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Most solid-state DC motor controls presently in production and in use today utilize SCRs as the major solid-state element for the control circuit. Since an SCR is by nature a latching device and since most motor control circuits utilize some type of switching technique, generally pulse width modulated, it is necessary to switch the SCRs at a high frequency. In order to accomplish this, a complex commutating circuit is required and usually includes capacitors and extra control logic to develop the turn-off drive to the SCR. SCRs, although widely used and quite reliable, operate in a basically hostile environment because of their latching characteristic when used with DC sources, and are not the ideal element for a DC motor control circuit. Their major limitations are that they are relatively slow, which limits the switching frequency, they require more complex circuitry, and have greater sensitivity to Di/dt and Dv/dt transients, which can cause false triggering. The major reason that SCRs have been used and are being used is that transistors with adequate current handling capability and capacity to handle highly inductive loads were not readily available in the past.
Upon the development of a 10-ampere transistor, many circuit designers had used large, paralleled arrays of 10-ampere transistors for their high current switching requirements. This approach, while appearing attractive in design, also had some major limitations which did not necessarily provide an advantage over the SCR system, which they were trying to replace. Since all of these transistors had to be in parallel, emitter current sharing resistors were required to be sure that all transistors shared an equal load. These resistors reduced overall efficiency and added complexity to the circuit. In addition, arrays to accommodate the many devices necessaryto achieve the current capability required for high starting currents became cumbersome to handle. Since many devices had to be used, complete testing of all devices was too expensive and failures were prevalent. Although the SCR had limitations, as mentioned previously, their usage was the preferred solution and their limitations for a DC application were circumvented by whatever techniques were available.
In the past, motor controls with high current capability were necessary for high reliability military applications, where the additional circuit complexity and the possibility of false triggering of SCR circuits could not be tolerated and the extra space required for commutation was not available. In addition, paralleled arrays of power transistors and their inherent unreliability and space requirements could not be tolerated; thus, newer ultra-high current transistors were required to fulfill the control function.
Providing a transistor with high current capability, which is capable of handling the energies that high inductive loads present, involves more than simply utilizing standard semiconductor technology with large silicon chips. Transistors were designed with the sole objective of switching high current in the order of 200 to 500 amperes while being able to withstand inductive energies in excess of 6 joules. These transistors utilize large silicon chips, 527 mil. in diameter, for 200-ampere capability and 820 mil. In diameter for 500-ampere capability. The desirable characteristics of these devices for motor control applications are not just attributable to their large diameter but mostly to their basic construction.
The silicon wafers are processed by a single diffused technique which results in the wafers having extremely wide base widths. Wide base widths are a prime factor in providing resistance to second breakdown, both forward biased IS/b (Fig. 1) and reverse biased ES/b (Fig. 2). High second breakdown capability is the most important factor in the utilization of transistors in motor controls since the motor presents a highly inductive load to the transistor. The ability of the transistor to withstand the energies caused by this inductance is measured in terms of its second breakdown capability.
Since transistors inject carriers mainly along the periphery of their emitter fingers, it follows that the greater the total periphery, the higher the gain a particular transistor will have at a specific operating current level. A transistor designer will, therefore, normally strive to maximize the emitter periphery per unit area of silicon in order to achieve the highest current gain (hFE) at a specific current level. In order to ensure that these transistors would have the greatest possible safety margin, even though they would not normally be expected to dissipate great amounts of power, they have been designed to be able to dissipate substantial power and thus have low thermal resistance Ojc. It is for this reason, among others, that the chip area has been made large, and that the emitter periphery per unit area was not optimized. Limitation of total periphery is the trade-off that must be made because of the constraints of photolithography and metallization techniques. Most transistor manufacturers use aluminum metallization since it has many attractive advantages; among these are ease of application by vapor deposition and ease of definition by photolithography.
A major problem with aluminum is that only a thin layer can be applied by normal vapor-deposition techniques. Thus when high currents are applied along the emitter fingers, a voltage drop occurs along them and the injection efficiency on the portions of the periphery that are furthest from the emitter contact is reduced. This limits the amount of current each finger can conduct. If copper metallization (Fig. 3) is substituted for aluminum, then it is possible to lower the resistance from the emitter contact to the operating regions of the transistors, which is the emitter periphery. This will help to keep all areas of the emitter periphery injecting at the same level. These devices successfully utilize a relatively thick copper grid which makes contact with the entire emitter as well as the base. Since copper is an excellent conductor, all areas of the emitter are at the same potential and working well, which is why the high current capability is achieved. The effectiveness of this copper contact manifests itself by the much lower saturation voltages, Vce(sat), obtained at high currents. In general, saturation voltages of these devices are well below one volt at anywhere between 50 to 300 amperes (Fig. 4).
