A high-performance replacement for mechanical relays

15th March 2016
Nat Bowers

Renesas Electronic has announced six Intelligent Power Devices (IPDs) for automotive motor and heater control applications. Integrating protection and self-diagnostic functions with MOSFET switching elements, the IPDs are compact, lightweight, power efficient and not subject to the contact wear and tear that affects mechanical relays.

Therefore they provide an extremely reliable, high-performance replacement for mechanical relays that switch the current flow on and off in ECUs.

Previously, most ECUs used relays as the switching elements in their drive circuits, with the average automobile using around 100 relays. The transition from mechanical relays to semiconductor switching began in applications like lighting control and the trend is expanding to other automotive systems, such as motor and heater control, driving demand for highly reliable and high-performance semiconductor switching devices.

Recently, stricter environmental regulations in Japan and other countries have spurred accelerated efforts to improve fuel efficiency. This has heightened the need to address demands for reduced vehicle weight, more compact and lighter ECU and low power consumption.

However, creating lightweight vehicles has been made difficult as routing of the wire harness is determined and the configuration location is being restricted due to the mechanical relay maintenance in the relay box - or junction box - that is responsible for power distribution within the vehicle and in which many of the mechanical relays are housed.

This has led to the growing demand to free placement constrains in the relay box and to allow greater flexibility of the wire harness routing by configuring the relay box with a semiconductor switch with self-protection function.

However, conventional semiconductor switches have been unsuitable for handling large currency, which has impeded efforts to develop semiconductor-based replay boxes previously. Therefore, it is difficult to deal with high current loads when using conventional semiconductor switches.

The IPDs combine Renesas’ low-loss technology and a new fabrication process with a cell size of 2µm to achieve an on-resistance of 1.6mΩ (RAJ280002, typical value when Tch = +25°C). This also enables adoption in high current applications and it further allows the devices to replace conventional mechanical relays and provides a reduced mounting area, contributing to smaller and more lightweight ECUs. As solid state devices, the IPDs deliver extremely high reliability and performance levels, operating at up to 100m switching cycles with no degradation, which is significantly higher than the approximate 10m cycles expected from a typical mechanical relay.

An earlier Renesas device, the µPD166033, would transition to the off state when the power supply voltage dropped to 4.5V as a protection function to prevent malfunctions. These IPDs incorporate enhancements that provide support for continued operation when the voltage obtained from the battery drops temporarily, such as during starter motor cranking. The guaranteed on-resistance characteristics at a power supply voltage of 3.2V allow the IPDs to be used in applications where starter motor cranking might otherwise be an issue, such as cases where the driver turns off the engine at stoplights to prevent the engine from idling. The IPDs also feature built-in diagnostic functions and proportional load sensing. This allows for clearly defined fault signals that flag the system/controller when an abnormal load condition is detected. Since the analogue current sense feedback is available, no additional load current sense elements are required.

In applications that use inductive loads (L loads) such as motors and solenoids, it is possible that counter-electromotive force from the energy that has accumulated in the L load when the device transitions from the on state to the off state could damage or destroy the device. The RAJ280002 increases the ability to withstand this energy (active clamp tolerance (EAS)) from the 260mJ of comparable earlier Renesas devices to 1,700mJ. This expands the range of large-current applications for which the IPDs can be used. In some applications it may be possible to eliminate the need for flywheel (regenerative) diodes as external protection elements, thereby contributing to more compact ECUs and reduced overall system cost.

Samples of the IPDs are available now. Mass production is scheduled to begin in January 2017 and is expected to reach 850,000 units per month by January 2018.

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