E910.27A View Datasheet(PDF) - ELMOS Semiconductor AG
Part Name
Description
MFG CO.
'E910.27A' PDF : 50 Pages View PDF
- Preliminary -
E910.27A
IRS RMS = IOUTMAX ⋅
DMAX
⋅ 1 +
r2
12
[15]
For 100% duty cycle we get:
IRS RMS = IOUTMAX
[16]
The current sense resistor's power rating should be:
P _ RS = VCLIM MAX 2
RS
[17]
8.1.3 Choosing the Choke Value
The E910.27 operates with a wide range of inductance values. The inductance mainly determines the choke
current ripple. Lower values are chosen to reduce physical size of the choke. Higher values allow more out-
put current because they reduce peak current seen by the internal P-channel MOS transistor. Higher values
also reduce output ripple voltage, and core loss.
When choosing a choke you might have to consider maximum load current, core and copper losses, allow-
able component height, output voltage ripple, EMI, fault current in the choke, saturation, and of course, cost.
Since the choke value determines the output ripple current, you have to decide the ratio of the ripple current
to the output current. The ripple current is depending on the duty cycle. Take the worst case ripple current
value of your application.
r=
∆I L
= I RIPPLE
IOUTMAX IOUTMAX
[18]
The core-material losses (magnetic hysteresis loss, eddy-current loss), skin-effect and proximity-effect
losses in the conductor and radiation losses increases substantially with increasing “r”. Calculations show,
the most optimum choice happens when this ratio “r” is set between 0.2 and 0.4 at the maximum output cur-
rent. Often r=0.3 is used. Note that this is just a guideline for an economic design.
I RIPPLE ≈ 0.3 ⋅ IOUTMAX
[19]
If the converter should work in continous mode, even with the lowest output current, than the ripple ratio
must yet be lower:
I RIPPLE ≤ 2 ⋅ IOUTMIN
[20]
The first step in choke design is to determine the appropriate operating mode of the E910.27. Calculate the
minimum inductance value of the choke in continuous mode as follows.
ELMOS Semiconductor AG
Specification 27 / 50
03SP0357E.00 05.09.2006
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