Understanding a mosfet gate driver circuit !

Discussion in 'General Electronics Chat' started by Zakaria Fadli, Jun 12, 2015.

  1. Zakaria Fadli

    Thread Starter New Member

    Jun 12, 2015
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    I need to understand how exactly this circuit works, there is not much references and books on those circuits, they're used as "power" oscillator for DC AC conversion for wireless.

    [​IMG]

    here is something i found on how it works (i'm not sure you need that to answer my question) [​IMG]

    i also know that the mos control voltage is arranged in such a way to perform switching at ZERO BIAS VOLTAGE. what i really need to understand first is what are the capacitors C4 and C5 on the gate drive and how are they calculated, also what are the zener diodes for exactly? and i think the shottky is for have a fast turn off right?

    here are different other circuits of the royer i found, if this may help, it seems like there are two types,

    [​IMG]

    [​IMG]

    thank you very much.
     
  2. Alec_t

    AAC Fanatic!

    Sep 17, 2013
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    Not so. The FET switches when its Vgs goes above/below a threshold voltage which is generally in the range ~1V-10V, depending on FET type.
    C4/5 are 'speed-up' capacitors to reduce the switching time. The zeners prevent the gates being over-driven beyond their spec limits.
     
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  3. ScottWang

    Moderator

    Aug 23, 2012
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  4. ian field

    Distinguished Member

    Oct 27, 2012
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    Probably the most important thing is charging and discharging the gate capacitance as quickly as possible to minimise linear region dissipation. Bipolar transistor totem pole drivers are very popular.

    Its also important to drive the gate high enough to get well above the linear region, a typical power MOSFET needs at least 6 - 8V to switch hard - care is needed though, the upper limit is only about 18 - 20V or so.
     
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  5. Hypatia's Protege

    Distinguished Member

    Mar 1, 2015
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    FWIW: Such is a widespread misnomer owed, in no small part, to the common (and, often, incorrect) description of certain SMPS topologies as 'ZVS' -- A particularly egregious example being the so called 'Mazzilli ZVS Oscillator':rolleyes: --- Said arrangement being quite similar to that shown in the OP...

    Best regards
    HP:)
     
    Last edited: Jun 12, 2015
  6. Zakaria Fadli

    Thread Starter New Member

    Jun 12, 2015
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    thank you all
     
  7. ian field

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    Oct 27, 2012
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    The TO92 variety MOSFETs like the BS170 and 2N7000 etc have threshold voltage somewhere around the 1 - 3V mark, logic level MOSFETs obviously have to be compatible with at least one logic family to get away with the advertising. In most cases, a TO220 power MOSFET will need around 6 - 8V to achieve the headline RDSon. Any that didn't get the job done at 8V, and I'd be scrutinising the data sheet to see if its an exception.
     
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