dc dc boost 12v to 18v, 4A [need help]

THE_RB

Joined Feb 11, 2008
5,438
If you go back to your original idea of a 34063, it's datasheet shows how to use an external NPN or NFET as the main power switch in a boost converter.

A 4A boost is not a hard circuit although you will need a good inductor capable of 10A or so without saturating.
 

bountyhunter

Joined Sep 7, 2009
2,512

http://www.ti.com/lit/an/snva552/snva552.pdf


I pretty much copied that but with 68uH with thick wire, you telling me it doesn't work?
Actually, I think you are telling us it doesn't work.;)

One of the things I learned in my 20 years working at Nat Semi, where that part was designed, is they always try to come up with slick trick circuits to put on the data sheet for all their parts...... some of them work, some don't. In most cases they are not fully tested.

As for this particular part, I think maybe it's a current mode control scheme inside it (?) If so, that would make it theoretically possible to parallel the drivers. In reality, paralleling power devices sometimes make them fight each other as the internal limiter of one kicks in before the other.

I don't know exactly what's up with this circuit because somebody else designed it and I never tested it.

The fact that it blew up is certainly a reason I would not use it......
 

Thread Starter

mdagli1

Joined Dec 4, 2012
17
Well after buying and blowing up several sets of LM2587, I'm decided to give up on it as I don't really have the experience to balance large current. All that current going through just 1 internal transistor just doesn't seem right. So going to drawing a line under this now and move back onto MC34063A.
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The before bit: (clarification)
The current draw will jump between 0.1A and 4A at 18v and the driving circuit has very low driving impedance. (4 Ohms with soft start) So need a way to ensure that the surge won't effect other circuits being driven off the 12v input. There is 1F, 0.1uF, 0.1uF capacitors currently being used and want to know if that would be enough.

The starting bit: (working)
Using: http://www.nomad.ee/micros/mc34063a/
Output: Ct=147pF, Ipk=12655mA, Rsc=0.024Ohm, Lmin=3uH, Co=1324uF, R=180 Ohm, R1=5.6k R2=75k (17.99V)
Been using 50uH for L.

The middle bit: (need help)
Used both a push/pull 3A transistor set up and then a 9A MOSFET (IRF540 or IRF520) but seem to be unable to get enough gain to drive the MOSFET above 1A. I've got lots of spare little transistors here to increase the gain further but need help to arrange them. 1A from the switching chip to 8A at the MOSFET. Don't really know what that would mean in terms of gain.

The end bit: (N/A)
Low-pass filter using: http://www.wa4dsy.net/filter/hp_lp_filter.html
I don't think it's necessary as frequencies above 100KHz won't effect it.
I'm more concerned about turn on/off surges as anything above 22v would cause damage to the load.
 
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