Re: Feedback for low frequency PWM regulator

On Mon, 14 Jul 2008 12:45:59 GMT, "JMini"
<j.minihane@xxxxxxxxxxxxxxxx> wrote:

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




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.

This is one of the few applications where the average and rms voltage
are identical.If you averaged the voltage before it hit the resistive
load, there would be an issue. You're not doing this.

In both cases regulation is possible.

If you can't get the simpler arrangement working using an RC filter in
the feedback path, adding an rms converter will not improve the
situation.

RL
.

Relevant Pages

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