AN2649 datasheet(5/20 Pages) STMICROELECTRONICS

5 / 20 page

AN2649

Power MOSFET

5/20

Power MOSFET

Since the MOSFET device has to sustain a minimum blocking voltage value of 500 V

(VDSS = Vout + VOUT - ripple + Vout), then the most important parameter for the selection is

the RDS(on) for its relation with the power dissipation.

The device STP12NM50N with its 500 V BVDSS and the RDS(on) (RDS(on)max = 0.38 at

T= 25 °C), is the best choice for the application. The losses at turn-on depend on the

selected boost diode and on the choice of the RG chosen to reduce the di/dt and therefore

the levels of EMI of the converter. As described in AN628 (please refer to 2) a gate

resistance of 15

Ω has been selected for turn-on, while a diode is used for a fast turn-off.

●

The maximum "on state" power dissipation evaluated at the minimum input mains

voltage is:

Equation 1

●

The switching (on + off) losses can be estimated as:

Equation 2

where, Pcrossover are the switching losses due to the crossover time of the power MOSFET

while PREC is the contribution due to the diode recovery.

In general PREC depends on the di/dt value of the current MOSFET at turn-on (and this

depends on the RG value selected and the intrinsic capacitance of the MOSFET) because

this di/dt sets the value of IRM on the boost diode recovery current. To take into account the

boost diode recovery effect, for the silicon diode, an easy approach is to compute two times

the current value (at turn-on). This means that PSW is 1.5 times the Pcrossover value, (see

AN628), but for the SiC diode we can suppose (thanks to superior switching performances)

that the PREC value is negligible.

Equation 3

The capacitive losses at turn-on to be added are:

Equation 4

where Coss is the drain capacitance at VDS= 25 V.

To reduce the switching losses at turn-off, a RCD snubber is used and in order to keep the

junction temperature at a safe level at worst case condition, low-line input voltage (88 V) and

full load (200 W), a small heatsink is used.

MAX

–

Qrmsmax

max

–

2.15

()

0.38

1.76 W

⋅

crossover

REC

out

rms

REC

⋅⋅

⋅

15ns 400V 100kHz 2.15A

⋅⋅

⋅

() 1.3 W

capacitive

------ C

OSS

1.5

out

------ 230pF

400

()

1.5

100kHz

0.6 W

⋅⋅

⋅

⋅⋅

⋅

≈

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