AN3303 Application note Secondary-side rectification for an LLC resonant converter featuring the SRK2000 Introduction The EVLSRK2000 is a family of demonstration boards designed for the evaluation of the SRK2000 in LLC resonant converters with synchronous rectification (SR). The first part of this application note is a brief description of the IC features while the second is dedicated to the board description. Finally, some considerations regarding circuit optimization and performance are given.
Contents AN3303 Contents 1 SRK2000 main characteristics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5 1.1 Drain MOSFET sensing and driving logic . . . . . . . . . . . . . . . . . . . . . . . . . 5 1.2 Drain sensing optimization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7 1.3 Blanking time . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9 1.4 Light load operation and sleep mode . . . . . . . . . . .
AN3303 List of tables List of tables Table 1. Table 2. Table 3. Table 4. Table 5. Table 6. Demonstration board ordering codes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 EVLSRK2000-L-40 bill of materials . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22 EVLSRK2000-L-60 bill of materials . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22 EVLSRK2000-S-40 bill of materials . . . . . . . . . . .
List of figures AN3303 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. 4/27 EVLSRK2000: smart driving control for an LLC resonant converter. . . . . . . . . . . . . . . . . . . 1 Block diagram of an LLC converter with synchronous rectification. . . . . . . . . . . . . . . . . . . .
AN3303 1 SRK2000 main characteristics SRK2000 main characteristics The main features of the SRK2000 are described below. The values of the following parameters are reported in the SRK2000 datasheet (see Reference1). The SRK2000 implements a control scheme specific for secondary-side synchronous rectification in an LLC resonant converter that uses a transformer with center-tap secondary winding for full-wave rectification.
SRK2000 main characteristics AN3303 where IDVS1,2.on is the current sourced out of the DVS1,2 pins (50 µA typ.) and VDVS1,2_TH is the lower clamp voltage of the DVS1,2 pins (-0.2 V typ.). This may enable the ON threshold to be set according to the SR power MOSFET body diode VF chosen for the application or the external diode connected in parallel to the power MOSFET drain-source (e.g. Schottky rectifier).
AN3303 SRK2000 main characteristics Figure 3. Power MOSFET drain voltage sensing and typical waveforms A 65. '96 5* 'UDLQ VRXUFH YROWDJH 9'96 B37 5' 65 C ,VU 7R ;IRUPHU ,65 B 9'96 B2Q 9'96 B2II *' *DWH 'ULYLQJ 73'B2Q !- V Power losses are certainly much higher during phases a) and c), when the secondary current flows through the SR power MOSFET body diode, than during phase b), when the current flows through the power MOSFET channel.
SRK2000 main characteristics AN3303 capacitance, but it may be necessary to add an external capacitance; in particular when using power MOSFET packages with a high associated stray inductance, such as the TO220. In several cases it can be advantageous to over-compensate with the external capacitor, therefore introducing an additional delay to the power MOSFET turn-off.
AN3303 SRK2000 main characteristics Figure 6. Effect of parasitic elements on power MOSFET turn-on $FWXDO 5 %4 PO YROWDJH ,VU '96 SLQ YROWDJH 9'96 B2II *DWH 'ULYLQJ !- V 1.3 Blanking time One peculiarity of resonant converters and in particular of LLCs which differentiate them from hard switching topologies, like flyback or forward, is that secondary currents have a sinusoidal shape.
SRK2000 main characteristics AN3303 peak current and, consequently, the turn-off point may jump from one half-cycle to the next between the 50 % limit imposed by the blanking time and the desired turn-off point. The result is a sort of sub-harmonic oscillation in the duty cycle of the SR power MOSFET. This has no significant effect as long as the oscillation amplitude is limited but could cause instability in the case of wider oscillation.
AN3303 SRK2000 main characteristics Figure 9. Conduction time sensing during normal operation 'UDLQ 6RXUFH YROWDJH 65 'UDLQ 6RXUFH YROWDJH 65 ,VU ,VU 9'96 B37 9'96 B2II *' *' &/. +DOI &\FOH 7&21'8&7,21 !- V Once in sleep mode, gate driving is re-enabled when body diode conduction time of both MOSFETs exceeds 60 % of the half-cycle. Figure 10 shows details of time measuring for sleep mode exiting. Figure 10.
