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MOSFET + Driver = True?

  1. Jan 12, 2010 #1
    Dear Oracle,

    I'm designing a boost converter where the input is a current limited (~500mA) voltage source that varies from 3.5V to 6V. The output (load) varies from 7V to 9V. I intend to drive a MOSFET, IRF8707PBF [1] from IR, with a 250kHz PWM control signal via Maxim's driver IC MAX4420 [2].

    Do you see any unforeseen mis-happenings in your crystal ball, if I chose to mate the two devices at the given frequency and voltages?

    Other comments/questions on driving MOSFET's or on the boost topology in general are welcomed.

    [1] https://ec.irf.com/v6/en/US/adirect/ir?cmd=catSearchFrame&domSendTo=byID&domProductQueryName=irf7807apbf [Broken]
    [2] http://www.maxim-ic.com/quick_view2.cfm/qv_pk/1486
    Last edited by a moderator: May 4, 2017
  2. jcsd
  3. Jan 12, 2010 #2


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    Staff: Mentor

    Why is your output voltage going to vary? Or do you mean that you are going to make the output adjustable?
    Last edited by a moderator: May 4, 2017
  4. Jan 12, 2010 #3
    It's the power stage of a maximum power point tracking solar cell to lithium-ion battery charger -- i.e., the output is the battery voltage with a (float voltage) Vmax =~8.2V, (and not 9V as I said in the OP). I'll sense the output current from the converter and battery voltage into a uC ADC and implement a software MPPT algorithm that will generate the PWM output to the mosfet driver. I'm thinking a state machine with a MPPT constant (max) current state, and a constant voltage state where I'll move away from the MPP as necessary, as a start. Having said that, placing a uC in the feedback loop muddies the already murky waters for me, so I'm struck with bursts of confusion from time to time..

    I was just looking to bounce some ideas of someone with a little more than my non-existing experience. Thanks.
  5. Jan 12, 2010 #4


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    Science Advisor

    What made you choose 250kHz for the switching frequency. Have you considered the trade-off's of going higher/lower?
  6. Jan 12, 2010 #5
    If your source voltage is 3.5V, that won't be enough to turn your mosfet completely on. Looking at the datasheet, you need at least 4.5V on the gate to completely turn the mosfet on. It will work, but you will be loosing a lot of power at low input voltages, especially if your battery is low and drawing a lot of current.
  7. Jan 13, 2010 #6
    I need the system to be fairly small while still being efficient. Basically I've tried to squeeze the frequency down as much as possible to minimize switching losses, while at the same time not letting the inductor and capacitor occupy too much pcb real estate. By using the expression for Lmin (see page 11, http://focus.ti.com/lit/an/slva061/slva061.pdf" [Broken]) with fsw = 250kHz, I've decided to go with a 82uH (torodial for less EMI?) inductor in an attempt to gain some foothold somewhere from where I can proceed.

    Yes, maybe http://www.national.com/pf/LM/LM5112.html#Overview" [Broken] from National is a better choice. Similar switching speeds and current consumption at 250kHz from what I can see.

    Thanks for the questions/comments. It helps, so don't be shy now! :)
    Last edited by a moderator: May 4, 2017
  8. Jan 13, 2010 #7
    I just realized the Output Resistance is higher on the LM chip than the one from Maxim (30 ohm vs 1.5 ohm), but I was prepared to place (as a saftey measure) a small resistor on the gate input anyway to dampen any ringing oscillations that might occur.

    As long as fsw is relatively low at 250kHz, the bigger Rout of LM5112 should not pose a big problem, right?
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