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1、电流源与电压源驱动网络变压器电流源与电压源驱动网络变压器 Current and Voltage Source drive transformer Content 1. Current and Voltage Source drive 2. Bob Smith Termination 3. The function and classification of transform 4. Integrated and discrete magnetic 5. 8-Core and 12-Core Magnetic 6. CMC(common-mode choke) on the cable and
2、 PHY side 2 1 Current and Voltage Source drive The drive type of PHY contains voltage and current source drive. The center tap should connect to cap when use voltage source drive. Meanwhile the center tap should connect to power when use current source drive. But which kind of drive type to be used
3、is determined by the PHY-ic . 3 Current Source Drive 4 Center tap to power: Provide the needed drive current 5 Voltage Source Drive Center tap Cap: Provide low impedance path for common-mode current Comparison of Power Consumption 6 Current Source power consumption To drive 1V output voltage and 10m
4、A output current, PHY have to sink 40mA, 10mA going down and 30mA going down, current from the center tap. Finally the transformer have 20mA going down in N/2 turns of coil, so 20mA is wasted The total power consumption of 4 pairs differential in current mode driver. 40 mA/ch * 4 ch *1.8 V=288mW/por
5、t to drive +1V on all 4 pairs. 7 8 Voltage Source Power consumption To drive 1V output voltage and 10mA output current, the voltage source has to apply 2V because of the series termination. Typically the voltage source is at lest 2.5V assuming 1:1 turn ratio transformers. The needed current is only
6、10mA to output 10mA. For 2.5V operation: 10mA/ch * 4 ch * 2.5V =100mW/port to drive +1V For 3.3V operation: 10mA/ch * 4 ch * 3.3V =132mW/port to drive +1V 9 10 Conclusion Current Source driver needs 4 times current as much as Voltage Source driver and more power than Voltage Source driver, so newer
7、generation of Marvell PHY-ic use Voltage Source Driver to reduce power consumption. (Below for reference). For example, 88E1318 uses current source driver, but newer PHY-ic 88E1543 and 88E1518 use voltage source driver. Note: It describes the basic difference between current source driver vs voltage
8、 source driver and the difference in term of the voltage, current, and power between the two approaches. It only describes the theory not actual situation. 11 Transform and Bob Smith Termination 12 3 Bob Smith Termination Robert W. (Bob) Smiths patented idea proposes the way to reduce the common mod
9、e currents on a multi-pair system (e.g. CAT5 network cable). It is based on the fact that the pair-to-pair relationships of a CAT5 cable form transmission lines in themselves. Due to the approximate symmetry in the construction of the CAT5 cable, each pair has the same relation (150)with any other p
10、air. Function of Bob Smith Termination Providing those pair-to-pair transmission lines with termination matching their characteristic impedance in order to absorb the reflected wave and prevent standing wave from occurring significantly lowers parasitic interference radiation. The high voltage capac
11、itor between the common point of all the four resistors and the Chassis ground is not a part of Bob Smith termination but helps in filtering the noise residue managing to leak out through the transformer. 13 Transform Function: Connect PHY and RJ45 for isolating cable interface and low common mode e
12、missions, protect PHY IC from high voltage, strengthen signal for farther distance, and EMC (Electro-Magnetic Compatibility) Classification: Integrated and discrete 8-core and 12-core 14 4 Integrated and Discrete 15 Integrated Discrete Integrated Advantages: Lower component count and cost Better EMC
13、 Smaller footprint, easy for layout Disadvantages: Higher production cost, hard to rework during production test. Due to the space limitations, the magnetic cores are smaller and closer to each other thus degrading crosstalk and EMC characteristics, increasing non-linear distortion and losses. 16 La
14、yout for Discrete Layout requirements are slightly different when using discrete magnetics. These include: Ground cut for HV installation (not required for integrated magnetics) A maximum of two vias Turns less than 45 Discrete terminators(Bob Smith termination) The ground plane beneath a magnetics
15、module should be split. The RJ45 connector side of the transformer module should have chassis ground beneath it. A minimum of 50-mil spacing from high voltage (1500 pF/2 KV or 1000 pF/3 KV)capacitor to traces and components should be maintained. 17 18 A reference layout for discrete magnetics 19 8-C
16、ore Magnetics Transformer + CMC=2-Core for 1pair For 4 pairs = 8-Core 20 5 12-Core Magnetics Transformer + CMC + Autotransformer = 3-Core for 1pair For 4 pairs = 12-Core 12-core and 8-core Magnetics comparison The 8-core magnetic are cheaper, but riskier in many cases and cautiously recommended to skilled designers to meet stricter EMC guidelines. The Bob Smith termination becomes less
