UC3845 12v -> 24V 3A

Discussion in 'Electronic Projects Design/Ideas/Reviews' started by arvinfx, Feb 25, 2009.

1. arvinfxMember

Joined:
Nov 5, 2008
Messages:
174
Likes:
0
Hi

I need Schematic for converting 12V to 24V With minimum 3A Current

I use UC3845

Last edited: Feb 25, 2009
2. Ubergeek63Well-Known Member

Joined:
Apr 16, 2008
Messages:
1,886
Likes:
37
you can start with the realization that you need a 20A inductor and a 50A FET...

3. arvinfxMember

Joined:
Nov 5, 2008
Messages:
174
Likes:
0
20A Inducator!? how mach µH of it? 200µm is enough!

I need Schematich for this IC uc3844-45 Boost Step-up

Joined:
Jan 12, 1997
Messages:
-
Likes:
0

5. Ubergeek63Well-Known Member

Joined:
Apr 16, 2008
Messages:
1,886
Likes:
37

well it was a quick head calculation.

12V means you are probably trying to run off a lead acid battery which drains to 10V unloaded and will be lower under load.

the voltage ratio times the current delivered divided by efficiency equals average current:

24/10*3/0.8=9A average

since the inductor current is a ramp you will have at least double the average for the peak current, or 18A so 20A is not even conservative.

Also you want to use a 3842, not a 44 since it will likely run over a 70% duty cycle.

look at the data sheets, to make it a boost you use an inductor instead of a transformer, and a diode from the drain to the output capacitor.

the inductor value depends on the frequency. Using a tapped inductor would allow a lower peak current rating...

Dan

Last edited: Feb 25, 2009
6. arvinfxMember

Joined:
Nov 5, 2008
Messages:
174
Likes:
0
thanks Ubergeek

But i still like use my UC3845 IC (lets try to explosion it )

So , I not have many time and must have a schematic to finish this project.

Do you have this one ? in datasheet not a good example for one boost output !!!

I use the MC34063 and current is very low about 700ma Maximum power output for this IC and it not usable ...

Best Regard

Joined:
Apr 16, 2008
Messages:
1,886
Likes:
37
8. arvinfxMember

Joined:
Nov 5, 2008
Messages:
174
Likes:
0

look at this is good sample?

9. Ubergeek63Well-Known Member

Joined:
Apr 16, 2008
Messages:
1,886
Likes:
37
seems reasonable. What is it rated for?

Dan

10. arvinfxMember

Joined:
Nov 5, 2008
Messages:
174
Likes:
0
rate of output voltage? or rate of useable?

Joined:
Apr 6, 2006
Messages:
14,902
Likes:
79
Location:
England

12. arvinfxMember

Joined:
Nov 5, 2008
Messages:
174
Likes:
0
I don`t now but must be 1 or 1.5A some one give to me and say its work ! but i now 10uH in self is very low for me!

Last edited: Feb 28, 2009
13. Ubergeek63Well-Known Member

Joined:
Apr 16, 2008
Messages:
1,886
Likes:
37
you asked for 3A at 24V froma 12V battery. I was asking what the circuit that you posted puts out.

If you want to use less than a 10uH you need to either reduce the frequency or the rated power, of course that is assuming that it is doing doing what you want otherwise.

Dan

Last edited: Mar 1, 2009
14. arvinfxMember

Joined:
Nov 5, 2008
Messages:
174
Likes:
0
I only find upper schematic for example and it`s in accident near to my circuit ...

I think high frequency provide to us low reple in output and also reduce efficiency! and i like to use bigger self in my circuit but How much uH? what is the formula ?

15. Ubergeek63Well-Known Member

Joined:
Apr 16, 2008
Messages:
1,886
Likes:
37
Sorry I did mean a larger inductance... the formulas depend a lot on how the chip runs... in general you need to get enough energy into the inductor with out saturating it to get the amount of energy you need out of the converter.

J=½I²L and I=tV/L and if memory serves W=Jf in this case (W=J/S).

Dan

Last edited: Mar 2, 2009
16. arvinfxMember

Joined:
Nov 5, 2008
Messages:
174
Likes:
0
I build upper circiut and my output is 24V and current maximum is 1.5A ! its work ,i think if my source was stronger 1.5A can be added...

17. Claude AbrahamMember

Joined:
Mar 17, 2006
Messages:
197
Likes:
23
Location:
NE Ohio, USA
You will need to add current limiting. The boost topology is not protected against a shorted output.

Regarding the inductor current rating, if the average inductor current is 9 A, then the peak will be greater. But, not 18 A if continuous conduction mode is used. The double value occurs if boundary mode is used. With a large enough inductor value, with 9A average, the peak can be 12A, while the valley can be 6A. As the L value increases, the ripple decreases, i.e. 11A peak w/ 7A valley, etc.

Since the duty factor will exceed 50%, instead of a 3845, use the 3843. It is identical in all respects except there is no toggle flip-flop which limits duty factor to 50%. I'd use the newest CMOS version, the UCC28C43 part. Have I helped?

18. OblongNew Member

Joined:
Aug 2, 2008
Messages:
50
Likes:
0
Why don’t you use a modern controller like TPS40210 these are only about two bucks. There are lots of online tools to use here is TI’s. For your input / output.700kHz.

Ratings for power stage components

Just select the external components from your preferred supplier. You can get select a better mosfet Alpha & Omega Semiconductor make nice logic level ones low Qt Rdson good prices etc and pick a better Rectifier.

TPS40210 product folder, with online tool.Spice models etc.

Non-Isolated Switching DC/DC Regulators - DC/DC Controllers (External Switch) - TPS40210 - TI.com

another online tool
Design of switch power supplies

Last edited: Mar 11, 2009
19. arvinfxMember

Joined:
Nov 5, 2008
Messages:
174
Likes:
0
Hi, Oblong

Thanks about your remark, At end my UC3845 has been explosions and i buy another UC3843 it make for me about 21 volt and 2.8 AMP to! u think i must use TPS40210 and UC384x can`t handle my Load!?

I see the sites its perfect! thanks about that too.

AW! forget this-> IS RIGHT? : THE WATTS OF BOOST TOPOLOGY IS 150W AT MAXIMUM AND PERFECT DESIGN ?!!!!!!

20. OblongNew Member

Joined:
Aug 2, 2008
Messages:
50
Likes:
0
I’m not saying you have to use the TPS40210, I’m saying there are better controllers out there for the same price or little difference. This one has internal slope comp, has a fraction of the current sense resistor power req etc. The reason I suggested that controller is; it is well supported with spice models the online tool and a very detailed design for a boost converter in the data sheet. The part is also widely available. Ultimately your choice will depend on what you can get. I live in North America so sourcing components for me is easy. Where you live this may not be the case.

The above was meant as an example of what’s available to assist you in your design. If you go to other semi manufacturers they usually have tools that will recommend a controller for your design parameters. You still have to apply common sense obviously you want higher voltage rated caps and some of the other components should be upgraded. I use them as a general guide,to give me a rough idea of what I'll be needing.

The controller doesn’t set the power output, the power stage components do, unless the controller has an integrated switch.