Infineon XDPL8218, Design Manual

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Design Guide 11 of 48 V 1.0
2018-06-06
XDPL8218 design guide
For high power factor flyback converter with constant voltage output
MOSFET maximum current cycle-by-cycle limit and start-up phase design
6
MOSFET maximum current cycle-by-cycle limit and start-up phase
design
Under the single-fault condition of second-stage CC converter MOSFET drain and source pins being shorted, the
connected output LEDs could clamp the flyback output voltage below its constant voltage regulation set-point.
To limit the flyback output power to the LEDs below 100 VA as per UL1310 requirements under such fault
conditions, especially at high input voltage, XDPL8218 regulated mode features the CS pin voltage level 1 for
MOSFET maximum current cycle-by-cycle limit V
OCP1
(V
in
), which is adaptive based on the estimated input
voltage V
in
, as shown in Figure 7 .
Note: Regulated mode is a controller operating state, which is entered after the start-up phase, to
regulate the output based on the FB voltage mapping, as shown in
Figure 8 .
Note: The estimated input voltage V
in
used for V
OCP1
(V
in
) adaptation is in rms value, which is assumed as
0.707 of estimated peak input voltage V
in,peak
, regardless of whether actual input voltage is AC or
DC. As the input voltage is estimated based on ZCD pin and CS pin switching signals, proper ZCD
resistor selection (see
Section 12 ) and R
in
parameter fine-tuning (see Section 18.1 ) are needed.
V
OCP1
(V
in
)
V
in,high
V
in,low
V
in
V
OCP1,at,V,in,low
V
OCP1,at,V,in,high
Figure 7 Regulated mode CS pin voltage level 1 for MOSFET maximum current cycle-by-cycle limit
V
OCP1,at,V,in,low
parameter defines the value of V
OCP1
(V
in
) variable when the estimated input voltage V
in
is same as or
lower than the lowest operational input voltage parameter V
in,low
.
V
in,low
and V
OCP1,at,V,in,low
parameters can be defined and calculated as:
𝑉
𝑖𝑛,𝑙𝑜𝑤
= 𝑎 ∙ 𝑉
𝐴𝐶,𝑚𝑖𝑛
(15)
Where a is the ratio recommended to be between 0.9 and 0.95.
Taking a = 0.91,
𝑉
𝑖𝑛,𝑙𝑜𝑤
= 0.91 ∙ 90
𝑽
𝒊𝒏,𝒍𝒐𝒘
= 𝟖𝟐 𝑽
𝒓𝒎𝒔
𝑉
𝑂𝐶𝑃1,𝑎𝑡,𝑉,𝑖𝑛,𝑙𝑜𝑤
= 𝐼
𝑝𝑟𝑖(𝑝𝑘),𝑚𝑎𝑥
∙ 𝑅
𝐶𝑆
(16)
𝑉
𝑂𝐶𝑃1,𝑎𝑡,𝑉,𝑖𝑛,𝑙𝑜𝑤
= 2.606 ∙ 0.2