Problem of high voltage drop at the input terminals and excess heating of power diode

Discussion in 'The Projects Forum' started by kautilya, Apr 14, 2010.

  1. kautilya

    Thread Starter Active Member

    Apr 26, 2007
    Hi there.

    I am working on a project called 'Soft- Switching of Buck-Boost converter' and I am facing the following problems:

    1. There is an excessive voltage drop at the input terminals of my circuit. I am providing a dc voltage from a rectifier board. At the rectifier output terminals, the volage is fine but as soon as I connect it to my circuit, it registers a huge drop. The voltage at my input terminals doesnt seem to go above 5 V however much I increase the supply voltage through the variac.

    2. There is an excessive heating of one of the power diodes. I feel that the voltage which is dropping at the input terminals is appearing at the diode. I have checked for this condition using a DMM. For eg. if I provide 15 V through a variac, I get a fixed 5V at the input and about 7-8V across the diose in question.

    I have tried to debug the circuit but I can't find any flaw in the connection. The continuity test through the DMM also appears fine.

    The load resistance I have used is 100 ohms, the resonant inductance is 180uH, the resonant capacitance is 50nF and the output capacitance is 1000uF.

    I have attached the circuit diagram along with this thread.

    Pls. go through it and help me correct the problem.


  2. kubeek


    Sep 20, 2005
    First, please don´t post pictures inside a document, post just the picture itself.
    Second, you don´t say how are the two transistors switched. Please post the entire diagram, including the drivers for the transistors.
    Are you sure that S1 or S2 aren´t kept on?
  3. rjenkins

    AAC Fanatic!

    Nov 6, 2005
    I'm guessing you have a major problem with your driver circuit.

    The switching transistors are shown as NPN/N Channel, (IGBTs?) so they need a few volts above the emitter to turn on. For that to work correctly, the gate drive needs to be above the positive supply as drawn or the switching losses will be very high.

    If the control supply is actually taken from the same source as the main power input, the transistors will not start to turn on until the supply reaches a few volts. If a device is normally On rather than normally Off, the thing will act somethng like a shunt regulator and clamp the supply somewhere near it's gate threshold voltage.

    In other words, it will go up to a few volts and then draw high current if you try to increase the supply beyond that point.
  4. Bychon


    Mar 12, 2010
    I started out suspecting Cr as leaking but then I thought, "any diode that has 5 volts during forward current is defective". That's as far as I can get right now.
  5. kautilya

    Thread Starter Active Member

    Apr 26, 2007
    The driver circuits for the two transistors are provided through 2 totem-pole transistors (npn and pnp) which are isolated from the PWM IC (3524) through an optocoupler. SInce I have two switches I have provided two isolation circuits (i.e. 2 optocouplers and 2 totem-poles). The gating pulses of the switches are perfectly synchronized. The two switches turn-on and turn-off at the same time. Initially the switches are off.

    Can you spot anything in the circuit operation which might be leading to the voltage drop?


  6. kubeek


    Sep 20, 2005
    Diagram, please. With actual values.
  7. SgtWookie


    Jul 17, 2007
    Your circuit diagram is woefully incomplete.

    Submit a complete circuit diagram, with all component values specified; ie: resistance, capacitance, inductance - and the actual part numbers of IGBT's. Include reference designators on the schematic (ie: R1, C3, Q2) so that it will be easy to reference the items in shorthand. All reference designators must be unique.