How to increase the amount of current a buck converter can put out

Thread Starter

uraniumhexoflorite

Joined Oct 23, 2016
204
I've been working on a 12 to 5 volt buck converter designed to charge mobile devices and I've run into a bit of a snag. The current output of the converter is usually about 2 milliamps. The voltage is around 5 volts and the phone says that it is charging. I have a zener stabilizer on the converter's output, but no current is going through it. I'll post a picture of the circuit in a few minutes. Thanks.
 

ronv

Joined Nov 12, 2008
3,770
Here's an up to date schematic of the buck converter and the BJT multivibrator, which is acting as a PWM.
There are a few problems:
The 200 ohm resistors are to small, so the transistors don't turn on very good.
The frequency is to low for only a 50 Uh inductor.
A PFet would be a better choice for the switch. As it is, it is a source follower.
Have you thought about using a 555 for the multivibrator?
How much current are you looking for?
 

Thread Starter

uraniumhexoflorite

Joined Oct 23, 2016
204
There are a few problems:
The 200 ohm resistors are to small, so the transistors don't turn on very good.
The frequency is to low for only a 50 Uh inductor.
A PFet would be a better choice for the switch. As it is, it is a source follower.
Have you thought about using a 555 for the multivibrator?
How much current are you looking for?
I'll try these out. I initially tried to use a 555, but inverting it was a pain and I had worked with BJT multivibrators before, so I tried one out and it worked much better than the 555 did. I'm unsure what R1 and R4 change, so I tried different resistances and stopped changing them when I got close to 5 volts. I'm looking for a current output of 0.7 to 2 amps (the more, the better). I have made several phone chargers before, but this is my first SMPS, so I might have some questions that may seem obvious. I really appreciate the suggestions. I'll test them out and tell you how it goes. Thanks.
 

ronv

Joined Nov 12, 2008
3,770
I made a few changes to get it in the ball park. But I'm not sure you will be happy with it. The biggest problem is that there is no regulation. I know you were thinking of a zener to clamp the voltage, but it would have to be very big at 5 volts and 2 amps.
So I also attached a buck regulator. This is the simplest type I think. I just picked parts for the simulation so if you want to build it we should pick some different ones.
 

Attachments

tcmtech

Joined Nov 4, 2013
2,868
So why not just buy a common automotive 12 - 24 :5 VDC converter unit for the price of a cheap burger and fries and not bother with the DIY design work altogether? I guarantee you won't build your own SMPS based buck converter unit for less or smaller either.
 

Obli

Joined Aug 7, 2016
22
So why not just buy a common automotive 12 - 24 :5 VDC converter unit for the price of a cheap burger and fries and not bother with the DIY design work altogether? I guarantee you won't build your own SMPS based buck converter unit for less or smaller either.
It seems like a worthwhile learning experience. unraniumhexoflorite stated that this is not their first DIY charger, but it is their first SMPS-based charger.
 

Thread Starter

uraniumhexoflorite

Joined Oct 23, 2016
204
Tomorrow I'm going to see if I can salvage a coil from a dead CFL. Using a CFL coil should allow me to lower the PWM frequency. Will lowering the frequency have any effect on the current? Thanks.
 

Thread Starter

uraniumhexoflorite

Joined Oct 23, 2016
204
It seems like a worthwhile learning experience. unraniumhexoflorite stated that this is not their first DIY charger, but it is their first SMPS-based charger.
Yup. In the future, I want to build a variable SMPS power supply, so this will give me a bit of experience before trying to build something really complicated.
 

ronv

Joined Nov 12, 2008
3,770
Tomorrow I'm going to see if I can salvage a coil from a dead CFL. Using a CFL coil should allow me to lower the PWM frequency. Will lowering the frequency have any effect on the current? Thanks.
Make the changes like I posted in the first attachment. Then try your coil.
I would not hook it up to a phone until you see the voltage.:D
Don't forget you need a P channel FET.
 

Thread Starter

uraniumhexoflorite

Joined Oct 23, 2016
204
Make the changes like I posted in the first attachment. Then try your coil.
I would not hook it up to a phone until you see the voltage.:D
Don't forget you need a P channel FET.
I don't have a PFET on hand (to my knowledge), but I have a good deal of circuit boards that I've salvaged stuff from. I may be able to find a PFET on one. Is there a way to identify one with a multimeter, or are datasheets the only way to tell FETs apart? Thanks.
 

ronv

Joined Nov 12, 2008
3,770
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