FEATURE SWITCHES & RELAYS Using PMOS relays in automotive
Richard Thornton, Managing Director at Panasonic Electric Works UK explores the use of PMOS relays in automotive applications and how this is gaining new ground for power systems in cars of tomorrow
N
obody knows how long conventional power trains will continue to dominate
the automotive market, but it is certain that the era of alternative power trains has long begun. For the electric vehicles of the future and their highly complex energy storage systems, intelligent and above all safe battery management systems are of utmost importance. To meet stringent demands for reliability and quality, modern semiconductor relays are used in the areas of isolation monitoring, cell balancing and wake-up switches. DC voltages ranging from 100V to 1000V
typically found in hybrid and electric vehicles pose a high risk for people. Therefore safely insulating the passenger compartment from the high-voltage system has top priority. For safety reasons, both of the battery's poles must be completely isolated from the auto body's ground. A multi-redundant system that continuously monitors the isolation status would initiate a complete shutdown of the power supply in case of failure. High demands are not only placed on the system as a whole but also at the component level. Panasonic PhotoMOS relays are a special
kind of semiconductor relay using MOSFET technology. In the PhotoMOS input circuit a low level operating current of just a few mA triggers an LED to emit an infrared signal. After passing through a translucent resin the light is detected by an array of solar cells integrated in an optoelectronic device that converts light into voltage. This kind of non- conducting electrical connection insures that the input and output circuits are isolated from each other. The photovoltage provide a trigger that in turn controls the gates of two source-coupled power DMOSFETs (Double Diffused MOSFETs) on the output side. These power transistors are located directly in the component's output circuit. The integrated trigger circuit is activated at a certain photovoltage threshold and switches the output on and off to produce a defined switching behaviour analogous to the operation of an electromechanical relay. Depending on the PhotoMOS type, the I/O
isolation voltage can be as high as 5000Vrms, which is sufficiently high to guarantee the isolation demanded between the HV and LV sides. For DC, this voltage can be switched by each of the output transistors individually while for AC loads by the bidirectional anti- serial switching alternating from both
24 JUNE 2015 | ELECTRONICS
analyses. This procedure guarantees optimal performance of the cells and prevents them from being overloaded, which in turn positively affects lifetime. PhotoMOS relays function to turn on and off parallel resistances and hence control the charge current. The fast, quiet and bounce-free switching combined with low contact resistance of the low-voltage types give PhotoMOS relays ideal specifications for cell balancing. Depending on the application,
transistors. To guarantee breakdown voltage on the PC board, automotive PhotoMOS relays in a DIP5 package are used to monitor isolation. In comparison to the standard DIP6 package, the output pins have an increased gap of at least 5mm.
PHOTOMOS RELAYS PhotoMOS relays can be used as a fast electric switch for detecting leakage current or monitoring isolation. If the batteries are connected so as to be electrically isolated from ground, leakage current will activate a PhotoMOS relay.
communication between the HV and LV sides may be essential. In the simplest cases a mere wake-up signal from the LV electronics suffices to activate a microprocessor in the HV system. In this area PhotoMOS relays possess the ideal requirements for such applications due to their high I/O isolation of 5kV and the linearity of the MOSFET transistors at the output. Regardless of the communication direction the systems remain isolated from each other solely due to the opto-electrical isolated coupling within the PhotoMOS. The rigorous demands placed on
components for automotive applications naturally also apply to PhotoMOS relays used in electric vehicles. This is especially true for applications relevant to safety such as isolation monitoring. Many parameters must be considered in order to ensure reliable design. The individual requirements required by various applications cannot be summarised in a single, general data sheet. For this reason Panasonic offer application-specific special types that have been thoroughly tested for compatibility so that they can meet the high demands placed on them in the automotive sector. Although choosing the best component
A further area of use for PhotoMOS relays is for cell balancing. In current hybrid and electric vehicles the NiMH batteries used so far are being increasingly replaced by more powerful lithium-ion batteries. Since lithium-ion batteries are extremely sensitive to overloading and exhaustive discharge the cells must usually be controlled individually when being charged. Batteries with many cells have each cell equipped with its own connection so it can be charged separately. PhotoMOS relays can be used in such cases to make sure all cells receive an identical charge. Typically to achieve an equivalent charge level for the individual battery cells, units are balanced on the basis of charge level
Figure 1:
Schematic representation of a PhotoMOS relay
requires due consideration PhotoMOS relays that qualify for use in automotive applications are characterised by strict, special manufacturing processes. Only double- moulding types are used because their internal design allows them to cope with the harsh ambient conditions in automotive applications such as those subjected to continuously fluctuating temperatures. Even choosing which components to use at the end of line testing is subject to stringent control requirements. Finished products are put through special cycles in 100% manufacturing end tests to guarantee maximum possible quality. Distinctions range from packaging to date coding on the components that permit optimal traceability of each part.
Panasonic Electric Works UK
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