Some more hacking to my demo board... I worked out that start circuit would need 1uF capacitor in order to give about a couple amps soft start charge into about 10,000uF capacitance. So to be on the safe side I just used 2.2uF. This gives about 300ms start up time. I guesstimated that this will give about 1 A soft start charge.
In any case, thankfully this is solve the problem and I have added on a 4,700uf low ESR capacitor on the power supply had no problem starting up with this value. Before he could not start up with even just 1,500uf.
Oddly even know this new chip should have slightly better efficiency than the old one, it is actually drawing a little more than expected so I will investigate this and see if I can find out why...
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9V 1.17A 10.53W 94.21% 10V 1.05A 10.50W 94.48% 11V 0.96A 10.56W 93.94% 12V 0.87A 10.44W 95.02% 13V 0.81A 10.53W 94.21% 14V 0.75A 10.50W 94.48% 15V 0.70A 10.50W 94.48% 16V 0.66A 10.56W 93.94% 17V 0.62A 10.54W 94.12% 18V 0.59A 10.62W 93.67%
Okay, so, my previous best efficiency was 94.70 @ 15V input. So the efficiency loss was not as bad as I initially thought. In actual fact the best efficiency is now at 12 V at 95.02% efficiency! So this new IC is capable of marginally better efficiency than the previous one overall efficiency figures are averaging out around 94.50% anyway so there really is not much difference with any input voltage really..
So now am now confident that this circuit can ultimately be used to replace my previous power supply design. Also as stated before, this design does not need any electrolytic's on the low-voltage side. It can just use ceramics. This will also save a lot of PCB space and will also save on the assembly time. This circuit does need a few more extra components, but at least these are still surface mount, it will be a lot quicker and easier to assemble anyway. Of course the bottom line figure is I was looking for better efficiency.
So now I am happy with this design, I now need to look at 12V switch mode mode circuit's and trying find one which has the best efficiency at around 15 V input. As mentioned before, likely 24 V would give the best efficiency with a lot of chips, but I'm sure I found one somewhere the best efficiency at 16 V. So I just need to wade through millions of datasheets again until I can narrow down my candidates list to just a handful of chips to try