High Voltage Fully Variable Output Power Supply

Discussion in 'The Projects Forum' started by Uriah, Jul 14, 2007.

  1. Uriah

    Thread Starter New Member

    Jul 13, 2007
    I am in the process of designing a lightweight power supply using all surface mount components. A driving circuit powers a piezoelectric transformer using halfwave DC at 65.5kHz. The driving voltage is 1.25V to 41V, while the current is adjusted with the voltage to be the only constant. The AC output (75-2460V) is rectified and stored in a capacitor. A special IGBT is used to discharge the capacitive discharge circuit at a variable frequency of 1-100Hz.

    My problem is stepping the voltage up further. I need a final output of around 50kV and at the high current from the capacitive discharge circuit a cockroft walton multiplier type circuit would be WAY TOO BIG AND HEAVY. Because of the relatively low frequency it is actually extremely hard to nearly impossible and expense to use a custom multiplier.

    I need suggestions on how to solve my problem please. This is for aerospace application so I'd say weight is one of the biggest considerations. The second of course is a reliable system with a long life span.

    This does entail that a wire wound transformer will not fit the application either, as they are bulky and heavy and sometimes not reliable systems during constant operation.

    Attached is the schematic for the circuit without a secondary circuit to step the voltage up further to that needed for application.

    C1 - 0.1μF
    C2 - 1μF
    C3 - 10pF to 1nF 5kV protective isolation capacitor
    C4 - (presently undefined value) 5kV discharge capacitor
    D1 - 100V rectifier diode (regulator protection)
    D2, D3 - SM3F 3kV rectifier diode
    D4 - Freewheeling diode
    R1 - 240Ω
    R2 - 200Ω
    S1 - NPN
    S2 - MOSFET - IRF9Z24NS
    S3 - IGBT - IXEL40N400
    78M05B - 5V Positive Fixed Voltage Regulator
    OM7646 - 1.25V-57V Positive Adjustable Voltage Regulator
    AD8400 (R-8 package) Digital Potentiometer

    The max dissipation of the piezoelectric transformer (PT) is 7 watts, but because of the capacitive discharge circuit the output is much greater then the input. The frequency of 67kHz is the resonant frequency of the PT.

    PT INPUT: 6.88watts - 1.25V to 41V @ 0.168amps (constant current when voltage is adjusted)

    PT OUTPUT: 2.46kV @ 0.0028amps

    I do not know how to calculate the capacitive discharge output (dependant of frequency), but I think that is all the information that I know at this time.


    Uriah George
  2. mrmeval

    Distinguished Member

    Jun 30, 2006
  3. Uriah

    Thread Starter New Member

    Jul 13, 2007
    That is my problem. A wire wound transformer and a voltage multiplier are just too big because of the power output of the Piezoelectric Transformer and the frequency of the discharge capacitor. Voltage multiplier "strips" (also called a cascade multiplier) do not work well or at all using low frequencies, even low RF. They are usually optimized at 50 to 100kHz.
    To get them to work efficiently and reliably capacitors with quite large values and expensive diodes must be used or the current must be very small. Wire wound will just not work for my application because of its size and weight, and also the large EM field it puts out. I am looking for something unique in order to keep the weight to a minimum and the reliability maximum.

    One idea I had was making my own capacitors out of a special material (nicknamed Hkx) which has an extremely high dielectric constant of around 50,000K. These capacitors would be around 1cm x 5mm (approx size of the diodes as well) and could be fabricated as surface mount, not through hole components. This would bring the size and weight down considerably or at least to a point where it would be acceptable for my project.

    Although I know nothing on how to fabricate a capacitor that is meant for high speed charge/discharge sequences.

    The capacitor listed as C4 in my diagram is a 5kV surface mounted component. For a cascade multiplier of any type these capacitors will need to be able to withstand up to 70-100kV and have a fairly large value to withstand the high current output of the capacitive discharge circuit.

    Of course a lot of calculating will need to be done. So, for now as a novice, I am just looking for general suggestions to point me in the right direction.

    Thank you for the reply though. I do appreciate any help given.

    Uriah George