Engineer's Asylum

Isolated Switching Power Supply

I like to design an AC/DC Isolated Switching Power Supply with multiples outputs. Maybe a device with 3v3, 5v, 24v with 300mA each. With optocoupler. I using a starter schematic, but I don´t know how I to make the changes for multiple outputs. The big challenge is, made this project with PTH components and no drive.

Thanks so much!

Hi Prof_Telles,
First of all welcome to Engineer’s Asylum. Sorry for the late response.

I request you to provide us more details of power supply you are planning to design, like are you planning to build a AC to DC or DC to DC power supply? Do you need design with an opto-coupler or with no opto-coupler? What about the current and voltage rating you expect your device could operate?

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Use computer power supply, there you have 3v3, 5v, 12v,-12v… You can use bith 12v to have 24v

Theodil I wanna learn and project by myself.

Hello, try this link i think you can start from

Hi tks for help, but that´s not a switching power supply

Hi @Prof_Telles

It’s very hard to make one if you dont have access to components. I assume you already know the basic of switching power supply. Let me give you a my thought though. If you can make a buck converter using what you have, you just need to make a PWM signal generator. Make sure the PWM signal duty cycle is variable (you can change it using potentiometer). I can suggest you make PWM signal using 555 timer.

Btw may i see your starter schematic? Or you could just explain it. Just wanna see where you are on the progress. Then i can give better suggestion.

I started with this schematic. But it have one output.

For starters, the optocoupler is backwards. It should be LED to the right side and phototransistor to the left. Also, you would use multi tapped secondaries because otherwise your linear 5V and 3V3 regulators would be dissipating more power than what they are delivering.

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Ok, Prototype! I inverted the phototransistor and swap the transformer. But I don´t know, how a I dimensioning the flyback return, because now I have 3 outputs. I imagine I need 3 more flybacks, one each for output.

I calculated any secundary spirals each 30% turn plus
L1 = 100 turns
L2 = 3 turns
L3 = NC
L4 = 4 turns - 3,3v
L5 = 6 turns - 5v
L6 = 28 turns - 24v

The regulators I use for shortcut protection!

Look closely, the primary circuit is still wrong. Paralleling diodes is often used in switch-modes as it is not DC, the pulse waveshape of the current pulses in the diodes goes a long way to ensure balanced operation - even better to have them close together and on the same heatsink … equal length leads. It would be better to have one tightly regulated 24VDC o/p then a buck to 5v1 then a buck to 3v3 - much better overall efficiency … and the 3v3 will then be very stable - as will the 5v1

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I try to build the schematic with only PTH components. A Buck circuit I will need SMT components.
Sorry, but what´s means by o/p??

It means output. The key is to achieve snap action of the control device. A home-brew switch-coil converter does not require a commercial IC. Control can be done by the op-amp, logic gate, or two transistors arranged so gain is multiplied. These change state quickly in response to a slight change at the input. Example, invert-gate senses load voltage dropping below supply V/2 (adjustable by potentiometer). It goes high, turning on transistors. When load V rises above supply V/2 the gate goes low. This method is able to control 24V supply using IC’s which cannot endure more than 15V.

The 100pF capacitor is an integrator causing a slight delay in changes at the input. This introduces hysteresis in the action.

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