MC34063 boost with external NPN transistor. Regulator 1, Lasse 0 :)

[lk]

Greetings,

I wanted to play around with the MC34063 in the boots configuration with external darlington, boosting from 3.5 volts, to 12 volts. And as long i dont apply anyload, it gives me 12 volts, although it does seem to have significant ripple.
if i load it with 10ma the output drops to 2 volts, this has me puzzled. But my thinking is this is a common part, so someone else might have had experience with the part in a simliar configuration.


I use the MJD122 as the external switch, i am however unsure about how to get the value of R3, 100Ohm is just choosen at random, i only found one datasheet where they had an example with the external darlington, something with its not allowed to get saturated.

I use a 3300uf cap on the output, this was the only one i had at hand that was had more capacitance than what was needed, i can see that it draws a lot of power for an instant during power up, but then it settles at 1 to 2 ma.

I get 56khz when looking at Ct, and the sawtooth looks very sharp

on pin 6 +7 i see 21 volts, looks fairly stable,  pin 8 has some ripple between 16 and 21 volts.


and on the output with my scope i see 12 volts raising to 20. volts aswell, its all very odd:)


-lasse

[Paul Price]

It is always best to post a schematic to explain, rather than using just words, since your circuit is unique.

All circuits that work are pretty much the same, but those that don't can usually be unique.

[c4757p]

There is one...

[c4757p]

Quick observations: You want to boost to 12V, but you say you see 21V on VCC? And your feedback network gives 19V, not 12V. I think you've got some calculations to redo.

I built up the circuit in SPICE (yeah, yeah, I know) exactly as you have drawn it, and it's oscillating at 600 kHZ (!) and can barely hit 12V, let alone the 19V you've actually configured it for, with a 10mA load because it's going so fast. The datasheet and my experience seem to agree, that timing capacitor is too small. The fact that you've probed it and saw 56 kHz makes me suspect either your numbers are wrong, or something else is seriously wrong with that circuit.

And if the converter buckles under load, I'd always suggest making sure the inductor is not saturating.

Also, how much current are you actually looking to get out of it? All these "current boost" MC34063 configurations are usually bodgey at best, tricky to get just right, and quite inefficient. There are better ways. Hell, you could do better with just a 555 and a MOSFET.

[lk]

C4757p,
I have a pot on the feed back, its a 100k one, i forgot to write that it seems but anyway i can dial in the 12 volts on the output no problem. so that 20 volts i see are on the anode side of the cathode.

If we suspect that my inductor is not saturating, then its too large, perhaps maybe   so maybe my problem is hidden in the 330uh inductor i have choosen, instead of the 1.8uhish something the calculations called for, i just stuck the one i had with the highest current rating in there.

i would like 600 to 800 ma out if it if possible at 12 volts.


-lasse

[c4757p]

The calculation table might say "L_min", but exceeding it greatly is rarely a good thing to do. 600-800mA is tricky but doable.

What is the current rating of the inductor?

[lk]

Hi  c4757p,
The inductor was rated for 3amps. and i just replaced it with a 500ma rated 3.3uh to see what happened, but now the unloaded voltage does not go above 4 volts.

My soldering on the protoboards gets messy, the error could easily be there, but anyway i made a pcb of the design and send it off to ITEAD, lets see if it behaves better when its not build from solderblobs.

-lasse

[c4757p]

Stick a scope on the collector. How low does the voltage go when it switches on?

[lk]

Hi again,

i have attached a trigger of the power on. One curious thing is that it with no load draws 600ma, but when i put the scope probe on the collector it drops to 1ma, the plot thickens after poking around, i seem to have some sort of short with as no matter where i appliy pressure where the transistor is the current draw drops to 1ma.

and the transistor gets hot when the circuit burns 600 ma, i guess the pwer has to go somewhere.

-lasse

[c4757p]

Ah, dammit, I wasn't clear enough. Sorry, a trace of the collector as the converter is running, to look at the behavior each time the transistor switches. Not the whole device power-up.

I shouldn't have expected you to read my mind and know what I wanted to see.... I'm interested in V_CE(sat), how low the transistor can pull the inductor when it switches on. I've seen a ton of variation between real parts and simulated, so I wanted to compare your real circuit to my SPICE one.

But investigate that poor connection first, it may be the cause of all your problems.

[Marco]

Why are you using a darlington as an external switch? Essentially you're building a three transistor darlington together with the output transistor inside the IC ...

Oh I just realized something ... you misinterpreted the datasheet. They were never suggesting using a darlington external transistor to keep it from saturating. They were suggesting using the external transistor in a darlington CONFIGURATION with the internal transistor. Because of the shared connection of collectors the output transistor in a darlington configuration can't go into saturation.

You're not supposed to use an actual darlington transistor Just a plain NPN transistor (the 100 ohm pull down resistor on the base is fine BTW).

[lk]

Hi Marco,

that makes sense, now i just have to find a suitable non darlington NPN transistor with the same footprint as the so i can use it with the pcb that shows up at some point in the future. I prefer the PBCs over the protoboards as its neater and its easier to see what is going on.

I will revive this thread when i have the board and assebled it

Thanks everyone for now, i will recommence the bugging soon.

-lasse