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Selecting ADuM5230 Buffer Transistor

Hello,

Are you able to provide any advice on how I should select appropriate buffering transistors to sit between the ADuM5230 outputs and my MOSFET gates?

There aren't any example part numbers given in the datasheet or in any application note that I could find, and I don't have the background to be able to pick from among the many thousands that are available.

Thanks,

Doug McClean

Mass Bay Engineering

  • Doug: I can give you part numbers you can use for buffer transistors: NPN: BC817, PNP: BC807.

    Regards, Brian

  • I wants to use ADUM5230 with AUIRGB4062D1 in new industrial device.
    Max switching frequency: 20 kHz.
    Gate capacitance: about 1,5 nF

    Questions:
    1. Can I use it ADUM5230 with AUIRGB4062D1 without output buffer transistors with good reliability?
    2. What output internal circuit is in ADUM5230 outputs A and B. What output resistance have outputs A and B?
    3. There are parameters "output short-circuit pulsed current" "sourcing" and "sinking" in datasheet on ADUM5230. (100/300 mA, "short-circuit duration is less than 1 sec") What difference between "...sourcing" and "...sinking"? Can I do output current 2 A with time 1 us with frequency 12 kHz whithout damage from A and B outputs? What voltage loss between internal dc/dc-converter output and output A when output current 2 A (1 us current pulse from 10 uF external capacitor betwean Viso and GNDiso)? Also what voltage loss betwean Vddb and output B when output current 2 A (1 us current pulse).

  • Alex: I don't think you can use ADuM5230 with AUIRGB4062D1 without output buffer transistors and have good reliability. I'll review your questions and reply.

    Regards, Brian

  • Brian: Thanks.

    Next questions, if you allow me:

    1. Max Output Short-Circuit Pulsed Current (Sourcing) = 100 mA with duration is less than 1 s. But it would be logical to assume that Max Output Short-Circuit Pulsed Current (Sourcing) can be more than 100 mA and more than 1 A if pulse duration less then 1 ms or 1 us (1 s/ 1 us = 1 000 000). Is there an error here or can I increase the current at the driver output in this way? There is no information about it in datasheet.

    2. Why ADUM5230 doesn't have any control logic between inputs and enable pin (protection functions)? It does not take up much space on the crystal. Or I don't understand something.

    3. For reason in previos question I need to organize deadtime, shoot-through protection and enable after fault event in half bridge circuit with Xilinx PLD. But it more difficult, takes extra place on plate and increases the probability of error. Do you know compact IC chips, that can organize protection functions without PLD? 

    Thanks a lot.

    Regards, Alex.

  • Hello Alex,

    The ADuM5230 has and output current capability based on the size of the internal output MOSFETs. When driving the gate of the IGBT the gate driver is connected to, the amount of current available to drive that gate is limited by the combination of the internal resistance of the gate driver, and the external series gate resistor. When driving the external IGBT gate high, the gate driver is "sourcing" current. When the external IGBT gate is being pulled low, the gate driver is "sinking" current. The peak current of the internal MOSFETs are around 100 mA when sourcing, and 300 mA when sinking. If you attempt to get more current than that, you may be able to, but the internal MOSFET will eventually plateau and not give more than what the saturation current is. The current rating on the gate driver can be confusing.

    The ADuM5230 is special in that it delivers both isolated power and isolated gate drive in one, very small package. The available power that the ADuM5230 is limited mainly by thermal considerations, and for some low gate charge applications, this driver is a great fit. When the gate charge (or equivalent gate capacitance) is larger, the ADuM5230 can be limited in the switching frequency it is able to provide in the application. The 100 mA and 300 mA current ratings translate into slower rise/fall times than higher current gate drivers, but an external buffer stage can help increase the peak currents in sourcing and sinking.

    The ADuM5230 does not include deadtime control (or overlap protection). In a half-bridge topology, it is required that the user does not request that both IGBTs be on at the same time. This has to be ensured by external circuitry, or by the controller, as you have already noted. There are methods to produce deadtimes based on NAND gate structures, using RC time constants on the inputs. There are also some dedicated chips to convert a PWM signal into two signals with deadtime. I don't believe ADI has any of these deadtime generator chips right now.

    For the AUIRGB4062D1, the equivalent gate capacitance is around 5.1 nF. I got this from the datasheet by taking the maximum gate charge, and dividing by the driving voltage (15 V). Calculating estimated power for switching the AUIRGB4062D1 at 20 kHz, we get:

    C * V^2 * Fs

    5.1 nF * 15^2 * 20 kHz = 23 mW

    This is within the output power of the ADuM5230. In order to get faster rise and fall times at the IGBT gate, you can add a buffer as shown in the datasheet.

    RSchnell

  • Thank you very much for your answers! It was very interesting to read them!