AN3276 STMICROELECTRONICS | Alldatasheet
Document overview
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Technical content
Datasheet sections
- 1 Existing solutions
- 1.1 Diode switch-on losses
- 1.2 Soft switch-on method
- 1.3 Active recovery circuit
- 1.4 Passive recovery circuit
- 2 The new ST solution - BC 2: energy recovery circuit
- 2.1 Concept description
- 2.2 Phase timing description
- 2.2.1 Phase before t0
- 2.2.2 Phase t0 to t1
- 2.2.3 Phase t1 to t2
- 2.2.4 Phase t2 to t3
- 2.2.5 Phase t3 to t4
- 2.2.6 Phase t
- 2.3 Electrical voltage stress in BC
- 2.4 Calculation of m2 and m1 ratios
- 2.5 Calculation of L
- 2.6 Range of products
- 3.1 BC 2 design
- 3.2 BC 2 typical waveforms
- 3.3 Efficiency comparison
- 3.4 Thermal measurement
- 4 Conclusion
- 5 References
- 6 Revision history
November 2010 Doc ID 17975 Rev 1 1/22 AN3276 Application note ST solution for efficiency improvement in PFC applications, back current circuit (BC2) Introduction The challenges for modern high efficiency switching power supplies are to minimize power losses and increase their power density without raising the cost. The goal is to reduce both power conduction and power switching losses. Minimization of power conduction losses is difficult to achieve without considerably affecting the cost and power density, since more material is required (bigger active and passive components). Unlike the conduction losses, it is easier to reduce the power switching losses without significantly increasing the power supply cost. There are two main ways to achieve this improvement: ■ working on the dynamic behavior of the semiconductor technologies ■ working on circuit topologies Novel diodes using technologies such as SiC and GaN materials significantly reduce the switching losses. However, their high price makes them not so attractive for applications such as desktop server power supplies, solar inverters and µinverters. The patented circuit [see Section 5: References, 1.], described in this Application note is based on the soft switching method and meets market expectations since its efficiency/cost/power, and density/EMI trade-offs are better than high voltage SiC Schottky diodes. www.st.com
1 Existing solutions
This section describes some existing areas for efficiency improvements in PFC applications.
1.1 Diode switch-on losses
PN diode as shown in Figure 1. Figure 1. Switch-on losses in PN diode behavior turn-on by improving component performance only.
Figure 2. Switch-on losses in SiC or GaN diode behavior
1.2 Soft switch-on method
Figure 3. Switch-on losses in current soft switching behavior
- Reset the current in the inductor L at each switching period, whatever the variations of the current, and input and output voltages.
- Recover the saved inductive energy without losses.
- Limit any overvoltage and overcurrent stress in the semiconductor devices.
- Keep cost down when adding any device.
- Maintain a similar power supply density. There are many circuits that are classified in two families of recovery circuits:
- active
- passive
1.3 Active recovery circuit
switch-on and switch-off power losses to be removed. Figure 4. Zero voltage transition (ZVT) active recovery circuit electrical stress across the semiconductors. their higher price makes these circuits less than ideal for mass market applications. Therefore, the passive recovery circuit can be more attractive.
1.4 Passive recovery circuit
Figure 5. Passive recovery circuit on both boost diode recovery current and the external electrical conditions. the application without affecting the five criteria listed in Section 1.2.
2 The new ST solution - BC 2: energy recovery circuit
designed to reset the energy stored in the small inductor L. Figure 6. Novel energy recovery circuit: BC 2
2.1 Concept description
the AC line voltage is low, corresponding to the maximum value of the inductor L current.
