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  1. #1
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    Synchronous Two trasnsistor forward goes wrong....too high reverse inductor current

    Hi,

    The attached is a picture of the inductor current in a synchronous two transistor forward converter just after going from full load to no load. (its an LTspice sim)
    As you can see the inductor current suddenly reverses to a badly high level due to the fact that the LT1681 has “malfunctioned”….the LT1681 holds the “catch” synchronous rectifier ON for far too long.
    If the Vout had been higher, then this inductor current reversal would have been even worse, if not catastrophic.
    The LT1681 is a $6 chip and for that price its amazing to see poor performance like this.
    Do you know of other synchronous rectifier control chips which don’t allow this behaviour?
    Attached is the LTspice schematic and the LTspice simulation of this part.
    If we were to use this part in real life, we would have to buffer the SG output of the LT1681, (and add other circuitry) then take the buffered SG output high if it went low for more than one switching period.


    LTC1698
    https://www.analog.com/media/en/tech...eets/1698f.pdf
    LT1681
    https://www.analog.com/media/en/tech...eets/1681f.pdf

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    Re: Synchronous Two trasnsistor forward goes wrong....too high reverse inductor curre

    Large value of C19 produces inappropriate sync waveform and prevents the time-out circuit from turning off the rectifier MOSFETs. Try with 100 pF or similar.


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    Re: Synchronous Two trasnsistor forward goes wrong....too high reverse inductor curre

    Thanks, i am actually wondering whether it would just be better to use a digital isolator to port the SG signal across.



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    Re: Synchronous Two trasnsistor forward goes wrong....too high reverse inductor curre

    Would you agree that the Two Transistor Forward Converter with synchronous rectifiers (2TFC+SRs) is potentially a death trap….if not dealt with properly?

    In a 2TFC+SRs, if the output is suddenly no-loaded, then the output inductor current will try and reverse and flow “the wrong way” through the synchronous rectifiers. This is because the “catch” Sync FET has an opposite drive to the main primary fets, so if the main primary fets go continuously off, then the ‘catch’ sync fet goes continuously ON…..disaster!

    The attached LTspice sim and schematic is the best way to counter this. Would you agree?
    ....The sync FETs are only permitted to ever turn ON if the load current monitor indicates that the load current is above 33%. However, there is more to it than just that. Supposing the 2TFC+SRs had been no-loaded for some time, and then suddenly went into full-load…..the output capacitor would then supply high current to the load before the PWM controller had chance to start switching, and so the ‘catch’ FET would be continuously ON if then re-enabled…resulting in massive current reversal through the sync fet……..so the ‘catch’ sync fet must be disabled …until the COMP pin voltage on the PWM controller has built up high enough to mean that the 2TFC+SRs has started switching again. Also, it must be switching with a long enough on-time…because otherwise the “Forward” sync FET wont really turn ON, and so the “catch” sync fet will be continuously ON……which leads to disaster.
    Some 2TFC+SRs actually have expensive controllers which can manage it so that the inductor current can indeed reverse in times of no-load…..the output inductor current is simply made to oscillate positive and negative such that its average is zero….however, controllers capable of managing this are far too expensive for most Power supply budgets.

    One other problem of reversing output inductor current in a 2TFC+SRs is that the current in the primary side sense resistor also reverses, causing a slight negative voltage into the PWM controller current sense pin…which will kill the controller immediately….a Schottky cant be used to counter this as its leakage current at high temperatures is too great, and it will form a divider with the current sense filter resistor.
    …A current sense transformer can however be used to counter this.

    One other problem of ever allowing the output inductor current to reverse in a 2TFC+SRs, is that of the problem if the sync fets somehow both get turned off, and the converter also stops switching….because in this case the reversed output inductor current has nowhere to flow and so will overvoltage the fets and diodes severely.

    The above also applies to a Flyback with sync rects, would you agree?


    Really, to play safe, a microcontroller is needed to manage the situation. ….When the load current goes below 33%....then the micro should really disable the sync rects until the load current has been above 33% for 10 seconds or so…the exact amount of time depends on how long it would take for the output capacitor to discharge, and for the converter to resume switching following re-connection of the load.

    The attached sim shows the 2TFC+SRs starting up, then getting no loaded, then getting fully loaded again…….the combo of output current monitor and COMP pin monitor both prevent disaster….take either of these out of the sim and you soon see disaster unfold.



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    Re: Synchronous Two trasnsistor forward goes wrong....too high reverse inductor curre

    Do you know why onsemi do many sync rect controller ICs...
    https://www.onsemi.com/products/powe...de-controllers

    ...but not one single one for the two transistor forward converter.
    Why is this?



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    Re: Synchronous Two trasnsistor forward goes wrong....too high reverse inductor curre

    a proper synch rect controller only turns on the fets when the Vin to the "diode" is > Vout ( Vload )

    a work around here is to turn off the fets at light loads - assuming they will handle the light currents OK ....

    - - - Updated - - -

    the Tx output of the 2 xtor forward is really pretty identical to the single ended version ...


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    Re: Synchronous Two trasnsistor forward goes wrong....too high reverse inductor curre

    a proper synch rect controller only turns on the fets when the Vin to the "diode" is > Vout ( Vload )
    I would agree with this……however, it seems the only way to do this is to “roll your own” sync rect driver.
    All the off the shelf ICs for sync rects actually sense reversed current before turning it off….so the bad thing has already happened before you can deal with it.
    Take the LTC3901, it senses the VDS voltage of the sync fet, and when it goes negative by more than 10.5mV, it turns off the sync fet…but by then the reverse current could be as high as 5 Amps or even more in reverse….which is going to give a destructive kick-back spike voltage to the sync fet….and more than likely destroy it.

    I believe Off the shelf sync rect controllers are all bad, and if used, need a micro to manage the situation and disable them before disaster strikes.......also, a TVS across the sync fets is mandatory i believe when offtheshelf sync rect controller ICs are used....Am i wrong?
    TVS voltage goes up though with TVS current, so what you end up needing to do , when offtheshelf sync rect ICs are used, is use a sync fet of far higher voltage rating than the circuit asks for…..but then that gives you a high rds(on)…..So I think we’re always stuffed when using offtheshelf sync rect driver ICs.

    LTC3901
    https://www.analog.com/media/en/tech...eets/3901f.pdf
    Last edited by treez; 31st December 2019 at 14:24.



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