NCV8871
www.onsemi.com
13
Rds(on)
V
d
GND
ISNS
VFB
GDRV
VC
R
i
C
OUT
V
OUT
C
1
R
2
V
CTRL
OTA
V
IN
r
Cf
C
2
R
OUT
R
ESD
R
0
R
1
R
low
L
p
1:N
V
REF
Figure 13. NCV8871 Flyback Converter OTA and Compensation
The following equations may be used to select compensation
components R
2
, C
1
, C
2
for Figures 12 & 13 power supply.
Required input design parameters for analysis are:
V
d
= Output diode V
f
(V)
V
IN
= Power supply input voltage (V)
N = N
s
/N
p
(Flyback transformer turns ratio)
R
i
= Current sense resistor (W)
R
DS(on)
= MOSFET R
DS(on)
(W)
(R
sw_eq
= R
DS(on)
+ R
i
for the boost continuous conduction
mode (CCM) expressions)
C
OUT
= Bulk output capacitor value (F)
r
CF
= Bulk output capacitor ESR (W)
R
OUT
= Equivalent resistance of output load (W)
P
out
= Output Power (W)
L = Boost inductor value or flyback transformer primary
side inductance (H)
r
L
= Boost inductor ESR (W)
T
s
= 1/f
s
, where f
s
= clock frequency (Hz)
R
1
and R
low
= Feedback resistor divider values used to set the
output voltage (W)
V
OUT
= Device specific output voltage (defined by R
1
and
R
low
values) (V)
R
0
= OTA output resistance = 3 MW
S
a
= IC slope compensation(e.g. 53 mV/msfor NCV887100)
g
m
= OTA transconductance = 1.2 mS
D = Controller duty ratio
D’ = 1 − D
Necessary equations for describing the modulator gain
(V
ctrl
-to-V
out
gain) H
ctrl_output
(f) are described next. Boost
continuous conduction mode (CCM) and discontinuous
conduction mode (DCM) transfer function expressions are
summarized in Table 1. Flyback CCM and DCM transfer
function expressions are summarized in Table 2.
