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Bench CC/CV PSU Based on Daves uSupply (Not Anymore)

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Kleinstein:
Starting with 18-20 V and 1 A maximum current the maximum power would be slightly below 20 W. This is about the practical limit for the TO220 case. At this power level one may not need a pre-regulator and may get away without a fan.

With relatively slow OPs there is no real need for a really fast transistor. The question of a fast transistor would be something if it's about getting the output capacitor really small (e.g. 10 µF range). The speed of the current regulation is more limited by the speed of the OPs.

KC0PPH:
The TIP-120 lists a max current of 5A, so absolute outside max current of my PSU of 1A gives me a decent bit of headroom.

If I am reading the datasheet correctly it also lists 60W of dissipation. With an absolute max voltage drop across the transistor of around 4V this gives me 4W.

I honestly do not know how to calculate the capacitive or inductive but would be hard to believe that it can increase my dissipation from 4W to 60W. I am thinking about this in terms of Power Factor (Im an industrial guy) Regardless I will put the fixed regulators (5V, 3V3, 2V5, 1V8) on heatsinks and any transistors in this design that might get a bit toasty.

Just by guessing how far am I pushing my luck if I only use 1 TIP-120? What If I lower my current down to say 800ma?

KC0PPH:

--- Quote from: Kleinstein on March 12, 2019, 07:02:26 pm ---Starting with 18-20 V and 1 A maximum current the maximum power would be slightly below 20 W. This is about the practical limit for the TO220 case. At this power level one may not need a pre-regulator and may get away without a fan.

With relatively slow OPs there is no real need for a really fast transistor. The question of a fast transistor would be something if it's about getting the output capacitor really small (e.g. 10 µF range). The speed of the current regulation is more limited by the speed of the OPs.

--- End quote ---

I do want to pre-regulate - if for no other reason other than to dabble in a switching reg. Depending on performance I dont plan to keep more than about a 4V drop across the transistor.

I may look into some different OPs that are DIP so I can swap them easily (I plan on socketing them for easy tinkering).

In my design ill leave in the 2nd transistor to supply base current to the pass element if I decide that a Darlington is not a good option. Hopefully I win one of those new Keysight scopes and take some nice picrures of what I am doing.

 

Kleinstein:
The power ratings of transistors are more theoretical numbers for 25 C case temperature. So the more practical limit is often something like half of that (except for some modern FETs with unrealistic theoretical numbers to start with).
The current limit is not a problem, its more the question on how much heat sink is used. If the heat sink is not that large it may help to have a temperature sensor to turn of if too hot.

With a linear regulator a capacitive  or inductive load would not change the worst case heat load. The worst case would be a short anyway.
Capacitive and inductive loads are the more difficult cases for the loop stability.

With a pre-regulator a highly variable load (like the infamous file test) could increase the worst case loss, though usually still lower than without a pre-regulator.

iMo:
I've tried with LT1363, 2SC2078+2SC5200, 47pF comp, 1uF ceramic output cap. It simulates ok, and it is faster, indeed.

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