AN75
Free-wheel diode calculation
The free-wheel diode D
F
shown in Figure 1 is chosen based on its maximum stress voltage and total
power loss. The maximum stress voltage rating of the free-wheel diode is the same as the MOSFET. It
is advisable to use ultra-low reverse recovery time T
RR
(<35ns) diode as D
F
to reduce the MOSFET’s
switching ON loss. In the design example, 1A 600V rectifier, MUR160, is selected.
The worst case average current through the diode occurs at V
LED(max)
and V
IN(min)
.
⎛
V
LED(min)
⎞
⎟ =
240mA
× ⎛
1
−
42V
⎞ =
202mA
I
D(avg)
=
I
LED(nom )
× ⎜
1
−
⎜
⎟
⎜
V
IN(max)
⎟
⎝
373 V
⎠
⎝
⎠
Assuming a constant forward voltage drop V
F
across the diode, the conduction power loss can be
calculated,
P
D _ COND
=
I
D(avg)
×
V
F
=
202mA
×
1.1V
=
222mW
Finally, the diode junction temperature without using the heat sink can be calculated from,
T
j
=
P
D _ COND
×
θ
thJA
+
T
AMB
=
222mW
×
32
o
C W
+
80
o
C
=
87
o
C
The internal ambient temperature within the LED converter, T
AMB
, is assumed to be 80ºC.
θ
thJA
=
32
o
C W
is the thermal resistance for DO-201 package. For practical design, it is recommended to
keep the junction temperature below 110ºC to avoid temperature stress on the device.
Issue 1 – January 2011
© Diodes Incorporated 2010
6
www.diodes.com
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