Choosing mosfet for boost converter

[ece_student]

hi

im undergraduate student.

We ask to design a Smps boost converter with 12v to 48 v 3a. f=500khz and i having a difficulty of choosing the mosfet can someone give me a hint. and the choosing for the right L and C.

 

[karthick1987]

Hi mate,

For the FET the most important things that you need to look at are the absolute maximum characteristics, typically for the N-type MOSFET for boost converters look up IRF1405 or IRF3205, etc... See if they match up. With L and C you need to find an inductor that can handle the current. The L and C that you choose also determines the current ripple. As a general rule I would go with a large capacitor (with proper voltage rating possibly 63V or more). For more information on calculating the values look up Power supply cookbook by Marty Brown. Its a good book.

Hope it helps
K

 

[uoficowboy]

hi

im undergraduate student.

We ask to design a Smps boost converter with 12v to 48 v 3a. f=500khz and i having a difficulty of choosing the mosfet can someone give me a hint. and the choosing for the right L and C.

I'd choose the L first. For most situations, you design to be in continuous mode. For your parameters, you need at least a 750nH inductor to remain continuous. The size of the inductor controls the ripple current. Your average inductor current, regardless of inductor size, will be 12A. The larger the inductor, the smaller your peak to peak current will be. A 750nH inductor will give you a max current of 24A and a minimum current of 0A. I normally shoot for ripple to be 50% or less of my average current, so in this case that'd suggest a 3nH inductor (peak current 15A, minimum current 9A).

Once you know how large your peak current will be, you should size your FET to handle that much current plus a safety margin. Further, you need to make sure the FET can handle any ringing on the switch node, so a safe point would be to choose a FET that has a max Vds rating of double your output voltage.

Make sure your DCDC controller can drive the gate fast and high enough (some DCDC controllers can only put 5V on the FET gate, while some FETs need more than that).

If you're using a specific controller the datasheet should have lots more detailed info about choosing the FET - it's not an immediately obvious part to choose.

Best of luck!

 

[Yaser43082]

Hello,

I am new to Boost Converters but here are what my calculations show:

1) Your Duty Cycle (D) = 75%
2) Inductor (L) = 0.75uF (I may be off by a few zeros....2:34 am in the morning)

Please let me know if this helps

 

[Tahmid]

Go through these:
**broken link removed**


By the way, is fsw really 500kHz, or is that a typo?