inductor for buck converter

Discussion in 'The Projects Forum' started by mah, Feb 19, 2014.

  1. mah

    Thread Starter Active Member

    Mar 15, 2010
    276
    2
    i have toroidal inductor and i want to use it in dc-dc buck converter but i don't know its value , how to measure it ?
    what would happen if i use this coil schaffner RB series 3 mhhttp://sa.rsdelivers.com/product/sch...h/7690578.aspx in 17-9 volt buck converter 55 watt ? will it be ok or should i use smaller inductance ? also could i use any type of inductors or just toroidal coil ferrite core?
    how to calculate the inductance needed for buck converter?
     
  2. crutschow

    Expert

    Mar 14, 2008
    13,033
    3,241
    You can measure the inductance by applying a pulse of DC and measuring the rate the current increases with time (by measuring the voltage across a small resistor in series with the inductor). This requires an oscilloscope for the measurement.

    The amount of inductance you need depends upon the particular converter configuration you choose. That data sheet for the buck converter controller you want to use will tell you how to calculate the inductance required.

    The inductor doesn't have to be toroidal but it should have a high frequency core such as ferrite.
     
  3. mah

    Thread Starter Active Member

    Mar 15, 2010
    276
    2
    would you propose a company to buy from it ?
     
  4. crutschow

    Expert

    Mar 14, 2008
    13,033
    3,241
    Coilcraft is one that has a large assortment of inductors.
     
  5. Papabravo

    Expert

    Feb 24, 2006
    10,152
    1,793
    Pulse Engineering makes great inductors for SMPS
     
  6. mah

    Thread Starter Active Member

    Mar 15, 2010
    276
    2
    toroidal inductor isn't available in my area is there any other type could be used ?
     
  7. crutschow

    Expert

    Mar 14, 2008
    13,033
    3,241
    To help resolve this guessing game, what type of inductors are available in your area?

    What is your area?
     
  8. mah

    Thread Starter Active Member

    Mar 15, 2010
    276
    2
    EGYPT is my area
     
  9. crutschow

    Expert

    Mar 14, 2008
    13,033
    3,241
    That wasn't my question -- What type of inductors are available in your area? What suppliers are available to you?
     
  10. mah

    Thread Starter Active Member

    Mar 15, 2010
    276
    2
  11. crutschow

    Expert

    Mar 14, 2008
    13,033
    3,241
    Your inductance value seems to be too high for your requirements. Here's an inductor calculator you can use.
     
  12. mah

    Thread Starter Active Member

    Mar 15, 2010
    276
    2
    these are my requirements and i think it is normal , the output of the calculator was close to the value i bought 100uh .my
    input power is 55watt if i set v out to 9 volt and load current 3.6 amp to suite the inductor i bought the output power will be 3.6*9=32.4 watt however input power was55 w so did i miss any thing, i think that i should modify the current to be 7 amp consequently i will parallel to inductors
     
    Last edited: Feb 25, 2014
  13. crutschow

    Expert

    Mar 14, 2008
    13,033
    3,241
    In post #10 you said 500μH so that's why I questioned the value.

    Here's a 7A 100μH inductor.
     
  14. mah

    Thread Starter Active Member

    Mar 15, 2010
    276
    2
    I want to make it suits my toroidal 100uH,3.6A could i do that or i won't feel difference in output as input will be 3A,17V
     
  15. crutschow

    Expert

    Mar 14, 2008
    13,033
    3,241
    It's the output current, not the input current that determines the required current rating of the inductor.
     
  16. mah

    Thread Starter Active Member

    Mar 15, 2010
    276
    2
    so i could use this toroidal to get 3.6 Amp and the output voltage should be 55/3.6=15.2 as 7A is not available.
     
  17. mah

    Thread Starter Active Member

    Mar 15, 2010
    276
    2
    i used the attached circuit with atmega 8 to drive dc buck converter and it was working very well , then i increased the input voltage on the drain to 20v and the input current was 4 A (from a power supply ) suddenly the output on Ho of IR2112 was zero when i removed the converter the output on Ho was 7.6 v dc constant when the volt on drain Vd was 9.6 . and when i connected power resistor to the source of the mosfet Ho was zero ,did i lost the driver ?
    also I noticed that when I increased Vd to 20 volt the output on IR2112( Ho) increased too ,the pwm was amplified to 25 v on the Mosfet gate how could the IRfz48 mosfet survive while Vgs from (10-20 v )?
     
  18. crutschow

    Expert

    Mar 14, 2008
    13,033
    3,241
    Since there is not inductor in the circuit I assume this is just a test of the switch(?). What is the value of R5?

    Sounds like you may have lost the driver, but I don't see why it would have failed. :confused:

    The transistor gate to ground voltage will be high but the bootstrap voltage between the transistor gate and source is less (which is the critical voltage).
     
  19. mah

    Thread Starter Active Member

    Mar 15, 2010
    276
    2
    there was inductor when the driver was working then it stopped so i simplified the circuit to know what happened . R5 is ranging from 4 to 10 ohm i don't remember exactly its value when i raised the input voltage to 20 V , i don't know why does the voltage on Ho raised when i raised the drain voltage . i guess that the driver has been damaged but why ??:confused:
     
    Last edited: Feb 27, 2014
  20. crutschow

    Expert

    Mar 14, 2008
    13,033
    3,241
    Was there a free-wheeling diode from the MOSFET inductor interface to ground. I don't see such a diode in your circuit. Without that there can be high voltages induced into the circuitry.
     
Loading...