Mosfets used in ideal diode circuits for Lundell alternator rectifiers

Abstract

Low voltage power applications suffer from losses in diode voltage drops. For example, the Lundell car alternator has a low efficiency, partly due to a high-current diode voltage drop of 1.1V, being 2.2V in a bridge configuration, resulting in 15% of output voltage and corresponding losses. Schottky diodes have a lower drop, but are quite fragile and seem not to be preferred in that application. This paper proposes a two terminal circuit with a MOSFET, which emulates a diode while having a very low voltage drop. The main item is that the MOSFET is turned on at a small negative drain-source voltage. This could be done using an op-amp circuit, but the used transistor array circuit can have a lower current consumption. At full load, some 73% voltage drop reduction is possible, for example while using a MOSFET of max. 0.003 ohm on-resistance at 125°C, hence the voltage drop at 100 A can be limited to 0.3 V. At a rather typical 30A DC-current load, some 87% voltage drop reduction is possible, 100mV drop compared to 0.8V for a diode. The solution costs hardly more than the usual “press-fit” diodes. The circuit has a short paid back time, by the lower use of aluminum for the heat sink and the fuel saving. In large quantity such MOSFET can cost less than 0.5 Euro, and the cost of the circuit is not larger.