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UCC28C51: +24V to -5000V low ripple boost converter

Chris van der Aar
Intellectual 570 points
Part Number: UCC28C51
Other Parts Discussed in Thread: UCC28C50

Tool/software:

For a new project we have to design a DC/DC converter from +24V to -5000V (adjustable).
The challenge here is to meet the requirements from the customer (stable and <20mV ripple voltage).
And we do not have a lot of space for a PCB  or module that can perform this task. The load current is nihil (1uA min according spec) and has to do with a metal piece that we have to keep at a stable -5kV voltage (within a vacuum environment) but there is in fact no current flow to ground at the output.
We can choose between a  complete module (adjustable output) but most of them have too much ripple( and expensive) or something we can design by our selves.
I found an application note from Analog Devices AN118 based on a Royer oscillator and a LT1534 (low noise switching regulator) and a CCFL transformer for generating -1000V with very low ripple (<1mV) but we need -5000V
When I use your webench designer tool the max voltage I can select for the output is +1000V. When I select +24V and +1000V out and a load current of 10uA it shows some designs with the UCC28C51 and similar devices. However 1000V is still too low for our requirements (-5000V and adjustable).
Does T.I. has a device that could be used for such an application that could meet these requirements?  Or a boost converter (Royer principle or flyback?) from +24V to-1000V with voltage doublers at the output? However I do not know if this would be stable enough and would meet these low ripple requirements?
Best regards
Chris van der Aar
Sr PCB design Engineer
NTS Group Eindhoven The Netherlands
  • 0
    TI__Guru 73345 points

    Hi,

    A boost requires much bigger duty cycle for 24V to 5000V, somewhere about 0.995 which is difficult to do.

    A flyback is a boost + transformer so the transformer turns ratios can help to change the duty cycle in a reasonable range. For example, Nsp = 200, then duty cycle =  0.65, then Vo = 5794V from Vin = 24V. The duty = 0.65 is much more doable than 0.995.

    Then you can use UCC28C50. 

  • 0 in reply to Hong Huang
    Intellectual 570 points

    Thanks for your answer. Is it also possibte to get a negative DC DC output voltage at the output and to make a feedback control loop by measuring the output voltage? And do you think low ripple could be possible with the UCC28C50? Are there any application schematics based on this or can I use the design from webench with some modifications?

  • 0 in reply to Chris van der Aar
    TI__Guru 73345 points

    Hi,

    The negative voltage in flyback is easy just swap the flyback output connections to the load.

    Ripple design is external not on ic side.

    You can look at the datasheet application section for flyback design example.

  • 0 in reply to Hong Huang
    Intellectual 570 points

    Some days ago I asked for support for a +24V to -5000V with very low ripple (<20mVrms). You suggested the UCC28C50. In that case we would need a transformer with Nsp=200 which results in a duty cycle 0f 0.65. However it seems to be very difficult to find small PCB mount fly back transformers with this winding ratio and suitable for 5kV? Or do you have any idea about that? In the past high voltage transformers were used for CRT and for CCFL but this becomes less and besides these transformers were big and we want to use a very small PCB mount transformer with low profile (we do not need power). Or do you think it could be an option to generate a lower voltage like 1000V and using voltage doublers to increase the output voltage to 5000V or more? Or do you expect too much ripple with that principle? We do not need power at the output (1uA is enough) so that is not the problem. We only need a stable and low ripple voltage at the output.

  • 0 in reply to Chris van der Aar
    TI__Guru 73345 points

    Hi,

    A transformer with over 5000V is not common in our experience. Please talk to a transformer vendor or vendors to get help how to minimize the size.

    A voltage doubler idea looks ok since only 1uA load is required. The potential ripple may be still concerned since leakage current could be higher than 1uA. 

    We do not have much experience to help.