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The Highs and Lows of Automotive Power Management
Hybrids require higher voltages, ignition off requires lower current
Randy Frank, Contributing Editor -- Design News, August 11, 2008
Power has always been a problem area for automotive electronics with integrated circuits (ICs) both causing the problem and solving it. With the earliest application of semiconductors in the mid 1970s, the presence of high-voltage transients in excess of 100V, as well high-energy spikes, most notoriously, the automotive load dump, were discovered. These transients caused failures that previously had not occurred with electromechanical components. Today, two major power management challenges occur with the need for lower current draw and the ability to handle higher voltages in hybrid vehicles. However, improved efficiency in the power supply is also required to reduce heat and avoid the weight increase from heatsinks.
Efficiency, Heat and Weight
One approach to increased efficiency and less heat uses a switching regulator/converter instead of a linear low drop out (LDO) regulator. “We have a number of automotive customers working on a number of switching power supplies and quite often efficiency is one of the deciding factors,” says Kevin Daugherty, field applications engineer, National Semiconductor.
While automakers like the idea of the efficiency of switching power supplies compared to a linear voltage regulator, the switching noise and potential electromagnetic interference (EMI) problems, as well as the increased cost, have been issues. “It is really coming down to the fact that they have too much heat,” says Daugherty. “They are really forced to go to a switcher.”
While it still takes a fair amount of experience and skill to deal with the switching transients, IC suppliers have field application engineers at auto suppliers to help. “With the proper layout, you can solve those problems without too much difficulty,” says Daugherty. He has helped with board design and the selection of components that has simplified the transition from linear voltage regulators to switchers.
One of the possible choices is National Semiconductor's LM5642, a dual controller that is quite efficient in driving external FETs. It has an external oscillator sync that allows the design of a spread spectrum for higher power and shifts the frequency away from the tuner. This minimizes the EMI emissions without having to go to a shield, according to Daugherty. PCB layout considerations include using Kelvin traces for sense lines, separate power and signal grounds and minimizing the copper in the switch node, while maintaining a low impedance power path.
A Key (off) Issue
“With all of the additional electronics going into cars now, the key-off current drain is a key issue,” says Kevin Anderson, analog automotive strategy and product definition manager, Freescale Semiconductor.
As a result, the current draw from every module is extremely important. “They basically always have three things that are always on in the module, the communication protocol whether it be LIN or CAN, you have the regulator supplying the voltage to keep those alive and you have a micro of some type that also has a sleep mode or low quiescent current mode,” says Mark Scholten, automotive field applications engineer, ON Semiconductor. Today, a low drop out regulator provides an answer to power lower current loads. “With an NCV8660, with 100 µA of load current, we're doing 30 µA or less of quiescent current on the regulator,” says Scholten. “And we are constantly pushing that down.”
However, higher current loads necessitate a transition from LDOs to switching regulators/controllers. “Under 100 mA is in the LDO range, above 1A is in more of the controller range,” says Art Eck, senior product marketing manager, portable applications, Microchip Technology.
A transition in microcontroller (MCU) technology is one thing that is pushing the requirements up. Freescale's Anderson says there is a definite trend to move from 16- to 32-bit MCUs for applications such as body control and air bags. “Once you get to 32-bit solutions, you normally have to go to a switch mode topology,” he says. “You have to have it for the efficiency.”
Willie Fitzgerald, manager of Microchip Technology's Automotive Products Group, also points out that for modules that must remain functional even when the ignition is off, such as the keyless entry receiver, the sleep mode current drain is critical. “Some of our products that we have spec'd in the market actually are operating down around 0.1 µA in the sleep mode,” says Fitzgerald. This level would have been closer to 1 µA just a few years ago.
“Most of the car manufacturers are coming up with power management schemes across the vehicle,” says Freescale's Anderson. By taking a complete electrical/electronic architecture approach, appropriate nodes can be put to sleep. “In order to do that, it's more than just the power management piece, it's how the network works with the power management,” he says.
ICs called system base chips (SBCs) have either a CAN (the primary vehicle bus) or LIN (a low-cost vehicle bus) transceiver with voltage regulators and sleep mode, wake up and other features. With this approach, a part can be commanded to go to sleep and enter a very low power mode.
Some of the requirements from automotive electronic suppliers go down to around 10 µA per module. “This is a very difficult target if you are going to maintain any kind of wake up because something has to be alive to wake up,” says Anderson. While the lowest current-draw parts that meet these targets are not released products, currently available units are in the 50 to 60 µA range such as Freescale's MC33989 CAN and MC33689 LIN SBCs.
“LIN is definitely being driven by the European community,” says Microchip's Fitzgerald. “But the U.S. is moving in that same direction.” He expects that from eight to 15 nodes could be LIN in a vehicle where they are used extensively.
Hybrid Voltages
In hybrid vehicles, the issues occur when the vehicle is on and power management is quite different. “There, you have runtime current issues,” says Anderson.
In the high-voltage area, hybrids are pushing the voltage limits of the insulated gate bipolar transistors (IGBTs) qualified for automotive applications. On a full hybrid, the battery voltage is around 350V. “When you have the regenerative mode, braking the car through the generator into the battery, the voltages are going way over 400V,” says Dennis Stephens, principal staff engineer, Continental Automotive. The voltage could go as high as 460 or 470V and today, the high voltage automotive modules are only rated at 600V.
“You have di/dt's across the inductors inside the module causing voltages on those IGBTs in excess of 600V,” says Stephens.
While the motor control industry uses 1,200V IGBTs in addition to 600V units because of the different line voltages, industrial IGBTs are not qualified for automotive applications. Equally problematic, intermediate 900V IGBTs that would be more appropriate for automotive applications do not exist. With today's low hybrid volumes, the 900V part would be unique to automotive, so unlike the lower voltage issues with low current, semiconductor suppliers are not rushing to solve the high-voltage problem.
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Highs and Lows of Power Management
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