AN3142 datasheet(19/44 Pages) STMICROELECTRONICS

19 / 44 page

AN3142

Designing a fixed-off-time PFC

Doc ID 17005 Rev 3

19/44

Figure 10.

Conduction losses and total losses in the STP12NM50FP MOSFET

couples for the 400W FOT PFC

Figure 10 shows the trend of the total losses (Equation 35) on the line voltage for the two

selected STP12NM50 MOSFETs.

4.3.6

Boost diode selection

Following a similar criterion as that for the MOSFET, the output rectifier can also be

selected. A minimum breakdown voltage of 1.2·Vout

(4) and a current rating higher than

3·Iout (

Equation 8) can be chosen for a rough initial selection of the rectifier. The correct

choice is then confirmed by the thermal calculation. If the diode junction temperature works

within 125°C the device has been selected correctly, otherwise a bigger device must be

selected. The switching losses can be significantly reduced if an ultra-fast diode is

employed. Since this circuit operates in the continuous current mode, the MOSFET must

recover the boost diode minority carrier charge at turn-on. Thus, a diode with a small

reverse recovery time (trr) must be used.

In this 400 W application an STTH8R06, (600 V, 8 A) has been selected. The STTH8R06

offers the best solution for the continuous current mode operation due to its very fast reverse

recovery time, 25 ns typical. This part has a breakdown voltage rating (Vrrm) of 600 V,

average forward current rating (Ifave) of 8 A and reverse recovery time (trr) of 25 ns. The

rectifier AVG (

Equation 8) and RMS (Equation 17) current values and the parameter V

(rectifier threshold voltage) and Rd (dynamic resistance) given in the datasheet allow the

calculation of the rectifier losses.

From the STTH8R06 datasheet, Vth is 1.16 V, Rd is 0.08, neglecting the recovery losses:

Equation 37

From

(13) and Equation 37 the maximum thermal resistance to keep the junction

temperature below 125°C is then:

Equation 38

MOSFETS total losses

110

135

160

185

210

235

260

285

Vi n_ac [Vr ms]

Plosses(Vi)

Range Limi ts

rms

out

diode

⋅

()

diode

⋅

Ω

⋅

diode

ambx

125

−

125

−

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