AN3276

18/22

Doc ID 17975 Rev 1

3.4

Thermal measurement

The soft current switching method allows the power dissipation in the switch transistor to be

reduced. Figure 18 shows that a high thermal temperature difference (18 °C) occurs

temperature in the power transistor, the size of the thermal heatsink could be reduced. In

this way, the space saving balances out the space from the small L inductor required by the

thermal rising. Figure 18 shows that 0.75 W should be removed from the 5.4 W measured in

efficiency criteria than the SiC diode.

Another consideration is that, for the same thermal heatsink, the Power MOSFET size could

be reduced to further reduce the PFC cost.

Figure 18.

Thermal measurement comparison

STP20NM50

STP20NM50

STTH8BC065DI

TR

DB

STTH8BC065 diode in the BC² circuit

Power MOS thermal junction measurement

BC² circuit

Tj(avg)= 72 °C

Rdson STP20NM50

Rdson= 0.26

Ω

TR Cond. losses

(0.26/2)x4.28² =2.38W

Power MOS thermal junction measurement

SiC diode

Tj(avg)= 90 °C

Rdson STP20NM50

Rdson= 0.34

Ω

TR Cond. losses

(0.34/2)x4.28² = 3.12W

+0.75W

STP20NM50

STP20NM50

STPSC806D

TR

DB

STPSC806D diode in the Boost circuit

PFC : V

= 90 V

mains

Fs = 140 kHz , P

= 450 W ,

OUT

I

= 4.28 A , Rg = 47

TRRMS

1

Ω