Control Module of a PWM step up (boost) circuit. 3.7 li-po cell to 5V regulated PSU

Thread Starter


Joined May 17, 2008

I'm trying to build o rock solid battery bank and usb charger.
I'm NOT trying to develop anything with ready IC for control.

I have the basic step up concept on Multisim 12. Circuit Attached.
I have all the knowledge base about usb pinout. I can charge apple and android devices with the proper signals on D+ D-.
I have simulate the circuit on the charging side. Works ok with current limiting on 0,7A and voltage Limit on 4,2V. I think is proper handling of li-po cell.
I didn't dimension correctly the power circuit at all. for example i know that 2n2222 is not ok... so as other parts on the power circuit.

I read that 741 can do square generation and another 741 can integrate that to make a triangle wave. So for now i just put a triangular source.
For feedback and pulse on transistor another 741 used as comparator. Compare the triangular and the reference voltage (i don't know how to create this, maybe is feedback from output) and then we have the PWM signal that goes to transistor gate. Capacitor in series used to remove DC offset.

Please help on:

1.the power section dimensioning
2.comment on charging method (from usb)
3. comment on triangle generation using 2 741
4. comment on PWM generation using 741 as a comparator.
5. the feedback generation for regulation on 5V.

Thanks all.



Joined Oct 2, 2009
Just so we are clear, you want to make 5V regulated out of 4.2V to 3.5V (as the LiPo discharges after being disconnected from the USB) using no exotic ready-made ICs? You are building your own boost SMPS using a restricted list of components that you happen to have, or can get in your locality?

What is the Peak current output at 5V?

To build the SMPS yourself, you still need some rather special components:
Ferrite inductor (special part designed for this purpose)
Schottky catch diode
Low Rds NFET (not a 2N2222)
Low ESR filter capacitor(s)
special low-voltage rail-to-rail opamps (not 741).

btw-your method of charging the Li-Po is marginal. It will damage the battery if left plugged in too long...

Thread Starter


Joined May 17, 2008
Hello MikeML,

yes, you are correct. The reason is i'm in Cyprus, our suppliers have only basic and classic things. Only 1 electronics store in my town and 4 in whole Cyprus. If you think is better to use a ready made IC that's very welcome.

Ferrite Inductor : is no problem. I can make any winding. I know a guy he is special.
Schottky catch diode: usually i can find in the store.
Low Rds NFET: i can find. (already have some pieces take out from power supplies)
Low ESR filter capacitor(s): What kind of capacitors?
special low-voltage rail-to-rail opamps: please give some example you may have in mind.

The charging method needs cut off maybe??

Thanks for your reply. Please tell me what you suggest and how we can proceed.
can do PCB using toner transfer method and i'm good on integration in box. i will send you a copy of the device if we can work it out :)

P.S: i'm electrical. i always admire electronics and the beauty of a working circuit. :)