IGBT-the leader in automotive ignition systems The new generation ignition system IGBT is tailor-made for the coil of the spark plug system and is rapidly becoming the mainstream ignition topology. Advances in geometry and doping profiles can enable smaller die and package sizes without sacrificing the robustness of the most important latch resistance and avalanche energy capacity. Today, IGBT products already have high-value protection and adaptability features, such as active clamping, ESD protection, logic-level gate thresholds, and gate resistance networks. In the medium term, additional functions will be integrated into the IGBT chip or implemented as a separate controller chip in a multi-chip concept. These functions include overtemperature detection / shutdown, current detection / limiting, and watchdog timer , Non-sparking shutdown and ion detection interface. The single-coil (pen-shaped coil) concept for each cylinder can fully utilize the advantages of proven effective driving of key performance and cost reduction: mechatronics and modularization. This is a difficult long-term evolution for early mechanical contact circuit breakers and mechanical high voltage distribution cap ignition via distributorless transistor ignition, and later dual-spark coils (used today) to current plug-in coil solutions. process. The "passive coil on the plug" only integrates the coil on the spark plug connector, and the switch and pre-driver (one for each cylinder) are located in the engine control module (ECU) or in a separate box between the ECU and the coil. As for whether the switch is allowed to be located inside the ECU module, each supplier of the ignition system has different internal regulations. Principle of car ignition "Active coil on plug" contains the coil, pre-driver and switch on the extended spark plug connector, one for each cylinder. They only need 4 low voltage connections to the ECU's pencil coil, so the ignition system has more functions to provide extremely high modularity, mechatronics and flexibility, so as to achieve the true "plug and play" expected by car manufacturers point". This principle is to generate a voltage equal to LdI / dt on the primary side of the transformer, and then become the spark voltage of the secondary coil. Figure 1 shows a typical pen-shaped coil circuit of a cylinder. As long as the rising edge of the trigger pulse from the ECU exceeds the threshold voltage of the IGBT, the circuit opens. The current in the primary coil ramps up according to the following formula: dICC / dt = -Vbat / Lcoilexp (t / (), where (= Rcoil / Lcoil In fact, the range of Lcoil is 1 ~ 3mH, and the range of Rcoil is 300 ~ 700m (, the result will be a primary current ramp of 5-10A / ms. Under normal operating conditions, the coil charging time depends on the application-is 1 ~ 3ms, and the peak range of the primary current before shutdown is 7 ~ 15A. When the IGBT is turned off by the falling edge of the trigger signal, the coil shaft is released. The induced voltage (-LdI / dt) in the primary coil forces the IGBT into avalanche conduction. When the counter-voltage of the gate-collector active clamp diode (VBRR, 350-450V, a safe voltage, which is below the avalanche breakdown voltage of the CE structure) is reached, the IGBT is turned on, and the feedback energy is evenly and reliably distributed in the IGBT In the entire active area. At the same time, the required spark voltage (about 40kV) is generated in the secondary coil, and its value is determined by the transformer turns ratio (generally 1: 100 to 1: 150). The basic waveform is shown in Figure 3. Selection of primary current switch Bipolar Darlington transistors are still used for primary current switching, although usage has been greatly reduced. Almost all new ignition system designs use IGBTs. The IGBT was invented by Frank Wheatly in the former RCA 19 years ago. It combines the advantages of bipolar and split-gate transistors, and has obvious advantages in a specific voltage / switching speed domain. Table 1 compares the Darlington tube and IGBT in the ignition application concept in detail. The main electrical parameters of ignition IGBT IGBTs are very suitable for ignition switches and require a large number of pulsed forward currents and avalanche energy capabilities at low switching speeds. For example, according to fmax = nmax / 120, the pen-shaped coil used for four-stroke engines must ignite at a frequency lower than 100 Hz. Therefore, at least in today's single-cycle single-spark system, the switching speed has little effect on the system. Even under the harsh conditions of up to 64 sparks per cycle, the use of IGBTs can easily be used to improve the multi-spark system of engine starting. Primary switches mainly require low Vceon (Iceon) forward characteristics. In normal operation, the energy is mainly dissipated when the primary coil is charged, and the value is Eon (t) = (Ic (t) Vceon (IC) dt. For the coil, approximately 130 ° C) determines the average junction temperature. Suppose there is a small temperature ripple whose value is determined by the heat of the circuit chip, and partly by the heat of Rthj-c and the package label. The second driving force of low Vceon is the low reciprocating voltage of the cold start of the 12V battery at minus 40 ° C, which will drop to a minimum of 6V. Because the peak voltage of the primary coil can be expressed as Ipeak = (Vbat-Vceon) / Rcoil, the lowest Vceon value is determined. Of course, this can be achieved by the steep rise in the active component area, but it has a counterproductive effect on the cost reduction plan that is generally promoted by the automotive industry. ON Semiconductor's new third-generation ignition IGBTs, especially fourth-generation ignition IGBTs, have improved side characteristic dimensions and vertical doping profiles to compensate for the significantly reduced chip area. In addition, as Ic increases, the temperature coefficient of Vceon is optimized from a negative value to a slightly more positive value, improving critical low temperature operation. Another main parameter is the threshold voltage. It must be low enough to fully turn on the output voltage provided by the 5V drive MCU (VOUT drops to 3.7V). On the other hand, the gate oxide must be able to withstand the potential failure mode when the 12V grid and the gate are shorted. The new generation of IGBT has optimized the transmission characteristics of VGE, reduced the same Ic level by about 400mV, and can ensure complete saturation when the logic control signal level. Main reliability parameters The reliability of the ignition application is the most important, although because of its inherent redundancy, if a pen-shaped coil fails, it will not endanger the life. Since they are in close contact with the cylinder