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What Mosfect acts at Unsmooth DC Voltages

Discussion in 'General Electronics Discussion' started by pyrodigy, Oct 10, 2012.

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  1. pyrodigy

    pyrodigy

    3
    0
    Oct 10, 2012
    Hello all
    I will have couple of questions;
    I have a similar design as below (except transformer) convert high voltage 230VAC mains electricity to a high voltage 400VDC smooth DC supply.
    I use series 2 pieces 200V 470uF while converting AC to DC.
    [​IMG]


    [​IMG]

    I think i have quite smooth 400VDC after series capacitors but if source need more current probably this Smoothing is not perfect due to the capacitor voltage falling a little as it discharges, giving a small ripple voltage.
    My questions are;
    1- What Mosfet acts while driving with a small ripple 400DC Voltage? Do they Burn?
    2- How could i able to supply Smoothly 400VDC as regulated ? Do i need to apply more capacitors? Or any filter devices? Or boost covertors?
    Note: Source need maximum 400VDC 2 Amps and I use mosfets capable 400VDC
    Best Regards
     
    Last edited: Oct 10, 2012
  2. duke37

    duke37

    5,364
    771
    Jan 9, 2011
    1. Do you mean that the circuit is connected directly to the mains without a transformer?
    2. Two 200V capacitors in series will not be safe at 400V. The voltage may not be divided equally even with balancing resistors. Why not use 500V capacitors which are available.
    3.To smooth 2A a resistor/capacitor filter will lose quite a bit of power. A large heavy choke instead of the resistor would be technically better.
    4. You do not say what the fets do or whether the circuit needs a super smothed supply.
    5. Running a 400V maximum fet on 400V is asking for trouble.
     
  3. pyrodigy

    pyrodigy

    3
    0
    Oct 10, 2012
    Dear Duke
    Thank you for your reply
    1- Yes Circuit directly connected to Mains without transformer.
    2- After passed rectifier 230VAC will be approximately 325VDC. So i think 400V Capacitors will be safe.
    3- As your recommendation To smoth DC Results LC filter is better. What kind of choke do you recommend? What A large and heavy means?
    4- i am asking what will fets do when i pass through to rippled (unsmooth) 400VDC between Drain to Source!
    5- Mosfets which i use are capable of 400vDC,
    Best Regards
     
  4. BobK

    BobK

    7,682
    1,688
    Jan 5, 2010
    Ripple in the voltage going into the MOSFET is not going to harm it. Using a 400V MOSFET at 400V likely is likely to harm it. All you need is a little spike to exceed its breakdown voltage, and at those voltages and current it is not likely to survive. Use a MOSFET rated for 600V to be safe.

    Bob
     
  5. pyrodigy

    pyrodigy

    3
    0
    Oct 10, 2012
    Hello BobK
    Especialy in load i have 325 VDC and i use FQP6N40C mosfet which capable at 400Volt 6A
     
  6. duke37

    duke37

    5,364
    771
    Jan 9, 2011
    You have still not said what the fets are doing. As Bob said, 600V fet would be better.

    Computer power supplies run with a ripple on the supply and the fet control circuit adjusts at high frequency to reduce output ripple.

    I do not know what the choke would be like, possibly 100*100*100mm and a weight of 2kg. You are unlikely to find one to suit you so will have to make a special. The design is not easy.
     
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