MOSFET Body Diode during ON state

Thread Starter

k1ng 1337

Joined Sep 11, 2020
85
Hey I am working on a synchronous boost converter as shown in the attached image, I am trying to understand the body diode in the high side p channel MOSFET (M3) during ON and OFF time. If I am understanding this right: while M3 is OFF the MOSFET is conducting with a voltage drop equal to the internal body diode, when the MOSFET is turned ON the voltage drop of the junction is equal to Rds(on) provided sufficient gate voltage. So as the MOSFET switches there are two distinct voltage drops as the current ripple is forced through it.

I tried proving this with an N channel MOSFET on the breadboard but was unable to eliminate the .5 voltage drop while reverse conducting and ON, my reasoning concluded (aside from poor wiring) I did not apply enough gate to source voltage because the source sees VCC, OR the diode voltage drop always exists while ON or OFF
Also I'll note I'm just starting with p channel MOSFET.. perhaps my understanding is wrong when applied to P channel?

Thanks
 

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dl324

Joined Mar 30, 2015
12,871
If I am understanding this right: while M3 is OFF the MOSFET is conducting with a voltage drop equal to the internal body diode, when the MOSFET is turned ON the voltage drop of the junction is equal to Rds(on) provided sufficient gate voltage. So as the MOSFET switches there are two distinct voltage drops as the current ripple is forced through it.
When the MOSFET is off, the body diode is reverse biased. When it's on, the body diode is still reversed biased, but it can be close to 0V. It doesn't matter which polarity of MOSFET you're using; the body diode is normally reverse biased.
 

Thread Starter

k1ng 1337

Joined Sep 11, 2020
85
When the MOSFET is off, the body diode is reverse biased. When it's on, the body diode is still reversed biased, but it can be close to 0V. It doesn't matter which polarity of MOSFET you're using; the body diode is normally reverse biased.
So to clarify
When the MOSFET is off, the body diode is reverse biased. When it's on, the body diode is still reversed biased, but it can be close to 0V. It doesn't matter which polarity of MOSFET you're using; the body diode is normally reverse biased.
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When the MOSFET is off, the body diode is reverse biased. When it's on, the body diode is still reversed biased, but it can be close to 0V. It doesn't matter which polarity of MOSFET you're using; the body diode is normally reverse biased.
So during the ON state there is a conductive path (modeled as a resistor with a value of Rds(on)) created in parallel with the diode as implied by the schematic symbol?
 
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