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Selecting the proper BJT and IGBT

Discussion in 'General Electronics Discussion' started by pheni004, Jun 13, 2013.

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  1. pheni004


    Jun 13, 2013

    I have a design (made by someone else) for a ciruit that I want to build, and I want to make sure I buy the right transistors so they'll work in the circuit.

    I'm looking at these BJTs and IGBTs on Mouser and Digikey, and I have no clue what the various parameters mean. Is there anything I specifically need to look out for to make sure I don't break the transistors in the ciruit?

    The BJT will get a 20mA pulse at the gate which then (hopefully) pulse a 10V signal at 20mA through the BJT. Would any old BJT be able to take this, or do I need something specific? (I think the point of the BJT is to give a voltage pulse to another part of the circuit, not amplify current, but that's not what I'm worried about.)

    The IGBT will take a 10V pulse at 20mA and (hopefully) discharge a capacitor that was held around 200V. Do I need to be worried about the Maximum Gate Emitter Voltage since the gate will be 0 or 10V and the emitter will be at 200V? Do I need to worry about the alleged "tens of amps" coming through the IGBT from the capacitor as it discharges? Or would any old IGBT suffice here, as well?

    Thank you so much!!
  2. duke37


    Jan 9, 2011
    Post the circuit diagram.

    Define the power supply, input signal and load.
  3. pheni004


    Jun 13, 2013
    I think I have posted an image of the circuit diagram to this entry. I would like to have the 100V power supply at the top right to be able to go as high as 200V.

    Is there more that you need? I'm sorry, I'm very new to electronics so I'm not exactly sure what you mean by "define the power supply, input signal, and load". The power supply could be either a battery or something I plug into the wall, I'm not very particular. The input signal will be a function generator for now, a pulse from a photomultiplier tube in the future. I'm not exactly sure what load means in this context, but the pulse from the capaictor on the middle-right will be going to a transformer.

    I'd be happy to clarify anything further! Thanks!

    Attached Files:

  4. duke37


    Jan 9, 2011
    The pnp transistor puts a pulse of the right polarity to the output device.
    I have only used FETs and SCRs in this application so do not know about IGBTs.

    The load is not defined so that the output device can not be specified. I would be inclined to simulate this part of the circuit to see the peak currents and inductive voltages.

    The source/gate will not exceed 10V in this circuit.

    I do not see the purpose of the diodes unless they are connected at the crossroads. If they are, then it is better to use two T junctions or a blob.
  5. pheni004


    Jun 13, 2013
    I believe the diodes are in there as some sort of safety measure, as are the capacitors from the power supply to ground. Fortunately, I'm fine just following this design without trying to improve it.

    What I'm really interested in is purchasing the the right transistors for the circuit.

    Would I be correct in saying

    IGBT: Anything from Mouser as long as Maximum Gate Emitter Voltage is more that 10V

    BJT: Anything from Mouser

  6. john monks

    john monks

    Mar 9, 2012
    The schematic does not have any dots where the wires are connected. So I am assuming that where wires meet they are connected.
    You have not stated what the load is going to be so I will assume 50 ohms.
    The diodes prevent the collector of the IGBT from swinging outside of 0 to 100 volts in order to protect the IGBT.
    A 2N2905 should be fine for the PNP transistor. You have not stated what the load is going to be but if it is 50 ohms then just about any N-channel IBGT rated for for 10 volts or more gate to emitter and a collector to emitter voltage of about 200 volts or more. To be on the save side and because IGBT's are cheap I would choose on rated for about 50 amps.
    I don't use IGBT's because they are slow. MOSFETS have much faster transition times but are more easily blown out so greater care is required.
    Last edited: Jun 14, 2013
  7. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    Jan 21, 2010
    You want a small signal PNP transistor. A BC558 should work.

    For the IGBT you need something that switches on with 5V or so of gate voltage. It also needs to be able to withstand above200V (try for something like 400V or higher). Most IGBT's are rated for pretty high voltages, so that's not an issue. R5 needs to be 1W. C3 and C4 should be rated for well in excess of 200V. Oh, 400V diodes would be a good idea (even though anything above 200V would technically suffice).

    The current is minimal. The resistor R5 is the one which will dissipate the most power.
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