AN525 O PTIMIZED C LASS 1 PD D ESIGNS U SING T H E S i 3 4 0 2 1. Introduction The Si3402 is designed to support up to 20 W of input power with over 15 W of power delivered to the load. For this reason, the standard reference designs have been optimized for high-power situations. The IEEE standard for PoE (802.3 clause 33) specifies the PD classes as listed in Table 1. Table 1. PD Classes PD Classification Input Power Maximum Output Power Allowing for 80% Conversion Efficiency Class 0 or Class 3 13 W 10.4 W Class 1 3.84 W 3.07 W Class 2 6.49 W 5.19 W Class 4 25.5 W 20.4 W Even the 3 W of output power that can realistically be derived from a Class 1 interface is adequate for many applications. This application note shows how the standard reference design is modified and simplified to support lower power situations. 2. Optimized Reference Designs Figure 1 shows the completed schematic of the Class 1 reference design for the isolated case, and Figure 2 shows the reference design for the non-isolated case. Tables 2 and 3 are the bills of materials corresponding to Figures 1 and 2. Rev. 0.2 7/11 Copyright © 2011 by Silicon Laboratories AN525 Figure 1. Optimized Class 1 Design (Isolated Case) AN525 2 Rev. 0.2 AN525 3. Bill of Materials (Isolated Class 1 Designs) Table 2. Bill of Materials for Isolated Class 1 Designs Item Qty Ref Value Rating Tol PCB Footprint Mfr Part Number Mfr 1 1 C2 12 µF ±20% C2.5X6.3MM-RAD EEUFC2A120 Panasonic 2 1 C3 1 µF ±10% C1210X7R101-105K Venkel 3 1 C5 1000 µF 4 1 C6 22 µF ±20% C0805 C0805X5R6R3-226M Venkel 5 1 C7 470 pF ±10% C0603 C0603X7R101-471K Venkel 6 1 C8 560 pF ±10% C0603 C0603X7R160-561K Venkel 7 1 C9 15 nF ±10% C0603 C0603X7R160-153K Venkel 8 9 C10,C11, C12,C13, C14,C15, C16,C17, C24 1 nF ±10% C0603 C0603X7R101-102K Venkel 9 1 C18 0.1 µF ±20% C0603 C0603X7R101-104M Venkel 10 2 C19,C20 1 nF ±10% C1808 C1808X7R302-102K Venkel 11 1 C21 0.22 µF ±10% C0603 C0603X7R100-224K Venkel 12 1 C22 0.1 µF ±20% C0603 C0603X7R100-104M Venkel 13 1 C23 1 µF ±10% C0603 C0603X7R100-105K Venkel 14 1 C25 0.1 µF ±10% C0805 C0805X7R101-104K Venkel 15 1 D1 1N4148W SOD-123 1N4148W Diodes Inc 16 1 D2 DFLT15A POWERDI-123 DFLT15A Diodes Inc 17 1 D3 MBRS1100 1A DO-214AA MBRS1100T3 On Semi 18 4 FB1,FB2, FB3,FB4 330 1200 m A L0603 BLM18PG331SN1 MuRata 19 2 J11,J12 BND_POST 15 A BANANA-JACK 101 ABBATRON HH SMITH 20 1 L1 1 µH 2.3 A IND-ME3215 ME3215-102ML Coilcraft 21 1 Q1 MMBT3904 200 m A SOT23-BEC MMBT3904 Fairchild 22 1 R3 127 1/10 W ±1% R0603 CR0603-10W-1270F Venkel 23 1 R4 25.5K 1/16W ±1% R0603 CR0603-16W-2552F Venkel C1210 C3.5X8MM-RAD ±20% Rev. 0.2 3 AN525 Table 2. Bill of Materials for Isolated Class 1 Designs (Continued) Item Qty Ref Value Rating Tol PCB Footprint Mfr Part Number Mfr 24 1 R5 36.5K 1/16W ±1% R0603 CR0603-16W-3652F Venkel 25 1 R6 12.1K 1/16W ±1% R0603 CR0603-16W-1212F Venkel 26 1 R7 2.05K 1/16W ±1% R0603 CR0603-16W-2051F Venkel 27 1 R8 10K 1/16W ±1% R0603 CR0603-16W-1002F Venkel 28 1 R9 3.01K 1/16W ±1% R0603 CR0603-16W-3011F Venkel 29 1 R10 10 1/10W ±1% R0805 CR0805-10W-10R0F Venkel 30 1 R11 4.99K 1/16W ±1% R0603 CR0603-16W-4991F Venkel 31 1 R12 100 1/16W ±1% R0603 CR0603-16W-1000F Venkel 32 1 R13 5.1 1/4W ±5% R1210 CR1210-4W-5R1J Venkel 33 1 R14 10K 1/10W ±1% R0805 CR0805-10W-1002F Venkel 34 1 T2 86B-0021ADPC XFMR-EP7-SMT 86B-0021A-DPC Delta 35 1 U1 Si3402 QFN20N5X5P0.8 Si3402 SiLabs 36 1 U4 TLV431 TLV431-DBZ TLV431BCDBZR TI 37 1 U5 PS2911 OPTO-PS2911 PS2911-1 CEL 4 Rev. 0.2 Figure 2. Optimized Design (Non-Isolated Case) AN525 Rev. 0.2 5 AN525 4. Bill of Materials (Non-Isolated Class 1 Designs) Table 3. Bill of Materials for Non-Isolated Class 1 Designs Item Qty 6 Ref Value Rating Tol PCB Footprint Mfr Part Number Mfr 1 1 C2 12 µF ±20% C2.5 x 6.3 mmRAD EEUFC2A120 Panasonic 2 1 C4 1 µF ±10% C1210 C1210X7R101-105K Venkel 3 1 C5 560 µF ±20% C3.5 x 8 mmRAD EEUFM0J561 Panasonic 4 1 C6 22 µF ±20% C0805 C0805X5R6R3-226M Venkel 5 1 C7 3.3 nF ±10% C0603 C0603X7R160-332K Venkel 6 1 C8 0.1 µF ±20% C0603 C0603X7R100-104M Venkel 7 1 C9 0.33 µF ±10% C0603 C0603X7R100-334K Venkel 8 8 C10,C11, C12,C13, C14,C15, C16,C17 1 nF ±10% C0603 C0603X7R101-102K Venkel 9 2 C18,C25 0.1 µF ±20% C0603 C0603X7R101-104M Venkel 10 1 C19 150 pF ±10% C0805 C0805X7R160-151K Venkel 11 1 C20 3.3 nF ±10% C0603 C0603X7R160-332K Venkel 12 1 C24 1 nF ±10% C0603 C0603X5R250-102K Venkel 13 1 D1 MBRS1100 1A DO-214AA MBRS1100T3 On Semi 14 1 FB1 30 3000 mA L0805 BLM21PG300SN1 MuRata 15 4 