Special 9V supply

[Tarzan]

Hi,
I've been looking around and I've seen thousands of datasheets.
But not a single one could help me out on the following problem.

Here is the situation.
I need a stable 9v DC supply. Power is around 7.5 Watt (800mA).
The input voltage can vary between 6 and 15 volts DC.
A max of 30 Volts DC would be great.
Power comes from a DC source so an AC adapter or transformer is not an option.

Now that's a challenge I can't resolve.
Maybe one of you can.

I was thinking of going up first to eg. 48V and then back down to 9V.
But that implies to build two supplies.
I rather like to have one supply and one source of EMI.

Thanks,
Tarzan

 

[kipman725]

much easier to go up first and then down. You would be best going upto 24V and then down as you want to keep the duty cycle about 0.5

 

[Tarzan]

Hello Kipman725,
I'm afraid that's the way to go.
Done that but at the cost of twice the number of parts.
But nevertheless maybe a genious will come with a brigth idea.

But it just crossed my mind;
Maybe an isolated supply can bring a sollution.
A bit more complicated to build but still worth thinking about...

Tarzan

 

[sawreyrw]

You need a buck-boost supply. I know Linear Technology makes them and I assume National and other also do.

 

[Tarzan]

Yes, they do.
But with an input voltage that can vary from below the output voltage to way above it; these circuits do not automatically switch over from buck to boost.
That's the challenge with one psu.

 

[ratsept]

There are at least two ways to do it with just one supply. Either the older/simpler/lossy flyback converter or SEPIC. There are chips for both topologies available from manufacturers such as Maxim.

 

[RJM1]

From National Semiconductor Simple Switcher Web-bench.
9V-Reg 6 to 15V input.
9V-Reg2 6 to 30V input.

 

[Tarzan]

Here we have a classic example (me, myself, I) of getting lost in the multitude of possible sollutions after unsuccesfull searching and looking for the wrong topology.
The more people the more sollutions.
I was definitly on a wrong track.
Thanks for the sollutions guys,
Tarzan

 

[agdr]

How about this boost-buck supply?

Adjustable Auto Boost Buck Regulator Power Supply DC-DC - eBay (item 260728941144 end time Feb-27-11 13:00:13 PST)

input: 3.5V - 28V DC
output: 1.5 - 26V DC
input and output current: 1A
peak current: 3A
rated power: 13W
load regulation: 1%

$4 plus shipping and a long wait from China.

 

[agdr]

I forgot to say... if your max of 30V is required, just put 3 1N5401's in series with the input to knock it down 2V.

 

[thelenster]

LT is where I would go for the buck-boost. Just make sure the loop is stable, and the layout is optimized for mimimum R,C,L parasitics.

 

[jcx]

another seldom used topology is the cuk - non-isolated it inverts but can give continuous control without mode switching

~70 hits at linear.com for "cuk"

 

[thelenster]

That brings back memories. Lots of hype back in the 90's. Can't remember what the drawbacks were though? He did make a big stir at the time. White elephant?

 

[djQUAN]

SEPIC might work?

haven't tried it at high input voltages though. I made a spreadsheet from a nat semi app note to calculate values...

 

[Tarzan]

Thank you all,
I know enough now.
Cheers,
Tarzan