Switching control system to reduce coil output voltage when commencing coil charging

Description

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing aspects and many of the attendant advantages of this invention will become more readily appreciated by reference to the following detailed description, when taken in conjunction with the accompanying drawings, wherein:

FIG. 1 is a schematic diagram of a conventional inductive ignition circuit;

FIG. 2 is a schematic diagram of an equivalent circuit of the conventional circuit of FIG. 1;

FIG. 3 is a timing diagram example that illustrates a comparison of a gate voltage, a coil current and a collector voltage as a gate charging current is adjusted with a threshold detection input, in accordance with an embodiment of the present invention;

FIG. 4 is a high level schematic of a circuit for controlling a voltage collapse rate of a power transistor, in accordance with an embodiment of the present invention;

FIG. 5 is a timing diagram example illustrating a comparison of a gate voltage, mirror capacitor voltage, drain or collector voltage, and a threshold detection output when a transistor threshold detection circuit is utilized, in accordance with an embodiment of the present invention; and

FIG. 6 is a high level schematic of a circuit for dynamically determining a threshold voltage of a semiconductor device, in accordance with an embodiment of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

Exemplary embodiments are described with reference to specific configurations. Those of ordinary skill in the art will appreciate that various changes and modifications can be made while remaining within the scope of the appended claims. Additionally, well-known elements, devices, components, methods, process steps and the like may not be set forth in detail in order to avoid obscuring the invention. Further, unless indicated to the contrary, the numerical values set forth in the following specification, figures and claims are approximations that may vary depending upon the desired characteristics sought to be obtained.

Power semiconductor devices are commonly used to control the switching of current through systems including automotive ignition systems, solenoid drivers, motor drivers and power regulation systems. Switching control systems are employed to reduce coil output voltage. That is, a rapid voltage collapse rate across terminals of a semiconductor device appears across primary windings of an ignition coil. The change in primary voltage further produces a significantly higher voltage at a coil's secondary output terminal due to the turns ratio of the primary and secondary coil windings. The secondary voltage can produce an untimely spark. As discussed supra, conventional systems add circuit complexity, increase system costs by a non-negligible amount and can add switching control timing error.

Referring to FIG. 1, an inductive ignition circuit is illustrated including an ignition coil 110 having a primary winding 112 serially coupled to an automotive battery 114 and a transistor switch 116 shown as an IGBT. Other switches may be used, although it is important that the switch be lossy, that is, energy is absorbed in the switch as it is being turned off. The gate 118 of the transistor receives the spark command pulse to turn the switch 116 on or off. The ignition coil also has a secondary winding 120 connected across a spark plug gap 122. Upon initial closing of the transistor switch 116 a high voltage, in the opposite sense, is developed in the secondary winding 120 and typically is accompanied by oscillations or ringing of such magnitude that premature firing of the spark plug can occur, sometimes causing combustion in a corresponding cylinder.

An equivalent circuit for the coil turn-on condition is shown in FIG. 2 wherein the ignition coil 110 is replaced by an ideal transformer 124, and secondary impedances are reflected to the primary side and combined with primary impedances to result in resistance R and leakage inductance L in series with the battery and the transformer primary, and a capacitance C across the primary. The application of an ordinary dwell command to the gate 118 of the transistor 116 would initiate a primary current causing an energy buildup in the leakage inductance L which would oscillate with the capacitance C, causing the undesired oscillations or ringing. One approach to ringing, illustrated in U.S. Pat. No. 5,392,754, entitled “Method of Suppressing Ringing in an Ignition Circuit” assigned to Delco Electronics Corp, describes suppressing ringing by a short switch turn-on pulse to a transistor gate followed by a short delay prior to the main dwell period, causing a beginning build up of primary current and circuit energy followed by absorption of energy in the switch during switching to dissipate circuit energy needed for oscillation.

Controlling an IGBT collector voltage response by controlling the gate voltage is limited by the variation in actual IGBT threshold voltage. Slow charging of the IGBT gate voltage is effective in reducing the rate of change of the collector voltage, but slow charging from a fully discharged state introduces significant error in the coil turn-on timing. Further, slow charging of the IGBT gate does not give an indication of when the IGBT is fully turned on. Without such information, slow charging of the IGBT gate can result in limitation of the coil current charging rate due to the resulting linear control of the IGBT's collector current. Such limitation disrupts the overall ignition system timing, potentially reducing the energy available in the coil at the time a spark event is desired.

