DC cooling fan controller IC eliminates components and failure mechanisms Brad Marshall President Melexis, Inc. Ted Kawaji Vice president, Engineering Melexis, Inc. Abstract Cooling fans are a critical part of computer systems. Each generation of PCs provides CPUs with increased processing speed (clock frequency), which directly increases power dissipation. Cooling fans are required to control the increasing power levels, and add cost as well as reliability risks. Cooling fan problems account for a significant share of failures in Personal Computers, workstations and power supplies. Such failures are very costly in; lost data, lost working time, repair cost and frustration. There is tremendous pressure on cooling fan makers to improve quality, performance and reduce cost. Fan users want more for less! This means a new approach is needed, rather than simply trying harder. The solution presented here is a family of one-chip fan controllers, which replace up to 12 components used in conventional cooling fans, with 1 component. Creative mixed signal circuit design and High Voltage CMOS wafer technology has been used to enhance performance, improve reliability and reduce cost with a new single chip fan controller. Objectives of the project: Cost: The controller, with only 3 or 4 leads, eliminates 6 to 12 components, 12 to 20 solder joints as well as a large percentage of assembly, test, repair and inspection labor. The problems were solved using the newest Mixed signal, Analog/Digital design and sub-micron CMOS silicon technology allowing a complex function in a small chip keeping the cost low. The cost savings, in labor and components, has been estimated by some customers as 30 to 50% as well as improvements in quality, reliability and performance. Testing is simplified, since Dead Angle test is no longer needed. With only 3 to 4 solder joints and one component, in-process testing and inspection is greatly simplified and cost reduced. Performance, features: Using sub-micron CMOS silicon technology, Smart features are possible such as; locked rotor protection, locked rotor Alarm, Tachometer signal and slew rate control. Locked rotor protection enhances motor energy efficiency, since it is not necessary to add resistance to the motor windings to limit locked rotor overheating. Slew rate control reduces inductive transients, reduces audible noise occurring at abrupt on/off transitions and reduces emitted RFI. Alarm (L, R) and Tachometer (F, frequency) outputs are generally expensive additions to a basic fan motor. Reliability: The single chip Cooling Fan Controller virtually eliminates ESD failures. ESD is a major cause of electronic failures, universally estimated to cost the electronics industry $5B/year. Cooling fans are subject to ESD damage during fan assembly and as a finished component. Often the failure happens; hours, days or months following the ESD incident, so testing is not effective, only prevention is useful. While most fan makers protect PC boards during assembly operation from ESD, they wrongfully assume that the finished fan is immune to ESD damage, which is far from the truth. By eliminating the power supply pin of the fan controller, we have raised the ESD tolerance of the fan from 3,000V to 15,000V, and from 3,000V to 7,000V at the IC pins. Power is supplied through the motor coils, which provide a series ESD filter as well as protection from over voltage and reverse voltage by the series resistance of the windings. Information on ESD is available at www.esdsystems.com Figure 1 shows the US79 and US80/81 Fan Controllers and the discrete component version to be replaced. Figure 1, Discrete DC Fan and US79/80/81 Fan Controller family Discrete fan circuit US79 replacement 320 International IC China Conference Proceedings
High end discrete fan circuit High end replacement How does it work? Figure 2 shows the applications block diagram of the US79 and the equivalent filter formed by the motor stator coil. The coil series resistance limits the fault current during supply voltage transients or reverse supply voltage. The series inductance and parasitic capacitance of the coil winding, provides a filter to attenuate ESD from external sources and attenuates RFI generated by the inductive switching of the driver transistors. ESD considerations Most cooling fans are vulnerable to ESD when an operator or installer is able to touch the power wires. In a normal, conventional fan, the Hall IC is not protected by the windings and many failure are caused by the incorrect assumption that the finished fan assembly is not ESD vulnerable. Over voltage Protection is provided by on-chip 35V Zener diodes capable of dissipating supply voltage transients, switching transients and Back EMF. RF noise RF emission in a conventional fan may require filter capacitors. In the US79, slew rate is controlled on-chip to eliminate the need for capacitors. Reverse voltage protection eliminates the need for a diode. Reverse current flows through the coils and the chip. This is OK up to 300mA reverse current and T ambient=70 C. For higher currents a diode should be used, or ambient temperature derated. There is no power supply pin. The power supply voltage for the Hall plate and Hall amplifier are taken from the OFF output pin, through a switch. The digital controller synchronizes the Vdd switch and the gate drive of the power FETs to insure an un-interrupted Vdd. Figure 3 shows the block diagram of the US79. The Hall plate and Hall amplifier are chopped to provide a temperature stable, high sensitivity latch. Chopping dramatically reduces errors caused by Hall Plate and amplifier offsets. Chopping is a process whereby the Hall plate offset errors and amplifier offset errors are measured in a sample and hold circuit and cancelled with an anti-phase version of the same error voltage. 