AN2649
Application note
A power factor corrector with MDmesh
TM
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 v oltage drop, breakdown voltage and switching speed. In AC-DC
converters such as PFC circuits, efficiency is strongly related to the switch perf ormances
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 th e
diode current recovery behavior . In recent years the introduction of the Silicon Carbide (SiC)
Schottky diode has led to an effecti v e advantage in the switching transient losses reduction,
thanks to the very low re v erse reco v ery current with respect to the tr aditional fast diode. The
impact on the converter of the impro v ed ch aracteristics of bot h de vices lea ds to an increase
in efficiency.
In this application note the new generation of super-junction MOSFET (MDmesh
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 con v erter
efficiency. The presented experimental results allow analysis of inf ormation for the co nv erter
designers focusing on the determination of benefits and effectiveness of the de vices utiliz ed
in the considered application.
TM
II) and
September 2008 Rev 2 1/20
www.st.com
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
2/20
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
3/20
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 Vin
● 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 f
● Maximum Inductor current rip p le ∆IL = 35% of ILrms
● Worst-condition efficiency (at minimum input voltage) η= 90%
= 100 kHz
SW
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.
= 88 V to 264 V
rms
4/20
AN2649 Power MOSFET
2 Power MOSFET
Since the MOSFET device has to sustain a minimum blocking voltage value of 500 V
(V
= V
DSS
the R
DS(on)
+ VOUT - ripple + V
out
for its relation with the power dissipation.
The device STP12NM50N with its 500 V BV
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:
), then the most important parameter for the se lection is
out
and the R
DSS
DS(on)
(R
DS(on)max
= 0.38 at
Equation 1
P
ON MAX–
● The switching (on + off) losses can be estimated as:
2
I
Qrmsmax
R
on max–
2.15()20.38 1.76 W=⋅=⋅=
Equation 2
where, P
while P
crossover
REC
In general P
P
SWPcrossoverPRECtcrVoutfswIrmsPREC
are the switching losses due to the crossover time of the power MOSFET
is the contribution due to the diode recovery.
depends on the di/dt value of the current MOSFET at turn-on (and this
REC
+⋅⋅⋅=+=
depends on the RG value selected and the intrinsic capacitance of the MOSFET) because
this di/dt sets the value of I
on the boost diode recovery current. To take into account the
RM
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 P
is 1.5 times the P
SW
crossover
value, (see
AN628), but for the SiC diode we can suppose (thanks to superior switching performances)
that the P
value is negligible.
REC
Equation 3
P
SW
15ns 400V 100kHz 2.15A⋅⋅ ⋅()1.3 W==
The capacitive losses at turn-on to be added are:
Equation 4
P
capacitive
10
------
C
3
OSS
1.5
V
out
10
------
f
sw
230pF 400()
3
1.5
100kHz 0.6 W=⋅⋅⋅=⋅⋅ ⋅≈
where C
is the drain capacitance at VDS= 25 V.
oss
To reduce the switching losses at turn-off, a RCD snubber is used and in order to keep the
junction temperature at a saf e le vel at w orst case condition, lo w-line input v oltage (88 V) and
full load (200 W), a small heatsink is used.
5/20
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 so lution
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
⋅+⋅=
16 V
⋅
-------------------------- -=
3 π V
⋅⋅
I
2
lpk
Drms
out
with
Equation 6
P
DonVtoIoutRd
P
out
I
Drms
---------- -
V
lpk
The switching losses are:
Equation 7
P
swVout
Qrr f
⋅⋅=
SW
where
● V
● R
● V
● V
● I
● Qrr= total inverse recovery charge of diode
= threshold voltage
to
= differential resistance
d
= line voltage peak value
lpk
= DC output voltage
out
= RMS value of diode current
Drms
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 f or
every value of di/ dt of curr ent im posed b y 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
P
DonVtoIoutRd
2
I
Drms
0.9V 0.5A 0.065Ω 1.282A20.55 W=⋅+⋅=⋅+⋅=
Equation 9
Another important parameter to take into account for the choice of boost diode is the I
value. At startup the output capacitor sinks much current (it is discharged) and the boost
6/20
P
0W≅
SW
FSM
AN2649 Booster diode
diode must conduct high peak level current. In this application at startup, th e max peak
current in the diode is about 40 A, therefore, a bypass diode must be used, (1 N5406
standard diode low cost), with a high I
value, because the SiC's I
FSM
value guaranteed
FSM
in the datasheet is 30 A.