Rapid charging of the IGBT gate voltage to a predetermined sub-threshold voltage reduces the error in the coil turn-on timing, but due to the normal variation in IGBT gate threshold voltages the timing error is non-negligible. The threshold voltage region for an IGBT varies with factors including semiconductor manufacturing process variations and current environmental conditions. The gate voltage threshold levels depend on the operating temperature of the specific semiconductor device. For systems that experience a wide range of operating temperatures, the threshold voltage region can vary. Even if devices are sorted for specific threshold voltage ranges, system temperature variations will introduce error in finely tuned control circuitry. Temperature compensation circuitry can be added but again system costs are increased. The present invention addresses these issues and more.

A system and method are described herein for controlling a voltage collapse rate of a power transistor, thereby reducing a coil output voltage. The present invention slowly charges the IGBT gate during the time that the gate voltage passes through the threshold voltage region. After passing through the threshold voltage region, where the collector voltage would otherwise be changing rapidly, the invention resumes a fast charge of the gate, bringing the IGBT into full saturation, thereby enabling coil charging at the desired rate. Additionally, the present invention dynamically determines the threshold voltage of a semiconductor switching device during every switching event, providing immediate feedback and compensation for changes in threshold voltage. The threshold voltage can also be stored for future reference.

While the present invention is described with reference to an insulated gate bipolar transistor (IGBT), it is to be appreciated that other transistors can similarly be utilized with the present invention including, field-effect transistor (FET), metal-oxide semiconductor FET (MOSFET), and junction field-effect transistor (JFET). Further, while a switching control system to reduce coil output voltage is described herein, it is to be appreciated that the present invention may additionally be utilized to reduce other load output voltages besides a coil, inductive or otherwise.

Referring to the drawings wherein identical reference numerals denote the same elements throughout the various views, FIG. 3 is a timing diagram that illustrates a comparison of a gate voltage, a coil current and a collector voltage as a gate charging current is adjusted with a threshold detection input. Terminal voltages of an IGBT are shown passing through a sub-threshold, threshold, and above-threshold region of the IGBT operation. The gate voltage and collector voltage are shown with (solid line) and without (dotted line) the invention implemented.

The present invention controls the IGBT collector voltage collapse rate by controlling the gate voltage slew rate. The gate is slowly charged during the time that the gate passes through the threshold voltage region. The gate charging current source is switched to different levels to substantially reduce the rate of IGBT collector voltage collapse, without significantly affecting the overall system timing. The IGBT gate is initially charged at a rate defined by current level Ig1. The magnitude of the gate charging current Ig1 establishes the time required to charge the gate from a discharged state to the IGBT gate threshold voltage, and then later the time to complete the charge from slightly above the threshold voltage to the fully charged voltage.

When the threshold of the IGBT is detected (as discussed infra), the charging current is reduced to a substantially lower level Ig2. This reduced charging current Ig2 causes the IGBT to transition from the off state to a conducting state at a much slower rate than if the gate had been continuously charged by current Ig1. The consequence of this slow turn-on event is a reduction in the slew rate of the IGBT collector voltage. This reduced slew rate is desirable since it results in less voltage developed at the ignition coil primary and secondary winding terminals. While the current level Ig2 must be chosen to minimize collector voltage slew rate it must not add significant delay to the total charging cycle. The calculations to establish this gate charging current can be performed by one skilled in the art.

The gate charging current is subsequently returned to the current level Ig1 when the coil current has reached a predetermined level Ic1. Returning to the higher level charging current, after transition through the threshold region has occurred, ensures that the IGBT will be driven into saturation in the shortest possible time. The coil current at which the initial charging rate is restored may be detected by any of a number of means including direct sensing of the current via a low value resistor in series with the IGBT emitter, or by examination of the current flowing at a “sense emitter” terminal of the IGBT where a fraction of the total collector current is directed to current measuring circuitry. Other coil current detection methods may be utilized, as long as the detected current level is accurate. In setting a predetermined Ic1 current level, the Ic1 current level should be a level high enough to ensure that the collector voltage has completely transitioned to its low level, and yet not a high level as to excessively extend the low gate current charging period. In an embodiment, the desired current level is in the range of 1 to 2 amperes.

In another embodiment, the coil current is determined to have reached Ic1 by means of a timing function. The timing function includes a predetermined time period that it takes a particular IGBT to transition through a threshold region. When the threshold of the IGBT is detected the timing function starts, and when the predetermined time period expires then the gate charging current is returned to the current level Ig1.

