WEBENCH® Tools/TPS40425: Webench application question

Hi Bogey,

1) The WEBENCH optimizer has 5 dial positions and it proposes designs with differing priority on low footprint, low cost, and high efficiency by tuning the part selection and by adjusting the switching frequency. Higher frequency leads to smaller inductor, smaller output cap, but typically lower efficiency; and vice versa for lower frequency.

For your design conditions, the calculated design at knob 1 for smallest footprint/highest frequency is exceeding allowed junction temperature limits of the power stage and causing the optimizer issue. WEBENCH estimates the maximum allowed frequency of operation that will not violate design limits such as junction temperature, but in some cases like this one, the final temperature calculation in the model when operating at this max frequency can still violate the temperature limit.

I will look into the frequency calculation to see if it can be improved to avoid this.

2) WEBENCH Power Designer sets the recommended min and max inductance limits based on switching frequency and rules of thumb on inductor ripple as percentage of output current. However, you can override the limits by clicking on the pencil icon in the Alternate Part screen:

3) To set the design frequency, use the Advanced Options on the left of the screen. Check the 'User Preferred Frequency' box and type in the frequency that is within the recommended range. As I mentioned in 1), WEBENCH estimates the maximum allowed frequency of operation, shown below as 648kHz, that will not cause any design limit violations. In my testing, I could only create designs as high as 560kHz, because the power stage M1 and M2 junction temperatures are already at 124degC (limit is 125degC). Any higher switching frequency will push the temperature above the allowed limit.