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    Solid state relay vs mosfet + opto isolator + driver

    I understand that solid state relays integrate a driver + power mosfet + opto isolator in a single IC.
    So, when designing a high current switch circuit for motor control,
    Are there any good reasons not to use an SSR instead of 3 discrete components (driver/power mosfet/opto isolator) ?

    •   Alt24th February 2012, 09:30

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    Re: Solid state relay vs mosfet + opto isolator + driver

    SSRs often use Triacs or SCRs, not just MOSFETS. You MIGHT be able to design a less expensive discrete solution, but other nice features like zero-crossing, etc. will add complexity. You need to consider the tradeoffs.


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    Re: Solid state relay vs mosfet + opto isolator + driver

    I'd say, in general, only build it from discrete parts if you are building them for production quantities and need to minimize cost. Otherwise just buy an SSR.
    Zapper
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    Re: Solid state relay vs mosfet + opto isolator + driver

    What if the SSR's MOSFET has an RDS on resistance that's too high ?



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    Re: Solid state relay vs mosfet + opto isolator + driver

    Too high for what?



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    Re: Solid state relay vs mosfet + opto isolator + driver

    I meant, higher than a discrete MOSFET...



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    Re: Solid state relay vs mosfet + opto isolator + driver

    I still don't understand the question. What do you mean by "too high"? What if is IS higher than a discrete?

    If you have a specific requirement for minimum resistance (which is not really the way SSRs are rated) and the MOSFET doesn't meet that requirement, then it's not suitable for your purposes.


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    Re: Solid state relay vs mosfet + opto isolator + driver

    For the optoisolator + driver + FET you need a stiff power
    supply on the load side of the isolation boundary. For the
    SSR, you only need a control signal. You can find "photoMOS"
    SSRs where the gate is driven by photodiodes in a stack,
    from the source LED. These will be far slower for a given
    on resistance than using a direct gate drive from a supply.
    But you don't have to create that supply.


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    Re: Solid state relay vs mosfet + opto isolator + driver

    barry,
    I don't have any requirments yet. I'm evaluating the best solutions (performance wise)

    dick_freebird,
    I didn't understand this statement: "stiff power supply on the load side".
    For both solutions I can have a weak microcontroller signal actuating the switch...With the 3 discrete component solution the uC will actuate the isolated pre-driver wich will then actuate the power MOSFET. Where is the need for the stiff power supply ?



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    Re: Solid state relay vs mosfet + opto isolator + driver

    Quote Originally Posted by shaiko View Post
    I meant, higher than a discrete MOSFET...
    Well obviously if you can't find an SSR with a low enough ON resistance for your requirements, then you would need to do something different.
    Zapper
    Curmudgeon Elektroniker



    •   Alt24th February 2012, 21:59

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    Re: Solid state relay vs mosfet + opto isolator + driver

    Quote Originally Posted by shaiko View Post
    dick_freebird,
    I didn't understand this statement: "stiff power supply on the load side".
    For both solutions I can have a weak microcontroller signal actuating the switch...With the 3 discrete component solution the uC will actuate the isolated pre-driver wich will then actuate the power MOSFET. Where is the need for the stiff power supply ?
    Power MOSFET driver needs a power supply, and the supply has
    to fully charge the gate so the FET doesn't end up weakly "on"
    and burn up. FET capacitance is large so any kind of high rep
    rate will need a decent current.

    Without a supply of some sort, all you have is the photonic
    energy, which is very low.



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    Re: Solid state relay vs mosfet + opto isolator + driver

    Why can't the MOSFET driver be supplied from the same power source as the power MOSFET itself ?



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    Re: Solid state relay vs mosfet + opto isolator + driver

    Quote Originally Posted by shaiko View Post
    Why can't the MOSFET driver be supplied from the same power source as the power MOSFET itself ?
    Of course it can. But, you have to configure it as a low-side switch or use a bootstrap supply for using it as a high-side switch. Either way, you won't get isolation between the driver and the MOSFET and usually this is not a problem. Usually, you would want isolation between the driver stage (and the MOSFETs) and the control circuit (including microcontroller, etc).

    When designing a high current switch for motor control, first you have to decide if you need a DC or AC switch. Then, you need to find out if the solid-state relay you're looking at is AC or DC. I would use the solid-state relay if it were AC. If you need to do more than turn the motor on/off such as speed control, you should take a look at the speed of the solid-state relay such as maximum frequency, turn on / turn off, rise/fall time, etc

    If it was DC, I would use the discrete MOSFET with a driver as this would give me more control and you have a wide range of MOSFETs and drivers to select from. The solid-state relay can also be used, but I guess it would be more expensive. Again take care of specifications such as frequency, propagation delay, etc

    Either way, you can use the MOSFET (or triac/SCR for AC) or the solid-state relay. The solid-state relay may be easier to control/drive but if you can control the discrete components, I think that would be better as you have a plethora of components to choose from (as per your requirement) and it gives you a lot more versatility, that too at a lower cost.

    Hope this helps.
    Tahmid.


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    Re: Solid state relay vs mosfet + opto isolator + driver

    @ Tahmid,
    I have been lurking, You gave a great answer. I am also interested in using SSR,s.
    If you can find one that is fast enough, would there not be other advantages such as built in protections and lower component count?

    Regards

    pilko



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    Re: Solid state relay vs mosfet + opto isolator + driver

    The other advantages you mention can be had depending on the module. If you use one that is fast enough, then ease of driving itself can justify its use. However other factors can come into play here. Eg. you can always find discrete MOSFETs and drivers but can you guarantee that you will always find the SSRs.

    I guess it's a good choiceto use SSRs but I prefer discretes.

    Hope this helps.
    Tahmid.


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    Re: Solid state relay vs mosfet + opto isolator + driver

    So, when designing a high current switch circuit for motor control.
    Are there any good reasons not to use an SSR instead of 3 discrete components (driver/power mosfet/opto isolator) ?
    I think this debate would be more practical,
    if we all knew
    what is the type of motor in this case
    what is the operating voltage for the switch component
    and switching current / operating current.

    I find it hard to believe,
    that only by connecting components together and by simulating the circuit
    without comprehensive testing and test runs under different actual operating conditions,
    we could get any better and more reliable results than some of the major manufacturers.
    ---> Hike with wise men , then you will be wise <---



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    Re: Solid state relay vs mosfet + opto isolator + driver

    The motor is a brushless DC
    The input voltage is around 12V.



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    Re: Solid state relay vs mosfet + opto isolator + driver

    And current or power is ???

    BLDC motor needs usually 3-phase power driver
    and controller for speed setting.

    Are you planning to build a BLDC driver/controller circuit.
    Last edited by kak111; 26th February 2012 at 12:42.
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    Re: Solid state relay vs mosfet + opto isolator + driver

    The Current is ~8A.
    Yes, I'm building the whole control cicuitry - controller + power stage.

    The discussion at hand is about the power stage...As I can see there're 2 possible approaches:
    1.Use a pre-driver IC (like this http://www.toshiba.com/taec/componen.../379/28124.pdf) that will actuate simple power MOSFETs.
    2.The other approach will be to use SSRs that are directly actuated by the controller.



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    Re: Solid state relay vs mosfet + opto isolator + driver

    I don't think you want to use an SSR to control the motor, I don't think you're going to get ones fast enough (I could be wrong, though). mosfets and simple driver would be my choice.


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