Design Guide
17 of 48
V 1.0
2018-06-06
XDPL8218 design guide
For high power factor flyback converter with constant voltage output
Output OVP-related design
9
Output OVP-related design
Under the single-fault condition of the FB pin open, the main output voltage would rise quickly above V
out,setpoint
.
As shown in
, the output OVP would be triggered when the ZCD pin estimated output voltage V
out
is
higher than the output OVP level V
outOV
for longer than a blanking time.
Note:
The output OVP blanking time is typically a quarter of the half sine wave period, e.g. 2.5 ms for F
line
= 50 Hz with line synchronization established.
To prevent the output OVP from being triggered by the output overshoot during input voltage step-up, e.g.
from 120 V
rms
to 277 V
rms
, the output OVP level V
outOV
should be configured well above V
out,setpoint
. Therefore, the
V
outOV
parameter can be defined and calculated as:
𝑉
𝑜𝑢𝑡𝑂𝑉
≥ 120% ∙ 𝑉
𝑜𝑢𝑡,𝑠𝑒𝑡𝑝𝑜𝑖𝑛𝑡
= 120% ∙ 54 = 64.8 𝑉
Based on the above, V
outOV
= 65 V is selected in this design example.
Considering the ZCD pin estimated output voltage protection accuracy is subjective to the the sampled signal
accuracy, sampling delay, indirect sensing delay (e.g. output voltage cannot be estimated near AC input phase
angle of 0
degrees
and 180 degrees) and blanking time, the output capacitor voltage rating V
out,cap,rating
should be
selected well above V
outOV
. As a result, V
out,cap,rating
can be defined and calculated as:
𝑉
𝑜𝑢𝑡,𝑐𝑎𝑝,𝑟𝑎𝑡𝑖𝑛𝑔
≥
𝑉
𝑜𝑢𝑡𝑂𝑉
0.9
=
65
0.9
= 72.2 𝑉
(27)
Based on the above, V
out,cap,rating
= 80 V is selected in this design example.
Attention:
It is mandatory to ensure that V
outOV
is configured well below the actual output capacitor
voltage rating V
out,cap,rating
, while the V
out,cap,rating
is not exceeded in actual testing with all the
necessary test conditions.
V
out
time
Turn-on
Start of regulation
according to
feedback voltage
mapping
FB pin Open
Startup
phase
Regulated Mode
Auto-restart time
(based on t
auto,restart
)
V
outOV
protection triggered
after blanking time
V
outOV
V
out,start
Output setpoint
Reaction
OVP,Vout
= Auto-restart
Reaction
OVP,Vout
= Latch-Mode
V
out,cap,rating
Figure 12
Output OVP (based on ZCD pin voltage sensing)
The reaction of output OVP is configurable to latch-mode or auto-restart, based on the Reaction
OVP,Vout
parameter. Reaction
OVP,Vout
= Auto-Restart is selected in this design example.
Note:
The output OVP is not active in ABM.