Figure 1. Current diode ID at startup
The other components have been designed with the criteria already described in other
application notes and their values are given in the schematic (Figure 2).
7/20
Booster diode AN2649
Figure 2. 200 W evaluation board circuit
8/20
AM01041v1
AN2649 Booster diode
In Figure 3 a switching cycle of the MOSFET device is reported, while in Figure 4, 5, 6, 7
and Figure 8, 9, 10 and 11 are showed the turn-on and the turn-off MOS waveform for
several input voltage and in full load condition (400 V/ 0.5 A).
Figure 3. Switching cycle wave forms for MOSFET
In the Table 1 are reported the energy loss at turn-on and turn-off versus Vin.
Table 1. MOSFET energy losses using SiC diode
Vin [Vac] Eon [uJ] Eoff [uJ]
88 14.1 6.3
110 12 6
220 9 6
264 9 5.9
We observe that the value of switching losses in the worst case (Vin=88 Vac) is very close
with the value estimated in the design procedure equal to the sum of (Equation 3) and
(Equation 4):
Equation 10
P
SW
Eon Eoff+()fsw 14.1 6.3+()uJ 100kHz 2.04 W=⋅=⋅=
9/20
Booster diode AN2649
Figure 4. Turn-on switch (with SiC diode) -
Vin = 88 Vac
Figure 6. Turn-on switch (with SiC diode) -
Vin = 220 Vac
Figure 5. Turn-on switch (with SiC diode) -
Vin = 110 Vac
Figure 7. Turn-on switch (with SiC diode) -
Vin = 264 Vac
The di/dt value at turn-on measured in the application, due to the Rg value selected is
450 A/µs.
10/20
AN2649 Booster diode
Figure 8. Turn-off switch (with SiC diode) -
Vin = 88 Vac
Figure 10. Turn-off switch (with SiC diode) -
Vin = 220 Vac
Figure 9. Turn-off switch (with SiC diode) -
Vin = 110 Vac
Figure 11. Turn-off switch (with SiC diode) -
Vin = 264 Vac
For comparison purposes, the same measurements are performed using a fast silicon diode
used in this application (STTA5 0 6D, as described in AN628) as the boost diode instea d of
SiC. Figure 12, 13, 14, 15 and Figure 16, 17, 18, 19 show the turn-on and the turn-off MOS
waveform for several input voltages and in full-load condition (400 V/0.5 A).
11/20
Booster diode AN2649
Figure 12. Turn-on switch (with Si diode) -
Vin = 88 Vac
Figure 14. Turn-on switch (with Si diode) -
Vin = 220 Vac
Figure 13. Turn-on switch (with Si diode) -
Vin = 110 Vac
Figure 15. Turn-on switch (with Si diode) -
Vin = 264 Vac
12/20
AN2649 Booster diode
Figure 16. Turn-off switch (with SiC diode) -
Vin = 88 Vac
Figure 18. Turn-off switch (with SiC diode) -
Vin = 220 Vac
Figure 17. Turn-off switch (with SiC diode) -
Vin = 110 Vac
Figure 19. Turn-off switch (with SiC diode) -
Vin = 264 Vac
Table 2 gives the energy losses at turn-on and turn-off versus Vin.
Table 2. MOSFET energy losses usi ng Si diode
Vin [Vac] Eon [uJ] Eoff [uJ]
88 37.3 4.7
110 26.7 4.6
220 12.82 5.1
264 13.77 5.3
13/20
Booster diode AN2649
Figure 20. Turn-off switch (with Si diode) -
Vin = 88 Vac
Figure 22. Turn-off switch (with Si diode) -
Vin = 220 Vac
Figure 21. Turn-off switch (with Si diode) -
Vin = 110 Vac
Figure 23. Turn- off switch (with Si diode) -
Vin = 264 Vac
The turn-on switching of the MOSFET is strongly influenced by the diode recovery so the
SiC diode leads to a reduction of the turn-on losses of the switch. The turn-on switching
waveforms for the silicon diode case highlight the current peak at turn-on as shown in
Figure 24. In Figure 25 this is evident as the SiC diode allows a strong reduction of the
current peak.
