UC3843 laptop DC converter problem

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Bubba Zanetti

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Joined 2005
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#1
Hi,

I was making simple DC-DC converter for my laptop on proto board according to this schematic:

12-18V_DCDC.gif


It worked OK with constant resistive load , but not very good with my laptop, this is the problem:

1. when laptop is turned on, it starts normaly and draws at least 6 Amps according to my DMM,

2. then after few secs, current decreases slightly lower (around 5A) but then laptop usually shuts down - like there is suddenly no enough voltage.

I read 3843 datasheet throughly so I tried transient peak filtering on current sense input pin (IC pin 4), but with no much success - laptop always starts normaly but again it shuts down like IC went in some kind of protection mode.

Hm, then I remembered that I made all tests on my test bench with 5A regulated supply, so I thought logically - maybe it shuts down because it needs more powerful supply, so I connected it directly to car cigarette lighter connector - but the story is the same, it always shuts down.

So my question is - is my problem really in large peak current transients that possibly activate current limit or something else?
 
#4
What are your component values, and are you sure that the core of L1 is not getting close to saturation? I'm sure you have designed this boost stage to run in continuous-conduction mode at these currents, so have you checked the stability of the oscillation over the desired load range? It may be that after a short time, the oscillation may go unstable as the components heat up. I see you have got 'slope compensation' in the design, if you disable this does it make matters worse?
 
#6
Hm , I make stupid, unfinished post and accidentaly hit reply...this is actual post that I tried to write down:

2 Ouroboros:

First I must say that I don't have much experience with constructing and designing switching PSU units, only with rest of audio related equipment 🙂

So this particular DC converter isn't my design - I only copied design to my proto board ....this is component list (that I translated from original schematic that was in Croatian):

Semiconductors:

UC38543N IC1 1
BUZ11 Q1 1
B1045 D1 1

Resistors:

10k R1,R2,R3 3
15k R4 1
100k R5 1
62k R6 1
22k R7 1
1k R8 1
0,1R/5W R9 1
10k P1 1

Capacitors:

1n C1,C5 2
10n C2,C3 2
1000u/35V C4,C6 2

Misc.
Inductor 20uH* L1 1

" *Inductor must be wound on ferrite core (at least 20mm dia, thicknes. cca 7mm), with 19 turns of 1.2mm Cu wire."


Unfortunately, I don't have scope to check what's really happening with converter, only freq. meter from my DMM that's not very helpful.

I forgot to say that I also experimented with frequency and inductance - first I put original inductance (20uH) and original frequency (approx. 50kHz), but as current was around 5-6 Amps, and temperature of inductor coil rised very qucikly :hot: , I removed this coil and put some spare coil(from some old telecom equipment TX/RX PSU), that had around 15 turns of much thicker multistrand wire, on slightly bigger core...then I also incresad the frequency to around 100kHz, but there was no much improvement, - I only got increse in MOSFET temperature and standby current (with no load conected) was more then 1A , and it even rised to more then 9A when I tried to start the laptop :bigeyes:
 
#8
Bubba Zanetti said:

" *Inductor must be wound on ferrite core (at least 20mm dia, thicknes. cca 7mm), with 19 turns of 1.2mm Cu wire."
Click to expand...

No, no. Inductor must be wound on iron powder core ( you can use yellow toroids from pc power supply ). If you are using ferrite core, it should be gapped.
 
#9
Did you measure the DC output voltage, and see what happens to it when the problem occurs?

Have you tried a different inductor core, yet?

I am not familiar with your circuit, but do have a little bit of experience with a somewhat-similar boost-mode SMPS output stage.

It might be very difficult to know what is happening, and how to fix it, without an oscilloscope. But...

Did you try measuring the AC voltage of the output, just in case your DMM does go up to 50 kHz? (And even if it doesn't do an accurate job, at 50 kHz, maybe it will at least be able to tell you if you have made any improvement, with the ideas below.)

If you think that there might be some voltage or current spikes/noise that are causing your problem, you could try at least two or three things:

1) Try a snubber network in parallel with diode D1, in case you're getting high-frequency ringing on the high side of your mosfet. Without a scope, you can't easily "manually" tune the series-RC component values. But, you could just try putting something like 470 Ohms (maybe 1/2 Watt, or 2x 1K 1/4W) in series with 2.2 nF, and connect that across D1, to see if it helps. You can try changing the capacitance (but not too high without a higher-power 470 Ohm resistor). There might also be other places in your circuit where a snubber could be needed.

2) Try an LC lowpass output filter. You could add a 10uH to 100uH high-current inductor in series with the output, and then add another 1000 uF capacitor to ground, after that, so that it forms a Pi filter with C6. Also, experiment with taking your feedback from after the new output filter.

3) As a "last resort", you could try adding a high-current linear regulator circuit, after the filter, or maybe instead of the filter. But you would probably want to diagnose the SMPS, first.
 
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