Reduction of the turn-off delay of an output power transistor

Abstract: A fast-discharge switch (M1) is controlled by a comparator (M2) sensing the voltage difference between the output node and the input node of a driving integrator stage that controls the slew-rate of a power switching output transistor (MP). The fast-discharge switch turns off automatically when the output power transistor reaches (in the case of a MOS transistor) or exits (in the case of a bipolar transistor) saturation. The circuit of the invention accelerates the discharge thus reducing the turn-off delay and is insensitive of load conditions and does not affect the performance of the integrating (driver) stage that control the slew-rate.

Description: This invention concerns slew-rate control of output power transistors in general, and the reduction of turn-off delay in particular. In inductive load drive systems, as those of a DC brushless motor, for example, it is necessary to limit induced disturbances both in terms of electromagnetic emissions and of noise on supply lines. Usually, specific circuits for controlling output power transistor slew-rates are used for this purpose. Slew-rate control is usually implemented by the use of an integrating stage to prevent discontinuities in the load current characteristic so as to drive the output power transistor, as shown in Fig. 1. A driving signal is fed to an input node A of the integrating stage, which consists of a buffer B1, a feedback capacitor C and a discharge generator I(slew). The use of such an integrating circuit provides several drawbacks. It should be noted in particular that, when the ratio I(slew)/C (V/µsec) is small, the time taken by output power transistor MP to exit the linear zone and enter the saturation zone of its characteristic, during turn-off may be too long and consequently cause a switching delay that, under certain circumstances, may not be tolerated by the system as a whole. In order to avoid such a drawback, recourse is commonly made to circuits that permit one to "accelerate" the slew-rate during an initial phase of a turn-off process, before allowing the discharge to continue with an adequately greater time constant so as determined by the integrating stage, in order to curb disturbances. These known accelerating circuits are of the same type as that outlined in Fig. 2. A logic control signal IN is applied to the control terminal of a discharging current generator M1. At the instant t1, when the turn-off of the output power transistor MP is started, the transistor M1 (which may or may not be under current control) is switched-on. This makes the gate of the output power transistor MP to quickly discharge until the input node A of the integrating stage reaches the threshold set by the diode D1 (which may also be replaced by several diodes in series, by a MOS diode, or by another threshold device, as needed). At this point, the contribution to the discharge current provided by the transistor (generator) M1 ceases, and the turn-off process of the MP output power transistor thus continues with a time constant as set by the values I(slew) and C of the slew-rate control, integrating stage. One of the drawbacks of these known solutions is that they rely on a preset threshold to determine the moment in which the discharge generator M1 is switched-off. Moreover, since it is notorious that the polarization conditions of an output power transistor (MP) strongly depend on load characteristics (e.g. the electrical parameters of the driven inductive load, the working conditions of the driven motor, etc.), it often becomes impossible to set an optimal threshold value when designing the circuit. Depending on operating conditions, either this difficulty or the approximate character of even the most accurate design choice may often result in a failure to optimize switching delay or in an inadequate performance of the slew-rate control circuit. As a matter of fact, a retarded turn-off of the M1 switch would make the slew-rate equal to I(M1)/C and, being of course I(M1) > I(slew), the actual slew-rate would be much higher than desired. A system has now been found and represents the subject-matter of this invention, capable of effectively reducting the turn-off delay of an output power transistor. The system of the invention utilizes a slew-rate control circuit which, unlike known circuits, is capable of adjusting to changing load conditions. Basically, the turn-off delay reduction circuit employs a "comparator" that senses the potential difference between the output node of the power stage and the input node of a slew-rate control integrating circuit. The "comparator" produces, on an output node of the comparator, a signal that indicates the state of saturation or of non-saturation of the output power transistor, during a turn-off process. This signal, produced by the comparator, is used to enable and disable the transfer of the driving logic signal IN to the control terminal of the discharge switch of the input node of the integrating stage that drives the output power transistor. Therefore, the comparator acts as a "sensor", indicating the instant when the output power transistor (which may be a field effect transistor or a bipolar transistor) passes from a "linear" operating zone to a "saturation" zone of its characteristic during a turn-off phase, irrespectively of varying load conditions. Thus, the turn-off delay reduction circuit, object of this invention, exploits a variable and intrinsically optimized threshold for reducing the turn-off delay without affecting the function of the slew-rate control integrating circuit in case of varying load conditions. The different aspects and advantages of this invention will become more evident through the description of an important embodiment, herein incorporated by express reference. Figure 1 shows an output stage comprising a slew-rate control integrating circuit, according to the prior art, as already mentioned. Figure 2 shows a circuit diagram of an output stage provided with a turn-off delay reduction circuit according to a known technique, as described above. Figure 3 shows a circuit diagram of an output stage with a variable threshold, turn-off delay reduction circuit, according to the present invention. With reference to Fig. 3, it should be noted that a suitable comparator is realized by the transistor M2 and the biasing generator I2. This comparator senses the potential difference between the output node OUT of the power stage and the input node A of the slew-rate control integrating stage, which is made up of the buffer B1, the feedback capacitor C and the current generator I(slew). A switch M1 and a discharge current generator I1 form a functional discharge acceleration circuit capable of reducing the turn-off delay of the output power transistor MP, in accordance with the present invention. The signal on the output node of the M2-I1 comparator acts as an enable/disable signal for transferring a logic driving signal from the input terminal IN to the control terminal of the M1 switch. The illustrated circuit works as follows. When the output power transistor MP is on, nodes A and B are at the input voltage, whereas the drain of the power transistor MP (i.e. the output node OUT of the circuit) will have roughly the ground potential. The power transistor MP turns-off as follows. At the instant t1, the control signal, present on the input terminal IN, is transferred through the AND gate AN1 to the control terminal of the transistor M1, bringing it into conduction. The node A will start to discharge through the transistor M1 with a discharge current set by the generator I1. The discharge of the node A causes a drop in the gate voltage of the transistor MP as well as of the sensing transistor M2. As a consequence, the drain of the power transistor MP (that is the potential on the output node OUT) tends to rise owing to an increase of the resistivity of the power transistor MP. The sensing transistor M2 will switch-off when the voltage on node A reaches a value equivalent to the sum of the threshold voltage of the sensing transistor M2 and of the voltage drop across the power transistor MP (VthM2+VdsMP2). Therefore, by assuming that the threshold voltage values of the transistors MP and M2 are substantially correlated, the transistor M2 switches-off when the output power transistor MP reaches saturation. Of course in the case of a bipolar power transistor, the M2 would switch-off when the output power transistor exits saturation. The turn-off of the M2 transistor produces a logic zero on the output of the AND gate AN1, which switches-off the discharge switch M1 thus returning, from then on, the control of the slew-rate of the power transistor MP to the slew-rate control stage, according to its preset time constant (I/C). It is quite clear that the circuit of the instant invention achieves the aim of making the effect of the turn-off delay reduction circuit perfectly and automatically adjustable to real load conditions, without interfering with the performance of the integrating stage that controls the slew-rate.

