Multiplexed Load Control

25.5 Multiplexed Load Control

Another emerging application of power electronics in auto- mobiles is in the area of load control. In the conventional point-to-point wiring architecture, most of the loads are con- trolled directly by the primary mechanical switches, as shown in Fig. 25.1. In a point-to-point wiring architecture, each load has a dedicated wire connecting it to the fuse box via the pri- mary switch. Consequently, fairly heavy wires have to be routed all over the vehicle, as illustrated in Fig. 25.10a. The situation is made worse when multiple switches control the same load, as is the case with power windows and power door locks. The complete harness of a 1994 C-class Mercedes-Benz that uses point-to-point wiring has about 1000 wires, with a total length of 2 km, over 300 connectors and weighs 36 kg. The process of assembling the wiring harness is difficult and time con- suming, leading to high labor costs. Retrofitting, fault tracing, and repairing are time consuming and expensive. The bulky harness also places constraints on the vehicle body design, and the large number of connectors compromise system reliability.

An alternative wiring technique is to control the loads remotely and multiplex the control signals over a communica- tion bus, as shown in Fig. 25.10b and c. A control message is sent on the communication bus to switch a particular load on or off. This allows more flexibility in the layout of the power cables and could allow the pre-assembly of the harness to be more automated. Furthermore, with communication between the remote switches, it is practical to have a power manage- ment system than can turn off non-essential loads when there is a power shortage. One possibility is to group the remote switches into strategically located distribution boxes, as shown in Fig. 25.10b. A power and a communication bus connect the distribution boxes. Another possibility is to integrate the remote switches with the load, i.e. point-of-load switching,

as shown in Fig. 25.10c. In Fig. 25.10b the transceivers are also built into the distribution boxes, while in Fig. 25.10c each load and primary switch has an integrated transceiver. The point-of-load switching topology is attractive because of its simplicity, but raises cost and fusing challenges.

Multiplexed remote switching architectures have been under consideration since at least the early 1970s, when Ziomek investigated their application to various electrical subsys- tems [28]. The initial interest was dampened by cost and reliability concerns and the non-availability of appropriate remote switches. However, advances in semiconductor tech- nology and rapid growth in the automotive electrical system revived interest in multiplexed architectures. The SAE Mul- tiplexing Standards Committee has partitioned automotive communications into three classes: Class A for low data-rate (1–10 kbit/s) communication for the control of body func- tions, such as headlamps, windshield wipers, and power win- dows, Class B for medium data-rate (10–100 kbit/s) parametric data exchange, and Class C for high data-rate (1 Mbit/s) real- time communication between safety critical functions, such as between ABS sensors and brake actuators [29]. Although load control is categorized as Class A, lack of any widely accepted Class A communication protocols has lead to the application of Class B and Class C communication IC’s to load control. Class B has received the most attention due to the California Air Resources Board mandated requirement for on-board diagnostics (OBD II) and a large number of com- peting protocols, including the French vehicle-area network (VAN), the ISO 9141 and the SAE J1850, have been devel- oped [30]. Of these, the SAE J1850 is the most popular in the US. Another popular protocol is the controller area net- work (CAN) developed by Bosch [31]. Although designed for Class C with bit rates up to 1 Mbit/s, it is being applied for Class A and Class B applications due to the availability of inexpensive CAN ICs from a large number of semiconductor manufacturers.

Remote switching systems require remote power switches. An ideal remote switch must have a low on-state voltage,

be easy to drive from a micro-controller, and incorporate current sensing. A low on-state voltage helps minimize the heatsinking requirements, while current sensing is needed for the circuit protection function to be incorporated into the switch. To withstand the harsh automotive environment the switch must also be rugged. Furthermore, if PWM control is required for the load, the switch must have short turn-on and turn-off times and a high cycle-life. The traditional means of remotely switching loads in an automobile is via electrome- chanical relays. Although relays offer the lowest voltage drop per unit cost, they require large drive current, are relatively large, are difficult to integrate with logic, and are not suitable for PWM applications [32–34]. Therefore, their use will be lim- ited to very high current, non-PWM applications. The power levels of the individual loads in the automobile are too low for IGBTs and MCTs to be competitive. Bipolar transistors are

25 Automotive Applications of Power Electronics 653

Communication Bus

Alternator

S5

Power Bus

Communication Bus

Power Bus

FIGURE 25.10 Alternative control strategies illustrated for a simple automotive electrical system with six loads (L1-6) and six primary switches (S1-6): (a) conventional direct switching architecture with a single fusebox (F1); (b) multiplexed remote switching architecture, with remote switches and transceivers in three distribution boxes (D1-3); and (c) multiplexed point-of-load switching with electronics integrated into the loads and the primary switches.

also not very attractive because they are harder to drive than The benefits of remote switching electrical distribution sys-

a MOS-gated device. Because of its fast switching speed, low tems have been demonstrated by Furuichi et al. [35]. The voltage drop, relative immunity to thermal runaway, low drive multiplexed architecture they implemented had 10 remote requirements, and ease of integration with logic, the power units (two power units with fuses, power drivers and signal MOSFET is the most attractive candidate for remote switching. inputs, five load control units with power drivers and signal Smart-power MOSFET devices with integrated logic interface inputs but no fuses, and three signal input units with only sig- and circuit protection have recently become available. Use of nal inputs). To increase system reliability, each power unit was these devices for power electronic control of individual loads connected to the battery via independently fused power cables. has become economically competitive in some subsystems, and Although wiring cost decreased, the authors report an increase may be expected to become more so with the advent of higher in overall system cost due to the additional cost of the remote voltage electrical systems.

units. Intel’s CAN ICs with data rates of 20 kbit/s were used

654 D. J. Perreault et al. TABLE 25.5 Comparison of a multiplexed and the conventional

system, as reported by Furuichi et al. for a compact vehicle [35]. In the multiplexed system, the function of nine electronic control units (ECUs) was integrated into the remote units

Point-to-point Multiplexed Change (%) Harness weight (kg)

ECU weight (kg)

1.2 0.0 N/A

Remote unit weight (kg)

0.0 3.5 N/A

Total weight (kg)

Number of wires

Number of terminals 1195

Number of splices

FIGURE 25.11 Structure and circuitry of the conventional Lundell Length of wire (m)

brushes, and causes the two pole pieces to become opposing for the transmission and reception of control signals over an magnetic poles. A full-bridge diode rectifier is traditionally

unshielded twisted-pair ring bus. Intelligent power MOSFETs used at the machine output, and a fan mounted on the rotor were used as the remote switches and fusing was done with is typically used to cool the whole assembly. mini-fuses. The results of their work are shown in Table 25.5.

The dc output voltage of the alternator system is regulated Although weight of the wiring harness was reduced by 30%, by controlling the field current. A switching field regulator

the total system weight decreased by only 12.5% due to the applies a pulse width modulated voltage across the field. The added weight of the remote units.

steady-state field current is determined by the field-winding resistance and the average voltage applied by the regulator. Changes in the field current occur with an L/R field-winding