ST AN2649 APPLICATION NOTE

AN2649

Application note

A power factor corrector with MDmeshTM II and SiC diode

Introduction

The electrical and thermal performances of switching converters are strongly influenced by the behavior of the switching devices. Modern power devices design requires a trade-off in terms of forward voltage drop, breakdown voltage and switching speed. In AC-DC converters such as PFC circuits, efficiency is strongly related to the switch performances and the diode recovery behavior (please refer to 1 in Bibliography on page 18). In the past the benefits of the improved MOSFET performances have been generally spoiled by the diode current recovery behavior. In recent years the introduction of the Silicon Carbide (SiC) Schottky diode has led to an effective advantage in the switching transient losses reduction, thanks to the very low reverse recovery current with respect to the traditional fast diode. The impact on the converter of the improved characteristics of both devices leads to an increase in efficiency.

In this application note the new generation of super-junction MOSFET (MDmeshTM II) and SiC diodes has been used to design a 200 W continuous PFC converter. The dynamic characteristics of both super-junction MOSFET and SiC diodes, are investigated in the actual application and compared with the traditional components in order to carry out the qualitative and quantitative improvements in terms of switching performances and converter efficiency. The presented experimental results allow analysis of information for the converter designers focusing on the determination of benefits and effectiveness of the devices utilized in the considered application.

September 2008

Rev 2

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Contents	AN2649

Contents

1	Design consideration . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	. 4
2	Power MOSFET . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	5
3	Booster diode . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	6
4	Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	17
5	Bibliography . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	18
6	Revision history . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	19

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AN2649	List of figures

List of figures

Figure 1. Current diode ID at startup . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7 Figure 2. 200 W evaluation board circuit . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8 Figure 3. Switching cycle waveforms for MOSFET . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9 Figure 4. Turn-on switch (with SiC diode) - Vin = 88 Vac . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10 Figure 5. Turn-on switch (with SiC diode) - Vin = 110 Vac . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10 Figure 6. Turn-on switch (with SiC diode) - Vin = 220 Vac . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10 Figure 7. Turn-on switch (with SiC diode) - Vin = 264 Vac . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10 Figure 8. Turn-off switch (with SiC diode) - Vin = 88 Vac . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11 Figure 9. Turn-off switch (with SiC diode) - Vin = 110 Vac . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11 Figure 10. Turn-off switch (with SiC diode) - Vin = 220 Vac . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11 Figure 11. Turn-off switch (with SiC diode) - Vin = 264 Vac . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11 Figure 12. Turn-on switch (with Si diode) - Vin = 88 Vac. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12 Figure 13. Turn-on switch (with Si diode) - Vin = 110 Vac. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12 Figure 14. Turn-on switch (with Si diode) - Vin = 220 Vac. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12 Figure 15. Turn-on switch (with Si diode) - Vin = 264 Vac . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12 Figure 16. Turn-off switch (with SiC diode) - Vin = 88 Vac . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13 Figure 17. Turn-off switch (with SiC diode) - Vin = 110 Vac . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13 Figure 18. Turn-off switch (with SiC diode) - Vin = 220 Vac . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13 Figure 19. Turn-off switch (with SiC diode) - Vin = 264 Vac . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13 Figure 20. Turn-off switch (with Si diode) - Vin = 88 Vac. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14 Figure 21. Turn-off switch (with Si diode) - Vin = 110 Vac. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14 Figure 22. Turn-off switch (with Si diode) - Vin = 220 Vac. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14 Figure 23. Turn-off switch (with Si diode) - Vin = 264 Vac. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14 Figure 24. Turn-on switch comparison (Vin = 88 Vac) - Si diode. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15 Figure 25. Turn-on switch comparison (Vin = 88 Vac) - SiC diode . . . . . . . . . . . . . . . . . . . . . . . . . . . 15 Figure 26. Efficiency curve comparison . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16 Figure 27. Thermal maps comparison - Si diode . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16 Figure 28. Thermal maps comparison - SiC diode . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16

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Design consideration	AN2649

1 Design consideration

The following PFC design example is referred to as an experimental board, used for demonstration purposes as described in AN628 (please refer to 2 in Bibliography on page 18). The design target specifications are:

●UNIVERSAL AC input supply voltage Vinrms = 88 V to 264 V

●DC output regulated voltage VO = 400 V

●Rated output power PO = 200 W

● Full-load output ripple ∆Vout-ripple = ± 8 V

●Maximum overvoltage value ∆Vout = 50 V

●Switching frequency fSW = 100 kHz

●Maximum Inductor current ripple ∆IL = 35% of ILrms

● Worst-condition efficiency (at minimum input voltage) η= 90%

The guidelines for controller design (L4981A) and power component selection can be found in AN628 (please refer to 2 in Bibliography on page 18). In the next section instead we will discuss the choice of the power MOSFET and boost diode.

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AN2649	Power MOSFET

2 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

PON – MAX = I2Qrmsmax Ron – max= (2.15)2 0.38= 1.76 W

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

Equation 2

PSW = Pcrossover + PREC= tcr Vout fsw Irms + PREC

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

PSW = (15ns 400V 100kHz 2.15A)= 1.3 W

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

Equation 4

≈ 10 1.5 10 ( )1.5

Pcapacitive ------ COSS V out fsw= ------ 230pF 400 100kHz= 0.6 W 3 3

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.

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Booster diode	AN2649

3 Booster diode

The booster diode is selected to withstand the output voltage and current. Moreover, it has to be as fast as possible in order to reduce the power switch losses (please refer to 3 in Bibliography on page 18). The STPSC806D (600 V/8 A) SiC diode matches these specifications and is especially suitable for this application. This part offers the best solution for the continuous current mode operation due to its very fast recovery time, 15 ns typical. The diode power losses can be split in two contributions: conduction losses and switching losses.

The conduction losses can be estimated by:

Equation 5

PDon = Vto Iout + Rd I2Drms

with

Equation 6

IDrms =	Pout	16 Vlpk
IDrms =	----------	--------------------------
	Vlpk	3 π Vout

The switching losses are:

Equation 7

Psw = Vout Qrr fSW

where

●Vto = threshold voltage

●Rd = differential resistance

●Vlpk = line voltage peak value

●Vout = DC output voltage

●IDrms= RMS value of diode current

●Qrr= total inverse recovery charge of diode

At low-line input voltage the conduction losses are bigger with respect to the case of highline voltage while the switching losses are always negligible due to the small value of Qrr for every value of di/dt of current imposed by the MOSFET (at turn-on). The last instance is not true for the silicon diode, because Qrr is bigger and greatly depends on the di/dt value.

Furthermore the silicon diode performance are temperature-dependent (Vf, recovery current, etc.), while the SiC diode has the same behavior also for high temperature (please refer to 1 in Bibliography on page 18). In the worst case:

Equation 8

PDon = Vto Iout + Rd I2Drms= 0.9V 0.5A + 0.065Ω 1.282A2= 0.55 W

Equation 9

PSW 0 W

Another important parameter to take into account for the choice of boost diode is the IFSM value. At startup the output capacitor sinks much current (it is discharged) and the boost

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