Design Guide
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2018-06-06
XDPL8218 design guide
For high power factor flyback converter with constant voltage output
Secondary-side regulation FB circuit design
13
Secondary-side regulation FB circuit design
The FB pin filter capacitor C
FB
, optocoupler and the SSR FB circuit are connected based on the connections
shown in
The FB pin does not need any external pull-up as XDPL8218 has a fixed voltage reference V
REF
of 2.428 V, which is
internally connected to its FB pin via an internal pull-up resistor. The internal pull-up resistor value is
configurable based on the R
FB,pull,up
parameter.
C
VDD
Output
R
lower
FB
R
upper
C
comp
R
comp
R
bias,opto
C
FB
CV output
Secondary Side
Regulation (SSR)
feedback circuit
R
FB,pull,up
V
REF
N
a,sec
V
REF,SSR
R
bias,REF
+
-
C
bias,opto
Opto-
coupler
Op-Amp
V
supply,SSR
I
opto,SSR
N
S
V
out,sense,SSR
I
C
I
ib
I
KA
R
opto
I
opto,SSR
D
x
I
sense,SSR
V
supply,REF
Figure 16
FB pin internal pull-up, C
FB
, optocoupler and CV output SSR FB circuit connections
V
supply,SSR
from the secondary auxiliary winding rectified output is mainly used to supply the SSR circuit op-amp
operational voltage V
DD
and optocoupler LED current I
opto,SSR
via resistor R
bias,opto
. For V
supply,SSR
noise decoupling,
in this design example, a ceramic capacitor of C
VDD
= 100 nF with low ESR is selected and placed near the op-
amp V
DD
pin.
As shown in
, the SSR op-amp non-inverting input should be connected to the SSR reference voltage
V
REF,SSR
, while the inverting input should be connected to a resistor/divider formed by R
upper
and R
lower
for output
voltage sensing. In this design example, the selected op-amp part number is TSM103W, which has dual op-
amps, and the non-inverting input of one op-amp is wired to a voltage reference V
REF,SSR
of 2.5 V internally.
V
supply,SSR
can be used as the SSR voltage reference supply V
supply,REF
, to provide a minimum biasing current of I
KA,min
via voltage reference biasing resistor R
bias,REF
, for generating the V
REF,SSR
. However, in this design example, to
minimize the standby loss, V
supply,REF
is not supplied by V
supply,SSR
based on N
a,sec
= 3, but by another rectified
output with lower voltage, based on a partial secondary auxiliary winding turns number of N
a,sec,partial
= 2.
Hence, the recommended maximum voltage reference biasing resistance R
bias,REF,max
can be defined and
calculated as:
𝑅
𝑏𝑖𝑎𝑠,𝑅𝐸𝐹,𝑚𝑎𝑥
=
𝑔
𝐼
𝐾𝐴,𝑚𝑖𝑛
∙ [(𝑉
𝑜𝑢𝑡,𝑠𝑒𝑡𝑝𝑜𝑖𝑛𝑡
+ 𝑉
𝑑
) ∙ (𝑁
𝑎,𝑠𝑒𝑐,𝑝𝑎𝑟𝑡𝑖𝑎𝑙
𝑁
𝑠
⁄
) − 𝑉
𝑑,𝑎𝑢𝑥
− 𝑉
𝑅𝐸𝐹,𝑆𝑆𝑅
]
(38)
Where g is the ratio recommended to be between 0.75 and 0.85, and V
d,aux
is the auxiliary output diode forward
voltage.
Taking g = 0.8, V
d,aux
= 0.5 V and I
KA,min
= 1 mA based on the selected op-amp datasheet,
𝑅
𝑏𝑖𝑎𝑠,𝑅𝐸𝐹,𝑚𝑎𝑥
=
0.8
1 ∙ 10
−3
∙ [(54 + 0.7) ∙ (2 10
⁄
) − 0.5 − 2.5] = 6.35 𝑘Ω
