, I suggested that in a 4 V, 5 A system, two FETs can be both smaller and lower cost (or a very small cost increase) than a TRIAC, with significant performance benefits.

You don't say what the 4 V, 5 A signal is, other than it being AC. Is the distortion caused by the voltage drop across the TRIAC an issue, because that is something else that is mitigated with FETs.

ak

 

That certainly changes things. Posts #1 and #5 say you have a 4 V source. Is this correct?

That was a mistake. I'm switching a series pack. Therefore, the part must switch the voltage of the entire pack.

Any other thoughts about my last comments?