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ISO7740: abnormal output while input is normal

Part Number: ISO7740

Hi,

my customer is using ISO7740F as below in an isolated DCDC(phase shift full bridge).

F28069 is placed in the secondary side, and the PWM will drive half bridge gate driver in the primary side and the isolation is achieve through ISO7740F.

Primary side supply of ISO7740F is 3.3V, secondary side is 5V. 100ohm and 100pF applies to all four channels(I just draw one channel)

But we will observe below waveforms: 

light blue: INA, dark blue- OUTA, purple- gate driver output corresponding to OUTA.

We can see that OUTA does not respond to INA correctly. We measured the 5V and 3.3V supply of ISO7740 is stable during the period.

This may happen for all four channels.

What may be the cause?

  • Hi Howard,

    Sorry to hear about the issue.
    We do not expect the output of ISO7740F to not follow its input or cause any issues in following the input. There must be something external that could be causing this. Could you please share the schematic and PCB layout to start with? I need to see the actual schematic showing connections to MCU and gate driver along with component values and PCB layout. Thanks.

    Regards,
    Koteshwar Rao

  • The schematic is just as what I draw here.

    But the PCB layout is hard to get from the customer as confidential.

    What special requirement about the layout? Could you please give me some general advice?

  • Hi Howard,

    I understand the concern that has in sharing their schematic and PCB layout, if needed these can be shared over email as well.

    The diagram that you have shared doesn't show the decoupling capacitors and their values, it doesn't show which gate driver is used and doesn't show any other passive components in the schematic. Even these small things can cause issues and sometimes it could be an issue due to some other component which we may not have seen before. Hence, the very first step is look at the schematic to see if there is anything unusual is connected to device either directly or indirectly which could cause such an issue. Hence, I would need your help in sharing the actual schematic.

    In the PCB layout, it is critical for the decoupling capacitors to be placed within 2mm distance from device power supply pins. Just like schematic, there could something that doesn't look like is related to the issue but could be the one causing it. We need to look at PCB layout as well to rule out these two items.

    I would also appreciate if customer can share a waveform where ISO7740F input INA, output OUTA and VCC2 are captured at the same time in single screen and the issue is observed at OUTA. The absolute voltage at output depends directly on its supply voltage hence, there is a good chance this could be due to the VCC2 voltage dropping when OUTA is dropping.

    Please do share the requested info for us to debug this better and faster. Thanks.

    Regards,
    Koteshwar Rao

  • Koteshwar,

    We short EN2 pin to VCC2 and the problem is solved for now. Need further test in the product.

    Is EN2 short to VCC2 okay for this device?

    It seems that EN2 is easy to be interfered.

    At first EN2 is floating and there is such problem at very light load of the power stage.

    Then we test with EN2 pull up with external 4.7K resistor to VCC2, and the problem happens at heavy load like 400V, 10A output rating.

    Then we test with EN2 pull up to VCC2 with 0k resistor and the problem is solved.

  • Hi Howard,

    Glad to hear that customer could resolve the issue by connecting EN2 to its VCC directly which was floating earlier. Yes, floating EN pin is not recommended as the internal pull-up is weak. If the system is noisy, the EN pin can pick-up external noise very easily. This being a power supply application, transients noise due to inductive switching is expected.

    Using 4.7kΩ resistor (which strong enough) as pull-up for EN pin should have resolved it but I see it is not helping here. Did you use a leaded resistor to connect EN to VCC? Maybe the resistor leads picked-up the noise and part of the reason could also be the PCB layout. Connecting EN2 to VCC2 directly shouldn't be a problem.

    I stil strongly recommend that the schematic and PCB layout are reviewed from us to avoid any other issues which may not have showed up yet. Thanks.

    Regards,
    Koteshwar Rao