AN-2592: Inductor Selection Guide for Home Bus System

Introduction

In communication networks in industrial automation and building control systems, when the nodes have no access to power outlets, there is often the need of a solution that provides both data and power connections between multiple boards, such as a controller and one or multiple sensor or actuator boards. This is straightforward if all the boards are in one chassis, but often the boards in a typical controller-node network are many meters apart, and as the distance between nodes increases, and the number of nodes increases, the cost of cabling increases to become a significant portion of the overall solution cost and a major factor in system performance.

One solution is to use four-wire cabling in which one pair is used for data communication and a second pair is used for power delivery. This has the advantage being a relatively simple solution but has the disadvantage of extra cable cost and weight. A more elegant solution is to use a single pair of wires to provide both data and power, as with Power over Data (PoD) solutions.

Many different PoD solutions have been developed to address distinct types of end applications. This application note discusses PoD solutions targeted to more proprietary or ad-hoc networks commonly found in industrial communications or building automation control systems. These solutions are different than Ethernet- or Internet Protocol (IP)-based solutions, commonly referred to as Power over Ethernet (PoE), or, in the case of the newer Single Pair Ethernet (SPE) solutions, called Power over Data Lines (PoDL). For information on PoE or PoDL solutions, refer to ADI’s Chronous portfolio.

Power over Data Technology

Technology Overview

Power over Data or PoD technology uses a two-wire bus to transmit both power and data to connected devices across the network at the same time. Power is inductively coupled onto the two-wire bus while data is capacitively coupled. PoD solutions require inductors to “block” the AC data signals from DC power at each node. These inductors are commonly referred to as “AC-blocking” inductors. Capacitors are required between the two-wire bus and the transceiver at each node to couple the data signals on the bus. These capacitors are commonly called “AC-coupling” capacitors.

Figure 1 shows a typical multi-node PoD network on a two-wire bus. This is a half-duplex network using one pair of lines for the driver and receiver, which are capable of data transmission in either direction, but not simultaneously. Any node can transmit on the bus at any given time and all other nodes can receive the data. “Bus” is a two-wire cable, typically a twisted pair or parallel wires. This cable has a characteristic impedance (Z₀) and, in Figure 1, termination resistors (R_T = Z₀) are installed at node 1 and node N to achieve optimal signal quality. In a typical multi-node PoD network, one singular node is allowed to supply power to the bus and multiple nodes can sink power from the bus. In this example, node 1 provides the power source for the bus and the other n-1 nodes are loads or power sinks.

In many industrial PoD systems, engineers use a standard RS-485 (or CAN) transceiver for the physical layer and implement proprietary methods to deliver power over the data lines. Such transceivers are well understood and offer robust data transmission in noisy industrial environments. The challenge system engineers face is to develop a reliable and robust power delivery method, which is cost-effective and consumes minimal PCB space – “AC-blocking” inductors can vary significantly in terms of size and cost depending upon the network data rate and power capacity.

Since RS-485 is a physical layer standard (layer 1 of the OSI model), it has no inherent data link protocol (frames, packets, etc). For PoD networks, the implemented protocol software must include some sort of data encoding scheme (for example Manchester encoding or 8b/10b) to eliminate possible long series of 0’s or 1’s that could result in a DC imbalance when carrying power over the bus. A disadvantage of this type of encoding is that it requires extra processing cycles and potential for extra latency.

System interfaces with PoD architectures tend to have relatively low data rates (typically less than 5Mbps), being capable of supporting many nodes (64 or more), and can support long cable lengths (100s of meters). The proprietary communication protocols often used, however, which are fine for closed-systems, may otherwise limit equipment interoperability.

System Considerations

In a typical scenario, an engineer looking for a PoD solution has the following requirements checklist:

• Power delivery (24V DC typical at power source node, 2W to 5W per node at power sink node).
• Polarity agnostic to simplify field connections, reduces wiring errors.
• Data encoding scheme to keep bus DC-balanced.
• Minimize inductor size to save cost and PCB space.
• Robust performance including Transient Immunity such as ESD and EFT.

