An375 – Cirrus Logic AN375 User Manual
Page 14
AN375
14
AN375REV4
Step 12) Calculate Primary and Secondary Currents
Since the primary and secondary currents have a variable peak and duty cycle across the line voltage phase
angle, there is no practical closed-form solution to the RMS value. The RMS/I
PK
ratio depends primarily on
the ratio Z. Figure 6 provides a plot of the ratios P and S relative to primary and secondary, respectively.
The primary RMS/I
PK
ratio and secondary RMS/(N
I
PK
) ratio are functions of ratio Z as defined in Equation 19
on page 13. Calculate ratio Z then use the graph in Figure 6 to read out the ratios P and S. After calculating
values for peak current I
PK
and turns ratio N, calculate RMS currents I
P(RMS)
and I
S(RMS)
where
P = primary RMS/I
PK
ratio
S = secondary RMS/(N
I
PK
) ratio
Step 13) Select an Output Capacitor
For a single-stage LED driver, the output capacitor and LED effective resistance form an RC filter that defines
the amplitude attenuation applied to the output ripple current. To calculate the required output capacitance see
application note AN376 Single Stage Output Ripple Current and the Effect on Load Current in an LED Driver
for more details regarding the calculation and selection of the output capacitor.
16%
18%
20%
22%
24%
26%
28%
30%
0.4
0.6
0.8
1.0
1.2
1.4
1.6
1.8
2.0
2.2
2.4
2.6
2.8
3.0
Ratio
Z
P = Primary RMS / I
PK
S = Secondary RMS / (N x I
PK
)
P
S
Figure 6. Normalized RMS/I
PK
Ratios vs Z
I
P RMS
P I
PK
=
[Eq. 20]
I
S RMS
S N I
PK
=
[Eq. 21]