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Original question:
Hi,
I have a question about formula : ICINnorm(RMS)= Sqrt [(D-m/n) x ((1+m)/n -D)]
Is there phase limitation here? I assume the ICINnorm(RMS) will decrease when the phase count is increasing.
Below is my calculation:
D=0.275 (3.3V/12V)
n=2 ; m=0 ; ICINnorm(RMS)=0.2478
n=3 ; m=0 ; ICINnorm(RMS)=0.12665 (decrease)
n=4 ; m=1 ; ICINnorm(RMS)=0.075 (decrease)
n=5 ; m=1 ; ICINnorm(RMS)=0.0968 (*increase)
n=6 ; m=1 ; ICINnorm(RMS)=0.0794 (decrease)
n=7 ; m=1 ; ICINnorm(RMS)=0.0376 (decrease)
n=8 ; m=2 ; ICINnorm(RMS)=0.05 (*increase)
Hi Greg,
Adding additional phases doesn't always give a reduction in input current, but overall the trend is adding more phases will lower the RMS current. Depending on the exact operating conditions a lower phase count might have lower current than a higher phase count.
In Figure 4 from SLVA882 if you look at the 2 and 3 phase curves you can see that around the 50% duty cycle point three phases has a higher RMS current. This is because for those particular sets of conditions there is complete ripple cancellation with only two phases and not for three phases. What's likely happening in your calculations is that while sitting at a fixed duty cycle you're right around a similar operating region and adding/removing phases will show more or less current.
Please let me know if you have any questions.
Cheers,
Carmen