FIG. 4 shows a high level schematic of a circuit for controlling a voltage collapse rate of a power transistor in accordance with an embodiment of the present invention. It is to be appreciated that circuitry other than the circuitry shown in FIG. 4 can be used to control a power transistor voltage collapse rate. For example, the comparator and/or logic gate shown in FIG. 4 may be substituted for varied circuitry designs that still control the voltage collapse rate of a power transistor and are thus to be considered within the spirit and scope of the present invention. Now, turning to FIG. 4, at the start of the IGBT 202 turn-on cycle, the primary coil current 204 and secondary coil current 206 are zero due to the IGBT 202 gate having been discharged by transistor 222 (Q1). Since there is no coil current flowing through the IGBT 202 or current sense resistor 212 (Rsns), comparator COMP1218 outputs a low level. The output from COMP1218 is connected to one input of NOR gate 232 (N1). At the start of an IGBT turn-on cycle, the IGBT 202 threshold detection circuit 226 provides a low-level logic output to circuit 200. This low level signal is inverted by inverter 228 (I1) and applied to a second input of NOR gate 232. In this condition NOR gate 232 output is low, causing current source Igate 236 to source a current of magnitude Ig1.

The gate of the IGBT 202 is charged at Ig1 until the IGBT 202 threshold detection circuit 226 determines the IGBT 202 is beginning to conduct current (discussed infra). At this time, the threshold detection circuit 226 output switches to a high level. The output from COMP1218 is still at a low level. This combination of inputs to NOR gate 232 (0,0) result in NOR gate 232 output switching to a high level, causing current source Igate 236 to switch to a second current output level Ig2. Current source Igate 236 continues to source current at the Ig2 level until the output from COMP1218 changes to a high level due to the coil current having ramped up to a level (Ic1) that causes the voltage across resistor 212 to exceed reference voltage 216 (Vref). The change in the output from COMP 218 causes current source Igate 236 to revert back to the previous current level Ig1. The charging current remains constant through out the remainder of the coil charging period and engine timing dwell event. At the end of the dwell event, a spark is generated in the spark gap 208 defined by ignition plug 210 by rapidly turning off the IGBT 202 via transistor 222 (Q1), which is controlled by control circuitry 224. Subsequent to the spark event, the system is in the desired state for the start of the next IGBT turn-on charging cycle.

Referring to FIG. 5, a timing diagram illustrates a comparison of a gate voltage, mirror capacitor voltage, drain or collector voltage, and a threshold detection output when a transistor threshold detection circuit is utilized, in an embodiment of the present invention. The present invention determines the specific gate voltage at which a switching device begins to conduct current. The detection of this threshold voltage level is then utilized to adjust the gate charging current as discussed supra. A mirror capacitor is employed to mimic the behavior of the natural gate to emitter capacitance of a switching device. The mirror capacitance is charged via a scaled constant current source during the switching device turn-on time, such that the voltage on the mirror capacitor tracks the voltage appearing on the switching device gate. Since the gate is also charged with a constant current source, the voltages on the two capacitances remain proportionally or nearly equal until the switching device threshold voltage is reached. When the threshold voltage is reached, the switching device collector voltage begins to collapse. The collapsing collector voltage changes the rate that the gate voltage charges, due to the Miller effect multiplication of the apparent gate capacitance. The increase in apparent capacitance results in a reduction of the gate voltage rate of change. At the same time, the mirror capacitance continues to charge linearly, undisturbed by the collapsing collector voltage of the switching device. The change in the relative charging rates between the two capacitances causes the mirror capacitor voltage to eventually exceed the switching device gate voltage by a measurable amount. The difference in voltage between the mirror capacitor voltage and the gate voltage provides a signal and feedback that the switching device gate voltage has reached a threshold voltage. The threshold detection output signal results when the voltage on the mirror capacitor exceeds the gate voltage by a predetermined magnitude. It is to be appreciated that the charging rate of a transistor gate can be detected either by determining that a sample voltage of the mirror capacitor proportionally exceeds a sample voltage of the transistor gate by a predetermined magnitude or by determining that the mirror capacitor charging rate exceeds the transistor gate charging rate by a predetermined rate. By detecting the actual threshold voltage of a switching device in a dynamic manner, a variety of switching circuits (besides the circuit shown in FIG. 4) can be finely control tuned thereby increasing switching efficiency, output slew rate control, etc.

In another embodiment, the present invention dynamically determines the threshold voltage level of the transistor and subsequently stores the threshold voltage level in memory for later access rather than repeating the dynamic determining of the transistor threshold voltage level during a subsequent transistor turn-on phase.

