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AN2870 Datasheet(PDF) 23 Page - STMicroelectronics

Part # AN2870
Description  L6585DE combo IC
PDF  41 Pages
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Manufacturer  STMICROELECTRONICS [STMicroelectronics]
Direct Link  http://www.st.com
Logo STMICROELECTRONICS - STMicroelectronics

AN2870 Datasheet(HTML) 23 Page - STMicroelectronics

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AN2870
Designing with L6585DE
23/41
MOSFET selection
PFC MOSFET is to be selected considering the maximum current flowing into the switch,
the maximum voltage between drain and source and the maximum allowed losses.
Maximum allowed losses depend on maximum allowed junction temperature; an ambient
temperature equal to 70 °C – 80 °C is usually considered.
Power losses can be summarized as follows:
●
Conduction losses: due to ohmic resistance of the MOSFET channel during its on
state; these losses are prevalent at minimum input voltage.
●
Switching losses: experienced only during turn off transitions.
●
Capacitive losses: experienced only during turn on transitions when the MOSFET has
to discharge the parasitic capacitance present at its drain. These losses are very high
at higher input voltages.
Conduction losses are related to RDS(on) and RMS value of the drain current:
Equation 11
Considering a maximum conduction losses less than PCOND,max, the maximum RDS(on),
measured at 100 °C, can be found as follows:
Equation 12
Switching losses are directly related to frequency, to output voltage, to input current and fall
time of drain of the MOSFET. The frequency should be averaged over the half period of the
mains and the fall time of the MOSFET can be found on MOSFET datasheet.
Equation 13
Equation 14
Capacitive losses are present only if instantaneous input voltage is greater than half the
output voltage. In fact when the inductor current becomes zero the parasitic capacitances
seen at the drain node starts to resonate with parasitic inductances causing a damped
oscillation whose peak to peak amplitude is equal to VOUT - Vin.
When Vin<VOUT/2 the drain voltage at MOSFET turn on is almost zero.
⎥
⎥
⎦
⎤
⎢
⎢
⎣
⎡
π
−
⎟
⎟
⎠
⎞
⎜
⎜
⎝
⎛
=
OUT
rms
,
in
2
rms
,
in
in
2
RMS
,
MOS
V
V
9
2
4
6
1
V
P
8
I
2
RMS
,
MOS
max
,
COND
ON
,
DS
I
P
(max)
R
<
⎟
⎟
⎠
⎞
⎜
⎜
⎝
⎛
⋅
π
−
⋅
⋅
=
OUT
RMS
,
IN
in
2
RMS
,
in
SW
V
V
2
2
1
L
P
2
V
f
rms
,
in
in
OUT
sw
f
rms
,
in
OUT
sw
f
cross
V
P
V
f
t
I
V
f
t
P
=
=



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