Electronics > Projects, Designs, and Technical Stuff

Non-Zero Crossing AC SSR: Use with DC?

<< < (4/6) > >>

beduino:

--- Quote from: TimNJ on October 22, 2019, 02:43:29 am ---I think I'm getting the itch to feature-creep in a DIY MOSFET based solid state relay. Seems like a MOSFET based relay would be superior in this application. Plus it'll work with DC.

--- End quote ---

A few AC mosftes switches in parallel should do the job depending on how low resistance needed and voltage levels ;)

Zero999:

--- Quote from: TimNJ on October 20, 2019, 02:28:43 am ---Now, if I want to use this same tester to switch DC, is it possible?
--- End quote ---
Why not design a separate tester for DC? That would be much easier, rather than something which will need to work with both AC and DC.

If it doesn't need to turn off, a simple SCR will do. Once triggered it will turn on and not switch off again, until the power is interrupted.

It does need to turn off, a MOSFET can be used.

Isolation can be provided with an opto-TRIAC for the SCR and an opto-photovoltaic cell for the MOSFET.

TimNJ:

--- Quote from: Zero999 on October 22, 2019, 12:15:15 pm ---
--- Quote from: TimNJ on October 20, 2019, 02:28:43 am ---Now, if I want to use this same tester to switch DC, is it possible?
--- End quote ---
Why not design a separate tester for DC? That would be much easier, rather than something which will need to work with both AC and DC.

If it doesn't need to turn off, a simple SCR will do. Once triggered it will turn on and not switch off again, until the power is interrupted.

It does need to turn off, a MOSFET can be used.

Isolation can be provided with an opto-TRIAC for the SCR and an opto-photovoltaic cell for the MOSFET.

--- End quote ---

I agree with you that often times it's much better to make a single purpose device instead of making something that "does it all". However, if it's not too much extra work, it might be less work to tack on DC function to the AC tester. However, as I think about it more, I'll probably wind up making two separate testers because I might want to put some chunky caps on the DC input to get the source impedance down as low as possible.

I think I'll try designing a MOSFET SSR first (as a standalone module) and then integrate that into the full tester. There are some cool opto-PV gate drivers like Vishay's VOM1271 which seems reasonable. I don't really think switching speed is that big of a deal in this case, but they still are pretty slow...I'm thinking a little 1W isolated DC-DC converter + regular opto-coupler might have better performance without being overly complex or expensive.

Thanks!

MagicSmoker:

--- Quote from: TimNJ on October 22, 2019, 01:30:14 pm ---...
I don't really think switching speed is that big of a deal in this case, but they still are pretty slow...I'm thinking a little 1W isolated DC-DC converter + regular opto-coupler might have better performance without being overly complex or expensive.

--- End quote ---

The last sentence outlines the best approach - go with inverse series connected MOSFETs driven by a proper gate driver and a small isolated dc-dc converter to supply it because you do need decently fast switching speeds given that you are trying to maximize inrush current for testing purposes. You really don't want the MOSFETs taking >1ms to turn on while also trying to conduct a huge peak current.

TimNJ:
As I'm work on this MOSFET SSR design, I am beginning to understand why SCR based SSRs tend to dominate in the high power segment.

For one, zero-crossing switching, a freebie for SCRs, has to be added to a MOSFET design. Some preliminary modeling shows that switching off a 1KW load with 1 or 2mH of inductance will produce crazy ringing with 1-2KV peaks! That's not surprising, but still a problem.

My best solution so far is a current transformer + comparator which detects when the current is at or near zero. But of course, that makes the relay physically bigger extra and adds a few dollars to the BOM..

I'm curious: Is there any "real" application where a MOSFET based (high power) SSR would be advantageous? Any application where you need to control the turn on of something even faster, or with greater precision?

Thanks.

Navigation

[0] Message Index

[#] Next page

[*] Previous page

There was an error while thanking
Thanking...
Go to full version
Powered by SMFPacks Advanced Attachments Uploader Mod