FIG. 6 shows a high level schematic of a circuit for dynamically determining a threshold voltage of a semiconductor device in accordance with an embodiment of the present invention. The circuit shown can be utilized for the threshold detection circuit 226 as shown in FIG. 4. It is to be appreciated that threshold detection circuit 226 can utilize other means of determining a threshold voltage as well and is not limited to the circuit or method illustrated in FIG. 6.

IGBT 404 (Q3) is the device for which the threshold voltage is determined. The voltage on the mirror capacitor 416 (CMirror) is compared to the voltage on the gate of the IGBT 404 by comparator 410 (Comp1). The gate capacitance of IGBT 404 and the mirror capacitor 416 are initialized at near zero volts. The capacitors are charged with currents that are proportional to the relative capacitances, causing the voltages on each to increase at the same rate. The threshold is detected at the point that the voltage on the mirror capacitor 416 exceeds the gate voltage by a predetermined voltage magnitude Vos 414. Vos 414 may be set at a small voltage for example 100 millivolts, which allows the IGBT 404 to pass into the threshold region by some minor amount before asserting that the threshold region is reached, thus minimizing any capacitance variability in the IGBT 404 and mirror capacitor 416.

Current source 430 (I1) and mirror capacitor 416 are scaled proportionally to current source 432 (I2) and the gate capacitance of IGBT 404 in order to match the charging rates of the two capacitances. This proportionality can be set up by one skilled in the art. Transistor 420 (Q1) and transistor 424 (Q2) provide discharge paths for the gate of IGBT 404 and mirror capacitor 416, respectively. When the control signal 426 that is connected to the control terminals (bases) of transistor 420 and transistor 424 is at an operationally high level, transistor 420 and transistor 424 are switched into their conducting states, forcing IGBT 404 into its non-conducting state and discharging mirror capacitor 416. The voltage developed across resistor 438 (R1), defined by R1×I2, sets up an initial condition ensuring that the gate voltage of IGBT 404 is greater than the voltage on mirror capacitor 416, thereby establishing a low level output from comparator 410.

When the control signal connected to the control inputs (bases) of transistor 420 and transistor 424 switch low, the charging of mirror capacitor 416 and the gate of IGBT 404 commences. When the voltage across mirror capacitor 416 reaches the sum of the gate voltage of IGBT 404 and Vos 414 (at the threshold voltage level of the IGBT 404), then the threshold detector output switches to a high state. The high output state (threshold detected signal) is then inputted to inverter 228 (I1) and applied to a second input of NOR gate 232 (as described in FIG. 4).

Other features and advantages of this invention will be apparent to a person of skill in the art who studies this disclosure. Thus, exemplary embodiments, modifications and variations may be made to the disclosed embodiments while remaining within the spirit and scope of the invention as defined by the appended claims.

