Feedback for low frequency PWM regulator

[HarryD]

For less that $1 you can get a PIC in SO8 (and smaller) which has built in
5v regulator, built in voltage reference, built in oscillator, 10 bit ADC,
10 bit PWM generator, and enough processing power to measure the supply
voltage, calculate and apply the required PWM duty at several hundred Hz
(and still have 4 pins left over).

If you can't do software the lamp filament *is* a thermistor providing
direct feedback of what you are actually trying to control (the filament
temperature). You can connect the lamp as one leg of a wheatstone bridge
and make a bistable circuit controlling the MOSFET from a comparator which
flips off when the filament resistance exceeds a value set by the rest of
the bridge resistors. A second comparator can make a timer to flip the
bistable on again after a fixed dead time or an oscillator which flips the
bistable on at constant frequency.

Both are one chip (+ maybe something to drive the MOSFET hard) solutions.

You have three Flips which equals one Flop (see De Morgan)

Harry

 

[nospam]

Also, the circuit really needs to control RMS current or RMS voltage. The
bulb mary vary by application, so it's resistance would be different from
bulb to bulb.

The required current changes just as much as the filament resistance and
controlling current would be a terrible thing to do anyway.

Yes the target filament resistance would have to be adjusted for each type
of lamp and controlling filament resistance is a bit more sensitive to
lamp to lamp variation than controlling voltage.
--

 

[JMini]

He needs to regulate luminous intensity, controlling power may be one way.
A better way may be controlling current thru the lamp. VFF is a simple
current control. We need a plot of lamp current and power vises luminous
intensity. I'm betting on current being the more linear and best to
control intensity. Where is Don Klipstein when you need him? Sorry if this
is a rehash.

Cheers,
Harry

Luminous intensity is fine if the flashlight will only be operating in a
constant ambient luminosity level, however it almost definitely won't.

 

[HarryD]

He needs to regulate luminous intensity, controlling power may be one
way. A better way may be controlling current thru the lamp. VFF is a
simple current control. We need a plot of lamp current and power vises
luminous intensity. I'm betting on current being the more linear and best
to control intensity. Where is Don Klipstein when you need him? Sorry if
this is a rehash.

Cheers,
Harry

I see a chart on Wikipedia (SP?) of efficacy (Lm/W) for 120W lamps from
5W to 300W and the efficacy changes from 5 to 20.7. Power dissipated in the
lamp is not good intensity control, maybe current control is just as good or
better? It sure is simpler!
Cheers,
Harry

 

[JMini]

I see a chart on Wikipedia (SP?) of efficacy (Lm/W) for 120W lamps from
5W to 300W and the efficacy changes from 5 to 20.7. Power dissipated in
the lamp is not good intensity control, maybe current control is just as
good or better? It sure is simpler!
Cheers,
Harry

Regulating RMS current would eliminate the need for a soft-start. The bulb
pulls a lot of amps during startup. For example, a 100W bulb on 12V pulls a
50+ amp spike then settles in around 8.3A. If the circuit only allowed an RMS
current of 8.3A max, the bulb would soft-start itself. Good thoughts.
But how would controlling RMS current be easier than controlling RMS voltage?

 

[HarryD]

Regulating RMS current would eliminate the need for a soft-start. The bulb
pulls a lot of amps during startup. For example, a 100W bulb on 12V pulls
a
50+ amp spike then settles in around 8.3A. If the circuit only allowed an
RMS
current of 8.3A max, the bulb would soft-start itself. Good thoughts.
But how would controlling RMS current be easier than controlling RMS
voltage?

After further review, VFF (Voltage Feed Forward) would keep the voltage
across the lamp constant with changes of input voltage. The current would
vary with filament resistance. Now, is controlled voltage adequate for your
application? If yes, a simple VFF, one 8 pin IC, will do the trick. We still
need the curves of lamp Voltage, Current and Power vs. Luminous intensity
output to determine the best variable to control.
Cheers,
Harry

 

[nospam]

Which SO-8 one(s) have an internal voltage reference (ie. not Vdd) for
the ADC?

12F615 has a 0.6 and 1.2v bandgap reference but looking closer you can
measure it (to calibrate) but not use it as an ADC reference. The HV615
shunt regulator is derived from the bandgap so maybe Vdd is good enough
anyway.
--

 

[Spehro Pefhany]

On Sat, 05 Jul 2008 09:52:26 -0400, Spehro Pefhany

<hmm... repeated since my laptop clock seems to have been off by a
couple of days>
If you measure the voltage v(t), then the required PWM output %
is proportional to 1/v^2(t).

You'd need to calculate a voltage divider to give you the reference
voltage at maximum input, and specify the output % at that input.

Eg. 10V maximum input with 25% output at 10V in (say it's a 25 ohm 1W
bulb). Then at 6V in you'd have about 69.5% pwm %, for that same 1W
output. It would hit the end stop at 50% of the maximum input voltage
in this example, below which you could extinguish the light, flash it
or whatever, or simply allow it to drop naturally below that level.

Best regards,
Spehro Pefhany

 

[Spehro Pefhany]

12F615 has a 0.6 and 1.2v bandgap reference but looking closer you can
measure it (to calibrate) but not use it as an ADC reference. The HV615
shunt regulator is derived from the bandgap so maybe Vdd is good enough
anyway.

