n the latest generation of car exterior mirrors, the number of embedded functions has dramatically increased to include: the side repeater, or directional indicator, the door's outside lights, the defroster, and the folding in and adjustment of the mirror, to name a few. On top of these features, the electro-chromatic (EC) control has recently been adopted - this is required at night to dim headlight glare from cars behind the driver. EC mirrors automatically darken, preventing the driver from being dazzled by the reflected light.
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EC glass is a capacitive-resistive equivalent load; its control requires a sophisticated strategy. The voltage range of the EC characteristic, between transparency and maximum dipping, lies between zero and 1.2V, and the current profile is around 150-250mA at its highest point at the beginning of the dip. With a classic linear controller, starting from 12V battery voltage, too much high power can be dissipated (around 12-1.2V x 0.15A = 1.62W); here a switching controller is not recommended since it can lead to electro-magnetic control (EMC) issues. For these reasons the first EC control circuitries, embedded in automotive devices, were based on a shunt controller lying in parallel to the EC glass (Fig 1).
With this topology, a 6-bit digital-to-analog (D/A) converter, controlled by an external microprocessor, sets different (and accurate) voltage references for the shunt controller as a power output of the N-Channel MOSFET T2. The maximum power dissipation of the latter is 1.2V x 0.15 = 0.18W. The resistor R1 determines the peak current that the EC glass sinks to during the dipping phase. At maximum transparency, that is 0V control voltage, which means that no current flows into the saturated shunt controller, and T1 switches the current completely off under these operating conditions. The power needed to control the EC mirror, dissipated by the MOSFET T2 and the shunt resistor, comes mainly from the external component. The shunt resistor must be rated high wattage (its power dissipation can be around 2W in case of high battery voltage), and it must be a precision resistor (low tolerance) in order to ensure accurate current control of the EC material.
Obviously, this component has a big impact on cost and space of the mirror electronics. Additionally, the new generation of EC glass requires a "fast discharge" in order to implement a prompt brightening of the mirror. Another topology must be used to optimize the control and to make it possible for mid-end cars to adopt EC control.
Historically, several actuator drivers for car door loads have been offered. These devices are characterized by a scalable actuator driving concept that is also package and software compatible, to satisfy the multiplicity of door electronics variants. These drivers support all regular door zone loads such as door lock motors, mirror adjustment, mirror folder, heater (or defroster) and several lighting functions from incandescent lamps to LEDs.
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Nikkei Monozukuri |
Nikkei Automotive Technology |
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