2018 Thirteenth International Conference on Ecological Vehicles and Renewable Energies (EVER) MOSFETs used in ideal diode circuits for Lundell alternator rectifiers Alex Van den Bossche EELAB Ghent University Technologiepark 913 9052 Gent Belgium Email: Alex.VandenBossche@ugent.be Salim Haddad Département de génie mécanique Université 20 août 1955 Skikda Algeria Email: s.haddad@univ-skikda.dz Dimitar Petrov Vencislav Valchev Department of Electronics Technical University of Varna Email: muta4eto9303@gmail.com vencivalchev@hotmail.com com 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 a large quantity such a MOSFET can cost less than 0.5 Euro, and the cost of the circuit is not larger. Keywords—ideal diode; energy saving; automotive; Lundell alternator; synchronous rectifying. I. INTRODUCTION Although 48V systems are proposed now, still a lot of 12V circuits exist, but even 48V systems carry similar currents as the power requirements did increase as well. The reason is that, in start-stop systems also some braking energy is recovered and that in mild hybrids, some city traffic and also automatic parking can be done with it. The maximum DC output current of a Lundell alternator can be 100 A or even more, with an efficiency hardly exceeding 50% [1],[2]. The delivered voltage is about 14V at the terminals, to get some 13.8V at the battery. At a 2.2V diode bridge drop, about 15% of the output power is lost in the diode bridge. Usually press-fit normal diodes or Zener diodes are used in the rectifier bridge. Zener diodes are used to act as a “load-dump” if the battery is disconnected, but they have even more voltage drop in the forward direction than usual rectifier diodes. Normally, Schottky diodes could reduce losses, but large Schottky diodes are quite sensitive to local hot spots that may destroy them. In large size Schottky diodes, manufacturers have to add resistivity in the silicon to equalize the current density; this resistivity reduces a part of the benefit. A solution is using a MOSFET instead, that is turned on whenever the drain- source voltage gets negative. MOSFETs in the conduction mode do not show hot spot problems, as the channel has a positive temperature coefficient. It is an advantage if the stand by current is very low, so that the battery is not discharged by it. It is an additional advantage if the circuit only has two connections and does not need an auxiliary supply. Solutions have been proposed using only two driving transistors, but the estimate is a current drain for 6 transistors of 900 mA, this would empty a 30Ah battery in 33 hours [3]. Other circuits use about 1mA/ideal diode in stand-by mode, but are only good for battery swapping, so static switches [4]. Some IC’s exist with an internal charge pump, such as LTC4357 [5], giving a consumption of 3mA for a bridge, or about 5mA for a faster one LTC4218 [6]. In the proposed topology, there is no current in stand- by mode (only a few μA component leakage), so even a lower leak current than Schottky diodes, and about 0.1mA current drain/internal supply due to resistors. So, the proposed circuit does not reduce the self-discharge time of the battery. II. PROPOSED TOPOLOGY AND CIRCUIT Synchronous rectifying is widely used in other types of switched mode circuits [7]. In alternators some circuits propose the use of a reference diode, compared to the voltage of a drain sensing diode and a differential amplifier to control MOSFETs [8], or a lot of circuitry is used [9]. The gate driving can indeed be done by an op- amp, but if it has to sink at least some 50mA, it often draws 2mA or more from its supply in standby. A current of 2mA during for example 20ms is 40μC, this charge