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  1. #21
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    Re: Best topology DCDC adjustable output

    Ok, after searching a bit longer, I found some choke posts. One which you responded to was a good find. Some more time on the net and another pass, how about this.

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    Is the pi filter approach decent? A snub is on the tx output already, but does each rectifier diode need a snub?



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    Re: Best topology DCDC adjustable output

    take away cap1, put snubbers across all 4 diodes, not across Tx .... make sure the diodes can handle the PIV from the Tx ......

    - - - Updated - - -

    what is the leakage L of the Tx - I do not see it modelled or considered ...?

    - - - Updated - - -

    trying to do 1kV with just 4 diodes is ill advised unless you are at 20kHz with 0.2% leakage in the Tx and good quality 1200V SiC diodes.

    Else use 2 x 500V windings in series, and put TVS across each diode if worried about PIV, current mode on the pri is a good idea to protect against shorts on the o/p



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  3. #23
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    Re: Best topology DCDC adjustable output

    First of all, I want thank you for taking the time to guide me on this. Its much appreciated! Your suggested component changes have been updated. 2x 500V outputs looks like the right thing to do based on your imput and my Tx winding capabilities. So now the diagram is just one output. Any more suggested modifications?

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    I don't really know how or what to do to define the leakage. Can we assume its horrible and choose the best components?



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    Re: Best topology DCDC adjustable output

    Some additions have been made. Feedback and passive changes. If anyone wants to give guidance, much appreciated!
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  5. #25
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    Re: Best topology DCDC adjustable output

    You need 10uf across 3856, you need slope comp on the 3856 control ( plenty app notes on it ) for when duty cycle of A & B is greater than 45% of maximum...

    snubbers across all diodes, the TVS on the Tx o/p will just go bang if they are triggered,

    you need voltage sharing resistors across the o/p caps ...

    snubbers across each fet, turn on R = 33 ohm ( gate drive ) good for starters, a reverse schottky across each gate drive R ( 1A 40v ) will speed up turn off ...

    keep all wiring/leads on pin 1 very short...! ( 3856 )


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  6. #26
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    Re: Best topology DCDC adjustable output

    You have two diode bridges across a tapped transformer. It looks okay on paper but it needs close checking to be certain you avoid a short circuit through two (or more) diodes at some point in the cycle.



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    Re: Best topology DCDC adjustable output

    Respectfully - that can't happen unless a diode goes short - which would be bad news in a single diode bridge also - current mode on the primary limits the max current in the sec ...

    But Brad has just pointed out an error - the diode bridges must be connected pos o/p to neg on the upper one - the schematic is wrong ...

    - - - Updated - - -

    i.e. you need TWO SEPARATE o/p windings ...


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  8. #28
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    Re: Best topology DCDC adjustable output

    All suggested changes on this thread are now on the schematic. Well, hopefully... I have begun researching and reading up on comp, snubbing and feedback. That is some heavy reading! This will take some time. Defining those component values "?" is my next goal. In the mean time, do you have any more inputs for this latest schematic? Odds are, components are still missing or arranged wrong. Peace!

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  9. #29
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    Re: Best topology DCDC adjustable output

    Ok, so I'm drudging through feedback docs. There seems to be a concurrent motion that the smaller range, the easier/stable it is. I also found examples of FB from a single output. V regulation for this PS does not have to be perfect. +-10VDC fine. Sag acceptable, so long as it recovers and can maintain 400VA average. I was thinking of doing feedback off one of the bulk caps. So that is 50V to 500V feedback. Or use dividers and a low pass filter to buffer the feedback voltage before it gets to the 4430. Is this good logic? Any suggestions on choosing a good sample point?



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    Re: Best topology DCDC adjustable output

    3856: pin 6 does not go to gnd, link 6 & 7 for a voltage follower


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    Re: Best topology DCDC adjustable output

    also you don't need the lower o/p choke

    TVS should go directly across each bridge rect o/p - but if triggered they will likely go bang

    snubbers across all diodes, not the Tx

    film/foil decoupling caps right by the fets 1uF


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  12. #32
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    Re: Best topology DCDC adjustable output

    Good day! Some clarification needed.
    1.) "no lower op choke" Isn't the function of the chokes to lower Tx currents into the caps? Why wouldn't each output need one?
    2.) There is already TVS on each bridge o/p. Are you saying each diode should have a tvs and a snub?
    3.) Decouple 1uF caps near fets. Across D/S like snubs, or across Tx?



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    Re: Best topology DCDC adjustable output

    only one choke is needed at the top - you can put a choke in the neg for balance and more CM filtering if you wish - but not needed

    there is not a TVS across each bridge - only the lower one - in your picture above.

    across the HV supply - right near the fets ...


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    Re: Best topology DCDC adjustable output

    Ok, so my understanding of the function of the choke may need correction. Are we trying to limit DI/DT into the bulk caps via the Tx? When charging up or during sudden FB duty cycle changes, why wouldn't each section need one? It seems like both sections are being equally used and need the same components. If the bottom section does not have one, wouldn't it have much higher current draws from its respective tx o/p? Unless your talking about sudden load changes, then maybe just one choke after the top cap..?..



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    Re: Best topology DCDC adjustable output

    the two BR's go in series - when I say remove the choke I do not mean put a short in its place - just take it out entirely - the current flows thru the two bridges - as it would any way ...



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  16. #36
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    Re: Best topology DCDC adjustable output

    Ok, now I see what you are saying! Series bridges charge series caps. The idea behind two independent o/p was stability and easier feedback. A KV of series rectified o/p scares me considering my Tx winding capabilities.

    Quick revisit of o/p options! What about a 2 stage full wave multiplier? My perceived benefits: One center tapped 500V winding, still 8 diodes, 250V legs = easier snub/tvs/fb.



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    Re: Best topology DCDC adjustable output

    Quote Originally Posted by RandomElectrons View Post
    What about a 2 stage full wave multiplier? My perceived benefits: One center tapped 500V winding, still 8 diodes, 250V legs = easier snub/tvs/fb.
    Article about voltage doubler claiming it makes efficient optimum use of a transformer:

    'A genuine full wave voltage doubler.'

    www.kwarc.org/bulletin/99-04/tech_corner.htm


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    Re: Best topology DCDC adjustable output

    getting 400VA out of a voltage multiplier will be a difficult process with high peak currents in the diodes and caps - but you are welcome to try ...

    the o/p choke averages the Tx o/p and limits peaks currents in the diodes - which is a good thing ...

    By the way - you should be extremely careful with 1kV at 400mA - it can easily see you or someone else off this mortal coil ...

    I sense you are not totally familiar with HV - try reading online for safety precautions and use as much common sense as you can muster.

    Leaving such a unit ON, when any untrained people are about is a big no no.


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  19. #39
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    Re: Best topology DCDC adjustable output

    This has been a 3 year back burner project. HV safety definitely observed and we will be putting this thing in a grounded steel comp case when done. I made a 50khz 1.2KV 4 stage Cockcroft multiplier a few years back and it did ok... 16 diodes somewhat lossy but surprisingly stable with 30% sag at 400VA. This version has to do better given only two diodes and added chokes. Odds are the o/p topology will be revisited, but for now, lets run with it.

    This is my first pass at snubbing after reading up and doing some math. Are the 1u decouple caps where you recommended near the fets? Of course if anything else seems faulty, flame on! Inductor sizing and compensation(s) next to learn....

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