Filed under: Charge Controller. Tagged as: converter transformer, step down converter, step down power converter, transformer converter, voltage converters.
I finally got some time to put together a prototype of my proposed dual voltage converter. I wrestled with myself over how to wind the converter transformer. Depending on how I wind it, I could create a 48v battery charger for my electric bicycle, or I could step it down to charge a 12 volt battery. By carefully selecting the number of turns on the transformer, you should be able to make several different types of voltage converters from this circuit.
Sticking with the idea of a charge controller, I decided to wind a 16:15 ratio on the transformer converter. That is, the original coil had 16 turns and I wound 15 turns. I was actually trying for 16 turns, but missed one along the way.
I literally grabbed the MOSFET out of some N-channel ones that I had in the drawer. It’s an IRLB8748 and the Digikey number is IRLB8748PBF-ND. However, the point is that it shouldn’t make much difference what the specs on the MSOFET are. The important thing is to make sure it’s an N-channel MOSFET rated for the current you want to pass (5+ amps). You can also see the converter transformer wound with red, enameled wire.
Following the MPPT charge controller schematic, I added a rectifier circuit on the output of the transformer. I used two 3-amp schottkey diodes in parallel to create a half-wave rectification circuit. This probably isn’t as efficient a full diode bridge, but it cuts down on the number of parts needed as I only needed one diode instead of 4.
A note about putting diodes in parallel: Many engineers recommend against it. However, there is nothing wrong with it. You just have to make sure the diodes come from the same batch/same package. As long as they were manufactured under nearly identical conditions, then their device properties should be nearly identical. That being said, it would be better to use a single high-power diode. I just needed to use whatever I had on hand.
Now that I’ve got the prototype built, I just need to do a frequency sweep to determine its frequency response. I’ll attach a 4-ohm resistor on the output, which will load the ciruit down to 3 amps at 12 volts on the output. Once I get the load dialed in, I’ll adjust the frequency of a square wave into the MOSFETs gate pin to find out how the circuit responds to different frequencies. I’ll also adjust the duty cycle as well as frequency.
I’ll post oscilloscope images later!