Re: how to improve voltage reguration.
From: Terry Given (my_name_at_ieee.org)
Date: 10/19/04
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Date: Tue, 19 Oct 2004 16:33:44 +1300
KM wrote:
> I have use a LC3803 Linear regurator to design a flyback regurator. A
> tronsformer is wind with single primary and double secondary coil (one
> for 3volts and one 7volts) the FB was return from 3 volt output. It is
> found that the output of 7 volts when unloaded is floating high to 13v
> when only 3volts output are loaded. It is found that the overshoot is
> causing the raise. What is the possible solution to this?
basically your problem is due to the lack of coupling between the
regulated and unregulated outputs. Firstly, ensure tight coupling in
your transformer. Secondly, "stack" the windings - say you have 3T and
7T each for the 3V and 7V supplies. rather than having a 3T and a 7T
winding, use a 3T winding & a 4T winding - in other words, tap a single
winding.
Thirdly, the lack of load on the 7V supply is also a problem. Depending
on the 7V load, a simple solution is a series R such that the RC time
constant is longer than the "overshoot" (which is probably only 1us or
so). Or slap a minimum load on the 7V supply.
Fourthly, you could change your feedback circuit - instead of a voltage
divider connected to the 3V rail only, attach another resistor from the
divider to the 7V rail. select the resistors such that when 3V = 3 and
7V = 7 the voltage divider sits at the correct voltage (whatever that
happens to be). This method basically averages out the errors - instead
of one well-regulated output and one badly regulated output, you get two
moderately well regulated outputs.
cheers
Terry
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