SRK2000 main characteristics AN3303 time check is re-enabled. The number of ignored resonant converter switching cycles is 128 after entering sleep mode and 256 after exiting sleep mode. Figure 11 shows some examples of operation mode transition. F\FOHV LJQRUHG F\FOHV F\FOHV ! LJQRUHG F\FOHV F\FOHV LJQRUHG F\FOHV F\FOHV ! LJQRUHG F\FOHV F\FOHV Figure 11.
AN3303 2 Electrical diagram description Electrical diagram description The board schematic is shown in Figure 12. Components were dimensioned supposing an implementation of the SR on a 12 V output converter and using the converter output as supply bus for the SRK2000. If the board is used with a different supply voltage, some components should be modified accordingly.
Sensing optimization by waveform check 3 AN3303 Sensing optimization by waveform check The board has been tested on a 150 W 12 V LLC converter (see Reference 2). The following assertions refer to EVLSRK2000-L-40 but similar considerations can be made for all the configurations of EVLSRK2000-x-xx. Note that current and voltage probes can affect the IC sensing leading to malfunctioning. Signal probing must be accomplished carefully and with minimal modification with respect to the original circuit.
AN3303 Sensing optimization by waveform check performed by adding a capacitor on each DVS pin, as indicated before in Section 1.2. In Figure 15 and 16, IC behavior corresponding to different values of RC sensing circuit is shown. Figure 15. SR MOSFET turn-off CH3: SR FET current Figure 16.
Sensing optimization by waveform check 3.2 AN3303 MOSFET turn-on delay compensation As discussed in Section 1.2, the RC circuit added to fine tune the turn-off timing has the side effect of also delaying the turn-on. It was stated that to avoid this effect, a bypass diode plus a series resistor can be mounted in parallel to the sensing resistor. Figure 18 and 19 below show the turn-on delay improvement using this solution. Figure 18. SR MOSFET turn-on CH3: SR FET current 3.3 Figure 19.
AN3303 4 How to implement the board in the converter How to implement the board in the converter The demonstration board is intended to implement synchronous rectification in an LLC resonant converter with center-tap secondary winding. If the converter implements diode rectification, rectifiers must be removed and the board must be connected as indicated in Figure 21. Connect the transformer center-tap to the converter output.
Power losses and thermal design 5 AN3303 Power losses and thermal design SR dramatically reduces output rectification power losses enabling the design of more efficient power supplies and, even more significant, with a considerable reduction of converter secondary side size. To get a better idea of the improvement obtained by implementing SR with the SRK2000, power loss calculation in a 12 V-150 W application (see Reference 2) is illustrated below. 5.
AN3303 Power losses and thermal design In addition, the power consumption of the SRK2000 must be taken into account: for a rough estimate, consider the IC quiescent current indicated in the SRK2000 datasheet (Iq) and the energy required for SR MOSFET driving (EZVS).
Power losses and thermal design AN3303 Considering now the case with SR: again the thermal rise is 65 °C. Based on the power dissipation per MOSFET calculated in Equation 8, the maximum junction-to-ambient thermal resistance allowed is: Equation 14 65°C R th ( j – amb ) = --------------- = 245°C ⁄ W P MOS That means that a heatsink is not required, just some copper area is needed, calculated according to the SRK2000 datasheet indication.
AN3303 6 Layout considerations Layout considerations The IC is designed with two ground pins, SGND and PGND. SGND is used as the ground reference for all the internal high precision analog blocks. PGND, on the other hand, is the ground reference for all the digital blocks, as well as the current return for the gate drivers.
Bill of materials AN3303 7 Bill of materials Table 2. EVLSRK2000-L-40 bill of materials Ref Value/PN Description Supplier Case C501 4.
AN3303 Table 3.
Bill of materials Table 5. AN3303 EVLSRK2000-D-40 bill of materials Ref Value/PN Description Supplier Case C501 4.
AN3303 8 References References 1. SRK2000 datasheet 2. AN3233 application note 3. STPS20L45C datasheet 4. STL140N4LLF5 datasheet 5.
Revision history 9 AN3303 Revision history Table 6. 26/27 Document revision history Date Revision Changes 20-Jan-2011 1 Initial release. 20-Sep-2011 2 Updated Figure 12 on page 13.
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