Silicon Power Device Challenges Relays

A new solid-state relay from Motorola is a cool operator-so cool, in fact, that it could be a formidable competitor for many electromagnetic relays. Unlike typical SSRs, which generate a lot of heat while passing large currents, the MC33982 silicon power switch stays cool to the touch even without a heat sink. That means that the already small (12x12x2 mm) device takes up much less space than a conventional relay while also providing an SSR's superior reliability and long life.

The key to the MC33982's cool operation is an unprecedented low source-to-drain on resistance (RDSon) of only 0.2 milliohms. Typically, this resistance for solid-state relays is at least tens of milliohms, which leads to significant power consumption and heat generation when the device conducts a current of more than a few amperes. Motorola uses its HDTMOS version of MOSFET technology to reduce this resistance, making the MC33982 useful in systems that require up to two KW of power. The device operates at 6 to 27 volts; it withstands continuous currents of about 60 amps and short-term currents of 150 amps.

To demonstrate the new SSR's current handling at the recent Convergence exhibit in Detroit, Motorola wired six of the devices in parallel and used them to replace a car's starter relay. Starting the car, says Kevin Anderson, Design and Applications Manager for Motorola Semiconductor's Analog Products Division, resulted in a current surge of over 750 A, or 125 A per SSR. The SSRs remained cool to the touch, although Anderson cautions that starter relays are not an intended application for the devices. The MC33982 could, however, serve functions such as operating car door locks, and Anderson notes that because of its very long life, it could go into a door panel without the need to design access for later replacement.

The device adds to its appeal with the ability to emulate a slow-blow fuse. You can program it to withstand a certain amount of current for a certain amount of time before it turns off, a useful feature in applications like lamp loads where there's a large inrush current, followed by a lower operating current. "A relay doesn't have that capability," Anderson says, "so you've got to size the relay for the maximum inrush current."

The MC33982 does cost more than an electromechanical relay. In fact, its starting price of $3.90 in large quantities is easily double some relay prices or even beyond. However, because the device's on-chip intelligence provides control and self-protection features that relays have to get from added components, Motorola claims that its overall system cost, not just device cost, is competitive with electromagnetic relays. The company also notes that life-cycle costs are lower for solid-state relays, because of longer lifetimes and the possibility of not having to design in physical accessibility for device replacement.

The MC33982's primary market is the automotive field, and Motorola claims that a major automobile manufacturer has already signed on to use the device in its 2006 models. Motorola isn't naming the customer, or any customers, however, nor will it say how the customer is applying the device.

Motorola is also targeting applications other than automotive for the new device. "Because it was designed for the automotive market," Anderson notes, "it goes into overvoltage protection at about 26 to 28 volts." That voltage, Anderson adds, makes the MC33982 well suited for 18-volt battery-powered hand tools and many 24-volt industrial applications. In addition, he says, Motorola is designing other versions.