Z8F80483545 Input Capacitor (DCLINK) Calculation clarification / question

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Hgmt
Level 1
Level 1
First question asked Welcome!

Hi,

Input capacitor (DCLINK) calculation for single phase motor bridge (infineon.com)

I stumbled upon this application note, which is very well written and serves as a guide for sizing the DC link capacitor for a DC bus. 

There is a phenomenon that I cannot seem to justify in my head, and after doing my own analysis I feel I lack some understanding of why it is happening.

 

Page 14 shows a graph where VSpp (ripple) increases with duty cycle as you would expect, but when it reaches 70%, the VSpp starts to roll off. After doing my own analysis based on my own parameters and the set up described in the application note, I get the same results. VSpp starts to decrease when it reaches this threshold of around 70%. The calculated values are backed up by the real-world measured values, which adds a lot of credibility to this application as a design resource. 

Intuitively, I feel this has something to do with the interplay between D, (1-D), ESR and the energy balance between the DC LINK and the inductance of the motor. But I cannot quite put my finger on it, is there a name for this phenomenon? Why does it happen? 

 

Thanks,

 

 

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1 Solution
RupakC
Moderator
Moderator
Moderator
100 solutions authored 250 sign-ins 25 likes received

Hello @Hgmt 

Looking into the appnote you mentioned, I could find the following mathematical relations: 

RupakC_0-1701066954576.png

Now, we can focus on the last equation for CDClink. 
After the DC link capacitor has been chosen, the Left hand side of the equation becomes constant and now VSpp is directly proportional to the factor (1-DC)*DC. Vspp is also dependent on a second factor which is having the term IMOTavg. 

As Duty Cycle increases till 0.5, the factor (1-DC)*DC achieves maximum value at 0.5. Beyond duty cycle 0.5, this first factor reduces. However, as the duty cycle increases, the motor current also increases such that the second factor increases. Therefore, one of the factors reduce while the other increase as duty cycle goes higher than 0.5. 

An interaction of these two factor produces a resulting Vspp which achieves maximum value at around 70-80% of duty cycle. 

I hope the explanation was clear. Do let me know if you need further assistance. 

Best Regards

 

RupakC

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1 Reply
RupakC
Moderator
Moderator
Moderator
100 solutions authored 250 sign-ins 25 likes received

Hello @Hgmt 

Looking into the appnote you mentioned, I could find the following mathematical relations: 

RupakC_0-1701066954576.png

Now, we can focus on the last equation for CDClink. 
After the DC link capacitor has been chosen, the Left hand side of the equation becomes constant and now VSpp is directly proportional to the factor (1-DC)*DC. Vspp is also dependent on a second factor which is having the term IMOTavg. 

As Duty Cycle increases till 0.5, the factor (1-DC)*DC achieves maximum value at 0.5. Beyond duty cycle 0.5, this first factor reduces. However, as the duty cycle increases, the motor current also increases such that the second factor increases. Therefore, one of the factors reduce while the other increase as duty cycle goes higher than 0.5. 

An interaction of these two factor produces a resulting Vspp which achieves maximum value at around 70-80% of duty cycle. 

I hope the explanation was clear. Do let me know if you need further assistance. 

Best Regards

 

RupakC
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