STMicroelectronics AN2649 The electrical and thermal performances of switching converters are strongly Datasheet

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
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
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AN2649
List of figures
List of figures
Figure 1.
Figure 2.
Figure 3.
Figure 4.
Figure 5.
Figure 6.
Figure 7.
Figure 8.
Figure 9.
Figure 10.
Figure 11.
Figure 12.
Figure 13.
Figure 14.
Figure 15.
Figure 16.
Figure 17.
Figure 18.
Figure 19.
Figure 20.
Figure 21.
Figure 22.
Figure 23.
Figure 24.
Figure 25.
Figure 26.
Figure 27.
Figure 28.
Current diode ID at startup . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
200 W evaluation board circuit . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8
Switching cycle waveforms for MOSFET . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9
Turn-on switch (with SiC diode) - Vin = 88 Vac . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
Turn-on switch (with SiC diode) - Vin = 110 Vac . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
Turn-on switch (with SiC diode) - Vin = 220 Vac . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
Turn-on switch (with SiC diode) - Vin = 264 Vac . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
Turn-off switch (with SiC diode) - Vin = 88 Vac . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11
Turn-off switch (with SiC diode) - Vin = 110 Vac . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11
Turn-off switch (with SiC diode) - Vin = 220 Vac . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11
Turn-off switch (with SiC diode) - Vin = 264 Vac . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11
Turn-on switch (with Si diode) - Vin = 88 Vac. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12
Turn-on switch (with Si diode) - Vin = 110 Vac. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12
Turn-on switch (with Si diode) - Vin = 220 Vac. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12
Turn-on switch (with Si diode) - Vin = 264 Vac . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12
Turn-off switch (with SiC diode) - Vin = 88 Vac . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13
Turn-off switch (with SiC diode) - Vin = 110 Vac . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13
Turn-off switch (with SiC diode) - Vin = 220 Vac . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13
Turn-off switch (with SiC diode) - Vin = 264 Vac . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13
Turn-off switch (with Si diode) - Vin = 88 Vac. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14
Turn-off switch (with Si diode) - Vin = 110 Vac. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14
Turn-off switch (with Si diode) - Vin = 220 Vac. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14
Turn-off switch (with Si diode) - Vin = 264 Vac. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14
Turn-on switch comparison (Vin = 88 Vac) - Si diode. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15
Turn-on switch comparison (Vin = 88 Vac) - SiC diode . . . . . . . . . . . . . . . . . . . . . . . . . . . 15
Efficiency curve comparison . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16
Thermal maps comparison - Si diode . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16
Thermal maps comparison - SiC diode . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16
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Design consideration
1
AN2649
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
2
Power MOSFET
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
P ON – MAX = I
●
2
Qrmsmax
2
⋅ R on – max = ( 2.15 ) ⋅ 0.38 = 1.76 W
The switching (on + off) losses can be estimated as:
Equation 2
P SW = P crossover + P REC = t cr ⋅ V out ⋅ f sw ⋅ I rms + P REC
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
P SW = ( 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
P capacitive ≈ ------ ⋅ C OSS ⋅ V out ⋅ f sw = ------ ⋅ 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.
5/20
Booster diode
3
AN2649
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
P Don = V to ⋅ I out + R d ⋅ I
2
Drms
with
Equation 6
P out 16 ⋅ V lpk
I Drms = ----------- -------------------------V lpk 3 ⋅ π ⋅ V out
The switching losses are:
Equation 7
P sw = V out ⋅ Qrr ⋅ f SW
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
P Don = V to ⋅ I out + R d ⋅ I
2
Drms =
2 2
0.9V ⋅ 0.5A + 0.065Ω ⋅ 1.28 A = 0.55 W
Equation 9
P SW ≅ 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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AN2649
Booster diode
diode must conduct high peak level current. In this application at startup, the max peak
current in the diode is about 40 A, therefore, a bypass diode must be used, (1N5406
standard diode low cost), with a high IFSM value, because the SiC's IFSM value guaranteed
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
Figure 2.
AN2649
200 W evaluation board circuit
AM01041v1
8/20
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 waveforms 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 5.
Turn-on switch (with SiC diode) Vin = 110 Vac
Figure 6.
Turn-on switch (with SiC diode) Vin = 220 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
Figure 8.
Booster diode
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 (STTA506D, as described in AN628) as the boost diode instead 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).
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Booster diode
AN2649
Figure 12. Turn-on switch (with Si diode) Vin = 88 Vac
Figure 13. Turn-on switch (with Si diode) Vin = 110 Vac
Figure 14. Turn-on switch (with Si diode) Vin = 220 Vac
Figure 15. Turn-on switch (with Si diode) Vin = 264 Vac
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AN2649
Booster diode
Figure 16. Turn-off switch (with SiC diode) Vin = 88 Vac
Figure 17. Turn-off switch (with SiC diode) Vin = 110 Vac
Figure 18. Turn-off switch (with SiC diode) Vin = 220 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 using 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 21. Turn-off switch (with Si diode) Vin = 110 Vac
Figure 22. Turn-off switch (with Si diode) Vin = 220 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
Figure 25. Turn-on switch comparison
(Vin = 88 Vac) - SiC 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 efficiency). Furthermore a thermal
analysis has been conducted. The experimental results are summarized in Table 3 and 4,
where it is possible to compare the converter performances of the two cases of study.
Table 3.
Experimental measurements results of the PFC converter with SiC diode
MD2 and SiC diode
Vin [VAC]
Vout [V]
Iout [A]
Pin [W]
THD %
PF
η [%]
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
Table 4.
Experimental measurements results of the PFC converter with fast
silicon diode
MD2 and Si diode
Vin [VAC]
Vout [V]
Iout [A]
Pin [W]
THD %
PF
η [%]
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
Figure 26 compares the efficiency curve versus Vin for 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 boost diode decreases) while the 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
Tcase=65 °C
MOSFET
Tcase=79 °C
Figure 28. Thermal maps comparison SiC diode
SiC diode
Tcase=45 °C
MOSFET
Tcase=65 °C
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.
16/20
AN2649
4
Conclusion
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 MDmeshTM II
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. The no-reverse recovery for the SiC diode allows
using a lower gate resistance (high di/dt) optimizing the MOSFET power losses without
introducing high level EMI. In this case we have a high value of di/dt (the recovery current of
the SiC diode is smaller for every value of di/dt) , but the switching losses are reduced in the
MOSFET ( turn-on is faster with Rg = 0 Ω) with respect to the case of the silicon diode
where large values of IRM (dependent on di/dt) and the EMI problem limits the choice of Rg.
17/20
Bibliography
5
18/20
AN2649
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 power factor switching preregulator with the
L4981 continuous mode.
3.
Application note: TurboswitchTM in a PFC boost converter.
AN2649
6
Revision history
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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