Too bad, I was hoping I might have missed something. A built-in 2.50V
reference would be nice.
Best regards,
Spehro Pefhany

 

[Jamie]

Luminous intensity is fine if the flashlight will only be operating in a
constant ambient luminosity level, however it almost definitely won't.

I don't why you're making it so hard on your self..
Simply place a photo detector diode in the path of the light and use
that as a feed back to the PWM circuit..
the circuit can be operating on a low voltage fixed regulator ..
etc..

 

[legg]

In the design I had working with a 40kHz PWM controller, I level shifted the
RMS output using a resistor divider by +750mV because the Feedback voltage of
the TL5001 is 1V. I can do the same with this design. I can just level shift
the output by 600 mV. That puts the RMS converter output (in regulation) at
200mV. Right in the middle of that linear range.
Good catch on the linear range. I'll make a note.

Please give a little more detail regarding the use of snubbers. Snubbers
around the FET? In a previous design of a simple PWM soft-start, scope trace
of the output under a 12 Amp load was pretty clean. It runs at 175 Hz.

If you've checked it out with a scope and seen no funny stuff at
turn-on and turn-off of the switch, then there's no issue. Dedicated
gate rivers tend to be pretty quick, though ~ it's their job.

RL

 

[JMini]

After further review, VFF (Voltage Feed Forward) would keep the voltage
across the lamp constant with changes of input voltage. The current would
vary with filament resistance. Now, is controlled voltage adequate for
your application? If yes, a simple VFF, one 8 pin IC, will do the trick.
We still need the curves of lamp Voltage, Current and Power vs. Luminous
intensity output to determine the best variable to control.
Cheers,
Harry

There are at least 20 different bulbs that could be used. That's why I'm
hesitant to try controlling luminous intensity or something other than a
basic electrical property. Voltage or current. But would it be possible to
use VFF in a low frequency design and limit RMS current to the bulb?

 

[JMini]

I don't why you're making it so hard on your self..
Simply place a photo detector diode in the path of the light and use
that as a feed back to the PWM circuit..
the circuit can be operating on a low voltage fixed regulator ..
etc..

Because the output will change with ambient light conditions. I don't want
that.

 

[legg]

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

 

[HarryD]

There are at least 20 different bulbs that could be used. That's why I'm
hesitant to try controlling luminous intensity or something other than a
basic electrical property. Voltage or current. But would it be possible to
use VFF in a low frequency design and limit RMS current to the bulb?

VFF will control average voltage across the lamp. Since the waveforms are
almost constant, the RMS voltage across the lamp is just a constant factor.
This is the first circuit you should try and then see what else is needed.
Tell me your requirements, input voltage range, switching frequency,
dimming control voltage, lamp current, lamp control voltage range and any
other info necessary. If you have parts already selected, I will try to use
them. A simple pdf design on "alt.binaries.schematic.electronic" will be
posted for your viewing pleasure.
Cheers,
Harry

 

[HarryD]

Brings to mind, why not just regulate the bulb current?

...Jim Thompson

That was my first thought. Slightly harder than VFF because of current
sensing. IMHO the OP has not tried the simple things to determine what is
adequate for his application. Sometimes you have to put the pencil down and
grab the soldering iron. If the human eye is in the loop, it is very
insensitive to light intensity changes and control methods must be tried. So
the order of progression from simple to difficult control, voltage, current
and then power. In most lighting applications I have seen, current is the
best parameter to control.
Cheers,
Harry

 

[HarryD]

You make all of this sound so easy. It may be for you.
I'm going to readup on "Voltgae Feed Forward" control, but could you
elaborate on how the cap resets during Toff. If the cap is charged through
the resistor by Vin, its voltage will rise until it's at Vin. Or is the
resistor connected between the bulb and the FET drain?
Are there some shematic/examples on-line I can reference. Maybe I've just
been over complicating things. Regarding the laready available PWM
contriller
(UC42XX included). They operate at very high frequencies. I don't need
high
frequencies. Thanks for your input.

Schematic posted on "alt.binaries.schematics.electronics" as "VFF 10A
Lamp Driver".
Just a starting point. Need lots of added features the OP must specify.
Cheers,
Harry

 

[nospam]

Schematic posted on "alt.binaries.schematics.electronics" as "VFF 10A
Lamp Driver".
Just a starting point. Need lots of added features the OP must specify.

Over the supply range of 9 to 14v the lamp light output will vary by a
factor of about 3 and the lamp life vary by a factor of about 300.

So it isn't much of a starting point.
--

 

[HarryD]

Over the supply range of 9 to 14v the lamp light output will vary by a
factor of about 3 and the lamp life vary by a factor of about 300.

So it isn't much of a starting point.
--

Bitch, bitch, bitch, see updated driver at ABSE that controls Vrms to +/-3%
as input changes from 9VDC to 15VDC.
Different component values would control RMS power in a similar manner. What
the OP needs may be a different story.
Cheers,
Harry

 

[HarryD]

How badly is this design upset by a non zero source impedance? I ask
because you have included no supply bypass.

I believed that the OP stated batteries. The circuit will correct for noise
inside the 9V to 15V range on a pulse by pulse basis. If filtering is needed
it would be wise to increase the 260Hz switching frequency for filter size
reduction.
Cheers,
Harry