Feedback for low frequency PWM regulator
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Posted by JMini on July 4, 2008, 4:44 pm
Please log in for more thread options 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? | ||||||||||||||||||||||||||||||||||
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Posted by Martin Griffith on July 4, 2008, 4:59 pm
Please log in for more thread options Phew, Any chance of posting a circuit somewhere? martin | ||||||||||||||||||||||||||||||||||
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Posted by JMini on July 4, 2008, 5:20 pm
Please log in for more thread options On 7/4/2008 4:59:48 PM, Martin Griffith wrote:
> On Fri, 04 Jul 2008 20:44:37 GMT, in sci.electronics.design "JMini"
> >>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? > > > martin > OK. I've uploaded a quick drawing to imageshack. PLEASE forgive the hand drawing. I'm awful at getting this stuff into schematic software. http://img386.imageshack.us/img386/9920/pwmschemajx8.jpg | ||||||||||||||||||||||||||||||||||
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Posted by Mook Johnson on July 4, 2008, 5:39 pm
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> OK. I've uploaded a quick drawing to imageshack. PLEASE forgive the hand
> drawing. I'm awful at getting this stuff into schematic software. > http://img386.imageshack.us/img386/9920/pwmschemajx8.jpg 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. | ||||||||||||||||||||||||||||||||||
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Posted by Phil Allison on July 4, 2008, 8:23 pm
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"Mook Johnson" >
> 2) Why use Comverter. Average should be fine as you are only going to > create a DC average. ** 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 | ||||||||||||||||||||||||||||||||||
>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?