Synchronous boost converter pmos bulk connection to Vx or Vout ?

[areebaa]

Hi,
I am designing simple synch boost converter. i connected nmos bulk(body) to ground and how about the pmos side bulk(body) ? shud i connect it to Vx , Vout or Vdd ? thank you

 

[BradtheRad]

Be mindful of the biasing voltage for your pmos. If your output voltage is 100V, then your pmos will need 100V bias to shut it off. This is due to the node below the inductor reaching the same voltage as your output voltage.

 

[dick_freebird]

Are you using the PMOS as your sync forward diode?
Are you sure you wouldn't rather (say) use a NMOS
with a pulse xfmr gate drive, so that you get the
body diode conduction and a lower-forward "bonus"
when you assert it? Or maybe a bootstrap scheme
for gate drive (with the body diode likewise giving you
a crude forward path in the absence of established
gate control and bootstrap charge)?

I once designed an integrated boost converter with
a PMOS sync diode and that bit was the hardest
part - and that was on SOI where I had a free hand
with transistor terminals.

 

[areebaa]

Be mindful of the biasing voltage for your pmos. If your output voltage is 100V, then your pmos will need 100V bias to shut it off. This is due to the node below the inductor reaching the same voltage as your output voltage.

so, you mean we should connect bulk of pmos to >= 100V , am i right ?

 

[BradtheRad]

so, you mean we should connect bulk of pmos to >= 100V , am i right ?

This concept creates a paradox, where you should drive the pmos from the same voltage as the output stage. But that volt level is not available at startup. It may or may not be satisfactory to drive the pmos from the output stage. Its voltage starts at the same voltage as the power source, then rises as the converter operates.

So this means a boost converter is tricky to switch. The recommendations in post #3 may be your best bet. That is, if you really want to have synchronous instead of asynchronous.

 

[areebaa]

This concept creates a paradox, where you should drive the pmos from the same voltage as the output stage. But that volt level is not available at startup. It may or may not be satisfactory to drive the pmos from the output stage. Its voltage starts at the same voltage as the power source, then rises as the converter operates.

So this means a boost converter is tricky to switch. The recommendations in post #3 may be your best bet. That is, if you really want to have synchronous instead of asynchronous.

I tied pmos body to constant 100v, it operates well but the output voltage is harldy higher than 100V. i was expecting a big boost voltage at the output. but no

- - - Updated - - -

Are you using the PMOS as your sync forward diode?
Are you sure you wouldn't rather (say) use a NMOS
with a pulse xfmr gate drive, so that you get the
body diode conduction and a lower-forward "bonus"
when you assert it? Or maybe a bootstrap scheme
for gate drive (with the body diode likewise giving you
a crude forward path in the absence of established
gate control and bootstrap charge)?

I once designed an integrated boost converter with
a PMOS sync diode and that bit was the hardest
part - and that was on SOI where I had a free hand
with transistor terminals.

yes, i am using pmos as syn forward diode controlled by pwm of gate drive. sorry i dont understand about a pulse xfmr gate drive, what does this mean ? thanks

 

[mtwieg]

The pmos body must be connected to a potential which is always at least as high as the source and drain. Connecting it to Vout is sufficient.