Power Source vs. Power Sink

It is most common to have a system where one node is the power source (node 1 in Figure 1) while multiple nodes on the network are power sinks, where the power is supplied to them. Different considerations must be taken into account when looking at the “AC-blocking” inductor for these different types of nodes. The power source node’s inductors must be capable of delivering full power to the whole network, typically implying an “AC-blocking” inductor in a larger size. The power sink node’s inductors should be optimized to support the local power for each individual node.

Inductor Size

The function of the “AC-blocking” inductors on each node is to separate the AC data signal from the DC supply power. For the relatively low data rates used in PoD systems, these inductors are milli-Henries (mH), which requires bulky and costly inductor design. Figure 2 shows the inductance required in PoD system per channel data rate.

The inductor size becomes an even bigger factor when supporting higher current delivery and can become a major impediment to system design as form-factors continue to shrink.

Home Bus System

Home Bus System Architecture Overview

Home Bus System (HBS) is a popular PoD solution optimized for home and building automation applications. Analog Devices’ family of Home Bus products enables high-efficiency and high-performance PoD applications. For further information on the Home Bus standard and its operation, refer to the Introduction to Home Bus Application Note.

The MAX22288 transceiver is designed to operate on the power source node in a PoD system, in which an external “AC-blocking” inductor is required to supply the power. The MAX22088 transceiver features an integrated “AC-blocking” inductor, allowing it to be used in PoD systems on the power sink node, where power is delivered, without requiring an external “AC-blocking” inductor.

Figure 3 shows a 2-node Home Bus network, with one node using the MAX22288 acting as the power source, and the other node using the MAX22088 as the power sink. This application note discusses how to select an inductor for the power source node, shown as L1 in Figure 3. Note that the power sink node does not require an external inductor, as the MAX22088 integrates an active inductor circuit to simplify system design, reduce PCB space, and save cost.

Bus Termination

The Home Bus standard specifies the bus load resistance for different communication distances, as shown in Table 1. The bus load resistance is the minimum resistance (maximum load) on the two-wire bus necessary to maintain the minimum differential voltage for all drivers and receivers. In real application, the load resistance is largely determined by the termination resistors (R_T) at transceivers, and they should match the characteristic impedance (Z₀) of cable to achieve optimal signal quality on bus.

The MAX22288 and MAX22088 feature dynamic cable termination, configurable receiver thresholds, and transmit driver slew rate to ensure improved signal quality and flexible designs. See the MAX22288 data sheet for further details of these features.

Bus Inductance

The total load impedance of the bus (Z_BUS) is a combination of the termination resistance (R_T), inductance (Z_L) and capacitance (Z_C) seen by the bus; and it must be larger than the minimum bus load resistance shown in Table 1. Figure 4 shows a simplified equivalent model of a 2-node Home Bus network with MAX22288 on each node, where node 1 supplies power to the bus and node 2 sinks power from the bus.

To calculate the total bus impedance, consider all components on the bus and calculate Z_BUS as:

Z_BUS = Z_L1 || (Z_C1 + R1 + Z_C1) || Z_L2 || (Z_C2 + R2 + Z_C2)

Note that L1, C1, and R1 are the component values for node 1, and f is the data rate in hertz:

AC-blocking inductor L1 with impedance Z_L1 = 2π x f x L1.
AC-coupling capacitors C1 with impedance Z_C1 = 1/(2π x f x C1).
And, termination resistor R1.
Similarly, L2, C2, and R2 are the component values for node 2.

Assume the following system design parameters:

Termination resistance R1 = R2 = 1kΩ and the Home Bus transceiver node is high-impedance.
AC-coupling capacitors C1 = C2 = 22μF.
Data rate at 10kbps, or f = 5kHz.
The total impedance of the bus (Z_BUS) must be at least 100Ω to meet the Home Bus required load for the bus.

Z_BUS = 100Ω = Z_L1 || Z_L2 || (2 x Z_C1 + R1) || (2 x Z_C2 + R2)

In a 2-node network, the total power supplied is equal to the total power delivered. Thus, assume uniform inductors L1 = L2 and solving for Z_L:

100Ω = Z_L || Z_L || (2 x (1/(2π x f x 22μF)) + 1kΩ) || (2 x (1/(2π x f x 22μF)) + 1kΩ) = Z_L || Z_L || 1002.89 Ω || 1002.89 Ω
Z_L = 250Ω

At 10kpbs data rate, the required inductance L1, L2 is:

L1 = L2 = 250Ω / (2 x π x 5e3) = 8mH

Table 2 shows the minimum required total inductance in a typical 2-node Home Bus application.

