Might as well post some synchronous rectifier circuits

June 5, 2007

I’m pretty sure these will work. Their main innovation is that they are self-powered, using the amazing op-amps that appeared while I was napping for 20 years.

I realized, in the middle of a trip to Silicon Valley a couple of months ago, that I was thinking about this circuit wrong, and that I needed to be careful about just how the op-amps would behave (within the limits of the power supply). The main problem was that I needed to remember the assumption “zero volts across the inputs to the op-amp” once I had put the negative feedback in place. Thus, the 8-volt zener diodes. One thing missing from this diagram are the V+ and V- connections to the op-amp itself; those come from the DC power supply itself. The Shottky diodes in parallel with the MOSFets are intended to help the circuit get bootstrapped at low voltages — a better op-amp would allow lower voltages, yet.

I’m not working in surface mount, because I am a Neanderthal with big clunky fingers, even though the surface mount parts are better. Assuming this circuit works as it is supposed to, I’ll look into that next.

2 Responses to “Might as well post some synchronous rectifier circuits”

  1. wsanders Says:

    Dr2: You probbaly know all this already, but yuu might check out some of the puck power supplies at http://luxeonstar.com/, fabricators of insanely bright LEDs. They are not cheap, and less fun than building your own circuit, though.

    http://taskled.com/ is a garage-based business in Sunnyvale that seems to be making a similar item.

    These items, being microprocessor-based, might need clean DC, however.

    Allelectronics has the 1W Cree LEDS for reasonable prices:


  2. dr2chase Says:

    Ahoy WSanders: I am solving a slightly different problem. Bicycle “generators” are really alternators. To get DC to run all these nifty regulators, I’ve got to rectify the AC, and plain old diodes will lop 0.6 V off a 6 V RMS input (the nominal voltage from a bicycle generator).

    A synchronous rectifier instead turns power MOSfets on and off at the appropriate time to make them act like a perfect diode. New op-amps are will to turn on at very low voltages (these are 3.2, I’ve seen 1.8) and so it is possible to make the whole thing self-powered, if you bootstrap with ordinary diodes.

Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out / Change )

Twitter picture

You are commenting using your Twitter account. Log Out / Change )

Facebook photo

You are commenting using your Facebook account. Log Out / Change )

Google+ photo

You are commenting using your Google+ account. Log Out / Change )

Connecting to %s

%d bloggers like this: