As automotive engineers continue to implement more electronics on vehicles and tighten specifications and expectations for high reliability, the venerable mechanical relay has become endangered. A new power IC, an intelligent self-protected silicon switch (SPSS) designed for high-current grounded loads, may now provide an interesting displacement factor.
An extremely low (less than 2mΩ) on-resistance, this high side power IC integrates several features for replacing electromechanical relays, fuses and discrete power MOSFETs in power management applications.
Some major reasons for replacing relays include improved reliability, reduced audible noise, and low electromagnetic interference (EMI). Despite these shortcomings, the amount of relays on vehicles continues to increase — especially to handle infrequent loads such as seat positioning motors. Power MOSFETs with external circuitry or power ICs with built-in circuitry have been able to provide a technical solution for replacing relays for many years. The cost for the semiconductor solution, especially for handling higher loads has usually been a factor in favor of relays. However, cost targets — especially at the system level — may finally be achievable based on advances in the power IC integration and packaging technology, as well as tougher automotive application requirements.
When a passenger compartment is designed to limit engine and road noise and the number of relays increases, the audible noise of relays becomes even more apparent to the driver and passengers. The quietest passenger compartments are found in the high-end vehicle, but midrange and even low-end vehicles are also much quieter than they were a decade ago. The number of heating, lighting, and motor loads where a relay may be required easily exceeds 50 in a high-end vehicle, yet even a midrange vehicle has a minimum level of 10. Noise from the low number of relays can be at the very least annoying, but the higher count makes it unsatisfactory.
Since a relay has contacts that arc, EMI and wearout can be problems that reduce a vehicle's overall reliability. Wearout was a problem when vehicles were designed for 50,000 miles, yet today's vehicles can have 70,000 or even 100,000 mile warranties for certain models. EMI was a problem when only a few MCUs were used on vehicles. Today, high-end vehicles can have more than 80 MCUs, while midrange vehicles typically have more than 20 MCUs. As a result, relays are high on auto manufacturers Pareto list for impacting quality and reliability, adding to the cost beyond their purchase price.
As the vehicle usage of relays was increasing and relay-associated problems were multiplying, semiconductor technology advances allowed significant improvements in the efficiency of the power MOSFETs and functionality of power ICs. Advanced multidie packaging has brought together the semiconductor pieces with the current handling, heat dissipation and pinout requirements required for complex automotive applications without adding a large cost penalty.
Efficiency of the Switch
The benchmark for low on-resistance in automotive and many other applications is the lowest level that can be achieved in a TO-220 or D2PAK. For 40V rated MOSFETs, values as low as 2.3mΩ have been announced. In a D2PAK, the package (typically with three wirebonds) represents about 1mΩ of the resistance! The 40V power switch in the MC33982 is an N-channel MOSFET that is less than 2mΩ at the packaged level. Optimizing the die size to meet a 2mΩ specification required a layout that allowed a number of wirebonds to be attached to the die and to the external source connections. This reduced the packaging resistance and provided high current connections for the switch.
Current sensing capability was integrated in the power MOSFET to simplify control and protection schemes in the integrated circuit. A lossless current sensing technique is achieved by isolating a portion of the power MOSFET cells at the die level to provide a ratio indication of the total current flowing in the SPSS. To provide high current and low current sensing levels, two different current sense ratios are used. The high level for currents approaching 150A has a ratio of 1:6,000. For improved precision below 50A, a ratio of 1:40,000 is used allowing the accuracy to be 13% for currents in the 25A to 50A range.
Intelligence in the SPSS
The analog mixed-signal IC in the SPSS incorporates overvoltage, undervoltage, overcurrent, overtemperature, ground disconnect protection and open load detection found in many power ICs. However, a serial peripheral interface (SPI) communication protocol and many programmable protection and detection items enhance the capability of the SPSS for relay replacement and other high power load control applications. In addition, the SPSS has a configurable watchdog timer, Output-OFF open load detection, an internal regulator, 16V reverse battery VPWR protection and a 6V to 27V operating voltage with standby current less than 5.0µA. The SPI protocol reports and controls most of the functions. However, either parallel input or SPI control can be used for the SPSS.
As shown in Fig. 1, on page 58, the 3-MHz SPI interface has a full duplex, three wire synchronous data transfer with four I/O lines: SI, SO, SCLK and CS-bar). The SI/SO pins of the MC33982 follow a first in — first out (D7/D0) protocol with input and output words transferring the most significant bit (MSB) first. SPI communication is accomplished using 8-bit messages. Multiple messages can be transmitted in succession to accommodate those applications where daisy chaining is desirable, or to confirm transmitted data, as long as the messages are all multiples of 8-bits. This allows the user to control multiple outputs with minimum pins and send one message with a turn delay to minimize in-rush current.
Some unique features of this power chip are two levels of user programmable current limits as shown in Fig. 2. The SPSS has two programmable overcurrent-high detection levels and eight programmable overcurrent-low detection levels for maximum device protection. The two software selectable, overriding overcurrent detection levels (IOHC0 and IOHC1 in Fig. 2) are nominally 150A and 100A.
There are eight overcurrent-low detect levels (IOCL0 through IOCL7). Any one of the IOCLx levels can be selected by the user to determine a load current in nominal levels from 15A to 50A in 5A increments. If the selected level is exceeded for one of four selectable periods of time (tOCL0, tOCL1, tOCL2, tOCL3 with typical values of 155, 9.7, 1.2 and 0.15ms, respectively), it will latch the output off. If the load current reaches the selected overcurrent-low detect level it goes into current limit. The SPSS will latch off if it is still current limited after a period of time (tOCLx). Anytime the current reaches the selected IOCH level, the device will latch off immediately, regardless of the selected tOCLx. These selectable time and current levels can be used to emulate slow-blow fuses.
The output rising and falling slew rates and turn-on and turn-off delays are also adjustable. This makes the smart SPSS a flexible and adaptable switch for many power control applications. For example, the EMI can be minimized for specific applications by using the slower slew rates.
Chip scale packaging is allowing smaller package sizes and improved efficiency for many semiconductor products.
Managing the Inrush Current
One of the more interesting departures from the traditional relay applications is the use of the integrated SPI circuitry in the SPSS to create a load daisy chain. As shown in Fig. 3, on page 60, the CS-bar, SCLK, SI and SO pins of the IC and MCU only require three wires for the communication necessary to control several loads. All of the other pins are connected, as shown in Fig. 4. Only one CS-bar line is needed in the daisy chain configuration. All SPSS's are selected at the same time but each SPSS functions as a shift register moving data down the line from SO to SI until it arrives at the proper device. The last SPSS in the chain has its SO line connected to the SI line of the MCU to complete the loop and allow the diagnostics to function properly. The SPSS SPI bus speed of 3 MHz will be reduced as a result of propagation delay of moving data from the various SO to SI connections. However, this is still more than sufficient to control most mechanical loads.
Very few additional components are required to interface the SPSS from a grounded load to an MCU. Fig. 4 shows the circuit for a single load controlled by the SPI. For high current applications, the current that the traces on a printed circuit board can safely handle can be an issue. The MC33982 allows the turn-on delay to be programmed so that the entire inrush current does not occur simultaneously. An output switching delay register (OSDR) is used to configure the SPSS with a programmable time delay that can be used when turn on is initiated via the SPI. Whenever the input is commanded to transition from 0 to 1 via SPI, the output will be held off for the time delay configured with the OSDR register. This allows the user to minimize inrush currents when several daisy-chained MC33982s are controlled by a common CS and turned on simultaneously. Up to eight selectable output switching delay times can be used that range from 0 to 448ms.
This self-protected silicon switch technology has taken a significant step towards replacing relays. The combination of robust power die, advanced mixed signal technology, and innovative packaging offers compelling solutions for many high current switch applications where protection and diagnostics are required.
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