Inductor ripple rate r value selection problem

小太阳yy

Inductor ripple rate r value selection problem [Copy link]

 

The book on power supply design describes it this way. I have many questions about the inductor current ripple rate.

From this formula in the book, we know that r is related to the volt-second product, inductance, frequency, and inductor current value. In other words, the r value is not only related to the inductance, but also to the actual conversion conditions.

In general, the book says r=0.4 is the most appropriate. So under what conditions do we calculate this r value? Because if my input voltage is within a range, the boost and buck-boost are all calculated from Vin MIN. At this time, we calculate the required inductance according to r=0.4. But once the input voltage VIN reaches MAX or other values within the range, the selected inductance and the calculated r will definitely not be 0.4. Doesn't that violate the design rule that we want it to be 0.4?

maychang

"The book says that r=0.4 is the most appropriate value in general. So under what conditions do we calculate this r value?"

There are many factors involved. Usually we want the output ripple to be smaller. To achieve smaller ripple, we need to increase the inductance. To increase the inductance, we need to increase the core cross-sectional area and the number of winding turns, which will inevitably increase the cost, volume and weight. The book also says "the current stress of the power device and the loss of all power devices. Therefore, this r value is determined by comprehensive consideration.

小太阳yy

maychang posted on 2022-5-17 10:41 "In the book, r=0.4 is the most appropriate under normal circumstances. So under what conditions do we calculate this r value?" There are many factors involved. Usually I...

Teacher, suppose we use r=0.4 to calculate, I mean the inductance value calculated by VIN min, when VIN fluctuates, for example, I used 8V to calculate at the beginning, my input voltage is a range, and it reaches 14V. Then the r value is definitely not the original 0.4, how to consider this

maychang

小太阳yy posted on 2022-5-17 10:56 Teacher, suppose we use r=0.4 to calculate, I mean the inductance value calculated using VIN min, when VIN fluctuates, for example, at the beginning, we use 8V to calculate...

Not only the input voltage changes, but also the load changes will cause the r value to change.

The original book also said: r is more suitable for values between 0.3 and 0.5. Within the range of your input voltage variation, just keep r between 0.3 and 0.5. In this way, within the full range of input voltage variation, it will not be too far from the value of 0.4.