AN525: Optimized Class 1 PD Designs Using the Si3402

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
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