Higher frequency for a 555 pwm generator?

Discussion in 'The Projects Forum' started by dhevkumar, Sep 11, 2015.

  1. dhevkumar

    Thread Starter New Member

    Sep 11, 2015
    21
    0
    Sir i need an clarification acutally iam giving an duty cycle value like 0.6667 to pwm control block and its pulse is given to mosfet to boost up voltage of boost convert for an input 5v output 15 for 25khz frequency, but when i change my frequency to 50khz the output varies for same duty cycle . why? awaiting for ur suggestion


    Mod edit : please don't hijack other member's post.
    This thread was split from
    Increasing the frequency of a 555 pwm generator? ... replied to #8
     
  2. bertus

    Administrator

    Apr 5, 2008
    15,647
    2,346
  3. #12

    Expert

    Nov 30, 2010
    16,277
    6,788
    You have measured the duty cycle somewhere. At the 555 timer? At the MOSFET gate?
    You need to be clear about where in the circuit you found the duty cycle to be proper and where in the circuit the behavior changes. Then you will know which stage is not responding correctly to the new frequency.

    Please post a schematic of your circuit to obtain better answers.
     
    KJ6EAD likes this.
  4. wayneh

    Expert

    Sep 9, 2010
    12,101
    3,036
    You're surprised that the boost converter output is changed by doubling the frequency?
     
  5. GopherT

    AAC Fanatic!

    Nov 23, 2012
    6,015
    3,789

    If you are surprised by this change in voltage vs PWM frequency for constant duty cycle but change of frequency, then you need to look at the characteristics of the 555 and the MOSFET. Make sure the 555 maintains the same duty cycle. If you only change the resistor between pins 6 and 7 to change the frequency, and leave resistor between pin 7 and Vcc the same, then you will see a different duty cycle. Use an online 555 calculator to check your duty cycle at the two frequencies.

    For the mosfet, you have to check the rise time and fall time of the transitions to insure it can keep up with the new frequency. Often times you need higher gate current to slam the mosfet open and closed faster at higher frequencies.

    Third option, if you are measuring as a voltage I assume you are either the meter is averaging the values or you are using a low pass filter (capacitor and resistor) as a load. If you are measuring directly with your meter, it could be an issue that the meter cannot keep up with the higher frequency to give a true RMS reading (FLUkE 87 can only handle up to 30k HZ as a measured AC value and starts averaging or giving funky results after that). Some type of aliasing could also be occuring.

    If you are feeding into a filter, the low frequency may be giving you more of a sawtooth (shark fin) output than the higher frequency and your meter will see a different result.

    Sorry, no simple answer if you give a complicated question with limited detail.
     
    Last edited: Sep 11, 2015
    KJ6EAD likes this.
Loading...