Electronics > Projects, Designs, and Technical Stuff
Help with how this PSU works. (With revised design)
xavier60:
Also the CV and CC loops will likely need frequency compensation if instability is noticed during testing.
If purchasing the TIP35C BJT from riskier sources, the TO-247 variants are less likely to be fakes.
https://www.aliexpress.com/item/32539913063.html?spm=a2g0s.9042311.0.0.27424c4dOzQ9s9
duak:
I built a similar power supply years ago using MOSFETs. While the MOSFETs were rated for greater current and power dissipation, I went back to bipolar devices because they were easier to stabilize and didn't fail under some conditions. Since then, something called "Electro-thermal Instability" has been discovered to explain why MOSFETs can fail in linear power applications. However, don't let me stop you. Many people have used MOSFETs in linear power supplies. The important thing to look at is the Safe Operating area of the device, not just the maximum ratings.
With four TIP3055 transistors, I would consider larger emitter resistors, say 0R47 or 0R68. If each carries 1 A, they will dissipate 0.47 W or 0.68 W respectively. Each should be rated for 1 W although I would use 2 W so there would be less drift in the current limit as they heat up. It would also allow for an increase in ouput current if the TIP3055 transistors can handle the extra power.
aheid:
For fun I tried to recreate it in LTSpice. Made my own homebrew LM7808 implementation, probably doesn't behave exactly like the real deal but don't think the difference matters too much.
The sim as configured will simulate the voltage setpoint being changed between 3.3V and 10V, but the current limiter is configured so it kicks in at about 830mA, thus limiting the 10 Ohm load to 8.3V.
To change the current limit, change RcsetH and RcsetL while maintaining the constraint RcsetH + RcsetL = 1kOhm. To change the voltage limit, change Rvset.
In the simulation at least, the 220uF capacitor that buffers the V0 line (C7 in sim) limits the dynamic response. Lowering it to 22uF makes for a much more responsive output. However I guess in real life there might be a reason why you'd want to stick to 220uF.
Evangelopoulos Panagiotis:
--- Quote from: aheid on February 02, 2020, 09:46:36 pm ---For fun I tried to recreate it in LTSpice.
--- End quote ---
Wow you went the extra mile for this! Currently I'm away from my main PC but as soon as I get back I'll play around with it a bit to see how it behaves!
Evangelopoulos Panagiotis:
--- Quote from: xavier60 on February 02, 2020, 07:06:58 pm ---If purchasing the TIP35C BJT from riskier sources, the TO-247 variants are less likely to be fakes.
--- End quote ---
I'll probably be buying from LCSC so I think I'm good. Once I bought a few 2N3055s from AliExpress for like 5€ and when I opened one, the silicon was tiny. No wonder they got destroyed in no time...
--- Quote from: duak on February 02, 2020, 07:42:15 pm ---With four TIP3055 transistors, I would consider larger emitter resistors, say 0R47 or 0R68. If each carries 1 A, they each have to rated for 1 W although I would use 2 W.
--- End quote ---
Thank you for the wattage info. I was about to go overboard with 5W cement resistors.
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