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    What are the diodes for in this power supply sequencing circuit?

    Hi,

    What do the diodes do in this circuit?

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    It looks like some sort of hysteresis section, but simulating it and changing the resistor to ground or either of the resistors in series with the diodes seems to alter either the rise time and/or final fall time (similar to changing the capacitor to the right of the diode/resistor stage).

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    Removing the diodes didn't really change any voltage or current values, it only removed the slow tail after the fall from the NMOS load being on.

    power-supply-sequencing-in-spaceflight-systems.pdf

    Thanks.

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    Re: What are the diodes for in this power supply sequencing circuit?

    I'm guessing they are there to provide seperate on/off time.


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    Re: What are the diodes for in this power supply sequencing circuit?

    Hi,

    The document says:
    ...and the ability to delay both the power up and power down...
    Klaus
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    Re: What are the diodes for in this power supply sequencing circuit?

    Delaying power supplies by putting RCD networks in the current
    path seems like a pretty wasteful way to go. And to get a long
    delay at amperes of through-current probably means sub-ohm
    resistance and ridiculously-sized delay caps as a result (maybe
    the input filter of the POLs is big enough, but I've never put
    more than a couple of millifarad on any board).

    Gate control of a MOSFET would at least afford you the option
    that series R approaches zero, when the sequencing is done.
    Op amp control of gate can give you whatever risetime (and,
    if a second independent amp and shunt FET replaces the
    reverse diode and discharge shunt R, fall time) you want, as
    long as it respects the op amp bandwidth / slew rate.

    If you look at sequencer / supervisor ICs internal and/or
    application schematics, they don't put any series R in the
    power path.


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    Re: What are the diodes for in this power supply sequencing circuit?

    The 'POL' devices, whatever they are show the delayed signal going to the 'ENABLE' pin so it probably isn't the main current path but some kind of on/off control for the device. I would guess the main power path isn't shown. To place components in the main current path could involve some awkward values but at small signal level it would be quite easy.

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    Re: What are the diodes for in this power supply sequencing circuit?

    What's confusing is that the high side fet topology shown is often used for the main power path.


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    Re: What are the diodes for in this power supply sequencing circuit?

    Hi,

    Thanks for your replies. Sorry about this, what I'm failing to understand is what function those diodes serve that makes it necessary they be there. In simulations they make next to no difference whether included or not in each respective path. Are they blocking each path (blocking the lower path from the MOSFET to enable, and blocking the upper path from enable back to the MOSFET) for a reason or doing something else?



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    Re: What are the diodes for in this power supply sequencing circuit?

    As previously explained, the diodes are required to set on- and off-delay independently. Some parameters of your simulation schematic in post #1 should be corrected however.

    POL regulator enable input should have a threshold
    R13 << R12
    R11 and R12 (respectively R12+R13) should be sufficiently different to show a diode effect
    The simulation timescale should somehow fit the delay time order of magnitude, 10 ms in your circuit.


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