# Hybrid SMPS

Discussion in 'The Projects Forum' started by BrianHA, Apr 10, 2013.

1. ### BrianHA Thread Starter New Member

Apr 10, 2013
2
0
Dear friends,

I'm learning to make a hybrid power supply based on LM2577 and LM7815/7915.

I have a spare 9V-0-9V 2.7A toroidal power transformer that I can use for this project, and my intention with the LM2577 is obviously to step up the rectified unregulated DC voltage from 10V to 18V.

Based on the datasheet, the formula is:
VOut = 1.23 x (1 - (R1/R2))

Therefore, if my R2 is 2K, then R1 is:
R1 = ((18 / 1.23) - 1) X 2K = 27.2K

Am I correct?

I also understand that this step up process will lower the total supply current from 2.7A to 1.35A, which is ok, because my device draws a max current of 730mA.

I have also read the application note from LT written by the late Mr. Williams, about adding a ferrite bead before and after the linear regulators to help minimize the switching noise.

So before I proceed to make the board, I have just one question to ask.

Is it safe to use 1A inductors (eg: Murata 13R104C) and 1A Schottky diodes (eg: 1N5819)?

Thank you for the valuable assistance.

Regards,
Brian

2. ### BrianHA Thread Starter New Member

Apr 10, 2013
2
0
Hm, not sure why I'm not getting any answer. Perhaps I said something wrong or asked the wrong way.

Oh well... I guess I will just have to try it myself.

Thanks.

3. ### Dodgydave AAC Fanatic!

Jun 22, 2012
5,154
772
Yes R2 = 27K, to give 18V out.

As for the diodes yes use the 1n5819 instead its rated at 1 Amp, where as the 1n5821 is rated at 3 Amps, and the inductor is rated at 1 Amp also.

Also to minimise "ringing" of the oscillator use the star earth connection method ( all 0v connect to same point to pin 3) as listed on page 14 of the manual.