The circuit’s efficiency is quite good. At a 10-A dc output (7
V rms ac input), none of the components require a heatsink.
The power MOSFET case temperatures stay well below 50°C.
Due to a lack of equipment, I could not test the circuit at
higher currents. But beyond 10 A, it may be worthwhile to connect
two MOSFETs in parallel to reduce RDS(ON) even further.
But pay attention to the resistances of the PCB traces, since
they could be higher than the MOSFETs’ RDS(ON)!
The circuit was compared to a popular KBU8B silicon diode
rectifier. At an input voltage of 5 V rms at 50 Hz and a constant
load of 5 A dc, the KBU8B’s output was 4.45 V dc, average,
measured across C3 (15,000 μF). Under the same conditions,
the “greener” rectifier produced an output of 5.9 V dc, average.
Another comparison that may be even more meaningful
involves determining what rectifier input voltage is needed for a
given dc output voltage. For this measurement, a transformer with
several output windings (Ultron ULT2) was connected to the mains via a Variac. The desired output was 5 V dc, average. Measurements
were done at two constant load currents: 5 A and 10 A.
For the KBU8B rectifier and a 5-A load, the transformer’s 6-V
output winding was used. The Variac had to be adjusted for a
transformer output (secondary) voltage of VSEC = 5.55 V rms,
which had to be corrected slightly to 5.48 V when the rectifier
got hot. The measured input power was 47 W. With a 10-A
load, which is already beyond the specs of the KBU8B, the 8-V
output winding had to be used. The Variac was adjusted to 5.97
V rms (5.87 V rms when hot). Under these conditions, the real
input power of the Variac was 88 W.
Using the “greener” rectifier with a load current of 5 A, the
Variac had to be tuned back to a transformer output voltage of
VSEC = 4.34 V rms (off the 6-V winding). The Variac’s real input
power was only 36 W. At 10 A, the 6-V winding could still be
used, with the Variac tuned to 4.82 V rms. The real input power
was 69 W. Thus, the power MOSFET rectifier circuit saved
roughly 10 W at 5 A and 20 W at 10 A.
At high currents and low voltages, and especially when the
output ripple voltage increases, the two power MOSFETs on
the right get a little warmer than those on the left. The reason is
because the driver stages on the left have their own filter capacitors
(C1 and C2) that provide a smooth dc voltage, while the
driver stages on the right are directly supplied from the high-ripple
output voltage. Unfortunately, right at the moment when the
gates of the right-side MOSFETs should be taken high, the available
output voltage is rather low, since output capacitor C3 has
discharged to its minimum value (traces CH1 and R_A in Fig. 2).
The cure for this problem is simple. Add a diode and a capacitor
to supply the right half of the circuit (Fig. 3).
The whole circuit fits into roughly the same volume as a
conventional bridge rectifier. Considering that there’s usually
no need for heatsinks, the circuit should pay off quickly.
Also, in many cases, a smaller and cheaper transformer can
be employed.
REFERENCES:
www.linear.com (search for “lt1716”)
www.irf.com/product-info/datasheets/data/irf2804.pdf
www.vishay.com/docs/88658/kbu8.pdf