ON Semiconductor NCP1219PRINTGEVB, User Manual

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NCP1219PRINTGEVB
http://onsemi.com
10
-6 0
-5 0
-4 0
-3 0
-2 0
-1 0
0
1 0
2 0
3 0
4 0
1 1 0 1 0 0 1 0 0 0 1 0 0 0 0 1 0 0 0 0 0
Frequency (Hz)
Mag (dB)
-3 0 0
-2 6 0
-2 2 0
-1 8 0
-1 4 0
-1 0 0
-6 0
-2 0
2 0
6 0
1 0 0
Mag (dB)
− 17 dB
Figure 17. Frequency Response of the Power Stage
Phase ( ° )
PHASE (
°
)
The pole introduced by the optocoupler needs to be
considered. The pole location is dependant on the biasing
conditions of the optocoupler. The internal 16.7 k
W pullup
resistor and the output capacitance of the optocoupler set the
pole at 4.7 kHz, as shown in Figure 14. The location of this
pole limits the available bandwidth of the system.
The evaluation board design uses the k − factor approach to
pole and zero placement, and a phase margin of 65 ° is
chosen. For the type 2 compensation network, the k − factor
is found using Equation 27,
k + tan
ǒ
PM * PS * 90
2
) 45
Ǔ
(eq. 27)
where PM is the desired phase margin, and PS is the phase
brought by the power stage. For a crossover frequency, f
c
, of
1 kHz, the phase caused by the power stage is − 88 ° . The
resulting k value is 4.2. The pole frequency, f
p
, is calculated
using Equation 28.
f
p
+ f
C
@
k
(eq. 28)
The pole frequency for this design is equal to 4.2 kHz. The
zero frequency, f
z
, is calculated using Equation 29,
f
z
+
f
C
k
(eq. 29)
The zero frequency is set to 240 Hz.
The bandwidth of the optocoupler can be used to set the
pole location of the compensation network. In this case,
adding capacitance to satisfy the k
− factor calculations limits
the bandwidth of the system and causes slowing of the
transient response and increased output ripple. The
capacitance needed to place the zero is calculated using
Equation 30.
C
zero
+
1
2 @ p @ f
z
@ R
upper
(eq. 30)
For this design, a value of 33 nF is chosen for C
zero
.
The required gain boost (G
fc
) needed to compensate the
system and provide a crossover frequency of 1 kHz is
measured as 17 dB. The gain provided by the compensation
network is calculated using Equation 31.
G + 10
G
f C
20
(eq. 31)
The R
LED
value needed to produce this gain is calculated
using Equation 32.
R
LED
+
R
pullup
@ CTR
G
(eq. 32)
From the measurements and the resulting gain, R
LED
is
990
W
.
The open loop response is measured by injecting an ac
signal across R19 using a network analyzer and an isolation
transformer as shown in Figure 18. The open loop response
is the ratio of B to A.
Figure 18. Open Loop Frequency Response
Measurement Setup
The resulting loop response after compensation is shown
in Figure 19, where the crossover frequency is 1.3 kHz, with
a phase margin of 60
° , measured at low − line and nominal
load current.