Apparatus and method for slew rate control

The slew rate of a transistor is controlled. Upon a transition of a MOSFET control signal, an operating voltage of the MOSFET is measured and a determination of whether the voltage is between a predetermined set of values is made. Based upon the determination, a counter is incremented, and the count of the counter corresponding slew rate. The turn-on current of the MOSFET is controlled based upon the count.

TECHNICAL FIELD

This application relates to signal shaping, and more specifically to controlling the slew rate of signals used in various types of systems.

BACKGROUND OF THE INVENTION

Various types of transistors are used in control systems. For example, Metal Oxide Semi-conductor Field Effect (MOSFET) transistors are used in many control systems. In one example, a microprocessor may create the control signal and that control signal is used to control the MOSFET transistor.

Different operating parameters define the performance of MOSFET transistors. One of these parameters is the “slew rate” of the control signals that are used to control the MOSFET. The slew rate refers to the maximum voltage change allowed per unit time. If the slew rate is not controlled, negative effects can occur with respect to system operation. The higher the slew rate, the faster the signal transitions from one value to another.

MOSFETs can be used to control pulse width modulation (PWM) processes where the pulse width changes (is modulated) based upon input provided by the MOSFET. However, if the slew rate of the signal created by the MOSFET varies (due to a variety of factors such as MOSFET component variation, layout variation, temperature variation, and battery voltage) the PWM function will be inaccurate and a system might not perform in a satisfactory manner.

Present control approaches preset the slew rate to some preset value. Unfortunately, this means that variations of slew rate cannot be accounted for by the system. This has led to sub-optimal performance in some systems and some user dissatisfaction with previous approaches.

Skilled artisans will appreciate that elements in the figures are illustrated for simplicity and clarity. It will further be appreciated that certain actions and/or steps may be described or depicted in a particular order of occurrence while those skilled in the art will understand that such specificity with respect to sequence is not actually required. It will also be understood that the terms and expressions used herein have the ordinary meaning as is accorded to such terms and expressions with respect to their corresponding respective areas of inquiry and study except where specific meanings have otherwise been set forth herein.

DETAILED DESCRIPTION

Approaches are provided that vary the slew rate of a control signal that is used to drive or otherwise a control a MOSFET transistor. Although the approaches described herein are applicable specifically to MOSFET transistors, they may be used to control the slew rate of control signals that are applied to various types of transistors.

In many of these embodiments, a slew rate control approach is provided. In some aspects, at each MOSFET, during turn-on and turn-off cycles, the drain-to-source voltage (VDS) rise and fall times of the MOSFET is measured and the MOSFET gate turn-on and turn-off currents are adjusted accordingly in the next cycle to achieve a desired slew rate. Slew rates can therefore be changed according to varying conditions. Greater flexibility in alternate MOSFET sourcing is provided. Great PWM accuracy can be achieved. More predictable and consistent performance may be achieved for the MOSFET (and the system where the MOSFET is used) over a given and over the entire operating range. In other words, the present approaches takes into account system, temperature, and process variations to provide a variable and changeable slew rate for a control signal applied to a transistor.

In others of these embodiments, the slew rate of a transistor is controlled. Upon a transition of a MOSFET control signal, an operating voltage of the MOSFET is measured and a determination of whether the voltage is between a predetermined set of values is made. Based upon the determination, a counter is incremented, and the count of the counter corresponding slew rate. The turn-on current of the MOSFET is controlled based upon the count.

Referring now toFIG. 1, a system100that provides for slew rate control is described. The system100includes a controller102, a slew rate control module104, a MOSFET106, and an operation module108.

The controller102produces a control signal105that is transmitted to the slew rate control module104. The controller102, in one example, may be a microprocessor. The controller102may be part of (and be a controller for) a fuel pump, a motor, or a fuel injector to mention a few examples. Other examples are possible.

The control signal105controls operation of the MOSFET106. As described below, the slew rate control module104(during turn-on and turn-off cycles) measures voltages of the MOSFET106(e.g., drain to source voltages). Based upon these voltages, the turn-on and/or turn off current applied to the gate of the MOSFET106) are adjusted. This adjusts the slew rate of the control signal that controls the MOSFET106.