L2, L3 L4, L5 330 1200 mA L0603 BLM18PG331SN1 MuRata 16 2 J11,J12 BND_POST 15 A BANANA-JACK 101 Abbatron HH Smith 17 1 L1 33 µH 1.1 A 6.1 x 6.1 mm MSS6132-333ML Coilcraft 18 1 Q1 MMBT3904 200 mA SOT23-BEC MMBT3904 Fairchild 19 1 R3 127 1/10 W ±1% R0603 CR0603-10W-1270F Venkel 20 1 R4 25.5 k 1/16 W ±1% R0603 CR0603-16W-2552F Venkel 21 1 R5 2.87 k 1/16 W ±1% R0603 CR0603-16W-2871F Venkel 22 1 R6 8.66 k 1/16 W ±1% R0603 CR0603-16W-8661F Venkel 23 1 R7 30.1 k 1/10 W ±1% R0805 CR0805-10W-3012F Venkel 24 1 R13 5.1 1/4 W ±5% R1210 CR1210-4W-5R1J Venkel 25 1 R14 10 k 1/16 W ±1% R0603 CR0603-16W-1002F Venkel 26 1 U1 Si3402 QFN20N5X5P0.8 Si3402 SiLabs ±20% Rev. 0.2 AN525 5. Design Considerations Following are the detailed design consideration for adapting the standard high-power design for lower power situations. The Class 1 designs use smaller magnetic elements. In the isolated design, T1 is an EP7 core (10x10 mm footprint) vs. the EP13 core (13x13 mm) of the high-power design. In the non-isolated design, L1 is 6.1 x 6.1 mm vs. the 15 x 18 mm footprint required for full power. These components have been sized for the current level corresponding to 3 W of output power. For the isolated design, the magnetizing inductance and turns ratio of T1 was kept constant to avoid the need for feedback loop compensation changes and to keep the snubber and FET protection unchanged. Since the Si3402 is designed for short-circuit protection at approximately 15 W of output power, Q1 and R13 are added as an input current limiter to prevent the magnetic elements form saturating. The input current limit is Vbe/R or about 120 mA at room temperature. At elevated temperatures, this current limit falls to about 90 mA, which is the input current draw at full output power. Because this circuit limits input current, the circuit operates at constant input power. Under fault conditions, the output current increases as the output voltage decreases. As the output current increases, the magnetic elements start to saturate, reducing efficiency and limiting the maximum output current under short-circuit conditions. The input filter capacitor has been reduced from 12 µF electrolytic plus three parallel 1 µF X7R capacitors to 12 µF electrolytic plus one 1 µF X7R capacitor. The 5.1 sense resistor and 0.1 µF input capacitors, C18 and C25, further reduce ripple reflected to the input. For the isolated case, the first stage filter was reduced from 100 µF X5R to 22 µF X5R. The main output filter capacitor was not changed in order to avoid the need for feedback loop compensation changes and to give good load transient response. Finally, the classification resistor, R1, was updated to the value required for Class 1. 6. Layout Considerations While the circuits of Figures 1 and 2 have been tested, detailed layout data bases are not available. Due to the smaller magnetic element sizes and reduced filtering, it should be possible to substantially reduce the area encompassed by the input and output current loops so as to reduce EMI (see also “AN296: Using the Si3402 PoE PD Controller in Isolated and Non-Isolated Designs). The size of the thermal heat spreader for the Si3402 can be safely reduced from two square inches to less than one square inch. Even though the power level is substantially reduced, careful layout is highly recommended. Visit SiLabs support at www.silabs.com or submit layouts to [email protected] for schematic and layout review. Rev. 0.2 7 AN525 DOCUMENT CHANGE LIST Revision 0.1 to Revision 0.2 Updated Table 3, “Bill of Materials for Non-Isolated Class 1 Designs,” on page 6. Modified rating, footprint, and part number for inductor L1 in non-isolated Class 1 designs. 8 Rev. 0.2 AN525 NOTES: Rev. 0.2 9 Smart. Connected. Energy-Friendly Products Quality www.silabs.com/products www.silabs.com/quality Support and Community community.silabs.com Disclaimer Silicon Laboratories intends to provide customers with the latest, accurate, and in-depth documentation of all peripherals and modules available for system and software implementers using or intending to use the Silicon Laboratories products. Characterization data, available modules and peripherals, memory sizes and memory addresses refer to each specific device, and "Typical" parameters provided can and do vary in different applications. Application examples described herein are for illustrative purposes only. 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