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From: Tony Williams
Subject: Re: homebrew flyback transformer?
Date: Tue, 08 Oct 2002 09:37:09 +0100
NNTP-Posting-Date: Tue, 8 Oct 2002 10:03:28 +0000 (UTC)
User-Agent: Pluto/1.14i (RISC-OS/3.60)
In article <firstname.lastname@example.org>,
Larry Gagnon wrote:
> I want to construct a power supply with the characteristics of
> 11-15vdc input, 12vdc @ 2.5A output (to power a small handheld
> computer in a vehicle). I have elected a design from National
> Semiconductors website: a flyback switcher PS based around the
> LM2587-12V switcher regulator.
> I am a bit of a newbie with switchers. My question is : the design
> states the primary should be 35uH and then is says "Ns1 ratio" = 0.67
> or "Ns1toNp" = 0.67. I assume this means the secondary inductance
> ratio is 0.67 to the primary, i.e. 35*.67= 23uH??
The phrase "Ns1toNp" would usually be taken to mean
the ratio of the Sec1 turns to the Pri turns. In
this case Ns1= 0.67*Np. The inductance-ratio would
be to the square of the turns-ratio, Ls1= 0.67*0.67*Lp,
which works out to a secondary inductance of 10.3uH.
Flyback converters work by energy transfer... energy
put into the core via the primary (during the ON-time)
is delivered via the secondary (during the OFF-time).
An inductor with a decent energy-storage capability is
needed, and this means a core with an air gap. This
is not usual with ordinary toroidal cores, so you have
a gapped pot-core or E-core, preferably with the gap
in the centre leg.
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