LM5157EVM-FLY: Quickstart calculator for flyback regulator中关于Lm（原边绕组电感）计算的问题

Hi Rannie,

the quickstart calculator for the LM5157 Flyback only supports CCM operation mode.

As the application report spec was set to use CCM at the lowest input voltage and DCM at the highest input voltage (see comments in application report), the settings have to be made accordingly.

Best regards,

 Stefan

Thanks for your reply! As you mentioned, the quickstart calculator only supports the CCM mode. In this case, we should use the lowest input voltage(8V). However, when I key "8" in the cell of "minimum input supply voltage", the calculated primary inductance is 23.5uH. When I key "8" in the cell of "maximum input supply voltage", the calculated primary inductance is 13.1uH, which is as same as the EVM parameter. Besides, It seems that the calculated primary inductance is related to the data in the cell of "maximum input supply voltage", and it can't be changed by the value in the cell of "minimum input supply voltage". I am confused. Is the calculator wrong?

Hi Rannie,

the calculation of the inductor in the excel sheet is done on the inductor ripple and ripple ratio. 

In CCM operation, the maximum primary winding ripple current occurs when the supply voltage is at the maximum value.

Best regards,

 Stefan

Hi Stefan,

I still have 2 questions on the calculation of primary inductor.

1. How to select the primary inductor when I want to design a flyback using LM5157.

Should I use this formula (shown in the application notes) to calculate the primary inductor? The V_supply is the minimum supply voltage or the maximum supply voltage? which one is more reasnonable?

Why the inductor of EVM is calculated by the minimum input voltage? as you said, the maximum input voltage should be used at CCM.

2. When I use the excel sheet to calculate the inductor based on the customer's requirement, the calculated value (16.2uH) is much larger than customer's reality design (3uH). According to Pspice simulation, I noticed that the flyback works on DCM When the L_M is 3uH. 

So, I want to know what will happened if the primary inductor is smaller than the calculated. 

Hi Rannie,

In this report we wanted to describe how the EVM was designed which uses the DCM and CCM mode.

You can also check this report where a similar Flyback design is described but this one works in the full operation range in CCM:

https://www.ti.com/lit/an/snva866a/snva866a.pdf

So you select the inductor to be 3uH it will run in DCM mode - which could be OK just depends on your requirements.

Best regards,

 Stefan

Hi Stefan,

Thanks a lot for your reply. 

I wonder if the inductance value is too small, will it have any effect other than entering DCM mode? the efficiency? the volume of the transformer? or it's just the difference between CCM and DCM.

Best regards.

Rannie