Claims: A circuit for reducing the turn-off delay of an output power transistor of an output stage comprising an integrating stage (B1,C) for controlling slew-rate having an input node (A) driven by a discharge acceleration circuit that consists essentially of a fast discharge switch (M1) that can be turned-on by a control signal (IN) and turned-off when the potential on the aforesaid input node (A) drops below a certain threshold value, characterized by the fact that it comprises: a comparator (M2) sensing the potential difference between an output node (OUT) of the output power stage and said input node (A) of the integrating stage (B1,C) and capable of producing a signal indicative of when the output power transistor reaches or exits saturation during a turn-off phase; a logic AND gate (AN1) having a first input to which said control signal (IN) is fed, a second input to which said comparator-produced signal is fed and an output functionally connected to a control terminal of said fast discharge switch (M1); the control signal (IN) being transferred to the control terminal of said fast discharge switch (M1) when the output power transistor reaches or exits saturation during a turn-off phase. A turn-off delay reduction circuit, as defined in claim 1, characterized by the fact that said comparator is made up of a transistor (M2) having a first control terminal Connected to the input node (A) of said integrating circuit (B1,C) driving the output power transistor, a second terminal being connected to the output node (OUT), and a third terminal being connected to a supply node through biasing means (I2); the signal present on the said second terminal being fed to the first input of said AND gate (AN1) through an inverter. A circuit according to claim 2, characterized by the fact that said transistor (M2) making up said comparator is of the same type of said power transistor. A circuit according to claim 1, characterized by the fact that said output power transistor is a MOS transistor and said comparator (M2) produces a signal indicative of the power transistor reaching saturation, during a turn-off phase. A circuit according to claim 1, characterized by the fact that said output power transistor is a bipolar transistor and said comparator (M2) produces a signal indicative of the power transistor exiting saturation, during a turn-off phase.

Inventors: NESSI MAURIZIO, NESSI, MAURIZIO
Applicants: ST MICROELECTRONICS SRL, STMICROELECTRONICS S.R.L.
Application year: 1993
Main classification: H03K17
IPC codes: H03K17, H02P6