AN2782 datasheet(18/39 Pages) STMICROELECTRONICS

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Designing a fixed off-time PFC

AN2782

18/39

Doc ID 14763 Rev 2

Figure 10.

Conduction losses and total losses in the two STP12NM50 MOSFETs for

the 400 W FOT PFC

Figure 10

shows the trend of the total losses (48) 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 +

ΔOVP) and a current rating higher

than 3·Iout (21) 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 has to 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 (21) and RMS (30) current values and the parameter Vth (rectifier

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

rectifier losses.

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

MOSFETS total losses

115

140

165

190

215

240

265

Vac (V)

Pcond(Vi)

Plosses(Vi)

(50)

rms

out

diode

⋅

(

)

diode

⋅

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