A very simple synchronous rectifier?

On a sunny day (Mon, 02 Mar 2009 02:02:54 -0500) it happened legg wrote in :

Thank you for this very nice paper with real examples. I did change my circuit a bit and tested it: ftp://panteltje.com/pub/power_pic/power_pic_synchronous_rectifier_diagram_img_0968.jpg it works very nicely: ftp://panteltje.com/pub/power_pic/power_pic_synchronous_rectifier_working_img_0965.jpg

Did not have time yet to program a controller PIC, maybe not needed as this works OK, and has far fewer components. As somebody else here already pointed out, I am not sure the power gain (or better efficiency gain) is that big, over say the high current Schottky diode I used.

The diagrams in your paper also have a rather large number of extra parts. When efficiency is critical that would be the way to go, or just to keep temperature low, or if you simple do not have any high power Schottkys in the box.

Have to think about the advantages / disadvantages a bit more, it makes for nice experimenting.

ftp://panteltje.com/pub/power_pic/power_pic_synchronous_rectifier_diagram_img_0968.jpg

ftp://panteltje.com/pub/power_pic/power_pic_synchronous_rectifier_working_img_0965.jpg

works OK, and

better

the box.

nice experimenting.

When configuring a DC restorer in an active gate circuit, it is advisable to provide a DC shunt path to ensure that it discharges within a known time interval. This is most efficiently achieved if the coupling cap is no greater than 10xCgss.

The one situation that is often ignored when selecting part values is intermittent or 'last pulse' scenarios. You want the last blip into the gate (which will include an attempted core reset to zero) not to result in large surges of current through powertrain semicinductors in any particular direction, or even possible self-oscillation.

If this is a forward or flyback transformer-coupled application, be sure to examine operating conditions around conditions where the drive waveshape on the power magnetics may be sub-optimum.

RL

On a sunny day (Mon, 02 Mar 2009 08:44:41 -0500) it happened legg wrote in :

Wow, you got me there! If the load was a battery (for charging for example), and I switched the thing off, then the MOSFET could be in the 'on' state, and BANG. This is a good argument to use a comparator to measure the voltage across the MOSFET, and indeed lower cap values. As this one is a SEPIC hysteretic controller, the time between pulses can vary a lot too, to very long times with light loads, much longer then the PWM repeat frequency. An other reason to use a comparator. I was first planning to use the internal comparator of a PIC 12F629 for this. Seems like this is needed anyways (trying to avoid those special driver ICs). Good peer review!!!!

I did some practical tests, changed the circuit so it leaks away the gate voltage, but I could really make it lock up, so the MOSFET was always on. Does there exist a latchup effect for MOSFETS? Because when connected as here: ftp://panteltje.com/pub/power_pic/sepic_charging_self_img_1002.jpg an interesting test circuit, the SEPIC converter feeds back into the battery that powers it, via a car headlight (12V / 55W), so I can, by turning up the output voltage of the SEPIC, get a positive current back into the battery, or by turning it down, the battery feeds the light bulb directly, strange, but that is what it does! This is because the MOSFET is then still 'on' some of the time, and then a current can flow in reverse direction (so actually where the drain is positive, normally the right direction LOL), BUT when I remove the drive pulse, sometimes the MOSFET seems to say on, keeping a small gate voltage (a few volts), refusing to discharge the gate through the 4k7, and lighting up the light bulb. This I do not understand, maybe the MOSFET is damaged, but after removing all power, everything works normally again on power up.

For those who think this self-charging circuit is the answer to the quest for an unlimited energy supply, well, it puts a current back in the battery, but it draws more from the battery itself, so the difference says something about the efficiency, and that will never be greater then 100 % :-) But sure you could patent it, and try to get some investor money :-) Risk capital is hard to come by these days though...

through the 4k7,

power, everything

Ah, must be oscillating! the RF voltage is rectified and keeps the FET open. Pity I just removed the synchronous rectifier circuit, cannot verify that. In this application, where I also want to charge batteries, it causes more problems then it solves. A heavy duty Schottky is almost as efficient, and has far less problems and components. On what you save for components you can buy a bigger diode. But I learned a lot :-)