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ISOW7821: PMT circuit

Part Number: ISOW7821

Hi,

I am considering using ISOW7821 in the PMT circuit shown below.
The transistor is turned on and off at 600kHz to adjust the voltage of the PMT.
I want to use ISOW7821 to turn that transistor on and off.
Is it possible to use ISOW7821 with this circuit?
Please point out if there is a problem.

Best Regards,

  • Hello Kaji,

    thank you for using the E2E Froum!

    Could you give me a few more details on your circuit?
    If I get this right you want to use the ISOW77821 as an isolated gate/base driver in your control loop? Are you planning to connect GND2 to the Emitter of the Transistor?
    What Transistor are you using? Whats the needed gate Voltage and drive strengh? Do you want to drive this transistor from the digital outputs of the ISOW7781?

    Best Regards,
    Matthias
  • In reply to Mat1980:

    Thank you for your reply.


    > If I get this right you want to use the ISOW 7821 as an isolated gate / base driver in your control loop?
    ⇒ You are right. Drive the transistor with digital output.

    > Are you planning to connect GND 2 to the Emitter of the Transistor?
    ⇒ You are right.

    What Transistor are you using? Whats the needed gate Voltage and drive strengh?
    ⇒ Use Darlington transistors. Since the load is PMT less than 1 mA, the base current is about several uA.

    Best Regards,

  • In reply to Kaji@PAN:

    Hello Kaji,

    thank you for your clarifications! Are you planing to use a 5V or 3.3V output voltage on the secondary side?

    Normally this digital isolator is not intended to directly drive a transistor.

    However taking the values you specified the drive strength should be sufficient. You can find this information in the datasheet:

    Are you planning to connect other devices to the secondary side power supply?
    Are there other system care abouts we need to be aware of? Like certain certifications/parameters/noise requirements you will need to meet?
    I assume that the transistor will have a sufficient HV rating?
    As the bias current will come from the isolated secondary supply we also recommend to follow the general guidlines for layout and decoupling specified in section 12 in the datasheet.

    Best Regards,
    Matthias

  • In reply to Mat1980:

    Hello Matthias,

    >Are you planing to use a 5V or 3.3V output voltage on the secondary side?
    ⇒i will output 5V.

    >Are you planning to connect other devices to the secondary side power supply?
    ⇒There is no plan to connect other devices.

    >Are there other system care abouts we need to be aware of? Like certain certifications/parameters/noise requirements you will need to meet?
    ⇒For now, there are no specific requirements.


    >I assume that the transistor will have a sufficient HV rating?
    ⇒This circuit is required to transmit a signal exceeding the potential difference of 1 kV between the primary and secondary of the digital isolator.
        However, as you can see from this circuit configuration, in practice the digital isolator does not require insulation.
        Between the primary and secondary of the digital isolator, it is connected as follows.
        Transistor + attenuator, or transistor + PMT, or transistor + capacitor

    I would like to hear your opinion as to whether such a circuit configuration is established or not.

    Best Regards,

  • In reply to Kaji@PAN:

    Hello Kaji,

    I am trying to better understand your system. Could you please help my with answering these questions:

    what is the exact application of this circuit? And what is the isolation barrier needed for?
    What is the voltage at the node marked HV? The Emitter voltage will be -1100V?
    How are you generating the voltage for the ISOW?
    Would it be possible for you to share a more detailed block diagram?

    Thank you for your inputs!
    Best Regards,
    Matthias
  • In reply to Kaji@PAN:

    Hello Kaji,

    I hope your project is going well!

    I saw you did not get a chance to reply to my questions yet..

    Regarding your question raised last time:

    I would like to hear your opinion as to whether such a circuit configuration is established or not.


    We want to make sure to provide you a detailed and profound answer, in order to avoid future issues within your project.
    Thats the reason I was following up with furhter questions. Taking the limited information we have, I see no reason why this should not work (as your drive requirements are within the output specifications of the device). But it is not the typical way our digital isolators are used, so to get a more detailed opinion we would require you to answer the questions I raised.

    If you have concerns about sharing specific details of you circuit online, I can also offer you an offline discussion.


    I hope that this helps you!

    Best Regards,
    Matthias

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