Application Note AN-1150 - International Rectifier

V
4.7V
⋅
=
3.1
( 1−
0.69)
ISNS ( MAX )
= 0. 47
Note: if the calculated V ISNS(MAX) is higher than the cycle-by-cycle peak
overcurrent limit specification of the IC, the V ISNS(PK) value should be used to
determine R SNS . In this example, V ISNS(MAX) is lower than 0.68V V ISNS(PK)(MIN) thus
0.47V is used for R SNS calculation.
V
Next the peak inductor current at maximum peak AC line current, derated with an
overload factor (K OVL =10%), is calculated.
I
IN PK ) OVL
= I
L(
PK ) max.(1 + K
I
( OVL
IN ( PK ) OVL
= 7.4×
1.1 = 8. 2
From this maximum current level and the required voltage on the current sense
pin, we now calculate the maximum resistor value that can be used for the PFC
converter.
R
R
SNS , MAX
SNS , MAX
V
=
I
SNS (max)
IN ( PK ) OVL
= 0.057Ω
A
0.47V
=
8.2A
It is noted that even though IR1152 operates in average current mode it is still
safer to use the peak inductor current for current sense resistor design to
guarantee avoiding premature fold-back.
Power dissipation in the resistor is now calculated based on worst case RMS
input current at minimum input voltage:
)
PR
S
= I
IN ( RMS )MAX
2
⋅ R
S
PR S
= 4.5
2 (0.057Ω)
= 1. 2
A standard 50mΩ resistor can be selected for R SNS for the PFC converter.
W
Peak Current Limit
The cycle-by-cycle peak current limit is encountered when V ISNS pin voltage
exceeds V ISNS,PEAK . For the PFC converter, this limit is encountered whenever the
inductor current exceeds the following:
www.irf.com AN-1150
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