Claims

1. A switching control system for a power switching application comprising: a first circuit for charging a transistor gate at a first current level during a turn-on phase of the transistor, for charging the transistor gate at a second current level when a threshold voltage of the transistor is detected, and to resume charging the transistor gate at the first current level when a load current reaches a predetermined current level, wherein the first current level is greater than the second current level; anda second circuit for determining the threshold voltage of the transistor.
2. The system as in claim 1, wherein the first circuit comprises: a comparator for detecting the predetermined load current through the transistor, via an offset voltage to an input of the comparator to set the predetermined load current level, wherein the comparator outputs one of a detected predetermined load current signal and an undetected predetermined load current signal; anda logic gate for receiving the comparator output and an output signal from the second circuit, wherein the second circuit output signal is one of a detected threshold voltage signal and an undetected threshold voltage signal, wherein the logic gate instructs charging the transistor gate at the first current level when the logic gate receives the undetected predetermined load current signal and the undetected threshold voltage signal, wherein the logic gate instructs charging the transistor gate at the second current level when the logic gate receives the undetected predetermined load current signal and the detected threshold voltage signal, and wherein the logic gate instructs charging the transistor gate at the first current level when the logic gate receives the detected predetermined load current signal.
3. The system as in claim 1, wherein the turn-on phase of the transistor commences when the transistor gate is discharged, the threshold voltage of the transistor is undetected, and the load current is less than the predetermined current level.
4. The system as in claim 1, wherein the predetermined current level is in the range of 1 ampere to 2 amperes, and is set at a level to drive the transistor into saturation in a minimized time.
5. The system as in claim 1, wherein the second circuit dynamically determines the threshold voltage of the transistor by dynamically detecting a reduction of a charging rate of the transistor gate, during the transistor turn-on phase.
6. The system as in claim 1, wherein the second circuit charges a mirror capacitor at a rate substantially matching a charging rate of the transistor gate, during the turn-on phase of the transistor, and generates a signal when one of the mirror capacitor charging rate exceeds the transistor gate charging rate by a predetermined rate, and a sample voltage of the mirror capacitor proportionally exceeds a sample voltage of the transistor gate by a predetermined magnitude.
7. The system as in claim 5, wherein the second circuit dynamically determines the threshold voltage level of the transistor and stores the threshold voltage level in a memory for later access rather than repeating the dynamic determining of the transistor threshold voltage level during a subsequent transistor turn-on phase.
8. The system as in claim 1, wherein the resumed charging of the transistor gate at the first current level is continued through a load charging period and an engine timing dwell event.
9. The system as in claim 1, wherein the transistor is one of a field-effect transistor (FET) and an insulated gate bipolar transistor (IGBT).
10. The system as in claim 1, wherein the transistor is connected to and affects one of an automotive ignition coil, a solenoid driver, a motor driver, and a power regulation system.
11. A method of dynamically detecting a threshold voltage comprising: charging a mirror capacitor at a rate substantially matching a charging rate of a switching device, during a turn-on phase of the switching device; andgenerating a signal when one of the mirror capacitor charging rate exceeds the switching device charging rate by a predetermined rate, and a sample voltage of the mirror capacitor proportionally exceeds a sample voltage of the switching device by a predetermined magnitude.
12. The method as in claim 11, wherein the charging rate of the switching device is a gate to source charging rate.
13. The method as in claim 11, wherein one of the mirror capacitor charging rate and the switching device charging rate, and the mirror capacitor sample voltage and the switching device sample voltage are compared utilizing a comparator.
14. The method as in claim 13, further comprising generating an offset voltage to an input of the comparator to set one of the predetermined rate and the predetermined magnitude.
15. The method as in claim 13, further comprising setting up an initial condition on the comparator to indicate that the switching device sample voltage is greater than the mirror capacitor sample voltage, prior to charging the mirror capacitor, to establish a desired output level from the comparator.
16. The system as in claim 11, wherein the switching device is one of a field-effect transistor (FET) and an insulated gate bipolar transistor (IGBT).
17. The system as in claim 11, wherein the switching device is connected to and affects one of an automotive ignition coil, a solenoid driver, a motor driver, and a power regulation system.
18. A switching control method for a power switching application comprising: charging a transistor gate at a first current level during a turn-on phase of the transistor until a threshold voltage of the transistor is detected, wherein the turn-on phase of the transistor commences when the transistor gate is discharged, the threshold voltage of the transistor is undetected, and a load current is less than a predetermined current level;charging the transistor gate at a second current level when the threshold voltage of the transistor is detected, to reduce a transistor slew rate, wherein the first current level is higher in magnitude as compared to the second current level; andresuming the charging of the transistor gate at the first current level when the load current reaches the predetermined current level.
19. The method as in claim 18, wherein the threshold voltage of the transistor is detected by dynamically detecting a reduction of a charging rate of the transistor gate, during the transistor turn-on phase.
20. The method as in claim 19, wherein dynamically detecting a reduction of a charging rate of the transistor gate comprises: charging a mirror capacitor at a rate substantially matching a charging rate of the transistor gate, during the transistor turn-on phase; andgenerating a signal when one of the mirror capacitor charging rate exceeds the transistor gate charging rate by a predetermined rate, and a sample voltage of the mirror capacitor proportionally exceeds a sample voltage of the transistor gate by a predetermined magnitude.
21. The method as in claim 18, wherein the predetermined current level is in the range of 1 ampere to 2 amperes.
22. The method as in claim 18, wherein the predetermined current level is set at a level to drive the transistor into saturation in a minimized time.
23. The method as in claim 18, wherein the transistor is one of a field-effect transistor (FET) and an insulated gate bipolar transistor (IGBT).
24. The method as in claim 18, wherein the transistor is connected to and affects one of an automotive ignition coil, a solenoid driver, a motor driver, and a power regulation system.
25. The method as in claim 18, wherein the resumed charging of the transistor gate at the first current level is continued through a load charging period and an engine timing dwell event.

Switching control system to reduce coil output voltage when commencing coil charging

Information

Publication Number

Date Filed

Date Published

Inventors

CPC

US Classifications

International Classifications

Abstract

Description

Claims