Help with dc/dc converter circuit

Discussion in 'The Projects Forum' started by herher, Mar 18, 2012.

  1. herher

    Thread Starter Member

    Oct 18, 2011
    Hi all,

    I have a new project with me now. It requires me to design and build several dc/dc converters such as boost, buck and cuk...

    I have read some relevant threads available in this site, and it did provide me the necessary information I need...but I still have some doubts over here, so please help. Thanks a lot.

    Starting with the boost converter...I will use microcontroller to generate a signal with high frequency and adjustable duty cycle to a N-type MOSFET for switching boost converter. The MOSFET I use is IRF540N and I also use a MOSFET driver (IR2110) in between the microcontroller and MOSFET to step up the switching voltage. Based on the IRF540N datasheet, it requires 10V voltage supplied to the gate in order to fully drive the chip. The voltage generated out from the microcontroller is 5V, thus a mosfet driver was used. but, I wonder how am I going to fully on the mosfet when I adjust the duty cycle of the pwm signal generated from the microcontroller??

    From my understanding, the pwm signal will be varied from 0V to 5V if I adjust the duty cycle from 0% to 100% (Is this concept wrong?). So, if the mosfet driver boost the voltage up to 10V at 5V pwm signal, then what will the mosfet driver output when the pwm signal is below 5V?? And if the voltage to the mosfet is below 10V, will it still work well??

    The IR2110 mosfet driver has low input, low output and high input, high output The input voltage I will use in my boost converter is <=12V. So, I just need to use the low I/O pins right for my application right? If yes, then how should I connect the pins of High I/O and its relevant supply and com pins?? Can I just leave the pins floating or do I need to ground them all.

    Please advice. Thank you very much

  2. bountyhunter

    Well-Known Member

    Sep 7, 2009
    You don't understand how a switcher works. The voltage applied to the gate drive is the same every time regardless of duty cycle, it must be switched fully ON every time even at narrow duty cycles.

    Yes, most FETs need 10V gate drive to fully crank them on and the gate drive circuit is normally run from a 12V bias supply to enable the drivers to do this.
  3. crutschow


    Mar 14, 2008
    As bountyhunter noted, PWM varies the duty-cycle of the signal, not the amplitude. It's the averaged value of the PMW signal that is the DC value being varied.

    You can buy "logic level" type MOSFETs which fully turn on with 3V or 5V Vgs. That may save you having to generate a higher voltage level to drive the MOSFET. But you still need a driver that will provide high source and sink drive current to rapidly charge and discharge the large gate charge current required to turn a MOSFET on and off.
  4. herher

    Thread Starter Member

    Oct 18, 2011

    Thank you very much for the replies.

    I think I get what you all mean. Although the signal I have measured from a logic 5V microcontroller as an example with the duty cycle of 10% is 0.5V, but actually to the mosfet driver, it do not see the average value, instead it sees that the signal is 5V for 10% of the period while 0V for 90% of the period, am I right? If this is the case, thank you very much for clearing my doubt.

    Actually I have bought several IRF540N mosfet and IR2110 mosfet drivers, I don't feel like wasting them so can I still use these chips for my converters?? Besides, I have some connection problems with the IR2110 mosfet driver chips over here. Please have a look at the attached image.

    This is my first attempt of testing the circuit but there is no output generated from the ''LO'' when I supply different duty cycle of signal to the ''LI'' pin. Is there any connection errors on my circuit. By the way the input of the boost circuit will be <=12V, so I use ''LI'', ''LO'' instead of ''HI'', ''HO''.

    Do I need to ground the pins that are not used?? Please advice, thank you very much.

    Last edited: Mar 19, 2012
  5. chimera


    Oct 21, 2010
    what I think ur looking for is to convert a pwm signal (with varying duty cycle) into an averaged DC voltage to control that MOSFET.

    1- U will need a low pass filter (a two stage one will do very nicely--- two stage means just have another RC filter after the first one)

    2- U might need to include an op- amp in your design to provide feedback.

    Frankly speaking, I think you should consider using a very concise IC designed your purpose--i.e buck, boost or buck boost

    I hope that helps
  6. herher

    Thread Starter Member

    Oct 18, 2011

    Thanks for the reply.

    Can I know what should be used as the load in these converter circuits?? I have tried with the normal 0.25 watts resistor, and have burned a plenty of them when I run the circuit...What type of load should I use in order to avoid burning them??? Can I use the resistor with higher power rating??

    Please advice, thank you very much.