One SMPS controller, two sets of MOSFETs

Discussion in 'General Electronics Chat' started by CraigHB, Feb 27, 2014.

  1. CraigHB

    Thread Starter Member

    Aug 12, 2011
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    I'm trying to work out a way to switch between two sets of MOSFETs driven by a single SMPS controller.

    I've looked at MOSFET drivers and analog switches, but haven't found anything that can do it. One problem is there can't be much additional impedance inline with the gate drivers. Also, there's an amount of current demand involved.

    The inactive MOSFETs have to be isolated from the driver so I need a switch that floats both sides similar to a relay. The switches need to be transparent to the gate drivers. I don't want to use an actual relay for size and reliability considerations.

    With no power or ground reference, I don't know of a way to bias a transistor so I'm stuck there. Is there a relatively simple circuit that will allow me to emulate a relay in a manner that both sides of the switch float?
     
  2. crutschow

    Expert

    Mar 14, 2008
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    I don't understand what you are trying to do.
    Floats both sides of what?
    What amount of current demand involved?
    What do you mean there's no power or ground reference? There has to be.
    Can you post a diagram?
     
  3. CraigHB

    Thread Starter Member

    Aug 12, 2011
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    I probably should have just posted a diagram in the first place, sorry about that. Here's a diagram below, pretty simple thing really. I might be missing something obvious, but neither side of the switch is tied to power or ground so I can't figure out how to bias a transistor to switch those signals.

    What I'm trying to do specifically is set up a switch array that allows me to reconfigure a multi-topology controller on the fly for either buck or boost depending on a variable output voltage requirement.

    These are not small signals since the equivalent gate capacitance is about 3nF and the frequency around 500kHz. Peak currents are around 500mA and signal levels are above supply voltage.

    [​IMG]
     
  4. crutschow

    Expert

    Mar 14, 2008
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    What is the gate driver voltage with respect to ground? (There has to be a ground somewhere ;)). It doesn't matter whether the MOSFETs are connected to ground.

    Edit: Why not use a buck-boost type converter rather than switching between outputs?
     
    Last edited: Feb 28, 2014
  5. CraigHB

    Thread Starter Member

    Aug 12, 2011
    127
    15
    There are four MOSFETs with this configuration. The gate voltage on them ranges from ground to output voltage which can be as high as 8V. Supply voltage is 3V for the logic circuits and 3.2 to 4.2V for converter input (high drain Li-Ion battery).

    There's a very limited selection of buck-boost controllers capable of outputting 50W with a voltage range 2.5 to 8V. Those are my output requirements. I tried one buck-boost controller that worked, but it couldn't handle startup transients at maximal voltage output. I was pretty miffed by that since I went to all the trouble to design and build a device around that controller. The data sheet said it was good for 10V, but it couldn't even handle 8V once I got into bench testing it.

    I have an unusually tight size limitation, the PCB is about the size of a credit card with only 4mm headroom available on top and 2mm available on bottom Anything with a transformer or coupled inductor is not going to work for me, I just don't have room for one. Those types of converters don't have the best efficiency anyway.

    I'm currently using a boost converter that works well providing a range 4 to 8V at 50W with excellent efficiency. I'm trying to extend the bottom end on that with a buck-boost setup.
     
  6. crutschow

    Expert

    Mar 14, 2008
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    You should be able to switch the gate drive using a CMOS switch. You need one that has low enough ON resistance to carry the Gate Drive current without significant distortion and can be controlled with a logic level signal, perhaps one of these.
     
  7. CraigHB

    Thread Starter Member

    Aug 12, 2011
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    Yes, that was my first thought, but I can't find one that will work. Analog switches typically don't allow voltages on switch inputs outside the supply rails. I could power the switch with output voltage to get around that issue, but then it has to have a supply range 2.5 to 8V. There's a few that can do switch inputs over the rails, but I could not find one with resistance under a couple Ohms.

    There's also a the possibility of using a solid state relay. Those can do the job nicely, but they're expensive and too large to fit on my PCB, especially since I need a number of them.

    I was hoping I could use a simple circuit with common components rather than an analog switch or solid state relay. Unless of course I can find one that fits my bill. I'll keep looking for an analog switch that can do the job.
     
  8. crutschow

    Expert

    Mar 14, 2008
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    What gate driver are you using? Can you use two gate drivers and just inhibit the driver that's not active?
     
  9. CraigHB

    Thread Starter Member

    Aug 12, 2011
    127
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    That's actually the switching controller's gate driver, not a stand-alone one. I had actually thought in terms of using two contollers which would essentially be the same as using separate gate drivers, but that has issues too. I also thought about putting a gate driver inline with the each MOSFET. The solution that is the least involved is to use one controller and switch around its configuration.

    There are analog switches with low on resistance and high voltage tolerance. Maxim makes a range of "beyond the rails" switches. They offer ones that are 25V tolerant with only a 3V supply and have only 1 Ohm on resistance. There are others that are still limited by supply voltage, but they have low on resistance and wide supply range. I could power a set of those with a charge pump to get input tolerance high enough.

    In any case, I'm probably going to give up on this idea for now. It's not something I have to have, just something I'd like to have. With such limited PCB real estate it makes doing something like this very difficult, there just isn't room for all the components.
     
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