Well, I have no idea why you could not get the circuits I posted to work.
I tested the exact circuit below on a plug-in breadboard which is not optimum, and it worked the first time. I changed a few values to maximize the efficiency at 500 mA out and had it running for more than an hour. The only thing that slightly heated up was the 470 uH choke.
The Simulator predicted an efficiency of 87.65 % and I measured it at 86.1 %.
I suspect that the difference is being consumed in the choke. I will try a larger current choke later and see if it improves things.
The switching waveform on the drain of Q1 is nice with rise and fall times under 50 nS.
I will post actual scope plots a bit later.
Well, I have no idea why you could not get the circuits I posted to work.
I tested the exact circuit below on a plug-in breadboard which is not optimum, and it worked the first time. I changed a few values to maximize the efficiency at 500 mA out and had it running for more than an hour. The only thing that slightly heated up was the 470 uH choke.
The Simulator predicted an efficiency of 87.65 % and I measured it at 86.1 %.
I suspect that the difference is being consumed in the choke. I will try a larger current choke later and see if it improves things.
The switching waveform on the drain of Q1 is nice with rise and fall times under 50 nS.
I will post actual scope plots a bit later.
This type of circuit is somewhat load dependent for stable operation. This means that you need larger inductance values with low current. One thing you must make sure is that the choke you use is rated for the current required by the load.
If you use exactly the same values and circuit as I did, I see no reason why you have bad efficiency at 500 mA.
OpAmp out:
View attachment 135173
As you can see, the output is no even close to the maximum ou minimum of the AmpOp, and since it's working as a comparator, this should be different.
The 741 output, at minimum is always a volt or two above its lower supply rail. You have it on a single supply. Therefore its output turns on an NPN continually. If you are still experimenting with this circuit, then you must turn off the NPN fully when it is supposed to be off.
Your resistor divider reduces the voltage a bit, but it's only a bit. Try putting a series diode or two at the op amp output. Try changing the resistor values. You need to make sure the NPN turns fully Off and fully On.
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I was referring to your post #36 schematic:
View attachment 135160
The thermal picture was taken after about an hour of operation. As you can see the choke is the only thing that heated up quite a bit.
I did find testing with BD139's from different manufacturers it does not always start up with the 18 k between the b-e of the BD140. So around 47k seems to be a happy median.
I found a 470 uH choke with a higher current rating and now the efficiency at 500 mA went up to 89.2 %. So this should make you aware to check that the choke is DC rated for the load current.
I replaced the UA741 with a LM393, I still need to put the series diodes?
Are your measurements to calculate the efficiency correct? Your figures indicate 96 %, which seems too good.
(12 * 0.5)/(48 * 0.13)*100 = 96.1 %
Since that is a comparator, it should turn off the NPN fully with no need for diodes to drop voltage.
However with its open collector, do you have a resistor network, etc., to turn on the NPN?
I tried to increase inductance to make it work with the 50mA load but no success so far.
You will need something like 4.7 mH for stable operation at 50 mA
Using a 4.7 mH choke at 50 mA will work well, but you will have to be sure that the choke can handle the larger currents without saturating.
The diagrams below show the response to a load changing between 50 MA and 550 mA.
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Your 393 needs a suitable output pull-up resistor to work.
You really need an oscilloscope to investigate what is going on. Without it, you are like the blind man on a busy street without a walking stick. You have to be able to see waveforms and measure switching frequency, etc.
The late **broken link removed** used to say something like: "It always makes me nervous when I find out that the customer I'm trying to help doesn't even have a scope".
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