On 05/12/2007, Apptech <apptech@paradise.net.nz> wrote:
>
> > Ah, but that's only part of the problem. The positive side
> > of the hysterisis
> > also changes with input voltage.
>
> Split R6 into two. Take a diode from it's midpoint (anode)
> to the reference diode (D11/D12). This clamps the midpoint
> at 1 diode drop above reference and "stabilises" the on
> drive through R6_left. Two diodes or lower R6_left = more
> drive.
>
> BUT somewhere about here you say enough is enough and
> Replace Q2 with two transistors in a long tailed pair. The
> reference feeds Q2_left_base  and the sense signal drives
> Q2_right_base with any hysteresis added to (probably)
> Q2_right. This was done by ?Natsemi? long ago. Different
> than either 'my' GSR or your/Roman's approach. Proper
> comparator should be much more regulated.
>
> *BUT* unfortunately, the thing that makes this circuit
> almost certainly unsuited to me need is that, as didn't
> occur to me in my initial enthusiasm, the "regulation" at
> present occurs on the input and not the output side. ie the
> ocillation occurs when Iin reaches a limit set by the
> voltage across R2. At that stage the inductor energy is
> dumped into the LEDs (and a series diode with a capacitor on
> the KLED side of it would make the LEDs' life more gentle)
> and the cycle then repeats. You COULD add LED current
> sensing to make the process more defined but for now it's a
> bit open loop. I need something with reasonably well defined
> LED drive current over the battery operating range and
> suspect that this would be hard to tame. Maybe not
> impossible though. My CD40106 circuit uses a $US0.08 (25000
> volume IC) and 3 transistors (2 are FET gate drivers, one is
> for LED current sensing) so is certainly dearer. Efficiency
> is excellent.
>
> > Coincidentally, I'm looking at trying to design a
> > "constant power" circuit
> > to discharge a capacitor (33mF, 75V) quickly . Somewhat
> > similar requirement.
>
> Why "constant power"?
> Would constant current do?
>
>
>         Russell


Fair comment re R6 but it's not the complete story either - there is another
mechanism involved I've yet to track down.

Re the load on the LEDs - the maximum current is only what current is
passing through the inductor at the trip point, so if tjis is set at a
sensible level, there should not be a problem.

Constant resistance is simplist but Since the current drops during the
discharge, the peak power is higher for a given discharge time.
Constant current is better, but the power disapation is shared between an
active device and  a load resistor (in the simplist case). The  power
dissipated also drops as the voltage falls - so a larger load capability is
still required.

Constant power minimises the peak power requirement, offers the chance to
use smaller active devices as they are operated `in switching mode and the
load power is constant so doesn't need to handle a high peak.

It's just an idea at present but looks like I could use a 10watt load
resistor instead of a 25watt one - or something like that. A big price
difference.

One of the specs for the product is an MTBF of >100years so I'm a bit wary
of overrating things too much, even on current pulses. Cost is less of a
problem in this case but profit is profit so a cheaper build price for the
same performance is preferred.

I should have time tomorrow to look at some of the details again - was too
busy today with an urgent quote.


Richard
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