10 Steps for Selecting The Right Film DC Link Capacitor

Ten Steps To Your Film DC Link Capacitor.

Step 1.  Specify your DC bus voltage

Step. 2 Determine the acceptable voltage ripple limit.

Normally the limits vary from 5-10%, however in some rectifier applications they could also be lower than 1% of the DC bus voltage. 

Step 3. Calculate the capacitance required to reach the Uripple level.

Step 4.Calculate the capacitance current

Step 5. Verifying Design Limits

Once the capacitor capacitance and capacitance currents have been identified, it is important to verify that the calculated capacitance is able to handle the calculated current. If not then increase the capacitance to match the current.

Step 6. Select the parts from the Databook.

Step 7. Thermal calculations

The thermal calculations and it's implication can be found in the PEC Application Notes and

Semikube Selection Guide from Semikron GmbH.

Step 8 Lifetime

The lifetime calculations and FIT rates would appear in later blogs however, the above mentioned links have sufficient information on this topic.

Step 9. Capacitance Characteristics

Do not forget to check the performance of the selected capacitor for your operating frequency.

Step 10.Mounting

This is the final but one of the most important steps in the selection process. Do it carefully.

Advantage of Film DC Link over Electrolytic DC Links

Traditionally Electrolytic capacitors have been used most of the DC Link applications. The main reason has been the low cost per capacitance for these capacitors. However, they have a limitation on the maximum voltage applied (600Vdc) which necessitates series and parallel combination, high ESR and low life expectancy. Further, they are one of the components most prone to failure. The main failure mechanisms include the loss of electrolyte through out-gassing and chemical changes to the oxide layer. All the degradation mechanisms are aggravated by ripple current heating.

Over the years Electronicon Kondesatoren GmbH, has developed and proven Film Capacitors as a an alternative of choice for DC- Link applications. Electronicon capacitors with film metallisation as core capability and the use of Secumet^TM metallisation technology leaves the conventional metallisation and segmented film solutions far behind.

The film DC links are now available also for smaller systems of less than 5kw, in PCB mounted construction.

Following are the advantages as a designer that you get while moving from Electrolytic to Film DC links, besides the cost advantage.

What is in it for You?
1.     Extremely long life greater than 150000hrs - High reliability of your equipment
 
2.     Extremely low losses compared to electrolytic caps.- Lower running costs, savings for the end user
 
3.     Extremely low self inductance-  lower parasitic
 
4.     Accurate capacitance values- eliminating the need for sharing resistors- Savings due removal of additional item and operations
 
5.     No losses due to sharing resistors- lower cooling costs Lower Running costs
 
6.     Reduced capacitance need- due to 5 times more current handling than electrolytic capacitors.- reduced risk by lowered  energy density inside the equipment-
 
7.     Voltage rating upto 5000Vdc available in single container.- no more headaches of series and parallel connections
 
8.     No forming ( pre- loading) of capacitors required- as needed for electrolytic capacitors.- reduced risk of failure due to negligence at final customer, increases robustness of your product
 
9.     Unlimited shelf life- no risk of scrapping
 
10.  PCB mounted, Bolt fixing or Clamp fixing both are possible.- covers all your needs and applications
 
11.  Strong Connections for direct mounting on the busbars-  ease of use and longevity of connections
 
12.  Dry construction- No risk of spillage, no aggressive electrolytes, helps you comply with stringent environmental regulations
 
13.   UL Approved capacitors –safety guaranteed