The MOSFET106is activated or de-activated by the control signal105. This controls the operation of the operation module108, which for example, may control be valves or solenoids to mention two examples. In one aspect, the operation module108performs a PWM function that is controlled by the MOSFET106. Other types of transistors may also be used.

Referring now toFIG. 2, one example of a slew rate control module200is described. The slew rate control module200includes a rise time counter202, a rise time register204, a digital-to-analog converter206, a first switch (S1)208, a second switch (S2)210, a fall time counter212, a fall time register214, a second digital-to-analog converter216, a first AND gate218, a second AND gate220, a third AND gate222, an inverter224a differential amplifier226, and a window comparator228. A MOSFET230is controlled by the module200.

It will be appreciated that the first AND gate218, second AND gate220, third AND gate222, inverter224, and differential amplifier226are used for gating, timing, and/or driving purposes as known to those skilled in the art and their functions will not be described further herein. Moreover, other combinations of these or other elements can be used to perform these functions.

In operation, every time the MOSFET control signal transitions from a low to a high value, the drain to source voltage (Vds) voltage of the MOSFET230is measured during rise time. This voltage is fed to the window comparator228. The window comparator228is configured so that its output will be at a level “high” when the MOSFET Vds voltage is between 10% and 90%. Other percentages may also be used.

The output of the window comparator228enables the rise time counter202, which is clocked by a high frequency clock signal. The counting will stop when the output of the window comparator228becomes “low”. The result (count) of the rise time counter202is a measure of the rise time slew rate, When the MOSFET control signal transitions from high to low, the value (count) of the rise time counter202is loaded into the rise time register204that is connected to the first digital-to-analog converter206(e.g., for current). The converter206generates the proper MOSFET Gate turn-on current based on the number (count) recorded in the rise time register. When the MOSFET control signal transitions from low to high again, the first switch (S1)208closes and the MOSFET Gate turn-on current will control the new MOSFET rise time. During this time the MOSFET Vds voltage is measured again to correct the MOSFET rise time slew rate for next cycle.

Every time the MOSFET control signal transitions from low to high the rise time counter202will reset to “0” waiting for the output of the window comparator228to start the count.

Every time the MOSFET control signal transitions from high to low, the MOSFET Vds voltage is measured during fall time; this voltage is fed to the window comparator228above that is designed so that the output will be at level “High” when the MOSFET Vds voltage is between 10% and 90%. Other examples are possible. The output of the window comparator228will enable that fall time counter212that is clocked by the same high frequency clock signal above. The counting will stop when the output of window comparator228becomes “Low”. The count or result of the fall time counter212is a measure of the fall time slew rate.

When the MOSFET Control signal transitions from Low to High, the value (count) of the fall time counter212is loaded to the fall time register214that is connected to the second digital-to-analog converter216(e.g., for current) that will generate the proper MOSFET gate turn-off current based on the number recorded in the fall time register212. When the MOSFET control signal transitions from High to Low again, the second switch210(S2) closes and the MOSFET Gate turn-off current will control the new MOSFET fall time. During this time the MOSFET Vds voltage is measured again to correct the MOSFET fall time slew rate for next cycle.

Every time the MOSFET Control signal transitions from high to low the fall time counter212will reset to “0” waiting for the output of the window comparator228to start the count.

Although one particular slew rate control module is shown, it will be appreciated that other types of circuitry and other combinations of circuitry can be used. In other words, the approaches described with respect toFIG. 2are examples only and other examples are possible.

Referring now toFIG. 3, operation of the switches208(S1) and210(S2) ofFIG. 2are described. At time302, the MOSFET control signal transitions from low to high. This action closes the switch S1 and the turn-on current will control the operation of the MOSFET.

At time304, the MOSFET control signal transitions from high to low. This opens S1 but closes S2. Closing S2 turns off the current. This controls the new MOSFET fall time for the MOSFET. It will be appreciated that when one of S1 or S2 is open, the other of S1 and S2 is closed. Also, in some examples the switches S1 and S2 can be omitted for instance when control involving only the rise time is desired.