In one such decentralized architecture, a 42-V integrated starter alternator (ISA) supplies 42 V directly to 42-V loads and indirectly to distributed 14-V and lower-voltage loads (Fig. 2). Many medium-power step-down converters, which are from several watts to a few hundred watts, will be utilized to supply power to 14-V loads located throughout the vehicle. These devices can be placed either remotely, at either the load or the control electronics, or in a central module that provides power conversion for several loads.
For example, ON Semiconductor's CS51022 pulse-width-modulation controller may be combined with the MTP16N25E power FET and other discrete semiconductors to create a medium-power point-of-use dc-dc converter for applications of up to 100 W. This circuit is ideally suited for powering high-power 14-V loads in a 42-V system. Lower-power loads—those less than approximately 20 W—can be powered by integrated supplies. This architecture has the additional benefit of providing a tightly controlled 14-V bias, which will extend the lamps' lifetime.
For 14-V automotive loads that extend up to a few hundred watts, it's possible to employ a dc-dc converter reference design based on the CS51022 (Fig. 3). This circuit could be applied in a distributed system, eliminating the "single point of failure" associated with centralized power systems.
The heart of the design uses a CS51022 enhanced current-mode PWM controller. It includes many features like externally programmable undervoltage lockout, programmable soft start, frequency operation at up to 1 MHz (although this design uses 150 kHz), overvoltage protection, and 1-A sink/source of gate drive. Among its other features are low start-up current, current slope compensation, a 5-V reference, and an externally programmable dual-threshold overcurrent protection. Under extreme overcurrent conditions, that last feature can reinitiate the soft-start sequence. The controller additionally features a sleep mode in which the device's current consumption is less than 100 µA.
The system topology is set up as a forward converter. In this topology, power is delivered to the output while the primary switch (Q2) is conducting. When Q2 is off, the stored energy in L2 delivers power to the load. The transformer windings must have zero volt-seconds (on average). Capacitor C18 (820 pF) provides the path to reset the transformer core (Fig. 4).
This dc-dc converter can supply up to 84 W to the load, making it suitable for many automotive applications including lamp drivers. The CS51022A has a voltage of up to 20 V and a temperature range that's between −40° and 85°C. The shunt regulator formed by the zener diode D2 and bipolar transistor Q1 enhances the system voltage capability for this 42-V circuit.
During normal operation this shunt regulator is turned off (back biased) by tying VOUT to the output of the shunt regulator. A future version of this controller will integrate many of the external components in this design, including the shunt regulator. Plus, the IC will be able to withstand a 60-V input voltage, so it won't require the external shunt.
The next generation of vehicles stands to benefit from power management implemented with power semiconductors and smart-power ICs provided that the system voltage is restricted to permit lower minimum breakdown voltages. These advantages are even more pronounced in a dual-voltage architecture. Cost reductions for existing products can be maintained and unnecessary cost increases for new products with higher-voltage requirements can be avoided by minimizing the system voltage transients. By working closely with automobile manufacturers and automotive electronic suppliers, semiconductor manufacturers will be able to provide power-management products that meet the 42-V specifications for the next generation of vehicles.
Reference:
- John M. Miller, Paul R. Nicastri, Shahram Zarei, "Automotive Power: Future System Architectures Face Formidable Hurdles," www.pcim.com/miller1/index.html.