It occurred to me last night that one of the failure modes of this power supply design could most likely be eliminated by using a mechanical limit instead of an electrical limit.
Let me 'splain.
One of the failure modes I was concerned about in post #30 was the failure of the voltage adjust pot due to too much current through it. The pot I spec'd was a single turn 250K unit, with 2W dissipation (i.e., the biggest unit I could source for reasonable cost). The problem I saw was that if you have a large cap only load, say 820 uF, and your starting condition is a fully charged cap (480V), and then if you very quickly turn the voltage adjust pot down to zero (if you are quick on the draw, you can make that rotation in what...1/2 a second). Because the load cap has no other discharge path, it will discharge through the adjust circuit (see post #30 for detailed description of the problem).
This is only an issue with a very sudden or abrupt change of selected output voltage. Since the supply can react extremely quickly--much quicker than the cap can discharge, the conditions are setup so that the full voltage change between voltage on the cap and the new selected output voltage must discharge through the supply's adjust circuit. This can be catastrophic for the pot and will overheat it and blow it up under the right conditions (I
KNOW because I blew up several before I was keen to this problem). So to solve it I put in place "current backflow preventer"--a series diode that would not allow the cap to reverse discharge through the supply's adjust circut.
The side effect of this "fix" is that supply regulation is not nearly as sharp as it could be without the diode. The diode always drops about 1 volt across it, so the best regulation you will see on a
fluctuating load is about 1V. Terrible considering what the supply is capable of.
As a (possibly) better solution, what if you were limited in how fast you could turn down the voltage adjust pot? Let's say you could physically only turn it 25K at a time. For a 250K adjust pot, a 25K difference in adjust pot value would adjust the output by 1/10th of 480V, or 48V (assuming linear relationship). In other words, the most voltage difference the supply could ever see under this condition would be a 48V change at any one time. With a large cap only load and the backflow prevention diode removed, instantaneous current discharge back through the adjust circuit would then be 48V over whatever the equivalent adjust circuit resistance was at the time.
In this scenario we need to consider both the steady state DC conditions and the dV/dt (fluctuating) condition. Let's take two boundary condition use cases:
Use case 1:
- Adjust pot is set at 250K, and supply is currently delivering 480V.
- You quickly move the adjust pot 25K downwards (so its new value is now 225K), which adjusts the output voltage to be 480V - 48V = 432V.
- Instantaneous current change through the adjust circuit (cap discharge through the adjust circuit) would be no more than 48V/225K = 0.213 mA. Wattage dissipation of the pot would be 10 mW.
- Steady state current through through the adjust circuit would be 435V/225K = 1.93 mA. Wattage dissipation of the pot would be 0.84 watts
With a 2 watt pot, the pot is easily able to handle these conditions.
Use case 2:
- Adjust pot is set to 50K, and supply is currently delivering 96V output
- You quickly move the adjust pot 25K downwards (so its new value is now 25K), which adjusts the output voltage to be 48V.
- Instantaneous current change through the adjust circuit (cap discharge through the adjust circuit) would be 48V/25K = 1.92 mA, Wattage dissipation of the pot would be 92.16 mW.
- Steady state current through adjust circuit would be 48V/25K = 1.93 mA. Wattage dissipation of the pot would be 92 mW.
Again with a 2 watt pot, the pot is easily able to handle these conditions.
So there is no reason in my estimation that the back flow preventer diode is needed, IFF we can somehow limit max change in the adjust pot to 25K at a time. And if we can eliminate that series diode, output regulation improves substantially to the spec of what the 317 chip is capable of.
Is there a way to limit adjust pot change to only 25K at a time? The answer is YES, and it's so simple I can't believe I didn't see this before now. The method is to use a 10 turn precision pot! A human hand is only capable (on a good day) of making a single turn in (let's say) 1/2 of a second. You then need to reposition your hand on the knob to turn it for another turn. About the fastest you can do that is ~1 second intervals. (unless you're Neal Peart or something).
Anyway, I speculate that there is enough time for the cap only load to dissipate 48V through the adjust circuit that this condition I've been worried about should not be a problem, if using a 10 turn adjust pot. And that removes the need for the "backflow preventer" diode, and thus substantially increases regulation capability.
Furthermore, I would probably fit a round knob on the pot shaft so that you will not be tempted to be able to turn it faster than say one full turn every 1/2 second. (A chicken head knob might be able to be turned faster if you use your finger and push it around).
Just a thought anyway...