AN75 Datasheet, PDF (6/12 Pages) Diodes Incorporated – High Power Factor LED Replacement T8 Fluorescent Tube

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).
I_D(avg)
=
I_LED(nom)
×
⎜⎜⎝⎛^1−
V_LED(min)
V_IN(max)
⎟⎞
⎟⎠
=
240mA
×
⎜⎛1−
⎝
42V
373V
⎟⎞
⎠
=
202mA
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
6
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