Re: Feedback for low frequency PWM regulator

On 7/8/2008 10:58:48 AM, legg wrote:
On Sun, 06 Jul 2008 13:38:03 GMT, "JMini"
<j.minihane@xxxxxxxxxxxxxxxx> wrote:

On 7/5/2008 8:50:29 PM, legg wrote:
On Fri, 04 Jul 2008 20:44:37 GMT, "JMini"
<j.minihane@xxxxxxxxxxxxxxxx> wrote:

I'm building a PWM regulator for an incandescent bulb. Some of thos was
described in a thread called "RMS Approximation of PWM/Square wave".
<snip>

Why ?

This sort of arrangement will hit the lamp with the mother of all
turn-on surges.

<snip>

What are you trying to do?


The input voltage will only be a few volts higher than the regulated output.
However, since the Vin is from batteries, the input voltage will be falling
the whole time, but I want constant RMS voltage to the bulb. This is for a
regulator for obscenely powerful flash lights. I recently built a
non-regulated version (PWM soft-start) that was 220W in a 3D Maglite size. It
has to be small (30mm round x 6mm high). The NCP102 has a built in
programmable softstart. I can stretch out the start-up over a full second or
more if I need to. I thought that the known min/max of the sawtooth would
provide a sort of reference. But the NCP102 looks like it might just be a
requirement That inrush current is monsterous. You're right. But
soft-starting will save the bulb. I plan on using the International rectifier
IRLR7843 for lower power applications and the IRF2804S for higher power ones.

By regulating the rms voltage across the lamp, you're not regulating
luminous intensity or power any more effectively than by regulating
the average voltage. For a resistor and pwm, these two voltage
quantities are the same.

Neither has a predictable relationship to power consumption in a load
with a positive temperature coefficient of resistance. PTC's with low
mass are strongly self-regulated for power. In the E^2 / R
relationship, as E increases, then so does R.

The use of the rms converter is simply complicating this circuit
unnecessarily. The use of PWM to reduce power loss in a control
element IS effective...you should be satisfied with this and
concentrate on simplifying the interface to a low-side switch and to
additionally reducing inrush current to values that will improve lamp
and switch operating life.

RL

As the input voltage falls, the PWM needs to increase to keep the RMS voltage
the same. I've tried regulating Average voltage. It doesn't work.
.

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