1,2,3 Slew rate control is used to prevent RFI emission. An anti-crossover logic circuit is used with the digital controller to prevent both drivers from being on simultaneously. Locked Rotor is a fault condition wherein a fan rotor is somehow jammed or locked such that normal rotation is prevented. This normally results in excessive driver current, failure of the cooling benefit of the fan and overheating as well as possible failure of the fan components. Typical fan motors are designed with a high resistance to allow Locked Rotor without fatal currents. The added resistance compromises efficiency by creating unwanted dissipation in the fan motor windings. The locked rotor shutdown contains a 1 second and 5 second counter. The 1 second counter is reset by any rotor motion which triggers the Hall latch. In a locked rotor state, the 1 second counter disables the gate drive to the power FETs for a period of 5 seconds. After 5 seconds of disabled power, the drivers are enabled for 1 second. If no rotation results, the cycle continues indefinitely. This feature prevents overheating during locked rotor testing or field conditions. It also allows the designer to make a more efficient fan, since it is not necessary to add resistance for current limiting to pass locked rotor testing. Figure 3, Block Diagram of the US79 Figure 2. Motor Coil Operates as a Filter The difficult testing and remedy for Dead Angle are solved. Dead angle is a point in rotation, where the combination of active torque, passive torque and friction result in a net 0 torque. At this 0 torque point, the fan will not start. Testing is very slow and labor intensive. With locked rotor shutdown it is not possible to turn the rotor to a position of 0 torque, since it will automatically turn off/on and restart. The power transistors are N-Channel MOS, providing low loss switching and no lost power in the base drive. Bipolar chips use a lot of power for base drive. The drivers are protected by 35V Zener diodes from drain to gate. International IC China Conference Proceedings 321
The use of advanced mixed signal CMOS technology allows reliability, cost and performance advances and opens the possibilities for low additional low cost features such as; tachometer signal, current sensing, fault signals, bridge drive for single coil fans, diagnostics, speed versus temperature control, etc. The power devices use most of the chip area and added CMOS logic for these features is low in cost. Figure 4 shows some more advanced features of the US80, 81 and 83. Tach output is sent to the host computer, as in Alarm, to warn the system of a cooling failure. Thermistor input is used to provide cooling on demand, by sensing the system temperature and controlling air flow as needed. Performance features are made available by the low cost of CMOS signal processing allowing the systems designed to solve problems in ways which were not available with Dumb sensors. 1 1. Melexis, Inc 1999 catalog, pages 5-11 through 5-15. 2. Sensors Expo proceedings, May 1999, Hiligsmann, Marshall, Riendeau and Pepin, Melexis, Inc, Innovations in Hall Effect Sensing pages 417-425. 3. Sensors Expo proceedings, May 1998, Marshall, Melexis, Inc, CMOS and Hall IC Evolution pages 299-306 Author s contact details Brad Marshall Melexis, Inc 33 Guinea Road Dunbarton, NH 03046 USA Phone: (1-603) 774 4569 Fax: (1-603) 774 2569 E-mail msjo1127@aol.com Figure 4, Block diagram of the US80, 81, 83 Series Extending Voltage and current. Figure 5 shows circuitry for applying the US79/80/81 to higher current and higher voltage fans. Figure 5, Driving High Current and High Voltage Fan Motors Conclusion A Smart Sensor has been presented which provides lower cost, higher performance and improved reliability a combination which is not normally available. Normally, one pays more for performance OR reliability enhancements. Cost reduction is a result of Eliminating 16 solder joints, 7 to 8 components and PC board replaced by one three lead component. Enhanced reliability stems from using the motor coils as an interference filter in series with the IC pins, thereby minimizing the major causes of failure in cooling fans such as; reverse voltage, over-voltage surges of power supply, ESD protection of IC and ESD protection of FAN. 322 International IC China Conference Proceedings
Presentation Materials International IC China Conference Proceedings 323
324 International IC China Conference Proceedings
International IC China Conference Proceedings 325
326 International IC China Conference Proceedings
International IC China Conference Proceedings 327