Feedback for low frequency PWM regulator

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. I got to thinking though. Question: Would it be possible to use a resistor divider between the MIC1557 and comparator to reduce the voltage of the sawtooth and feed it to the NON-inverting comparator input and send the RMS converter output directly to the INVERTING input of the comparator? That way, a falling output voltage would cause a reduction in voltage to the INVERTING input and increase duty cycle? I could reduce the component count. I realize there is no true reference voltage in the system, but since the

1557 is fed from a fixed 5V source, the sawtooth would be a constant 5*1/3 to 5*2/3 V. Thoughts guys?
Reply to
JMini
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Phew, Any chance of posting a circuit somewhere?

martin

Reply to
Martin Griffith

OK. I've uploaded a quick drawing to imageshack. PLEASE forgive the hand drawing. I'm awful at getting this stuff into schematic software.

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Reply to
JMini

Couple of questions.

1) Why are you driving the high side of the bulb instead of the ground side. Ground side is much easier becaust eh source is at ground so the gate driver can be referenced to ground. Unless it is a P-channel and VIN is less than 15V you're probabbly better off with a low side N-channel (they are more robust).

2) Why use Comverter. Average should be fine as you are only going to create a DC average. for average its as simple as a R-C if the pwm frequency is high enough or a multi pole active filter. if it is lower.

If you comare average voltage in with a variable reference, you can have a adjustable brightness curcuit that is linar with your pot adjustment andfixed from external changes in VIN or temperature.

Reply to
Mook Johnson

I'm not so sure about the error amplifier part, but I see why you are using an RMS converter, so you are controlling the "effective" voltage to the lamp. I take it you are after constant brightness. Of course, you could also measure the lamp light output so you compensate for it aging (and for the initial tolerance).

Are you very cramped for space, or is there some other good reason you don't want to average the voltage of the pulses with an inductor, so you can eliminate the RMS converter? Having the inductor would lower the current ripple from the power source and also the losses in the switch.

--
Regards,

John Popelish
Reply to
John Popelish

"Mook Johnson"

** Nonsense.

Lamps are rated for DC or rms AC voltage.

The average value of a PWM wave can easily be way under the rms equivalent value - hence you will wind up with a blown lamp.

...... Phil

Reply to
Phil Allison

I guess the real question is, why? The light output will still be very nonlinear on any control input. Why not feedback on the light?

John

Reply to
John Larkin

Anyone played with these?

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martin

Reply to
Martin Griffith

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

Reply to
legg

In reality, I will be using low side switching, but that would have required that I illustrate the voltage dividers feeding the differential inputs of the RMS converter. That's all.

Reply to
JMini

I am extremely cramped for space. the RMS converter is an 8-MSOP package. The inductor would be HUGE. This regulator will carry about 10-11 amps RMS. I'm also looking at keeping the frequency rather low to avoid a ton of switching noise an reduce the phantoms that pop up when using high frequency PWM in close proximity to other sentitive bits. I don't imagine 40+ kHz 10 Amps RMS would play bery nicely just millimeters away from my RMS converter.

Reply to
JMini

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.

Reply to
JMini

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

Reply to
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.

Reply to
JMini

Cut lots of bad ideas!

You are trying to control a resistive load so a low side N-MOSFET to ground is the simplest configuration. No need to look at the current waveform it is just Vin/Rl. You just have to control the pulse width as the input voltage changes, that is "Voltage Feed Forward Control". A simple comparator whose output is beefed up to drive the FET. One input has a cap to ground and charged thru a resistor by Vin. The cap is reset in the Toff time. The other comparator input is your DC control voltage. This voltage must have a soft start at power up. You might preheat the bulb with a resistor in parallel with the FET prior to soft start. Most of the above can be found in one controller chip or the ubiquitous UC42XX controllers can easily be configured into Voltage Feed Forward control.

Cheers, Harry

Reply to
HarryD

Cut another bad idea.

He needs to regulate the power in a resistive load. The required PWM duty cycle is inversely proportional to the square of the supply voltage. You don't square anything with an RC network.

You could get a square law by making the PWM on time inversely proportional to the supply voltage *and* the PWM period proportional to the supply voltage but there are simpler solutions.

Reply to
nospam

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.

Reply to
JMini

I'm all ears. I'm really looking for all input here. If someone has simple solution to keep the RMS voltage constant into a resistive load using PWM while the input voltage drops, I'd love to hear them. I showed you mine, now you show me yours. TIA, guys.

Reply to
JMini

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.

Reply to
nospam

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

Reply to
HarryD

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