Power MOSFET Bridge Rectifier

The
principle is simple: whenever the instantaneous value of the input AC
voltage is greater than the rectified output voltage, a MOSFET is
switched on to allow current to flow from the input to the output. As we
want to have a full-wave rectifier, we need four FETs instead of four
diodes, just as in a bridge rectifier. R1–R4 form a voltage divider for
the rectified voltage, and R5–R8 do the same for the AC input voltage.
As soon as the input voltage is a bit higher than the rectified
voltage, IC1d switches on MOSFET T3.

Just as in a normal bridge
rectifier, the MOSFET diagonally opposite T3 must also be switched on
at the same time. That’s taken care of by IC1b. The polarity of the AC
voltage is reversed during the next half-wave, so IC1c and IC1a switch
on T4 and T1, respectively. As you can see, the voltage dividers are
not fully symmetrical. The input voltage is reduced slightly to cause a
slight delay in switching on the FETs. That is better than switching
them on too soon, which would increase the losses.

Be
sure to use 1% resistors for the dividers, or (if you can get them)
even 0.1% resistors. The control circuit around the TL084 is powered
from the rectified voltage, so an auxiliary supply is not necessary.
Naturally, that raises the question of how that can work. At the
beginning, there won’t be any voltage, so the rectifier won’t work and
there never will be any voltage... Fortunately, we have a bit of luck
here. Due to their internal structures, all FETs have internal diodes,
which are shown in dashed outline here for clarity.

They allow
the circuit to start up (with losses). There’s not much that has to be
said about the choice of FETs – it’s not critical. You can use whatever
you can put your hands on, but bear in mind that the loss depends on
the internal resistance. Nowadays, a value of 20 to 50 mW is quite
common. Such FETs can handle currents on the order of 50 A. That sounds
like a lot, but an average current of 5 A can easily result in peak
currents of 50 A in the FETs.

The IRFZ48N (55 V @ 64 A, 16 mW)
specified by the author is no longer made, but you might still be able
to buy it, or you can use a different type. For instance, the IRF4905
can handle 55 V @ 74 A and has an internal resistance of 20 mR. At
voltages above 6 V, it is recommended to increase the value of the
8.2-kR resistors, for example to 15 kR for 9V or 22 kR for 12 V.