Energy storage - The Generator & Rectifier
A synchronous rectifier is an active device that replaces the good old diode bridge. The purpose is to reduce the losses caused by the forward voltage drop of the diodes. This voltage is the source of heat and raises the threshold where the generator begins to charge. Not what you want when you want to charge your batteries with maximum efficiency.
Synchronous rectifiers have been around for many years in all the switch-mode type power supplies used approximately everywhere these days. But, they are still rare in generators.
In the last post I had started the prototyping for this rectifier, as it deals with three phases, six active diodes are used. These diodes are typically referred to as ideal diodes because the behave as one would want a diode to behave. That is no forward voltage drop.
Below is picture showing a working fullwave rectifier using ideal diodes. These diodes are in fact not diodes at all but switched power MOS-FET´s controlled by a logic that ensures they are switched on only during the part of the cycle where the difference in voltage across it has the desired polarity.
The setup is very basic, the bridge is fed approximately 12 volts AC from the mains driven toroidal transformer seen in the background. The lightbulb serves as a load. So far so good.
You can see the scope probes hooked up across the load, + - output of the the bridge. Note that the bridge is readied for three phase but the last two FET´s with controllers are not fitted.
In the first scope picture we are looking at one single diode and you can see that the ON time, the positive going period, is just slightly shorter than the OFF time. This is exactly what we want. That is, a very small dead period when the diode is closed so that it does not open during negative periods.
Synchronous rectifiers have been around for many years in all the switch-mode type power supplies used approximately everywhere these days. But, they are still rare in generators.
In the last post I had started the prototyping for this rectifier, as it deals with three phases, six active diodes are used. These diodes are typically referred to as ideal diodes because the behave as one would want a diode to behave. That is no forward voltage drop.
Below is picture showing a working fullwave rectifier using ideal diodes. These diodes are in fact not diodes at all but switched power MOS-FET´s controlled by a logic that ensures they are switched on only during the part of the cycle where the difference in voltage across it has the desired polarity.
You can see the scope probes hooked up across the load, + - output of the the bridge. Note that the bridge is readied for three phase but the last two FET´s with controllers are not fitted.
In the first scope picture we are looking at one single diode and you can see that the ON time, the positive going period, is just slightly shorter than the OFF time. This is exactly what we want. That is, a very small dead period when the diode is closed so that it does not open during negative periods.
In the second image is the full wave rectification visible. Nice.
What became evident after some load testing was the lack of stability, when a rectifier does not switch as it should, it quickly becomes dramatic with lots of dumped current. I blew a few driver circuits and some exhibited questionable performance even before putting them into the rectifier path.
After ducking into the various data sheets it appears that the required gate voltage for the power
MOS-FET´s is in a greyzone. New devices are on order. Lets see if this solves the problem.
More will follow.
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