B32674 datasheet(29/43 Pages) EPCOS

29 / 43 page

29 3/23

Please read Cautions and warnings and

Important notes at the end of this document.

Metallized polypropylene film capacitors (MKP)

B32674 ... B32678

MKP DC link – high power series

Self Heating by power dissipation & equivalent heat coefficient

The IRMS and consequently the power dissipation must be limited during operation in order to not

exceed the maximum limit of T allowed for this series. Tmax given for this series is equal or lower

than 20

°C at rated temperature (70 °C), for higher ambient temperatures Tmax (T) will have the

same derating factor than IRMS versus temperature and then an equivalent derating as per:

Tmax (T) = (Factor)2

×T (70 °C).

For any particular IRMS the T may be calculated by:

T (

°C) = Pdis(mW) / G(mW/°C).

Where T (

°C) is the difference between the temperature measured on the box (see Figure 1) and

the ambient temperature when capacitor is working during normal operation;

T (

°C) = Top(°C) – TA(°C).

It represents the increasing of temperature provoked by the IRMS during operation.

G (mW/

°C) istheequivalentheat coefficientdescribedaboveandPdis(mW) is thedissipatedpower

defined by:

Pdis (mW) = ESRtyp (m

Ω) × IRMS2(ARMS).

Example for thermal calculation:

We will take as reference B32678G8156K (15 μF/875 V) type for thermal calculation.

Considering the following load and capacitor characteristics:

IRMS : 15 ARMS at 20 kHz

TA: 85

°C

× 50 × 57.5 box

G (mW/

°C): 145

Then we have to find the ESRtyp at 20 kHz what is approximately 4.0 m.

So according to

Pdis (mW) = ESRtyp (m)

× IRMS2(ARMS)

we have the following:

Pdis (mW) = 4.0 m

Ω × 15 ARMS2= 900 mW

And as per

T (

°C) = Pdis(mW) / G (mW/°C)

we have the following:

T (

°C) = 900 (mW) / 145 (mW/°C) = 6.2 °C.

What is below of the

Tmax (85

°C) = (Factor)2× T (70 °C) = (0.7)2× 20 °C = 9.8 °C.

On the other hand we may confirm that max IRMS at 20 kHz at 70

°C = 26.5 ARMS

And then max IRMS for 85

°C of ambient temperature is defined as follows:

IRMS (85

°C) = Factor × IRMS(70 °C) = 0.7 × 26.5 ARMS= 18.55 ARMS.

What confirms once again that IRMS (15 ARMS at 20 kHz at 85 °C) is below the max specified for

such frequency and ambient temperature.

B32674 Datasheet(PDF) 29 Page - EPCOS

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