Efficiency of SMPS design.

From: Denis Gleeson (dgleeson-2_at_utvinternet.com)
Date: 06/09/04 

Date: 9 Jun 2004 03:59:46 -0700

Hi Guys

Ive designed a switched mode PSU around the LM3578A (National Semi).

I need to go from +12V to -55V. My current requirement is 12mA.

I placed a 10k resistor for the load on the output of the switched
mode PSU
and tested the design that Ive created (on a PCB). I find the
following.

INPUT from bench Power supply = 5V @ 160mA = VI watts = 0.8Watts
OUTPUT from the switched mode PSU = -42V @ 4.2mA = VI watts =
0.176Watts

Now to work out the efficiency of this cct I calculate as follows.

0.176
------ * 100 = 22%
0.8

Is this right?
Do I need to source 160mA from 5V to get -42V at 4.2mA?

Is this the way it is or is there a more efficient way to do this
design?
My calculations show a Peak Inductor current of approx. 0.55A so Im
begining to conclude that this is correct.

There must be a better way!

Many thanks for any help.

Denis

________________________________________________________________
Ive designed as per figure 21 of the data ***.

http://cache.national.com/ds/LM/LM1578A.pdf

According to that figure I have:
R1 = 51k
R2 = 1k
R3 = 0.22
R4 = 860
R5 = 100
L1 = 2 * 220uH in series
C2 = 10uF 65V electrolytic
D1 = 1N5818 (schotky)
C1 = 1.1nF
C3 = 22pF
Q1 = 2N2905

I dont have C4 and R6 connected.

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