Infineon XDPL8218, Design Manual

Page: 9 / 48

Design Guide 9 of 48 V 1.0
2018-06-06
XDPL8218 design guide
For high power factor flyback converter with constant voltage output
Flyback MOSFET and secondary main output diode selection
The selectable MOSFET R
ds(on),25°C
can be defined as:
𝑅
𝑑𝑠(𝑜𝑛),25°𝐶
≤
𝑚 ∙ 𝑃
𝑜𝑢𝑡,𝑓𝑢𝑙𝑙
𝐼
𝑝𝑟𝑖(𝑟𝑚𝑠),𝑚𝑎𝑥
2
∙ ∆𝑅
𝑑𝑠(𝑜𝑛),100°𝐶
(10)
Where m is the desired ratio of MOSFET conduction loss over P
out,full
and ∆R
ds(on),100°C
is the ratio of
𝑅
𝑑𝑠(𝑜𝑛),100°𝐶
𝑅
𝑑𝑠(𝑜𝑛),25°𝐶
.
It is typical to select m in the range of 0.025 to 0.03 and ∆R
ds(on),100°C
in the range of 1.75 to 1.85. Taking m = 0.0275
and ∆R
ds(on),100°C
= 1.8, R
ds(on),25°C
can then be calculated as:
𝑅
𝑑𝑠(𝑜𝑛),25°𝐶
≤
0.0275 ∙ 43.2
0.838
2
∙ 1.8
= 0.94 Ω
Referring to the calculation results of equation (10) and Table 3 below, R
ds(on),25°C
= 900 mΩ is selected.
To utilize the PCB as a heatsink for the MOSFET, IPD80R900P7 with TO-252 (DPAK) package is selected.
Table 3 800 V CoolMOS
TM
P7 selection table
For the secondary main output diode selection, it is necessary to first estimate the maximum reverse voltage
V
r(diode),max
and maximum secondary main winding peak current I
sec(pk),max
, based on:
𝑉
𝑟(𝑑𝑖𝑜𝑑𝑒),𝑚𝑎𝑥
= 𝑉
𝑠𝑝𝑖𝑘𝑒,𝑑𝑖𝑜𝑑𝑒
+ 𝑉
𝑜𝑢𝑡,𝑠𝑒𝑡𝑝𝑜𝑖𝑛𝑡
+
𝑉
𝐴𝐶,𝑚𝑎𝑥(𝑝𝑘)
+ 𝑉
𝑚𝑎𝑟𝑔𝑖𝑛,𝐹𝐸𝑇
𝑁
(11)
Where V
spike,diode
is the diode reverse voltage spike.
Assuming
𝑉
𝑠𝑝𝑖𝑘𝑒,𝑑𝑖𝑜𝑑𝑒
≈ 35% ∙ (𝑉
𝑜𝑢𝑡,𝑠𝑒𝑡𝑝𝑜𝑖𝑛𝑡
+
𝑉
𝐴𝐶,𝑚𝑎𝑥(𝑝𝑘)
+ 𝑉
𝑚𝑎𝑟𝑔𝑖𝑛,𝐹𝐸𝑇
𝑁
)
,
𝑉
𝑟(𝑑𝑖𝑜𝑑𝑒),𝑚𝑎𝑥
≈ 135% ∙ (𝑉
𝑜𝑢𝑡,𝑠𝑒𝑡𝑝𝑜𝑖𝑛𝑡
+
𝑉
𝐴𝐶,𝑚𝑎𝑥(𝑝𝑘)
+ 𝑉
𝑚𝑎𝑟𝑔𝑖𝑛,𝐹𝐸𝑇
𝑁
) = 135% ∙ (54 +
√2 ∙ 305 + 90
3.2
)
𝑉
𝑟(𝑑𝑖𝑜𝑑𝑒),𝑚𝑎𝑥
≈ 292.81 𝑉
𝐼
𝑠𝑒𝑐(𝑝𝑘),𝑚𝑎𝑥
≈ 𝐼
𝑝𝑟𝑖(𝑝𝑘),𝑚𝑎𝑥
∙
𝑁
𝑝
𝑁
𝑠
= 2.606 ∙
32
10
(12)
𝐼
𝑠𝑒𝑐(𝑝𝑘),𝑚𝑎𝑥
≈ 8.34 𝐴
Based on the above, a secondary main output diode with repetitive reverse voltage rating V
RRM
= 300 V is
selected. To minimize its switching and conduction losses, the selected diode also has the properties of hyper-
fast recovery speed and low forward voltage drop at I
sec(pk),max
.
Additionally, a RC secondary snubber network, e.g. 10 Ω resistor in series with 150 pF capacitor, is deployed
across the secondary main output diode, to suppress the diode reverse voltage spike and the EMI.