If a MAX22088 transceiver is used in the power sink node, the integrated “active inductor” of the MAX22088 can be used to deliver power to the node. This active inductance is tunable and can be optimized for different bus conditions using the equation shown in the “Active Inductor” section of the MAX22088 data sheet.

Note that the transient response of the MAX22088 is different from a standard differential mode inductor because of the way the active circuitry of MAX22088 is implemented. Furthermore, to allow for dynamic cable termination, it is recommended to increase the value of inductance to compensate for the sudden loss of impedance on the bus when the dynamic termination is connected. For more information, see the “Dynamic Cable Termination” section in the MAX22088 data sheet. In a more complicated system with multiple connected nodes, the inductor value changes. Figure 5 provides an estimate of the required bus inductance for each node in a multi-node configuration, for a different number of nodes.

In a communication system with long cables, cable termination at both ends is important to reduce reflections and improve signal quality. It is important to terminate only the two furthest nodes. Otherwise, bus loading significantly increases. For higher speed applications, the required inductance is lower. However, adding more does not adversely affect its operation.

Inductor Selection

The selection of the “AC-blocking” inductor to use with the MAX22288 power source node is critical to ensure reliable operation in a multi-node network. The two-wire bus requires two inductors, or an inductor pair.

Inductor pairs can be configured in different modes depending on how they are connected in the circuit. The two most common configurations are differential mode and common mode. In differential mode, the current travels in one direction through one inductor and returns in the opposite direction through the other inductor to complete the cycle. In a common mode configuration, the current travels in the same direction through both inductors. These configurations serve different purposes.

A differential mode configuration impedes differential mode signals while passing common mode signals. For differential mode signals, the magnetic flux moves in opposite directions through the inductors and is canceled, allowing the differential signals to pass through the choke.

A common mode configuration becomes high-impedance when passing common mode signals, and is low-impedance when passing differential mode signals. Figure 6 shows two inductors arranged as a two-line common-mode choke. The magnetic flux is generated in the same direction from the common mode signals, thus impeding them.

In Home Bus applications, where data is passed differentially over a two-wire bus, the “AC-blocking inductor”, which is really a pair of inductors, must be configured to be high-impedance to these differential mode data signals. The differential mode inductance impedes these differential mode data signals; therefore, the “AC-blocking” inductor needs to provide differential mode inductance in operation.

Figure 7 shows normal communication (left) when the inductor pair in power source node (L1 in Figure 3) is configured in differential mode but abnormal waveforms (right) when the same inductor pair is configured in common mode.

The following criteria must be met when selecting a differential mode “AC-blocking” inductor for a Home Bus application:

• The turns ratio of the inductor pair must be 1:1. This is to provide balanced operation, as the data signals are opposite in direction, but symmetric in amplitude.
• The saturation current must be higher than the full operating current on the bus. Inductance reduces significantly once the core saturates.
• The inductor pair needs to provide at least the minimum required differential mode inductance. See the Bus Inductance section to determine how much inductance is required.
• The inductor pair needs to have a low DC Resistance (DCR), typically less than 10Ω. A low DCR reduces voltage loss in transferring DC power and improves overall efficiency in the network.
• The self-resonance frequency is recommended be at least 10x the data rate to ensure the inductor pair operates “inductively” in normal conditions.

Inductor Options

Analog Devices recommends using coupled inductor or two individual inductors as the “AC-blocking” inductor in Home Bus applications.

It is not recommended to use transformers, or common mode chokes, as the “AC-blocking” inductor.

Using Coupled Inductors

It is recommended to use differential-mode coupled inductors with a Home Bus System. Different vendors might refer to such coupled inductors as power inductors, or chokes, or even transformers. Coupled inductors are a group of inductors coupled through a common core with coupling coefficient (k) close to 1. The closer the k is to 1, the better the magnetic coupling, and the less the leakage. Traditionally, coupled inductors are used in power electronics circuits such as flyback converter circuits. Although the DC resistance and rated current may satisfy the Home Bus application requirements, the inductance typically is less than a few hundreds of micro-Henries (μH). To use coupled inductors in a Home Bus application, select those with enough inductance and pay attention to its test conditions. Often the specifications in the inductor data sheet are characterized at a different condition for non-Home-Bus applications. Thus, it is recommended to measure and characterize the differential mode inductance under the target operating condition, following the steps in the Measuring the Differential Mode Inductance section.

