VS - Electrical Systems From Stop-Start to Hybridization

The use of MOSFET’s to achieve the rectification function conventionally performed by diodes, improves efficiency, thermal performance, power density, and reliability. Higher efficiencies can be achieved by synchronously switching the power transistors to emulate diodes, taking advantage of the transistors’ low conduction losses. Synchronous switching means getting a transistor to turn on and off according to the polarity of the AC waveform so it acts as a rectifier. Choosing high performance MOSFET transistors reduces the forward voltage drop to a fraction of what any diode can achieve. Hence, the synchronous rectifier will have a much lower loss than a diode, improving the alternator’s efficiency. Synchronous rectification is one of the reasons for the outstanding 82% efficiency of the StARS starter alternator, which is a full 10% higher than the best conventional alternators on the market.

For a higher efficiency, conventional rectifier diodes are replaced by MOSFET transistors.

Implemented in Valeo StARS systems, the synchronous rectification allows a CO 2 reduction of about 2 g/km.

Active rectifier losses 70w@200A

Diode rectifier losses 350w@200A

Efficiency + 8 points

140 120 100

D

A

G

MOSFET synchronous rectifying Reduces voltage drops by factor of 5 versus diodes Generates less heat Gain in Amps.@Idle RPM Gain in efficiency by 8 points CO 2 reduction of about 2g/km

C

S

80 60 40 20 0

Current (A)

0,20V

1,00V

0

0,40V 0,60V 0,80V

1,20V

V drop (losses,...)

Thanks to their micro-electronic structure MOSFET’s are able to almost eliminate the voltage drop during current rectifying: 0.2 V instead of 1.0 V @200A

Given that: power equals voltage times current, power losses inside the bridge are divided by factor 5.

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