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Thread: Question about PWM

  1. #1
    Join Date
    Jan 2005

    Question about PWM

    Hi All,

    I have been looking into PWM lately. I understand the concept that a variable resistor generates a resistance and tells a controller what frequency to pulse the PWM output at; this pulsing signal can be fed to a MOSFET which will turn on and off at a rate to emulate the desired output voltage.

    I am confused about amperage available. If a MOSFET is implimented which is capable of 10 amps, how can you be sure 10A can really be provided? Is there an amperage rating for the output on the PWM controller that has to be same-or-higher than the desired Amps you want available from the output MOSFET?

    If someone can clear this up it would be appreciated, I'd like to understand this PWM concept fully!


  2. #2
    Raw Wave Rob Withey's Avatar
    Join Date
    Apr 2000
    Bedfordshire, UK
    More context required...

    Are you looking at this from the point of view of a switch mode power supply, or varying the speed of a motor. If the former then you have lots of configurations, such as boost, buck, boost/buck, or switching the primary on a high frequency transformer. There are inducters and diodes to consider, and not all the current supplied to the load goes through the switching transistor...!

    The variable resistor is just one input implementation. Often the pwm would be controlled from a comparator (in the case of a power supply), or programmed from a computer (in the case of a servo controller). The transistor bit is independent of the PWM bit, the PWM just describes the technique of having a waveform that you can vary the mark-space ratio of. Servos use this in non power-based applications to describe the servo's position for example.
    Old Systems retired due to new car
    New system at design/prototype stage on BeagleBoard.

  3. #3
    Constant Bitrate
    Join Date
    Dec 2006
    If you are looking at implementing PWM in power supplies, the following app note from Maxim will be a good start.

    An Introduction to Switch-Mode Power Supplies

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