Re: Flyback transformer design confusion!!
- From: Eeyore <rabbitsfriendsandrelations@xxxxxxxxxxx>
- Date: Tue, 07 Nov 2006 15:37:46 +0000
seegoon99@xxxxxxxxx wrote:
Eeyore wrote:
seegoon99@xxxxxxxxx wrote:
Hi to all.
I am looking to design a flyback transformer , for a small supply. I
have a couple design examples from ON-Semi that I'm looking at , and am
a little confused. Depending on what app note I use ,
I get wildly different results for the same design inputs. For example:
12V out 7W.
One app note shows a primary inductance of 3.3mH and the other app note
works
out at 1.8mH. I seem to come across this problem a lot. All the
different design procedures
seem to come out with totally different values for the same problem.
How do I know
which is the correct solution?
Just cheat and get the design calculator from powerint.com.
Graham
I have :0)
I would still like to understand what's behind the whole design process
though.
Ok, here's a quick example.
Your DC bus is ~ 160 V ? If Lpri is 3.3mH then dI/dt = V/L = 160/3.3.10^-3
= 48.4.10^3 A/s
Your f is 67kHz. which gives a period of 150 us. Allow a maximum conduction time
for the switch of ~ 75us.
This means that after 75us you'll have 0.36A flowing in the transformer primary
This gives 215 uJ of energy.
Multiply by 67kHz - this gives 14W max power transferred to the load @ 50%
conduction period.
Graham
.
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