Design Guide
16 of 48
V 1.0
2018-06-06
XDPL8218 design guide
For high power factor flyback converter with constant voltage output
VCC capacitance and output UVP design
Under the single-fault condition of second-stage CC regulator MOSFET drain and source pins being shorted, the
main output could be clamped by a low-output LED voltage, which could result in the flyback MOSFET
continuously operating in the saturation region due to low V
CC
and gate-drive voltages.
Hence, in this design example, EN
UVP,Vout
= Enabled parameter setting is selected to enable the regulated mode
output UVP, which could be triggered if the ZCD pin estimated output voltage V
out
is below the regulated mode
output UVP level V
outUV
for longer than a blanking time of t
VoutUV,blank
V
outUV
parameter can be defined and calculated as:
𝑉
𝑜𝑢𝑡𝑈𝑉
=
𝑉
𝑎,𝑈𝑉
∙ 𝑁
𝑠
𝑁
𝑎
− 𝑉
𝑑
(25)
Where V
a,UV
is the desired primary auxiliary winding demagnetization voltage when output voltage is V
outUV
.
V
a,UV
is recommended to be between 9.5 V and 10.5 V. So, taking V
a,UV
= 10.1 V,
𝑉
𝑜𝑢𝑡𝑈𝑉
=
10.1 ∙ 10
3
− 0.7
𝑽
𝒐𝒖𝒕𝑼𝑽
= 𝟑𝟑 𝑽
To prevent the regulated mode output UVP from being triggered by the output undershoot during input voltage
step-down, e.g. from 277 V
rms
to 120 V
rms
, t
VoutUV,blank
is recommended to be at least 100 ms. Hence, t
VoutUV,blank
=
500 ms is selected in this design example.
V
out
time
Turn-on
Output load current
too high causing
power limit and
Vout drop below
set-point
Startup
phase
Regulated Mode
(V
out
> V
outUV
)
Output under-
voltage protection
triggered after
t
VoutUV,blank
V
outUV
V
out,start
Output setpoint
Start of regulation
according to feedback
voltage mapping
Regulated Mode
(V
out
< V
outUV
for
t
VoutUV,blank
)
Figure 11
Regulated mode output UVP (based on ZCD pin voltage sensing)
The reaction of regulated mode output UVP is configurable to latch-mode or auto-restart, based on the
Reaction
UVP,Vout
parameter. Reaction
UVP,Vout
= Auto-Restart is selected in this design example.
Note:
The regulated mode output UVP is not active in Active Burst Mode (ABM).