This quality of metallization also allows the devices to have uniform current densities throughout the active regions. This is particularly important for second-breakdown (SOAR) capabilities. While particular transistors may have usable (Fig. 5) current gains at high currents, in order to effectively utilize the device, they must also have the capability of being switched through their active region without being destroyed. In the active region, the transistor will see short periods of a combination of high current and high voltage, which will result in high peak power levels. This occurs while the transistor is being switched from on to off, or vice-versa. Whether the transistor will be useful in a given circuit will be determined by its ability to withstand this combination. This ability can be shown in terms of the device's forward safe operating area.
As might be guessed, the key to success is to produce a large area junction free from defect, as well as to effectively solder the silicon to a suitable heat sink. It is difficult to solder a large area silicon device directly to copper without the silicon cracking due to thermal mismatch. To avoid this, a molybdenum slug is placed between the silicon and the copper. Both the molybdenum and the copper are brazed, and the silicon is then soldered to the molybdenum (Fig. 6). The most critical junction in the entire operation is that between the silicon and the molybdenum, which must be void free. Since this is the first interface between the silicon and the outside world, it is of utmost importance that this junction be of high enough quality so that uniform current and thermal density will result. This will limit hot spot generation under the silicon and provide and exceedingly high Safe Operating Area capability. The modular approach seems to be the most effective method of manufacturing large area devices since after the module is assembled, the device can be tested for its Safe Operating Area capabilities, as well as its high current static characteristics. At this point, those units successfully passing the SOAR test are categorized and inventoried. This technique provides the flexibility to place the module in any package or supply the module alone as a complete transistor for hybrid circuits. This modular technique is extremely cost effective since it allows us to only assemble into packages those transistors known to be of the highest quality.
When considering motor control circuits (for electric vehicles) we must realize that starting currents can run up as high as 1,000 amperes; therefore, making connections and packaging of devices a major consideration. Thus a modular approach, whereby the basic transistor modules with a capability to switch either 200 amps to 500 amps, was developed in such a manner that they would be completely flexible as far as packaging was concerned. A Powerblock Power System concept was developed, whereby the individual transistor could be supplied as a single element in a copper package with copper leads capable of being bolted to directly. Alternately, this same element could be supplied in a system, whereby a number of devices could be packaged in a "black box" configuration (Fig. 7).
Because yields are extremely important in processing devices such as these, great control is necessary over the process. This degree of control yields an extra benefit, in that the electrical characteristics (namely the Vbe/transconductance and the hFE) are precisely controlled. Thus arrays of these devices are easily matched. Current sharing between them, therefore, is excellent and large arrays capable of switching in excess of 1200 amperes are a reality and are, in fact, used in great quantities for military motor control applications with the utmost in reliability. Literally thousands of these devices are in the field and no field failures have occurred to this date although, admittedly, the devices have been conservatively specified. Systems utilizing the Powerblock Power System concept have an additional advantage of being repairable (Fig. 8), in that if any element within the system fails, only that element need be replaced. Higher voltage versions of these systems are available up to 600 volts at present.
At present, most of these devices are being utilized for Military-Aerospace applications since these applications require the utmost in performance and reliability, with cost being of secondary importance. Because of the market that they were designed to serve, their specifications are extremely conservative. In addition, to assure reliability, all systems are 100% tested at their maximum rated current for all major characteristics and burned-in at maximum rated power.
In order to reduce the cost of these devices so that they can be economically utilized for industrial and commercial vehicle applications, some changes will be required. It will be necessary to re-evaluate the safety factors built into the specifications as they now exist, as well as reduce the amount of reliability testing that is presently performed. In addition, re-designs, which could be incorporated with large volume production orders, could also reduce the cost. We are totally convinced that even when these devices are used to their upper limits, they will be more reliable and provide superior performance than SCRs or paralleled lower current devices. Usage of these systems in electric vehicles is still limited, however, by the fact that most electric vehicle manufacturers are more experienced at SCR systems due to their high cost at low volumes. Without a doubt, the technology for usage of high current transistors is readily available and proven. It is only the merging of design efforts and the prices which will result in a successful electric vehicle using this technology. We are looking to work closely with companies who have sufficient expertise in power semiconductor circuits who can specify their requirements to utilize these devices to their practical limits.
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