Re: Feedback for low frequency PWM regulator

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". In any
case. Since there is no inductor/diode/cepacitor in the output stage I'll be
using an RMS converter (LTC1968). For the PWM section I'm using the MIC1557
(SOT-23 size 555 equiv) for a R-C sawtooth to a comparator (TLV7211a)
inverting input. I can choose the frequency (probably in the 200-800Hz
range). The feedback is sent through the LTC1968 RMS converter to the FB pin
(0.8V) of a tiny (SC-70) 5mA voltage regulator (OnSemi NCP102). It's really
just a powerful error amplifier. The Output of that is sent to the
non-inverting input of the comparator. So if the feedback voltage drops, the
NCP102 increases voltage ot the non-inverting input of the TLV7211, thus
increasing duty cycle. I've tested this type of layout on breadboard using
different components.

Why ?

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

Once stable (tee hee), you've got an rms voltage comparison to a
buried reference that bears no constant relationship to anything else
in the circuit, save the NCP102's reference voltage.

What are you trying to do?

RL


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.

The output of the rms-dc converter has a very slow response time -
measured in the 100s of milliseconds. In order to get the NCP102 to
work with this inside the feedback loop, you're going to have to slow
it's regulator down considerably. There's a model available if you
want to see what a pspice-type simulator shows.

Although the converter has differential inputs, which will simplify
interface to the low-side driver actually being employed, it's linear
output is in the 0-400mV range. How are you matching this to the
regulator's 800mV internal reference?

Fast-rising and falling current transients will radiate, even in an
800hZ pwm cicuit. The resonant frequency is determined by your
battery, lamp and switch wiring loops reacting with the fet's output
capacitance. Check it out with a scope. At this low frequency, you can
probably be generous with snubbers.

RL
.

Quantcast