• Resolved

ISOW7842: Series-connecting output of ISOW784x?

Prodigy 175 points

Replies: 15

Views: 163

Part Number: ISOW7842

Hi,

If I want to series-connect the output of multiple ISOW784x converters, would it be sufficient to place a schottkey diode across the output of each converter (cathode to VISO, anode to GND2)? The idea is to provide a bypass to prevent reverse voltage and provide a path for load current in case one of the series converters starts before the other(s). Or would other/additional precautions be necessary?

Thanks,

Andrew

  • Additional details: The inputs of the ISOW784x's are all connected in parallel, and all are the same part number (ISOW7842 in this case).

  • In reply to Andrew Paquette:

    Hi Andrew,

    Thank you for reaching out and for the interest in ISOW7842.

    Just to make sure I understand your requirement correctly, could you please confirm which of the below do you intend to do?

    1. Series connection:
      1. VISO output of one device connected to VCC input of the other device.
    2. Parallel connection:
      1. VISO outputs of multiple devices tied together to form a single VISO point.

    From your description, it seemed like you want to perform a parallel connection. Could you please confirm again why do you want to do this? Is it to increase the overall output current capability or to provide redundant or backup power supply?

    Regards,
    Koteshwar Rao

  • In reply to Koteshwar Rao:

     Hi,

    I accidentally marked as resolved.

    I want parallel-in (VCC), series-out (VISO), to achieve a higher output voltage.

    Thanks,

    Andrew

    Edit: added example schematic.

  • In reply to Andrew Paquette:

    Hi Andrew,

    Thank you for the quick clarification and for sharing the schematic to understand your use-case.
    Yes, this approach of connecting outputs in series shouldn't be an issue. I do not remember us testing this approach ever but I do not expect any issues with this.

    Once you do test this configuration, please do share us results and your observations. Thanks.

    Regards,
    Koteshwar Rao

  • In reply to Koteshwar Rao:

    We initially did not include the schottkey diode across the outputs and had quite a few failures. We are still having occasional failures in this configuration, though we aren't sure of the cause. Failure mode is typically that the output from VISO to GND2 reads about 1-3 Ohms with a multimeter (chip unpowered), and outputs around 0.1 - 0.3V when powered, and sits 10C-15C hotter than its neighbors (of identical part number).

  • In reply to Andrew Paquette:

    Hi Andrew,

    Sorry to hear about the failures. We don't expect such a connection to be an issue as the two ISOW devices will be operating independently. I can't think of any reason for this to fail, not sure if external factors in the system are causing any issue.

    Could you please elaborate on what is the nature of failures you are seeing? What gets damage? Do you see see lower VISO votltage, VISO/GND2 short or anything else? Please do share more details. Thanks.

    Regards,
    Koteshwar Rao

  • In reply to Koteshwar Rao:

    Hi Andrew,

    Will you be able to share the information I requested in my post above? Thanks.

    Regards,
    Koteshwar Rao

  • In reply to Koteshwar Rao:

    Hi,

    I took a few waveforms of a functional set, but I didn't think to do that before I replaced two last week. Nothing abnormal looking in the functional set.The upper isolator has a little extra load than the lower isolator (about 4mA load + an extra 10uF decoupling that is only across the upper isolator), and there is about 8mA load and 40uF decoupling across the series combination. The waveforms below were consistent for two different (functional) sets that I measured.

    For the failed chips, I typically observed that they were sitting about 10-15C hotter than their neighbors. When I remove power, they measure about 1 to 3 ohms from VISO to GND2 (i.e., shorted output), and when powered VISO-GND2 would read 0.1-0.3V with a multimeter. When I remove the chip, then the VISO-GND2 on the board measures high impedance (no short on the pcb), and the chip measures 1 to 3 ohms across VISO-GND2.

    In one case previously (without the schottkey diode), I did measure it on the scope, and the VISO output was trying to startup, but was repeatedly starting and going past the datasheet OVT threshold and shutting down (but the multimeter reading was only around 0.3V). I don't have those waveforms though.

    - Andrew

    Startup: Ch1: VISO output of upper isolator, Ch2: VDD (input voltage to both isolators), Ch3: VISO output of lower isolator.

    Shutdown: Ch1: VISO output of upper isolator, Ch2: VDD (input voltage to both isolators), Ch3: VISO output of lower isolator.

  • In reply to Andrew Paquette:

    Hi Andrew,

    Thank you for sharing additional information along with the waveform.

    The issue that you are describing doesn't seem like related to the series connection of ISOW devices. It is probably due to not meeting the input and output capacitance requirement as described in the section 11 of ISOW datasheet. You mentioned of upto 40µF of additional load capacitance on one of the ISOW outputs, if there is an increase in the output cap I recommend to increase the input cap as well.

    It is important to have the input cap to be at least 100µF higher than the output cap so that the device stays in regulation during brown-out and power-up conditions. Once this is taken care, I believe the issue that you are seeing shouldn't occur.

    Please do consider these changes and let us know the results, thanks.

    Regards,
    Koteshwar Rao

  • In reply to Koteshwar Rao:

    Is the ISOW chip likely to damage itself if there isn't enough extra capacitance on the input? Or is it just that it may not meet its datasheet voltage regulation specs?

    When multiple of the ISOW supplies are powered from the same 5V input, how much relaxation from the "extra 100uF on input" and "600mA current limit available" recommendations can we take?