Re: crazy boost converter for LED use is too noisy!

On Sun, 13 Jan 2008 16:52:38 -0800 (PST), BW <bjorn.wesen@xxxxxxxxx>
wrote:

Hi,

I designed a, in retrospective, quite sick boosting DC-DC converter
for driving a LED-string. I boost using the LTC3783 controller and an
external FET, from 12V to around 80-90V at 0.5A!

Coming in at a duty-cycle of around the maximum the LTC3783 wants,
85%, it is needless to say that the inductor and associated PCB traces
are, well, magnetically active... (avg. current through the inductor
is around 3A, switching at +/- 1A approx. at 1 MHz)

The result works, somewhat, but is horrible - the output capacitor
makes this large clunking noise when the circuit is PWM'ed and the
current regulation is horrible as well, perhaps because the LTC3783
does delicate sensing of voltages around 100mV in the feedback-loop,
at the same time as there is an induced 100 mV in most leads due to
the magnetics it seems..

Should I thrown the design in the bin and start over with a more
prudent approach ? I guess the high-field high duty-cycle version will
never be "quiet" even with better PCB layout and better shielded
inductors ?

The LTC3783 demo-board only does 12V->36V, maybe for a reason!

Best regards,
BJorn

I'm just learning to make best guesses for pcb and component
parasitics in spice for smps design.
Throw in some Cs and Ls here and there and a sim'd smps gets really
ugly.
Ringing, spikes, oscillations, feedback, mistiming, heat, loss of
control, EMI,.... huhhhh :(

For even more fun, make a controller from scratch too :P


D from BC
British Columbia
Canada.
.

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