DC Power Supply (5V 10A) - Questions

[Dr.Ehsan]

Hello,
I'm planning to design a circuit for a DC Motor with these parts:
1* 7805
2 * 2N2955 (paralleling two transistors to increase current)
plus some capacitor and resistors.
(the circuit image is attached)
Here is my question:
If I use a 12volts-6Ampere transformer ( 12V * 6A = 72W), does it provide enough power to have 5V-10A as output?
theoretically: 72W/5V = 14.4Ampere


Thanks,

 

[Resqueline]

That transformer will give you only 15V & 4.24A DC (= 64W) since you lose a couple of volts in the rectifier.
So you can use that transformer and achieve those spec's only if you build a switch-mode (buck/step-down) regulator having an efficiency of 79% or better.
Using that linear regulator you'll need a transformer giving 7V & 14A (minimum), and a heatsink good for 30W (minimum). The rectifier will dissipate 20W btw..

 

[Dr.Ehsan]

Thanks, as far as I knew, designing a switching (SMPS) circuit is a little hard and needs many parts.
Is there any simple switching circuit to bu used?

 

[jackorocko]

Thanks, as far as I knew, designing a switching (SMPS) circuit is a little hard and needs many parts.
Is there any simple switching circuit to bu used?

If you look around you might be able to find a switchmode IC circuit that would only require a few external parts.

 

[duke37]

Your circuit is wrong, you have three resistors in parallel. you need only one.
You need a low value resistor in the emitter of each transistor to ensure current sharing, dropping say 0.5V at maxixmum current.

 

[Dr.Ehsan]

There's a simple switching circuit (including LM2576), which has 5V - 3A output.
(the circuit image is attached)
Datasheet: http://www.national.com/ds/LM/LM2576.pdf
Question: Is it possible to increase its current by power transistors?

 

[Resqueline]

There's also the 5A LM2678 (& LM22678) but I'm not so sure it's just a matter of parallelling & sync'ing them or that there's an easy way to add a transistor.
It might be better to use a low-power switcher chip that is made to drive an external MOSFET. Then there's (almost) no limit to the current that can be had out.
Or, if you can accept buying a ready-made 10A switchregulator module I found one example here.

 

[Dr.Ehsan]

Thanks all,
I've found a useful circuit: "Achieve up to 8 amps from LM2575/6", It is driven by a MOSFET IRF9540.
(the circuit image is attached)
here is the link: http://www.aca-vogel.de/DCDC-Converter/8AfromLM2575_en.html

 

[(*steve*)]

I'm a little concerned that the current peaks through the 1N4148 will be quite high.

I see a rated max of 450mA for repeditive peak current. I'd be far happier following the advice of selecting some other schottky diode in its place.

And you're pronbably better of not quoting the entire article. A link to it is fine.

 

[Dr.Ehsan]

I suggest to use 1N5822 or BYV28 instead of 1N4148.
I don't have BYV29, so I'll use BYV32 which has Average Forward Current of 20A.
Also, L2 is 100uH and should be similar to the attached image.
Question: may I use 1Amper inductor in this circuit?! OR I have to find a high-amper inductor?

 

[(*steve*)]

Your inductor carries the full current, so if your output current is > 1A... (and you have to consider that peak current will exceed the average current)

This pdf may help you.

 

[Dr.Ehsan]

Thanks steve, but my output current is at least 8A
What happens if I use 1amper inductor?

 

[Resqueline]

Overloading components will (as usual) make several things happen:
1st; the inductor core will saturate, which means the current will "spike" (rise uncontrollably fast) very early in each conduction cycle.
2nd; this will most likely destroy the MOSFET, and as a result you'll get the full 15V out on the 5V.
3rd; if (as in by magic) the MOSFET should survive, the inductor wire will burn to a crisp.