Using Two Individual Inductors

It is possible to use two individual inductors and create own “coupled inductor” on the PCB. Follow two rules when using this option:

Rule 1: Place them as close to each other as possible to reduce the flux leakage.
Rule 2: Pair them in the differential mode. Place the two individual inductors such that the magnetic flux is generated in the same direction when passing a differential mode signal.

In the following example, two 8.3mH coil inductors are used to create an inductor pair, coupled through their cores plus the space between them.

Rule 1: Place them as close to each other as possible to reduce the flux leakage.

The total inductance of the pair is more than L1 + L2, the serial combination of L1 and L2, when the two inductors are “coupled”. The total inductance L_TOTAL is calculated L1 + L2 + 2M, where M is the mutual inductance created by the magnetic coupling of L1 and L2. Mutual Inductance M is calculated as k x √(L1 x L2), where k is the coupling coefficient between 0 and 1. k = 1 indicates zero magnetic flux leakage and 100% magnetic coupling between L1 and L2. Figure 8(C) shows the magnetic coupling between the two inductors in red line; the closer two inductors are positioned, the more ideal the coupling, less the leakage, and hence, larger the mutual inductance M is.

With coupling coefficient k = 1, the final differential mode inductance of the two 8.3mH inductors configured as shown in Figure 8(A) is L1 + L2 + 2k x √(L1 x L2) = 33.2mH. In reality, the coupling coefficient k is always less than 1. When these two inductors are placed 1mm apart, the differential mode inductance is measured at 25.8mH. The reduction comes from the flux leakage. The mutual inductance M = 0.5 x (25.8mH − 8.3mH x 2) = 4.6mH, indicating a coupling coefficient of k = M/√(L1 x L2) = 4.6mH/8.3mH = 0.55.

When the two inductors are placed 1cm apart, the final differential mode inductance is further reduced and measured at 19.3mH. The coupling coefficient is reduced to 0.33.

When the two inductors are placed 10cm apart, the differential mode inductance is measured at 16.6mH, the same as two individual inductors connected in series without any magnetic coupling. This means the magnetic coupling is too weak to generate any meaningful mutual inductance.

Rule 2: Pair them in the differential mode configuration. Place the two individual inductors such that the magnetic flux is generated in the same direction when passing a differential mode signal.

When the coupled pair is positioned in the common mode configuration, the magnetic flux for both inductors generates in the same direction when passing differential mode signals and cancels out, as shown in Figure 9. The total inductance L_TOTAL is reduced and less than the serial combination of L1 and L2.

When two inductors are placed in the differential mode configuration, as shown in Figure 8(B), and 1cm apart, the differential mode inductance is measured at 20.3mH, more than 16.6mH, the serial combination of two individual inductors. The flux sums and creates a larger final coupled inductance.

When they are placed in common mode configuration, as shown in Figure 9(B), and 1cm apart, the differential mode inductance is measured at 14.7mH, less than 16.6mH. The flux cancels out and reduces the final coupled differential mode inductance.

In Figure 10 and Figure 11, two Home Bus nodes communicate at 50kbps and the “AC-blocking” inductor-pair is created using two individual inductors. Total bus inductance required at 50kbps data rate is 0.8mH from Table 2. Figure 10 shows distorted receiver waveforms when two 390µH inductors are used to achieve a total of 780µH less-than-minimum-required bus inductance. The inductor-pair is loaded with 200mA current, which further reduces the total effective inductance on the bus. Figure 11 shows undistorted and clean bus waveforms under the same operating conditions when the inductor-pair is constructed with two 3.9mH inductors to achieve a bus inductance larger than the required minimum.

