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Reverse current frying a boost converter?

Discussion in 'General Electronics Discussion' started by mun5, Aug 28, 2015.

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  1. mun5

    mun5

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    Aug 28, 2015
    Hi

    AFAIK, most boost converters incorporate a diode or some mechanism for ensuring that current flows from the battery to the output. For instance, TPS61030's datasheet says that

    > This device however uses a special circuit which takes the cathode of the backgate diode of the high-side PMOS and disconnects it from the source when the regulator is not enabled (EN = low).

    MPC6140's datasheet says that

    > With the EN pin pulled low, the output of the MCP1640B is isolated or disconnected from the input by turning off the integrated P-Channel switch and removing the switch bulk diode connection. This removes the DC path that is typical in boost con- verters, which allows the output to be disconnected from the input.

    Does that mean if current tries to flow from output, these chips (at least these two?) won't be damaged at all? That is, for instance, there are two possible power sources connected in parallel, boosted to 5V. If only one is hooked up, would the current flowing into the other booster in reverse fry it?

    What if both could be hooked up?

    Thanks
     
  2. Gryd3

    Gryd3

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    Jun 25, 2014
    Hold up.
    The datasheet claims true disconnect, and will actually prevent reverse current flow when operating in PWM only mode.
    This must be clarified though. The disconnect only happens when the device is off.
    If the device is on, and you connect Vout to a voltage source that is higher than the output, then there does not appear to be any protection against that.

    So. Is your questions regarding reverse current while the converter is on, or while it is off?
     
    Arouse1973 likes this.
  3. mun5

    mun5

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    Aug 28, 2015
    Are you referring to the MPC6140? By the 'device' do you mean the chip or with the circuit? S4.1.3 says 'With the EN pin pulled low, the output of the MCP1640B is isolated or disconnected...'. The EN will be pulled low if it's not hooked up whilst the other source is hooked up and powering the device.

    If we look at the block diagram, VOUT is connected to an amp - will the reverse current kill the amp?

    The TPS61030 seems to have VOUT connected to the regulator, bypassing the shutdown control logic.
     
    Last edited: Aug 28, 2015
  4. Gryd3

    Gryd3

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    Jun 25, 2014
    I am referring to the chip.
    The documentation states that the output is isolated from the input, and that the output is not discharged, so if there is 5V present on the Cout Capacitor, then 5V should remain regardless of what happens at Vin. Please note though that this is still only possible when the EN is pulled low.

    As far as the block diagram is concerned, take a look at the amp labelled Izero. If this is the amp you are talking about, then reverse current or no from the Vout will not be an issue, as all it's doing is acting as a comparator.

    What other power source will you be using? Why is this a concern for you?

    Will Vout have any more than 5 or 5.5V on it? Will the voltage be negative? What output voltage will you be configuring the device for?
     
  5. mun5

    mun5

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    Aug 28, 2015
    Indeed. Well, the particular booster is not hooked up to a battery, I'd expect both VIN and EN to be pulled low (even when the other converter is powered up by another battery), since each battery powers up only the converter boosting its voltage. So, if the chip is designed to allow for COUT to hold 5V after disconnection, does that mean it's protected against reverse current then, as COUT would also hold 5V? Or would it be -5V instead?

    Ah!

    I'm hoping to avail two power sources: one lipo boosted to 5V and one with 2x AAs boosted to 5V. It's configured for USB, so I'd keep it at 5V. The voltage will not be negative - but why?
     
  6. Gryd3

    Gryd3

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    Jun 25, 2014
    That would most certainly be correct, if you had another power supply operating that was paralleled with this devices Vout, then holding EN low would not allow the second power supply to feed back into the device, or to Vin on the device.

    I as about the - voltage, because reverse current could be looked at a couple different ways, and I wanted to make sure all the bases were covered ;)
    If you pulled Vout negative, then Vout would be lower than the ground used on the device and you could expect some weird things to happen!
    Thank you for clearing it up though.
    I think you'll be fine as long as you use the PWM output mode, and treat the EN pins properly.
    It's still unknown what will happen if the Voltage present on Vout is higher than what the device is set for. So if you use the same Cout for both devices ensure you do not run both device at the same time, and set them to the same voltage.
     
  7. mun5

    mun5

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    Aug 28, 2015
    Great! That's reassuring. :) In that case, does it mean that even if both sources are hooked up, it'll still be OK then?

    I think TPS61030 doesn't make the state of COUT as explicit as MCP1640 does though. Perhaps it'd be safe to assume that C2/C3 also hold 5V after disconnect?
     
  8. Gryd3

    Gryd3

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    Jun 25, 2014
    That's right. Any capacitor on the output side would hold the voltage until the self-discharge or load drains them.
    Both datasheets claim they disconnect the output from the input when turned off.
     
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