DC-Dc SSR vs High side Mosfet

Discussion in 'General Electronics Chat' started by shortbus, Jan 30, 2016.

  1. shortbus

    Thread Starter AAC Fanatic!

    Sep 30, 2009
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    Can a DC-DC SSR be used in place of a a high side mosfet switch in a circuit? Which one is capable of switching faster, SSR or high side mosfet? I know if the parts would need to be bought the mosfet route would be more cost effective, but I already have the SSR, from another project. The load will be a capacitor bank instead of an inductive or resistive load.

    The SSR is like this one - http://www.ebay.com/itm/Solid-State...c09e389&pid=100276&rk=1&rkt=4&sd=170961637557
     
  2. AnalogKid

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    Aug 1, 2013
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    Yes. Many DC-output SSR's use a power MOSFET for the switching element. If the input is fully isolated from the output, then it should work for you..

    ak
     
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  3. tracecom

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    Apr 16, 2010
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    Here is the datasheet for FOTEK SSRs; the DC-DC units are a couple of pages in. Note that the switching speed is listed as 1ms. I would guess that an SSR would be slower than a directly gated MOSFET due to the delay introduced by the optocoupler in the SSR.
     
  4. David Knight

    New Member

    Aug 4, 2015
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    1ms switching speed is really slow for a dc-dc converter. a high-side MOSFET can switch in a few nanoseconds with a good gate driver.

    An SSR used in a dc-dc converter would have to operate at very low frequency, which would require large inductors and capacitors. It seems like this would be an expensive and bulky solution.

    Note that when charging a capacitor bank, it's important to use current feedback to limit the current to a level that your power source, wires, and circuitry can handle. Without a current limiting mechanism, expect to let out the magic smoke.
     
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  5. Sensacell

    Well-Known Member

    Jun 19, 2012
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    SSR's are not design optimized for speed, they are more than 10 X slower than a typical switching FET.

    They are optimized for simple floating drive, high voltage and current.
     
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  6. Roderick Young

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    Feb 22, 2015
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    I agree with the others. Quoting the specs in the listing, "On-off Time: ≤ 10mS". When the units quoted are mS, that's a sure sign that the switching time is not in microseconds. For a switching converter, you will generally want an turn off time of just a few microseconds at most, and more likely, a fraction of a microsecond.

    The good news is that for the price of that solid state relay, you can buy a regular mosfet, and drive it with a pulse transformer, or capacitively coupled, or by some other conventional means, and still have a few coins left over.
     
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  7. shortbus

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    From all the stuff on the web, the old style DC-DC SSR were speed limited, to ~200Hz. The newer ones are/seem to be able to do up to 20kHz for motor PWM drives.
     
  8. AnalogKid

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    There is nothing in post #1 about the application being a DC/DC converter or needing any particular switching speed.

    ak
     
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  9. shortbus

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    You correct it's not for a DC-DC converter. It's for my ongoing electrical discharge machine. Was just thinking out loud. Thought if they, the SSR, would work, it would eliminate - 2 mosfets, a few resistors, a diode 2 caps, and a gate driver IC needed for a high side switch. Don't know how the DC-DC converter came into this?
     
  10. AnalogKid

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    Post #1: "Can a DC-DC SSR..." misread.

    For bulk power switching with relatively low frequency power cycles, should be no problem

    ak
     
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  11. crutschow

    Expert

    Mar 14, 2008
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    The main concern is not to exceed the surge current rating of the device from the capacitive discharge.
     
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  12. shortbus

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    The frequency would be varied, ~600Hz to ~6kHz.
     
  13. shortbus

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    The amperage charging the caps would be ~15A @ 95V So I was going to use a 40A @200V SSR.
     
  14. shortbus

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    After spending time reading about DC-DC SSR's, it seems the one that fits my requirements is only good to 950Hz when used with PWM. thanks for all the responses.
     
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