2.2 Phase timing description
Figure 7. Equivalent timing per phase
- +-= 1 2 1 +m mVV mainOUT ·- )m1( mV mains -·- )m1( mVV 2mainsOUT OUTmains mm1 )VV( ·+ )m1( )m1(I RM +·+- mainsOUT mmVV L)m1( VmV dt dI mains2OUTDB
- +-= 2112
- ·2 m mmmmI RM · V0UT I1-IRM (IRM +I0)(1+m2) tD1 tD2 ITR VTRVTR VLVL VDBVDB ID2 ID1 IDB Imains t t t t t t t t5t4t3t2t1t0
2.2.1 Phase before t 0
Figure 8. Equivalent circuit before t 0
2.2.2 Phase t 0 to t1
Figure 9. Equivalent circuit t 0 to t1 whereas the current flowing in DB decreases linearly down to -IRM. Figure 7 shows the behavior of these currents taking account of the m2 transformer ratio.
2 reduces switch-off losses too.
2.2.3 Phase t 1 to t2
Figure 10. Equivalent circuit t 1 to t2 At t1+, the boost diode DB turns off, and an overcurrent IRM is stored in the small inductor.
diode with an accurate trade-off between the IRM current value and its breakdown voltage.
2.2.4 Phase t 2 to t3
Figure 11. Equivalent circuit t 2 to t3 inductor increases up to I1 at time t3.
2.2.5 Phase t 3 to t4
Figure 12. Equivalent circuit t 3 to t4
2.2.6 Phase t 4 to t5
Figure 13. Equivalent circuit t 4 to t5 in the bulk capacitor and power saving occurs as the transistor turns on at t0.
2.3 Electrical voltage stress in BC 2
Table 1 summarizes the maximum voltage across each semiconductor versus the phases. The BC2 circuit needs to use a specific diode with a breakdown voltage higher than 600 V.
2.4 Calculation of m 2 and m1 ratios
m2 transformer ratio conditions. turn-on dI/dt at its maximum operating junction condition. Table 1. Maximum reverse voltage in the BC 2
2.5 Calculation of L
increase as well. Thus, m1 and m2 should be recalculated to get VRDB_reverse below 487 V.
2.6 Range of products
- STTH8BC065DI, STTH8BC060D, STTH5BCF060 for applications from 800 W to 2 kW
- STTH16BC065CT, STTH5BCF060 for applications from 400 W to 1 kW
- STTH10BC065CT + STTH3BCF060U for applications from 280 W to 600 W
Table 2. Inductor and size versus the PFC types
3 BC 2 design in 450 W PFC
have been compared with 8 A SiC Schottky diodes.
3.1 BC 2 design
provides the L inductance, m1 and m2 versus the switching frequency as given in Table 3.
3.2 BC 2 typical waveforms
Figure 14 shows the typical BC2 waveforms corresponding to a PFC working at 200 kHz. phases and switch-off power losses are saved. Figure 14. Typical BC 2 waveform at Fs = 200 kHz Table 3. NS1, NS2 and L versus Fs
3.3 Efficiency comparison
the BC2 have the same efficiency as shown in Figure 15. Figure 15. Efficiency comparison at 230 V rms
3.4 Thermal measurement
efficiency criteria than the SiC diode. be reduced to further reduce the PFC cost. Figure 18. Thermal measurement comparison
4 Conclusion
improve the CCM PFC performance as shown in Table 4. Table 4. BC 2 benefits in 450 W PFC, 140 kHz
5 References
- Benoît Peron, “Auxiliary switching circuit for a chopping converter”, Patent No: US 6,987,379 B2, June 2006 2. Bertrand Rivet, “New Solution to Optimize Diode Recovery in PFC Boost Converter”, PCIM 2000. 3. Jim Noon, UC3855A/B High Performance Power Factor Preregulator -Texas Instrument- application report- SLUA146A 4. Brian T, Irving and M. Jovanovic “Analysis, Design and Performance Evaluation of Flying-Capacitor Passive Lossless Snubber applied to PFC Boost Converter”, APEC 2002, pp. 503 - 508 vol.1.
6 Revision history
Table 5. Document revision history 10-Nov-2010 1 Initial release.