Beginner regulated buck converter, 20-30v in, 13.8v out

[bigwill]

Hi there,

I have (I presume) a fairly simple requirement for a switchmode DC-DC converter. I have some solar panels that generate around 20-30V in a good light, and I would like to design, hopefully with some help, a buck converter to charge a lead-acid battery, as well as power the load connected to it.

I'm hoping to be able to have a maximum output of 300W from this converter (solar panels permitting), so it'll have to be fairly robust. I'm sure the MOSFETs suited for these voltages / currents are a dime a dozen these days, and I understand the basic theory of a buck converter, but I'm not sure what kind of PWM controller chips / gate drivers I need to make this a reality.

I'd like to build this myself if possible, I have pretty good construction skills and I can fabricate neat single sided PCBs if necessary.

Any SMPS gurus around for some recommendations / circuit ideas?

[Th3 uN1Qu3]

For the power section i recommend you to use p-channel mosfets, they are a lot easier to drive in a buck converter. The inductor core can be recovered from a PC power supply, up to 30ish volts there is no problem on those -26 (yellow/white) toroids. For the PWM controller, especially if you're looking to learn something, i'd use the good ol' TL494. As for gate driver ICs, i'll let other chaps chime in... i don't like them. I use gate drive transformers driven by a push-pull BJT pair. But then, i've mostly built offline supplies, just a single buck converter, and that was for low power (5w), and was easily driven directly from the 494.

[EWorkshop1708]

P-channel mosfets are not as available as N-channel ones are, since this is a solar panel, he could have floating ground, common positive, and pulse the negative side of the batteries through the inductor and filter capacitors.

[!]

With 300W worth of solar panels, I would wonder if it is a good idea to earth ground (and non-floating DC ground) the panel frame(s).

[Th3 uN1Qu3]

If P-channel mosfet availability isn't a problem in here, then i'm sure it isn't a problem in the UK either. Try the IRF5305. Available in both TO-220 and D2PAK. I used the D2PAK version myself... i think that's the only surface mount package that i really like.

[bigwill]

Hi everyone, thanks for the responses

Th3 uN1Qu3,

What do you reckon on a simple design using the IRF5305 you recommended with the TL494 (which you also recommended )

I guess I could drive the gate with push-pull emitter followers, although I would have thought some kind of level shifting would be necessary as a P channel gate is relative to the positive side of things. Maybe some kind of simple common emitter amp then buffered by a pair of transistors....

[Th3 uN1Qu3]

Level shifting? One capacitor and one diode. I'll see if i can find my schematic... yours will be the same, only bigger.

[gmarsh]

Originally Posted by EWorkshop1708

P-channel mosfets are not as available as N-channel ones are, since this is a solar panel, he could have floating ground, common positive, and pulse the negative side of the batteries through the inductor and filter capacitors.

I'd do this.

You could probably make a TL494-controlled system work as it has two error amplifiers. You'll need both of them for two separate control loops:

- Battery voltage regulation (making sure the battery doesn't go above its maximum charge voltage)
- Solar panel voltage regulation (making sure the solar panel doesn't drop below its MPPT voltage)

A third thing to watch would be maximum charge current, but you can limit that by limiting the size of your solar panel.

[bigwill]

Originally Posted by Th3 uN1Qu3

I'll see if i can find my schematic... yours will be the same, only bigger.

If you could find it I would really appreciate it

[elmurry]

I would like to design my own inductor for my buck converter using the toroidal inductor from a damaged PC power supply but I dont know whether the wounded wire is capable to hold 25A current or not. And If it is not, what is the suitable gauge of enameled wire? and does the the core of the power supply is suitable for 50 KHz frequency?


Your help will be greatly appreciated

elmurry