14/20
AN2649 Booster diode
Figure 24. Turn-on switch comparison
(Vin = 88 Vac) - Si diode
The impact of the different device choices in the PFC converter performances has been
investigated at different values of the input voltage. The PFC demonstration board
performance has been evaluated, testing the following parameters: PF (power factor), THD
(percentage of current total harmonic distortion), η
analysis has been conducted. The experimental results ar e summarized in Table 3 and 4,
where it is possible to compare the converter performances of the two cases of study.
Figure 25. Turn-on switch comparison
(Vin = 88 V ac) - SiC diode
efficiency). Furthermore a thermal
Table 3. Experimental measurements results of the PFC converter with SiC diode
MD2 and SiC diode
V
[VAC] V
in
88 391.81 0.506 215.2 3.4 0.999 92.21
110 392.77 0.508 213.5 1.8 1 93.45
220 394.67 0.510 211.5 3 0.998 95.16
264 394.96 0.510 210.4 4.3 0.997 95.73
[V] I
out
[A] Pin [W] THD % PF η [%]
out
Table 4. Experimental measurements results of the PFC converter with fast
silicon diode
MD2 and Si diode
V
[VAC] V
in
88 396.51 0.512 224.5 3.7 0.999 90.42
110 398.26 0513 221.8 1.2 1 92.23
220 401.74 0.516 219.1 2.3 0.998 94.76
264 402.21 0.517 218.5 3.9 0.997 95.31
[V] I
out
[A] Pin [W] THD % PF η [%]
out
Figure 26 compares the efficiency curve versus Vin f or the two cases. At high-line input
voltage the difference of efficiency is smaller because the switching losses in the silicon
15/20
Booster diode AN2649
diode decrease (the current in the boo st d iode decr ea se s) while th e switching losses in SiC
diode are always negligible. Furthermore as the current diode decreases also, the losses
due to the diode in the MOSFET decrease.
Figure 26. Efficiency curve comparison
The difference in terms of efficiency is also evident if we consider the thermal behavior of
power devices. In Figure 27 we can observe the thermal maps for the MOSFET and Sic
diode compared with the same MOSFET and Silicon diode in the worst case in terms of
losses (Vin = 88 Vac). We note that the MOSFET and diode are compared using the same
heatsinks.
Figure 27. Thermal maps comparison -
Si diode
Si diode
=65 °C
T
case
MOSFET
T
=79 °C
case
The difference in terms of temperature is 15 °C for the MOSFET and 20 °C for the diode.
This is an important result for reliability as well as in terms of efficiency of the system.
These results allow using a smaller heatsink, saving cost and space.
Figure 28. Thermal maps comparison -
SiC diode
SiC diode
=45 °C
T
case
MOSFET
=65 °C
T
case
16/20
AN2649 Conclusion
4 Conclusion
In this application note an experimental investigation of the advantages and drawbacks
related to the use of new devices of the last generation has been carried out in a
continuous-current-mode PFC converter. In particular, the latest MOSFET MDmesh
and a SiC Schottky diode have been used. The experimental results show that the power
converter using the new devices gets better switching performances and increased
efficiency with respect to the case that uses the same MOSFET and an ultrafast silicon
diode. The better performance in terms of efficiency and thermal behavior allow using
smaller heatsinks, saving cost and space . Th e no-r everse recovery for the SiC diode allows
using a lower gate resistance (high di/dt) optimizing the MOSFET power losses without
introducing high lev el EMI. In this case we ha ve a high value of di/dt (the recovery current of
the SiC diode is smaller for every value of di/dt) , but the s witch ing losses are re duced in the
MOSFET ( turn-on is faster with Rg = 0 Ω) with respect to the case of the silicon diode
where large values of I
(dependent on di/dt) and the EMI problem limits the choice of Rg.
RM
TM
II
17/20
Bibliography AN2649
5 Bibliography
1. SiC Diodes and MDmesh™ 2nd generation devices improve efficiency in PFC
Applications; CIPS 2006 conference proceedings , pag.195-199
2. Application note 628: designing a high po wer factor switching preregulator with the
L4981 continuous mode.
3. Application note: Turboswitch
TM
in a PFC boost converter.
18/20
AN2649 Revision history
6 Revision history
Table 5. Document revision history
Date Revision Changes
10-Mar-2008 1 Initial release
12-Sep-2008 2
– STPS8600SIC replaced by STPSC806D
– Modified: Figure 2
19/20
AN2649
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