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?
<j.minih...@minihane.tzo.com> 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. 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?
> On Fri, 04 Jul 2008 20:44:37 GMT, in sci.electronics.design "JMini" > <j.minih...@minihane.tzo.com> 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. 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?
> Phew, > Any chance of posting a circuit somewhere?
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.
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.
<j.minih...@minihane.tzo.com> 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. 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?
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?
>>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?
>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?
<j.minih...@minihane.tzo.com> 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.
> 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.
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.
> 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.
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.
