Dc-dc boost converter

Discussion in 'General Electronics Chat' started by SABU31, Mar 7, 2011.

  1. SABU31

    Thread Starter New Member

    Mar 7, 2011
    4
    0
    Hi
    I am developing a boost converter rating I/P 150V-200V and O/P 350V , 370W. The circuit diagram is attached,switch is IRF840. When i am testing the circuit, the switch and output diode gets damaged (after about 2 min)even when input is 50V , 50%duty ratio ,50kHz. There is no heating in the heat sink Just a small smoke comes and drain and gate is shorted and diode is also shorted. How is the switch getting damaged. Before when I was using the switch IRF830 it was working till 300V o/p before getting damaged.
     
  2. newbies_hobbyist

    Member

    Jun 4, 2010
    67
    8
    What is your load (value, resistive, inductive, capacitive)? Maybe you overload your circuit. With the specification of your output your circuit is capable of handling a 331ohms max load.
     
  3. SABU31

    Thread Starter New Member

    Mar 7, 2011
    4
    0
    Hi

    The value of the load is 450ohms resistive.So at 350V o/p the power consumption would be 272W. The source is from 230V ac which is connected to auto-transformer and then transfromer isolated and there is full bridge rectifier to obtain dc. I am confused why the same circuit which was working for IRF830 atleast upto 300V ( for few minutes) is not working for IRF840 even at 50V I/P. Is it due to driver problem or source problem. The driver is ixys IXDN504PI and 6N137 opto isolation is used.
     
  4. SABU31

    Thread Starter New Member

    Mar 7, 2011
    4
    0
    Hi
    I have noticed that there is ringing in the gate waveform.Could this be the reason the switch is getting damaged?How do i avoid ringing.But even if losses is there due to ringing,it should heat the heat sink,isnt it? I have attached the waveforms, where waveform 2 is gate and 1 is drain to source voltage
     
  5. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    Sounds like a driver problem, in particular the MOSFET turn-off time.

    The IRF840 has a higher gate charge (63nC) than the IRF830 does (24nC)
    The Miller charge is three times as high (32nC vs 11nC). It'll take 3x the gate current to turn the IRF840 off as quickly as the IRF830.

    Ringing on the gate could do it as well. Try using a 10 to 22 Ohm resistor from the gate to the driver, keeping the leads as short as possible, and as close to the MOSFET as possible.
    You might use a Schottky diode across that resistor (anode to gate) to help decrease the turn off time.
     
  6. SABU31

    Thread Starter New Member

    Mar 7, 2011
    4
    0
    Hi
    I am using 10 ohm gate resistor but no shottky diode in anti-parallel. Should i keep lower value of resistor so that gate current is higher. Also if i keep the anti parallel diode, during turn off there would not be any limiting resistor, so will driver get damaged.? Also whats the cause of ringing.

    Thanks
     
  7. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    Ringing is caused by the effective capacitance of the gate (gate charge) and the inductance of the wiring between the gate driver and the MOSFET gate itself, as well as the bypass caps on the gate driver. It acts more or less like a series LC network.
    The wiring needs to be as short as possible.

    A slower turn-on time is not much of a problem; it's the turn-off time that's a killer. When the MOSFET starts turning on, the inductor has little-to-no current in it, so the supply voltage is dropped across the inductor.

    When the MOSFET is turning off, current through the MOSFET and inductor is at its' peak, so power dissipation in the MOSFET during turn-off will be high if it stays in the linear region.

    Try increasing the gate resistor to 22 Ohms, and add the Schottky diode. The resistor will still act to snub the ringing.
     
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