module, the environment of the pen-shaped coil is very strict: the ambient temperature is up to 140 ° C, the power consumption path is limited, and the vibration continues. There is also periodic electrical stress from forward pulse operation and reverse active clamping. Although the data sheet clearly lists the maximum Tj as 175 ° C, it is well known that certain operating conditions have far exceeded this limit. Unspecified short-term temperature excursions are up to 250 ° C, and the temperature excursions that may occur during the life of an ignition IGBT are greater. However, the field failure rate must be kept within a few ppm. Robustness can be specified by several SOA (Safe Operating Area) ratings, determined by different parameters in a complex and interactive way: P-tub doping profile, MOSFET geometry, carriers in N layer Lifetime, hfe of NPN / PNP structure, etc., vary. Forward-biased SOAs are limited by failure modes caused by high currents, where excessive main carriers in the P-tub bias on the NPN structure can cause latch-up of "parasitic" NPNP thyristors, which can be Avoid this effect, but it may still be caused by point defects in local areas. The way to fundamentally eliminate this effect is to eliminate the defect density in wafer production through continuous improvement projects. Pulse testing at the final test with continuous current that greatly exceeds the rated value can ensure quality. The reverse-biased SOA is limited by the continuity of the electric field of the N layer. When switching to the reverse condition, the MOSFET electron flow is quickly turned off, filling the N layer with minority carriers, which can effectively reduce the possibility of avalanche breakdown voltage. Another common SOA in ignition applications is UIS (Self-Clamping Inductive Switch). An open secondary (such as an open spark plug connection) will reflect 100% of the secondary energy (minus the coil losses) back to the IGBT. The data sheet specifies the "single pulse collector to emitter avalanche energy". ON Semiconductor can guarantee the maximum energy of 500mJ / 300mJ when the starting junction temperature is 25/150 ° C according to the chip size. The typical value is at least twice it. Even the smallest die size can maintain a UIS energy of 200mJ in all rated temperature ranges, with a maximum temperature of up to TJ = 175 ° C. The actual requirements of the current pen-shaped coil is 100 ~ 150mJ. Figure 5 shows the UIS function of the third-generation IGBT, which has a smoother temperature dependence, which can be obtained through careful optimization and precision design of wafer manufacturing parameters. In order to ensure quality, in the final test, each part needs to carry out 2 UIS tests with a peak current of 26A in order to rule out any potential damaged parts. And record the faults in the test as reliability monitoring. Robustness also means withstanding ESD events that mainly occur before the board assembly line. ESD damage can occur immediately, causing a large amount of detectable gate leakage. But more dangerous is the potential damage to the gate dielectric caused by ESD, which can cause field failures at lower overvoltage levels. With gate-to-emitter back-to-back polysilicon, you can ensure 8kV / 800V ESD protection that conforms to the human / machine model. Enhanced fault-free operation provides integrated VGE pull-down resistors to prevent IGBTs from accidentally turning on when no control signal is connected. Resistors can be customized to protect external components. You can choose to integrate a series gate resistor to limit the occurrence of excessive dVCE / dt, but in some applications may cause transient current and UIS failure. And this integrated Rg can avoid the negative effects of non-optimal pre-drive design, thus providing a low impedance path from gate to ground. Rg ensures that the IGBT can be safely turned on and off under clamping conditions. Development trend of ignition IGBT Plugging the coil will become the mainstream of recent development. High-performance systems will turn to miniaturized coils with a turns ratio of about 1: 100 and require a higher primary current (up to 18A) and a higher clamping voltage (about 400V) to provide high spark energy for the fuel-air mixture . The third-generation device NGX19N40 that meets these applications has a continuous current of 19A and a clamping voltage rating of 405V. It is available in TO-220 and D2PAK packages, and both have 0.9K / W steady-state thermal resistance junctions. Recently, the fourth generation (NGX820X series) has been further improved. The DPAK packaged IGBT can obtain the required functionality and robustness, thereby promoting a new degree of freedom in assembly technology, while also reducing board area (up to 60%) and cost . The medium-term development trend has not yet taken shape. For different ignition systems, there are great differences. The common feature is the enhancement of the function "intelligence". However, any additional circuits integrated in the IGBT chip must be compatible with existing components, and it will not change its optimized IGBT structure: a large number of N-channel FETs and the N layer of the IGBT (in the environment where the source and ground are close, the only possibility is Circuit), diode and resistor (P + and N + with different page impedance and TC) share the main body. The functions that can be easily integrated are: temperature sensing with 2 back-to-back diodes, which can provide the MCU with a voltage drop proportional to the die temperature. The current detection is the same as the PowerFET principle: the precise geometric ratio specifies a small mirror current (0.3 to 1% of the main current), which can be detected almost non-destructively by the integrated detection resistor and then transmitted to the MCU. The disadvantage of these two detection functions is that more connections are required, and the heavy load of a high-capacity, cost-effective 3-terminal power package cannot be used. It is very challenging to integrate more complex functions, or it is impossible to achieve at all. Here, the functions we discuss include over-temperature shutdown, over-current detection / signage / limit, optional clamping voltage, dwell time guard, ramp-up shutdown in failure mode, future ignition soft-open powered by 42V PowerNet, etc. Some requirements contradict each other, such as hard OTSD and ramp-up shutdown. And it is obvious that the application of each smart IGBT type is limited, so the economy of scale is lost. In short, the best solution is to use an optimized, non-intelligent IGBT and a linear bipolar or LinCMOS intelligent pre-driver as an interface between the MCU and IGBT to provide resident protection and control features. 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