# Buck . . . really ?

Discussion in 'The Projects Forum' started by Phaisit, Feb 24, 2015.

1. ### Phaisit Thread Starter New Member

Feb 23, 2015
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0
As I want to build a Buck converter circuit here I find the web calculator here

http://www.daycounter.com/Calculators/Switching-Converter-Calculator.phtml

So I tried to build with 20V 50kHz Input and 0.8A 7V output for my battery 7.2V (min 7V max 7.5V) 2800mAh charging

As I try to build it in the Orcad but I don't know why the current goes really high like that

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2. ### Dodgydave Distinguished Member

Jun 22, 2012
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Why do you need a 20V input, can you use lower voltage, and use an ordinary voltage regulator?

3. ### Phaisit Thread Starter New Member

Feb 23, 2015
26
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I use a 20v 1.12A Solar Cell as source to charge the battery so my prof. suggest me to use the buck converter and control by Microcontroller for Constant Current and Constant Voltage mode

4. ### Dodgydave Distinguished Member

Jun 22, 2012
4,762
705
You cant use both, only current or voltage is kept constant.., so you can use an Lm317t regulator set to 0.8amps constant current mode, it will take 35V input.

5. ### ronv AAC Fanatic!

Nov 12, 2008
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Right now your circuit has no feedback, so it the voltage is just a little high the current will be very high.
Make your duty cycle on for 30% and see if it drops.

6. ### Phaisit Thread Starter New Member

Feb 23, 2015
26
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It's still the same no matter how many duty cycle I change so I think it might be something wrong with my circuit

I was using the N-MOSFET before in Orcad and the output is kinda what I need but when I build it in real board it doesn't even work I don't know why . . . (The Voltage doesn't go through the MOSFET to load) So I tried simulate it with P-MOSFET here and this is what I got

7. ### jpanhalt AAC Fanatic!

Jan 18, 2008
5,536
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How are you driving the mosfet gate in real life? With the N-mosfet, you will need a gate drive considerably greater than 20V. The actual amount will depend on which mosfet you are using. For the P-mosfet, please post your circuit.

John

8. ### Dodgydave Distinguished Member

Jun 22, 2012
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okay, IF you get the software to work, which buck regulator will you be using?

9. ### Phaisit Thread Starter New Member

Feb 23, 2015
26
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Can I use the Logic N-Mosfet ? I found it need Vgs around 5V since battery is around 7V I only need for 12-15V (I don't know I kinda new about this)
With P-Mosfet I didn't build it yet just simulate it in Orcad as I added a pic in the head topic

Uhh I don't know what you mean (really sorry I kinda newbie about this topic since prof. just send it to me)

10. ### ronv AAC Fanatic!

Nov 12, 2008
3,212
1,064
Use the PFET, but turn it around from your simulation so the source goes to Vcc.
If you use NFET your gate signal needs to go to 30 volts.

11. ### Phaisit Thread Starter New Member

Feb 23, 2015
26
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Thx a lot! At least current is drop down to 10A

But in this case if I using PFET how much voltage for gate do I need to trig it ?

12. ### Phaisit Thread Starter New Member

Feb 23, 2015
26
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But the starting current kinda really high here (30A) so should I fix something ?

13. ### jpanhalt AAC Fanatic!

Jan 18, 2008
5,536
833
@ronv : I agree a P-mosfet is a better idea.

@Phaisit : To answer your question, is the source of the N-mosfet when it is turned on +7 volts or +20V? What would a 13-volt drop between the drain and source do? (Remember Rds(on) is very low.) Assume the voltage drop across the DS of the N-mosfet is very low, then you need to drive its gate above 20V (e.g., 25V). It can be done, but it requires a special drive circuit. The P-mosfet will source current, but you still need to show how you will drive its gate.

John

Jun 22, 2012
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15. ### Phaisit Thread Starter New Member

Feb 23, 2015
26
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I was going to use a Arduino for drive the MOSFET but since it only 5V output I find that I might need a gate driver TLP250 but I don't know if it work.
And I only have a 1 Solar Cell for a source

Thx I will look for it if this buck doesn't work I might have to change to another method (like IC or something) I will keep this in my bookmark

16. ### jpanhalt AAC Fanatic!

Jan 18, 2008
5,536
833
I thought this was some sort of an assignment. That is, you wanted to make the buck converter for reasons other than cost and simplicity.

Not only are there several chips available for doing it, SparkFun, Banggood, and eBay have complete units at reasonable prices. This one is based on the MP2307 chip (https://www.adafruit.com/datasheets/MP2307_r1.9.pdf). Up to 23V input and adjustable output from 1.0V to 17V at 1.8A. The size is about 5/8" X 1/2", which is very similar to SparkFun's devices.

John

17. ### Phaisit Thread Starter New Member

Feb 23, 2015
26
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So this work like a gate driver right ?

18. ### Dodgydave Distinguished Member

Jun 22, 2012
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No, these are deadicated IC's for power supplies,what your trying to achieve is the workings inside the chip, using components put together , they do the job alot better and more efficient,

we aren't trying to stop you designing your own, just don't try to re-invent the Wheel!

19. ### MikeML AAC Fanatic!

Oct 2, 2009
5,450
1,066
If the solar panel has an open-circuit output voltage of ~20V, then the output current will be tiny (few mA).

A 20V open-circuit panel puts out its maximum power when its output voltage is about 14V...

20. ### Phaisit Thread Starter New Member

Feb 23, 2015
26
0
I tested it with a resistance load around 15ohm by the specific of the solar cell is
OC 20V
SC 1.21A
MaxP 18V &1.12A

and after test with the load the output current at noon is around 0.8A