Using a Transformer

Power transformers are used to step-up or step-down voltages between the primary side and secondary side. When using a transformer in the Home Bus application, pay attention to the following requirements:

Make sure the turns ratio is 1:1. Most transformers typically have turns ratio not at 1:1. The primary and secondary sides are usually intended to be at different AC voltages.
Measure the differential mode inductance as the coil inductance is often not specified in the data sheet. The transformer is usually oversized and the footprint is usually giant.

Using a Common Mode Choke

A common mode choke is designed to filter common mode noise. Most common mode chokes are not suitable choices for Home Bus applications for the following reasons:

The differential mode inductance is much smaller than common mode inductance.
A common mode choke has relatively small current rating, and its typical operating frequency is most often much higher than the data rate used in Home Bus applications.

Measuring the Differential Mode Inductance

Many, if not most, inductor data sheets do not provide differential mode inductance information at the data rate and biased current used in Home Bus applications. Often, the easiest way to qualify the inductor pair of choice is to characterize it with an LCR meter. Measure the differential mode inductance and phase under the application’s data rates and bias current, taking note that:

A perfect inductor has +90° phase, meaning its current is behind its voltage by 90°.
When the inductor core saturates, the inductance drops quickly. There is sometimes a roll-off rate specified in the data sheet.
Each inductor has a self-resonance frequency and usually the value varies with the DC bias current.

Using an LCR meter is the easiest way to characterize the inductors of choice. Figure 12 shows a typical setup for measuring the differential mode inductance using an LCR meter. Table 3 shows four examples of measurements in the lab with an Instek LCR-6300 meter and their performance in a Home Bus application at 200kbps data rate and 200mA loading current.

The minimum required total bus inductance at 200kbps is 200μH, according to Table 2. The Wurth common mode power line choke (part number 7446323004) does not provide this minimum required differential mode inductance and thus, the bus waveforms are distorted.

Design Example

Example 1: 2-Node Network at 57.6kbps

Consider a 2-node Home Bus network operating at 57.6kbps data rate, as shown in Figure 3. Figure 13 is the simplified equivalent model. In the power source node, 100mA is supplied to the bus through the “AC-blocking” inductor L1 while the MAX22088 sinks power from the bus through its integrated “active inductor”. In this example the components are optimized for 57.6kbps data rate and a 100mH coupled inductor from Wurth Electronics, Inc. (part no. 750318652) is selected as the “AC-blocking” inductor at L1.

To qualify this coupled inductor in the application, calculate the total impedance on the bus Z_BUS = Z_L1 || (Z_C1+ R_IO1) || (Z_C1 + R_IO1) || Z_LAC || (Z_C2 + R_EQV + Z_C2), where:

L1 = 138mH at 20kHz and biased at 100mA, from bench characterization
L_AC = 22.4mH using the equation in MAX22088 data sheet
Static termination resistance R_TRM2 = 1kΩ
MAX22288 AIO/BIO input impedance R_IO1 = 30kΩ (typ)
MAX22088 AIO/BIO input impedance R_IO2 = 10kΩ (typ)
R_EQV = 952Ω, derived from the equation: 1/R_EQV = 1/(2 x R_IO2) + 1/R_TRM2
Coupling capacitors = 2.2μF
Data rate = 57.6kbps, or f0 = 28.8kHz

Thus,

Z_L1 = 2π x f0 x L1 = 25kΩ
Z_LAC = 2π x f0 x L_AC = 4kΩ
Z_C1 = Z_C2 = 1/(2π x f0 x 2.2μF) = 2.5Ω
Z_C1 + R_IO1 = 30kΩ
Z_C2 + R_EQV + Z_C2 = 957Ω
Thus Z_BUS = Z_L1 || (Z_C1 + R_IO1) || (Z_C1 + R_IO1) || Z_LAC || (Z_C2 + R_EQV + Z_C2) = 25kΩ || 30kΩ || 30kΩ|| 4kΩ || 957Ω = 713Ω, larger than the minimum required bus termination resistance of 100Ω.

This Wurth coupled inductor is designed in both MAX22088 and MAX22888 evaluation kits, and it supports Home Bus applications at 57.6kbps.

Example 2: 8-Node Network at 200kbps

In this example, consider an 8-node Home Bus system operating at 200kbps, as shown in Figure 14. L1 is the “AC-blocking” inductor in the power sourcing node supplying a total of 250mA to the bus, and there is a need to determine whether the coupled inductor from Wurth Electronics, Inc. (part no. 744851102) can be used.

