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I have been using the BSS138 level converter circuit for a while now and today it struck me I get how the voltage is stepped up by the FET or the BSS138 when you are applying say 3.3 V on the LV side.

What happens in the FET to reverse that and make it bidirectional?

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When the HV side goes low, the body diode becomes forward biased, pulling down the source to a lower voltage than the gate.

Provided the gate is pulled up to a sufficiently high voltage for that new source voltage to bias the channel into conduction, the gate pullup takes over and fully enhances the channel.

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  • \$\begingroup\$ Thanks that helps, but i feel stupid now I found the answer as soon as I posted: cdn.sparkfun.com/tutorialimages/BD-LogicLevelConverter/… that PDF has a great explanation on page 10 and 11 of what happens \$\endgroup\$ – Tjaart van aswegen Jan 7 '19 at 15:19
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    \$\begingroup\$ Interestingly I once plotted the BSS138 characteristics in the reverse direction and the threshold was lower than in the forward direction (~0.4V in my case) so the body diode never even conducted in this application. \$\endgroup\$ – Kevin White Jan 7 '19 at 15:23
  • \$\begingroup\$ I came across this brain tease today at work when I looked at one of my PCB designs and realized that my Drain and Source pins was switched and that got me thinking what actually makes it work says a lot about my PCB design skills LOL \$\endgroup\$ – Tjaart van aswegen Jan 7 '19 at 15:27
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When you apply 3.3V on the LV side the MOSFET turns off (Vgs ~=0) and the 10K resistor R4 pulls the drain up to 5V.

The only pull-up on either side is the 10K resistors so it can have relatively slow rise time compared to a voltage translator chip.

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