I'm still curious if the Spice sim can get an efficient SMPS happening at 5mA output (like I said in above post)? Maybe with the right inductor choice?
MrAl said:Isnt the 1nf cap the feedback cap? I mentioned it by value so that there would be no mixup
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MrAl said:Changing R1 and R2 probably wont have as much effect because the current through that branch includes the Q2 collector to emitter, which means the current will be regulated by Q2 more than anything else. ...
MrAl said:The inductor series resistance should be of less concern with the 5ma output.
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Hi Eric, thanks for simulating the design with 5mA output. It's good to see the sim still oscillating ok even at such low output current, but at a high freq now (140kHz) compared to the nice efficient sim you did before at 100mA out (40kHz).
If I read your info right, it is consuming 14v 4.8mA and supplying 5v 5mA? As a SMPS it's not efficient at all there and could be replaced by a linear regulator with low quiescent current.
It would be nice to see a setup offering the typical 80-90% efficiency that the circuit can produce, but "tuned" to produce that type of efficiency with a 5mA output. I was not able to do it with some parts swapping, although I admit not spending too much time on it as this circuit was tuned for a reasonable current range and I left it at that.
In SMPS ICs they usually run in "discontinuous mode" for very low output currents, this means the OFF period is very long with a short ON period, so inductor current actually reduces to zero for most of the OFF period. This gives higher efficiency as the inductor current is high enough to get good magnetic properties during the time it is on. I toyed with the idea of making a long RC delay OFF period to try to mini a proper discontinuous cycle but as I said never got it working very well.
Yep, the mistake was mine, I misread your post sorry.I didn't mean to sound as though you didn't understand how to get the efficiency up, it was more to draw attention to the way I "tuned" R1 and R2 for the Q1 saturation issues, and of course for the benefit of others who may read this and want to try tuning for efficiency. (Just for the record, I too have done a couple of commercial SMPS buck designs over the years. )
The big efficiency problem with the circuit Eric is simulating is that crappy high-ohms inductor, that was from my circuit 1 and was chosen as cheap and common. Some of the other circuits on my page had better inductors and much higher efficiency, and I was able to get >95% with a well chosen toroid and tweaking resistor values, which is a significant milestone in efficiency from a buck with no synchronous rectification.
Re getting the efficiency up, yep decreasing zener current is a start.
No, in the existing circuit during the ON cycle Q2 is quite saturated and has a low Vce, so R1 and R2 are the main things that affect current through that branch.
However if reducing the zener current with RZ far enough it might start to limit Q2 base current enough that Q2 won't saturate, so in that case you would be right Q2 might have more of an effect on the R2 current. I worry if that happens that Vin regulation will then be compromised.
Yes and no. As it is running now in continuous mode that is right, but if it ends up requiring discontinous mode to maintain a nice high inductor current then the inductor ESR will remain important. Either way if I was aiming for good efficiency as a goal I would lose that crappy little inductor.
Sure, if needed I can breadboard up a circuit I have no shortage of nice performing inductors, I even have some of the good Motorola glass schottky diodes left over from a commercial app, you can't buy good schottkys anymore.
perhaps we should also try to achieve the same with a boost topology which as im sure you know would be very useful in portable LED lighting applications.
I dont like many of the ones i've dealt with in the past as they are not that efficient and some of them would prefer to bang the little 20ma LED with 100ma peak like the Brinkmann circuit
Oblivion; That's a nice simple addition putting the zener there and would perform some level of Vout regulation.
You might want to put a resistor in series with the zener to improve efficiency
MrAl said:Oblivion, i like your mods there, so what are you seeing as far as efficiency with various loads so far?
Mr RB said:Good to see you added that R5, that should give more a "discontinuous" operation when Vout goes into regulation, which you reported as a "pulse by pulse" type operation.
Mr RB said:However I'm a bit converned by that problem where the initial Vout goes too high? I'm trying to understand why it happens. What should be happening is that the oscillator is switching based on the inductor and Q1 current sense, which should be reliably feeding pulses of power into Cout. This should slowly charge Cout until the zener starts to conduct which SHOULD just inhibit the oscillator turnon. So it should slowly charge Cout until it gets to the regulation point then stay there.
Mr RB said:Can you please post a screenshot of the output voltage, if possible showing oscillation too (like with Q1 Vc shown as well). And what current limit did you use in the oscillator vs what load current?
Mr RB said:I think the problem is largely due to an excessive current limit, with your R2 of 0.1ohm the current pulses will be 6 amps!! And then there is overshoot by the time R3 charges C1, it might be 10A peak. That is way past the sat current of the inductor!
A good choice for the current limit is at a rate just above the output current times Vout:Vin ratio, so with 3v in, 5v out and max 100mA out needed;
current limit = 5/3 * 100mA = 167mA
allowing for 70% efficiency;
167mA / 0.70 = 238mA
so you calc R2 as 0.6v over 240m
R2 = 0.6 / 0.240 = 2.5ohms.
There should be no voltage to turn Q2 base on UNTIL the voltage on R2 (0.1ohm) reaches 0.6v,
make sure it is oscillating based on R2 voltage hitting >0.6v.
I can't use online simulators as I have most of my internet Java features disabled for speed and security.
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With absolute respect, I was under a different impression of how it works. The voltage across R2 only needs to reach a level high enough for the difference from ground to cause the current going through R4 and R3 to drop, thus raising the effective resistance of the lower half of the voltage divider. It is actually this voltage divider that raises the voltage on Q2 base above 600mV like you said. But R2 doesn't need to actually see >600mV for this to happen. Point of fact I'm seeing 500mV across R2 and it's still getting 600mV on Q2 base without the zener. Here is an image...
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