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High Voltage A-synchronous Buck Converter (100V -> 28V)

  1. Jan 28, 2012 #1
    Hi guys,

    I'm new in here and hope I could get some advices.

    I am designing a 100V-28V DC-DC Buck converter (as per tittle) and has successfully tested on breadboard.

    The problem is when I've transferred everything to the stripboard, the circuit does not work. This is confusing to me because isn't connection made on the stripboard should perform better since parasitic components are lesser?

    From my schematic attached, you can see that the 12V Vcc for the 2 IC chips (IR2117 & TC4420EPA) are powered by 2 different auxillary supplies at different times. What I've done is basically construct 2 linear series reg. (one from the 100V V+ and other from the 28V Vout) as could be seen. My idea is, during start-up, Vcc is supplied via the 100V series linear reg for a very short time and once Vout achieved 28V, the supply of Vcc is then transfered and will be drawn from the 28V Vout instead. This should be easily understood by most experts here.

    Weird thing is everything worked out perfectly on breadboard but not on stripboard. WHY?

    Upon troubleshooting, I've found no zero short circuit between paths, all zener diodes and NPN transistors are also working fine. BUt when I do an o-scope on the base of the NPN transistor (100V linear series reg.), I've realised that the base voltage (or the zener voltage) did not have the chance to climb till 12V. In theory, as long as V+ > 12V, the 12V zener will hold at this voltage even if we ramp more than 12V to V+.

    In the o-scope graph attached, (V+ = 30V), it should already have a high enough voltage to ramp Vb > 12V? But the waveform on oscope doesnt seems to be the case (see VB_30.jpg attachment).

    The waveform tells me that Vb attempts to rise but dropped to zero immediately after ~3.2V like that.

    WHY? This could be the very reason why my circuit cant work because the IC doesnt have a 12V Vdd to start with.

    Attached are my schematic, stripboard layout and the o-scope readings.

    Could anyone help? Thank you in advance.
     

    Attached Files:

  2. jcsd
  3. Jan 28, 2012 #2
    Some additional photos of the real thing.
     

    Attached Files:

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