how to determine the PWM frequency for Buck Converter?

Thread Starter

kaneneville

Joined Mar 7, 2009
9
I am designing a buck converter controlled by ATmega8 for solar power panels.
What should I consider to determine the best PWM frequency for highest efficiency?
 

Thread Starter

kaneneville

Joined Mar 7, 2009
9
I found most of the DC-DC from maxim and TI are driving the mosfets at about 100k-200kHz. Some of them are constant frequency diving and some of them are not. Is it because of preventing high driving current at low duty cycle?
 

crutschow

Joined Mar 14, 2008
25,465
The PWM frequency is a tradeoff between output inductor and capacitor size, and the switching losses in the switch transistor. So just use the recommended or typical values in the data sheet and you should be fine.

Be aware that the higher the switching frequency, the more critical the PCB layout is. Use the recommended layout in the data sheet for the controller if you can.
 

bountyhunter

Joined Sep 7, 2009
2,512
The PWM frequency is a tradeoff between output inductor and capacitor size, and the switching losses in the switch transistor. So just use the recommended or typical values in the data sheet and you should be fine.

Be aware that the higher the switching frequency, the more critical the PCB layout is. Use the recommended layout in the data sheet for the controller if you can.
There is also a cost penalty for higher freq in more expensive core materials for the magnetics. Also, EMI typically gets worse. There is generally a cost increase in gate drive circuitry because the reduced time period forces you to switch the gate faster, takes drivers that can push more peak current. As for capacitors: it's not just size, higher switch frequency forces the use of lower ESR caps which are more expensive in order to keep safe power dissipation. Above about 200 - 300 kHz, ceramic caps have to be used.
 
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