Choosing MOSFETs for a synchronous buck converter

Discussion in 'The Projects Forum' started by minisystem, Mar 24, 2012.

  1. minisystem

    Thread Starter Active Member

    Dec 4, 2008
    I've been working on a digitally controlled synchronous buck converter that uses a driver chip to drive a high side and low side N-FET. It works as expected, but I'm now tweaking the design to maximize efficiency and am having a hard time choosing an optimum FET for the job. If I understand correctly, the two major parameters I should be looking at are turn-on/off times and Rds(on), both of which I want to be low, correct?

    My converter switches at 100kHz and the FETs I'm using now have a turn-on/off delay time of 2.1 and 13.1 ns, respectively and an Rds(on) of 65 mΩ. I can find FETs with lower Rds(on) but they are typically a little slower (25/40ns on/off for example), but still suggested for DC-DC switching applications.

    So, my question is: what is more important? Faster switching or lower Rds(on). My aim is maximum efficiency.
  2. shortbus

    AAC Fanatic!

    Sep 30, 2009
    Not real sure, but I would say faster switching is more important. The longer the turn on-off time the more energy is wasted in heat. Think slow on-off time would end up having more resistance in the long run because of the time spent in the 'resistive region' of the mosfet.
  3. crutschow


    Mar 14, 2008
    A rough estimate of the two losses can be made.

    For switching losses it roughly 1/2 times Vds (OFF) times Ids (ON) times the rise/fall time for each switch transition (twice per cycle) times the frequency.

    For the ON losses its roughly the ON duty-cycle times the average ON Ids times Rds(on).

    You add these two losses together for each transistor and, from that, you can estimate which transistor has the lower total losses in your application.
  4. minisystem

    Thread Starter Active Member

    Dec 4, 2008
    Ok, thanks for the rule of thumb. That's allowed me to do a relative comparison. Based on the 4 FETs I've analyzed, it does seem to be a fine balancing act between on/off times and Rds(on).
  5. DickCappels


    Aug 21, 2008
    One other factor can be important, and that is the gate charge. Gate charge x frequency = watts lost in gate drive. Simply put, if other factors are equal, use the transistor with the lowest gate charge (often given on the datasheet) or just choose the one with the lowest capacitance.
  6. minisystem

    Thread Starter Active Member

    Dec 4, 2008