Switching power supply

[Alun]

I agree Ante, more information would allow us to help,  but I was just pointing him in the right direction as well as asking the moderator to move it.

I've seen people getting confused about N-channel and NPN transistors too, I'd recommend a MOSFET anyway.

[soon1401]

Hi,

Glad I found this forum. I am trying to buld a AC-DC SMPS base on TI chip UCC28221 (see attachment) using the interleaved forward topology. The apps note provided by TI is configured for DC-DC at 36-75VDCwith the output of 12V, 16.67A . I am trying to modify the circuit to AC-DC at 195-265VAC with the output of 24V, 25A. Iam having some problem on the main transformer part. I really do not understand how to design the transformer for the system. Can any of you guys help out?

[Alun]

Here's an interesting simple switching regulator circuit the author says it's over 90% efficient but mine design was only 85% efficient at the most and it's step down (mine is step-up). I wonder if he could tweak my circuit to 90% - shame I couldn't find his email on the website or I would've sent him the design.

[SM2GXN]

Amazing that he could get that high efficiency with a ratsnest like that, guess there has been some tweaking. What I've read the layout is rader critical to get those last percent of efficiency out of a circuit. Must say that it was an interesting switcher.
Alun do you have LT,s switchercad? If so, try to simulate the circuit just to see how it match the real one.

Bjorn

[Ante]

Hi Guys!

I have built a”converter” once, without any transformer to produce 28Volts from 12Volts. But I never measured its efficiency! I think it was good though becurse it run cool at 2.5A! 

[Alun]

http://www.romanblack.com/a04.htm
Add an extra transistor and you can have currewnt limiting, in fact if you removed the zener and set RS to the right value for the forward cuurent you want, you'll have a switching constant current source which would be ideal for powering high intensity white LEDs.

[Ante]

Hi Alun,

I found it! Maybe you can calculate the efficiency, since you are good with figures?

[audioguru]

Hi Ante,
That's a cool charge-pump circuit.
The current through the zeners is about 20mA continuously, and the transistors will drop out of saturation if the total load current exceeds about 200mA for each transistor if they are weak.
Therefore the efficiency is about 95% before adding the 0.5V saturation loss of each transistor.

[Ante]

Hi Audioguru,

Thanks for the assessment, what’s the bottom line then? The darlingtons have more than 1000 in gain so there is no problem. The converter put out 2.5A for hours and stayed cool!

[audioguru]

Hi Ante,
Sorry I didn't notice that your transistors are darlingtons. They don't look like darlingtons.
In a half-wave charge pump the transistors must conduct twice the load current, right?
For your 2.5A load, each darlington conducts 5A and its max saturation voltage is 2V. Therefore their max dissipation is 10W each for half the time which averages to 5W each continuously. Your darlingtons were probably better than minimum spec, and you were lucky.

[Ante]

Hi Audioguru,

Yes, I might have been lucky. Don’t you think the minimum specs are very rare? I think so because the manufacturers like to have a good margin to play with! Can you honestly say you have ever found any semiconductor with minimum spec? Well I haven’t not even close to minimum! Maybe back in the germanium days, when I built my first variable bench psu with an AD149 on the back!  

[audioguru]

Hi Ante,
I always design circuits using manufacturers' guaranteed minimum specs and everything works fine.

I also still have my very first psu that has a BDY38 (I have its 1968 datasheet) silicon transistor replacing its original germanium AD149. I can hardly read the label on it since it is badly pitted from zapping shorted Ni-Cads.

[Alun]

Why not make it even better by using MOSFETs

[Ante]

Hi Audioguru,

I agree, caution is a virtue (better safe than sorry)! And I am sorry to have confused you with the wrong transistor symbols. At the time when I converted the old doodle to this drawing I couldn’t find the correct symbol for a darlington. Unfortunately I have lost my old psu along the way, 35 years is a long time for any psu! I still got my number two psu though; it has a 2SD92 on the back 0-20V@1A!


Alun,

Yes why not, go ahead make it more up to date and efficient!