How can I design a simple DC-DC buck step down converter with feedback

[crutschow]

Just the comparator alone pulls enough current to cause a spark

spark where?

 

[Fluffyboii]

spark where?

At the 12V lead acid battery terminals. I tried to just use the comparator and it was pulling unnecessary amount of current by itself. Got the LM393 today, I designed the PCB for it too since a quad comparator seemed overkill.

 

[Fluffyboii]

This is the PCB. I tried my best but this is my first self made PCB so it may have some issues. I tried adding some extra power lines to make it withstand 10A. The diode seems like two diodes in one packade so I parallel connected them. I am not sure if it is flawless.

 

[crutschow]

The right part of your schematic is rather a mess.
You don't normally use diagonal lines or run wires through a component on a schematic, and the grounds should be at the bottom, not run clear across the schematic to the ground on the left..
The schematic should be drawn neatly, not to follow the PCB layout.

D1 should be close to Q1 on your PCB as it carries high current.

 

[Fluffyboii]

The right part of your schematic is rather a mess.
You don't normally use diagonal lines or run wires through a component on a schematic, and the grounds should be at the bottom, not run clear across the schematic to the ground on the left..
The schematic should be drawn neatly, not to follow the PCB layout.

D1 should be close to Q1 on your PCB as it carries high current.

I fixed some of the issues with that schematic but you are right diode should be closer to mosfet. I also prepared the coil.

 

[Fluffyboii]

Big one is 200uH at 0.3 ohm the small one 300uH not gonna use the small one anyway.

 

[Fluffyboii]

crutschow Should I fully remake it or is it ok as it is. I think this will be the last version, I put a lot of effort into making it better.

 

[Fluffyboii]

Am I putting the mosfet pins in wrong direction. I keep loking the diagram and I can't be sure about it. LTspice mosfet diagram arrow location seems to be reverse of the real thing. So I think I made a mistake because of it in the EasyEda diagram. Can someone check it for the ease of my mind. Maybe I am getting confused because this is a P channel mosfet.

 

[crutschow]

Am I putting the mosfet pins in wrong direction.

In the LTspice schematic, the P-MOSFET source is on the left, the gate is in the middle and the drain is on the right.
Look at the MOSFET pin layout in the data sheet for the MOSFET you are using to determine if your layout is correct.
Be careful you don't confuse the top of the package from the bottom in the data sheet diagram.

 

[Fluffyboii]

The PCB mashine at collage was broken so I made it the usual way. It works. I wasn't able to test it with 10A but seems to work well at 2A. I want to make the voltage variable if I can. I tried to use my oscilloscope to look at the comparator output and mosfet gate but I wasn't able to get anything meaningful. Does anyone know why.

 

[Fluffyboii]

Ok I set my oscilloscope to AC from DC and I was able to see the comparator output and mosfet gate. It actually falls in the hearable frequency range when 1-2Amps is pulled from the circuit. More current = higher frequency and at some point you can't hear it, neat.

 

[Fluffyboii]

crutschow Hi, I wanted to say I finished the project and it was very successful. But I couldn't use the PCB I designed a the end because the PCB machine broke at the worst time possible. I was wondering if I can turn this circuit to a boost converter with minimal changes required to boost to 36V from 12V to power up my 100W led from lithium batteries. Also can I change the tl431 reference resistor values to get 4.2V to charge the lithium batteries.

 

[crutschow]

I was wondering if I can turn this circuit to a boost converter with minimal changes required to boost to 36V from 12V to power up my 100W led from lithium batteries.

No, that circuit cannot be readily converted to be a boost converter.

Also can I change the tl431 reference resistor values to get 4.2V to charge the lithium batteries.

Yes, you just change the divider resistor values to generate 4.2V, or any desired voltage.
The resistor values are selected to give 2.5V at the Ref input when the cathode voltage is 4.2V.
For example, R1=4.75kΩ and R5=6.89kΩ will work.

 

[Fluffyboii]

No, that circuit cannot be readily converted to be a boost converter.

Yes, you just change the divider resistor values to generate 4.2V, or any desired voltage.
The resistor values are selected to give 2.5V at the Ref input when the cathode voltage is 4.2V.
For example, R1=4.75kΩ and R5=6.89kΩ will work.

Thanks for the info. I guess I should open another topic to make a boost converter. I found 450V 3300uF capacitors for super cheap, wanna make a 900V power supply to charge them in series. Or can I use my 450V commercial boost converters to charge each capacitor independently.

 

[crutschow]

Or can I use my 450V commercial boost converters to charge each capacitor independently.

That should work.

What are you going to do with a 3300µF capacitor charged to 450V?
It might not be lethal, but it will sure give a heck of a shock (one that will remind you to never do that again ).

 

[Fluffyboii]

That should work.

What are you going to do with a 3300µF capacitor charged to 450V?
It might not be lethal, but it will sure give a heck of a shock (one that will remind you to never do that again ).

I once zapped myself with 330V film camera flash capacitor after accidentally touching it, then 3 seconds later I did it once again I wanted to make a coilgun for a long while. I have the thristors and boost converters but one of the boost converters broke at some point and shorted so I need to find a way to fix it first. The only reason I bought the caps was because they were unusually cheap. Got two of them for 6USD total.

 

[ChrisP58]

Thanks for the info. I guess I should open another topic to make a boost converter. I found 450V 3300uF capacitors for super cheap, wanna make a 900V power supply to charge them in series. Or can I use my 450V commercial boost converters to charge each capacitor independently.

Unless your boost converters have isolated outputs, or unless you use two isolated power sources for the inputs, you won't be able to connect their outputs is series to get 900 Volts.

 

[Fluffyboii]

Unless your boost converters have isolated outputs, or unless you use two isolated power sources for the inputs, you won't be able to connect their outputs is series to get 900 Volts.

I think they are isolated but not sure. The one on the right is dead I think the switching component shorted. They are rated for max 390V anyway so I need something new.