simple ic for 0-100% duty cycle PWM - diyAudio
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Old 1st June 2010, 05:44 PM   #1
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Default simple ic for 0-100% duty cycle PWM

most ic's , like sg2524/sg3525/tl494 are ment to work with inductors , cant reach 100% duty cycles .

i need to control the power to a heater , and cannot find a simple ic that will include sawtooth gen , error amp and comparator .want to keep parts count as low aspossible .
any suggestions ?
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Old 1st June 2010, 06:30 PM   #2
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If you're driving a resistive heater you only need a very low switching frequency. With 20k Rt and 2000P Ct you should get 20KHz switching frequency with less than .5uS dead time available.

The published 49% duty cycle limit for the 352x is for toggle/push pull mode.
Used in single-ended mode that translates to 98%.

The couple-few % reduction in max heater output could possibly be offset by raising the supply voltage, but in reality only means that the system reaches regulation threshold that few percent slower, and is not much of a consideration at running.
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Old 1st June 2010, 09:20 PM   #3
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We used to run heater controls often directly from 50Hz mains w/ Triac.
The control ICs were called <zero voltage switch with adjustable ramp> or so like:

U217B Datasheet pdf - Zero Voltage Switch with Adjustable Ramp - Temic
or

UAA2016 Datasheet pdf - Zero Voltage Switch Power Controller - ON Semiconductor
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Old 1st June 2010, 10:00 PM   #4
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Uc3823
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Old 2nd June 2010, 02:30 AM   #5
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Quote:
Originally Posted by Andrew Eckhardt View Post
If you're driving a resistive heater you only need a very low switching frequency. With 20k Rt and 2000P Ct you should get 20KHz switching frequency with less than .5uS dead time available.

The published 49% duty cycle limit for the 352x is for toggle/push pull mode.
Used in single-ended mode that translates to 98%.
and what about simple motor speed control ?
i used those ics for motor control , and at max duty cycle there is noticable decrease in motor speed , compared to wiring the mosfet gate directly to positive voltage.

Quote:
Originally Posted by darkfenriz View Post
Uc3823
i'm not sure this ic is really able to do 100% duty cycle . uc2843 was my alternative , but its got no "free" error amp.
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Old 2nd June 2010, 03:12 AM   #6
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If you take care to use the minimum dead time resistor on the 3524/5 you shouldn't have a big issue with max duty cycle. Refer to the Unitrode (TI) datasheet to get a complete rundown of what's possible with this IC.

How simple of a motor speed control? If open loop you can just use a 74AC14 with 100pF from gate to ground and a pair of signal diodes feeding the legs of a 470k pot. Reverse the polarity of one diode and connect the other ends of the pair to the inverter output. Use around 2K from the wiper to inverter gate. You should get wide duty range with fairly stable frequency above audible range. You'll probably need a driver or just a buffer if logic level FET. Tweak to taste.

Last edited by Andrew Eckhardt; 2nd June 2010 at 03:30 AM.
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Old 2nd June 2010, 02:34 PM   #7
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Use single ended
TL494 with grounded output control (pin 13).
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