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I am looking to implement reverse polarity protection into a 24V circuit that is currently using the LTC4357. I originally thought that the "ideal diode" that it forms with an N-MOSFET would act as reverse polarity protection, but when I tested reverse polarity, the N-MOSFET essentially shorted itself and pulled max current of my power supply. I then noticed on page 10 of the LTC4357 datasheet that they have reverse polarity protection circuit examples that utilize MMBD1205 which is just two general purpose diodes packaged together with a common anode. My N-MOSFET is IRFS4010TRLPBF

Here is a picture of the reverse polarity circuits from the LTC4357 datasheet:

enter image description here

How does D1 in the example circuits work with the LTC4357 to protect against reverse polarity?

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2 Answers 2

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During reverse polarity the body diode of the FET prevents current flow to the load.

The lower diode in the dual D1 package prevents reverse polarity being applied to the LTC4357 (preventing violation of the abs max negative voltage of -1 V).

The upper diode clamps the GND pin to Vin to prevent unwanted substrate or leakage currents from Vout to a negative Vin causing erratic operation with a floating GND pin.

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  • \$\begingroup\$ So essentially, without D1, what I was seeing was the FET body diode pulling all the current (and heating up rapidly)? \$\endgroup\$
    – Nick_Nack
    Mar 24, 2022 at 0:09
  • \$\begingroup\$ No, I don't think so. If you applied reverse polarity without D1, you violated abs max ratings on the LT part, and the FET probably turned on weakly, and operated in the linear region causing lots of power dissipation and finally failure. The body diode would have been reverse biased the whole time. \$\endgroup\$
    – John D
    Mar 24, 2022 at 16:28
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To put it another way an ideal diode circuit is a 3-terminal circuit. Vin, Vout and GND.

Vin can be greater than Vout (it will conduct). Vout can be greater than Vin (it doesn’t conduct).

But in both cases Vin and Vout need to be above (or very close to) GND (for this chip and most others).

The diodes protect against the reverse polarity case.

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