After a limited succes with the previous 0-15V 72A prototype, I'm working on this new one.
The old prototype was based on a PWM full bridge with synchronous rectification and worked fine as long as voltage control was used. However, I got a lot of trouble when I tried to implement average current control, mostly due to transformer saturation and probably also related to a lack of symmetry between secondary windings. Anyway, PWM full or half bridges with the buck inductor in the secondary side doesn't cope well with the quick duty cycle changes required for proper transient response.
I could have used peak current control, but it doesn't suit my needs. I need average current control because I want to connect several PSUs in paralell and obtain perfect current sharing by just ORing the current command buses (the one with the highest voltage preset will drive the rest).
That is why I'm about to try a different topology with the buck inductor in the primary side. It looks like a standard buck converter with a switch, an inductor and a diode in the primary side, but the output capacitors are placed on the secondary side and are transformer coupled. In this topology the transformer is always operated at 100% duty cycle and there is an inductor in series with it, so flux balance is guaranteed (I think...).
I've tried that topology in the past with succes, but I couldn't make PCBs at that time so the prototype was a mess of wires. Furthermore, I was using hard switching so there were lots of EMI and ringing. I was not able to improve EMI, so I abandoned the project.
The full bridge is essentially the same I was using in the previous prototype. Switching devices are MJE13009 bipolar transistors with proportional base drive.
I'm also going to try a magnetic snubber approach to achieve soft switching in continuous conduction mode. It works fine in the 2KW PFC prototype so it should also work fine here. L2 from the PCB is the center-tapped inductor that limits dI/dt.
This is the layout of the primary-side prototype PCB, IC1 is a CD4013 flip-flop for bridge sequencing and IC2 is a CD4049 hex inverting buffer, I will etch the PCB soon:
[IMGHTTPDEAD]http://eva.eslamejor.com/buckbrd0.gif[/IMGHTTPDEAD]
And this is a simplified schematic, don't trust all component values and note that the CMOS ICs are missing:
[IMGHTTPDEAD]http://eva.eslamejor.com/buckbrd1.gif[/IMGHTTPDEAD]
The secondary-side synchronous rectification PCB is already finished. This is the layout:
http://eva.eslamejor.com/100a_1.gif
And some pictures:
http://eva.eslamejor.com/100a_1_0.jpg
http://eva.eslamejor.com/100a_1_1.jpg
http://eva.eslamejor.com/100a_1_2.jpg
http://eva.eslamejor.com/100a_1_3.jpg
http://eva.eslamejor.com/100a_1_4.jpg
http://eva.eslamejor.com/100a_1_5.jpg
http://eva.eslamejor.com/100a_1_6.jpg
The rest of the stuff (SG3525 control IC, current and voltage amplifiers, etc..) is in bread boards so I can play freely with it.
I hope this could be useful for other people trying to design their own SMPS stuff.
The old prototype was based on a PWM full bridge with synchronous rectification and worked fine as long as voltage control was used. However, I got a lot of trouble when I tried to implement average current control, mostly due to transformer saturation and probably also related to a lack of symmetry between secondary windings. Anyway, PWM full or half bridges with the buck inductor in the secondary side doesn't cope well with the quick duty cycle changes required for proper transient response.
I could have used peak current control, but it doesn't suit my needs. I need average current control because I want to connect several PSUs in paralell and obtain perfect current sharing by just ORing the current command buses (the one with the highest voltage preset will drive the rest).
That is why I'm about to try a different topology with the buck inductor in the primary side. It looks like a standard buck converter with a switch, an inductor and a diode in the primary side, but the output capacitors are placed on the secondary side and are transformer coupled. In this topology the transformer is always operated at 100% duty cycle and there is an inductor in series with it, so flux balance is guaranteed (I think...).
I've tried that topology in the past with succes, but I couldn't make PCBs at that time so the prototype was a mess of wires. Furthermore, I was using hard switching so there were lots of EMI and ringing. I was not able to improve EMI, so I abandoned the project.
The full bridge is essentially the same I was using in the previous prototype. Switching devices are MJE13009 bipolar transistors with proportional base drive.
I'm also going to try a magnetic snubber approach to achieve soft switching in continuous conduction mode. It works fine in the 2KW PFC prototype so it should also work fine here. L2 from the PCB is the center-tapped inductor that limits dI/dt.
This is the layout of the primary-side prototype PCB, IC1 is a CD4013 flip-flop for bridge sequencing and IC2 is a CD4049 hex inverting buffer, I will etch the PCB soon:
[IMGHTTPDEAD]http://eva.eslamejor.com/buckbrd0.gif[/IMGHTTPDEAD]
And this is a simplified schematic, don't trust all component values and note that the CMOS ICs are missing:
[IMGHTTPDEAD]http://eva.eslamejor.com/buckbrd1.gif[/IMGHTTPDEAD]
The secondary-side synchronous rectification PCB is already finished. This is the layout:
http://eva.eslamejor.com/100a_1.gif
And some pictures:
http://eva.eslamejor.com/100a_1_0.jpg
http://eva.eslamejor.com/100a_1_1.jpg
http://eva.eslamejor.com/100a_1_2.jpg
http://eva.eslamejor.com/100a_1_3.jpg
http://eva.eslamejor.com/100a_1_4.jpg
http://eva.eslamejor.com/100a_1_5.jpg
http://eva.eslamejor.com/100a_1_6.jpg
The rest of the stuff (SG3525 control IC, current and voltage amplifiers, etc..) is in bread boards so I can play freely with it.
I hope this could be useful for other people trying to design their own SMPS stuff.