The total impedance on the bus Z_BUS = Z_NODE1 || Z_NODE2 || Z_NODE3 || Z_NODE4 || Z_NODE5 || Z_NODE6 || Z_NODE7 || Z_NODE8 , where:

Z_NODE1 is the total impedance of node 1, the power source node.
Z_NODE2 to Z_NODE6 are the total impedance of nodes 2 to 6, the data-only nodes.
Z_NODE7 and Z_NODE8 are the total impedance of nodes 7 and 8, the power sink nodes.

Consider the power source node and its equivalent circuit, as shown in Figure 15(A), Z_NODE1 = Z_L1 || (Z_C1 + R_EQV1 + Z_C1), where:

L1 = 358μH at 100kHz and biased at 250mA, from bench characterization
Static termination resistance R_TRM1 = 1kΩ
MAX22288 AIO/BIO input impedance RIO1 = 30kΩ (typ)
R_EQV1 = 983.6Ω, derived from the equation: 1/R_EQV1 = 1/(2*R_IO1) + 1/R_TRM1
Coupling capacitors = 1μF
Data rate = 200kbps, or f0 = 100kHz

Thus,

Z_L1 = 2π x f0 x L1 = 225Ω
Z_C1 = 1/(2π x f0 x 1μF) = 1.6Ω
Z_C1 + R_EQV1 + Z_C1 = 987Ω
Thus, Z_NODE1 = 225Ω || 987Ω = 183Ω.

Consider the data-only nodes and their equivalent circuits, as shown in Figure 15(B), Z_NODE2-6 = (Z_C1 + R_IO1) || (Z_C1 + R_IO1), where:

MAX22288 AIO/BIO input impedance R_IO1 = 30kΩ (typ)
Z_C1 = 1/(2π x f0 x 1μF) = 1.6Ω
Thus, Z_NODE2-6 = (30kΩ + 1.6Ω) || (30kΩ + 1.6Ω) = 15kΩ.

Consider the power sink nodes and their equivalent circuits, as shown in Figure 15(C), Z_NODE7-8 = Z_LAC || (Z_C2 + R_IO2) || (Z_C2 + R_IO2), where:

L_AC7 = 35mH at 50mA loading current using the equation in MAX22088 data sheet
L_AC8 = 16mH at 200mA loading current using the equation in MAX22088 data sheet
MAX22088 AIO/BIO input impedance R_IO1 = 10kΩ (typ)

Thus,

Z_LAC7 = 2π x f0 x LAC7 = 22kΩ
Z_LAC8 = 2π x f0 x LAC8 = 10kΩ
Z_C2 = 1/(2π x f0 x 1μF) = 1.6Ω
Thus, Z_NODE7 = 22kΩ || (10kΩ + 1.6Ω) || (10kΩ + 1.6Ω) = 4kΩ, Z_NODE8 = 10kΩ || (10kΩ + 1.6Ω) || (10kΩ + 1.6Ω) = 3.3kΩ.

The total impedance on the bus Z_BUS = Z_NODE1 || Z_NODE2 || Z_NODE3 || Z_NODE4 || Z_NODE5 || Z_NODE6 || Z_NODE7 || Z_NODE8 = 183Ω || (15kΩ / 5) || 4kΩ || 3.3kΩ = 157Ω, larger than the minimum required bus termination resistance of 100Ω. Figure 16 shows the bus waveforms at node 1, the power source node.

Conclusion

This application note outlines the criteria in selecting a suitable “AC-blocking” inductor for a Home Bus System. It is recommended to use a “coupled inductor” or two individual inductors configured as an “inductor pair”. It is important to qualify the choice of “AC-blocking” inductors on the bench based on the real application conditions using the standards and methods, as discussed in this application note.

Examples and bench measurements are also provided to assist readers when selecting proper inductors for their Home Bus applications.

References

MAX22088 data sheet and MAX22088 evaluation kit

MAX22288 data sheet and MAX22288 evaluation kit

Introduction to Home Bus, Introduction to Home Bus | Analog Devices

Inductor vendors:

• Wurth Electronics, Inc., www.we-online.com
• Sumida Corporation, www.sumida.com