Insulated gate enhancement mode field effect transistor with slew-rate control on drain output

Abstract: Voltage slew-rate control for inductive load (20) connected to the drain (18) of an insulated gate enhancement mode field effect transistor (12) is disclosed. The voltage slew-rate control is accomplished by incorporating a current integrator (26) between the gate and drain of the field effect transistor, thereby effectively annihilating the capacitance of the field effect transistor which has heretofore proven to be generally responsible for instability and oscillations when slew-rate control is attempted in such a circuit. The current integrator (26) includes a capacitor (28) and a current source (32) in circuit with a high speed unity gain buffer amplifier (34). The buffer amplifier (34) preferably has a low impedance through the frequencies of interest to prevent the natural resonances of stray capacitances with the inductive load.

Description: The present invention relates to an electronic circuit including an insulated gate enhancement mode field effect transistor, and, in particular, to a circuit including load voltage slew-rate control on the drain output of the field effect transistor. In many driving applications with inductive loads, such as, motors and coils, for example, it is of interest to limit the voltage slew-rate on the inductive load. This is particularly desirable at turn-off of the load in order to reduce electromagnetic disturbances and other related problems. Because of the structure and electrical characteristics of an insulated gate enhancement mode field effect transistor, this has been found difficult to achieve. The local feedback representative of the gate-drain capacitance is generally responsible for instability and oscillations when such a slew-rate control is attempted with an inductive load. The slew-rate voltage on a load may be defined as the maximum rate of change of the output voltage of the load. GB-A-2140996 describes a circuit for controlled switching of a field effect transistor by means of a feedback capacitor. In operation there are three successive stages, initially a rapid change up to the switching point, followed by a slow change due to the feedback capacitor during switching and finally a rapid change during which the feedback is again ineffective. According to a first aspect of the present invention there is provided an electronic circuit comprising an insulated gate enhancement mode field effect transistor means, including a gate, a source and a drain, and an inductive load means connected in circuit with the drain of the field effect transistor means, wherein said circuit includes load voltage slew rate control means connected to said gate and said drain of said field effect transistor means, cooperating with said field effect transistor means to form a current integrator receiving an input current and providing a voltage output at said drain defined by the time integral of said input current, the slew rate control means having capacitor means with one end connected in circuit between said load and said drain of said field effect transistor means, the other end of said capacitor means being connected 1. to a current source circuit and 2. to the input of a unity gain buffer amplifier means the output of which is connected to said gate of said field effect transistor means whereby local feedback at said gate of said field effect transistor means due to gate-drain capacitance is effectively annihilated. According to a second aspect of the present invention there is provided a method of controlling load voltage slew-rate at an inductive load using an enhancement mode field effect transistor comprising: providing a capacitor having a first terminal in circuit between said load and the drain of said field effect transistor, and a second terminal connected to a current source; providing a unity gain buffer amplifier in circuit between said second terminal and the gate of the field effect transistor, to form a current integrator for integrating the current from the current source; and driving said gate by said buffer amplifier such that voltage variations at said drain are coupled via the capacitor and buffer amplifier to said gate to annihilate the effect of gate-drain capacitance. The present invention provides a load voltage slew-rate control which includes a current integrator. The current may be constant or time dependent, for example, the current integrator is connected in circuit between the gate and the drain of the field effect transistor, whereby, the local feedback at the gate of the field effect transistor due to gate-drain capacitance is effectively annihilated by the current integrator. The current integrator includes a capacitor having one end connected in circuit to the drain of the field effect transistor and the other end of the capacitor connected in circuit to a constant current source. A buffer amplifier is provided having a predetermined operating frequency range. The buffer amplifier is connected in circuit between the other end of the capacitor and the gate of the field effect transistor. Desirably the buffer amplifier has a predetermined output impedance of less than 500 ohms over its operating frequency range. It has been found desirable that the operating frequency range of the buffer amplifier is such that the bandwidth of the buffer amplifier is at least ten times the resonant frequency of the inductive load including associated components and, said field effect transistor. For a better understanding of the invention, reference may be had to the accompanying drawings exemplary of the invention, in which: Figure 1 is a schematic diagram of the present invention; Figure 2 is a schematic diagram showing an equivalent circuit of the current integrator; and Figure 3 is a detailed schematic diagram of a preferred embodiment of the present invention. Referring to Figure 1, there is shown in combination a circuit 10 including an insulated gate enhancement mode field effect transistor 12. Such a transistor typically includes an insulated polysilicon gate 14, a source 16, and a drain 18. An inductive load 20 is connected as shown in Figure 1 in circuit with the drain 18 and a voltage source 22. As hereinbefore stated, it is often desirable to limit the voltage slew-rate on the load 20 especially at turn-off in order to reduce electromagnetic disturbances and other related problems. As already stated, this has proven difficult to obtain because of the local feedback represented by the gate-drain capacitance of the transistor 12 which often causes instability and oscillations when slew-rate control is attempted with an inductive load. The present invention utilizes a load voltage slew-rate control comprising a current integrator 26. The current integrator preferably comprises capacitor 28 having one end 30a connected in circuit between the load 20 and the drain 18 of transistor 12. A current source 32 is connected in circuit with the other end of the capacitor 28. The buffer amplifier 34 is connected in circuit between the other end 30b of the capacitor 28 and the gate 14 of the field effect transistor 12. The present invention utilizes the gain of the buffer amplifier for integration of the current, from current source 32. Figure 2 shows that the circuit is equivalent to an integrator, and the current, I in positive or negative, of the current source 32 is integrated by the capacitance C of the capacitor 28 into a linear output voltage variation, assuming I in is constant: dVOUT dt = I in C The amplifier 36 shown in Figure 2 represents the generic gain, -K, of the buffer amplifier 34 together with the gain of the field effect transistor 12, shown in Figure 1. Preferably, the buffer amplifier has a predetermined output impedance of less than about 500 ohms over its operating frequency range. It is desirable that the buffer amplifier 34 has a predetermined frequency range such that the bandwidth of the buffer amplifier is at least ten times the resonant frequency of the load including parasitics, resulting in a "high speed" buffer amplifier. Referring to Figure 3, it has been found that the buffer stage 38 of the buffer amplifier 34 is preferably a class AB-type amplification stage. This provides for a biasing current on the output stage of amplifier 34. The invention requires a "high speed" unity gain buffer amplifier 34. The term "high speed" means that the bandwidth of the amplifier 34 is much larger when compared to the resonant frequency of the field effect transistor 12 and associated components and load. As a first order estimate such resonant frequency, F R = 1 2π LC where L is the inductance of the load and C is the total parasitic capacitance of load and associated components and transistor 12. For example, if L is in the range of 0.5 to 10 mH and C is in the range of 0.5 to 5 nF, the resonant frequency, F R , is less than 500 kilohertz and a buffer 34 with a 5 megahertz bandwidth is appropriate. As stated before, the impedance of the buffer amplifier 34 has to be reasonably small over an extended range of frequency such as from 0 to 5 MHz. Referring to Figure 3, the gate to drain capacitance 40 of the field effect transistor 12 is shown. Typically, with zero bias, i.e., zero voltage between gate and drain, capacitance 40 is about 100 to 500 picofarads, for example, depending on the device. Similarly, the gate to source capacitance 42 is 50 to 500 picofarads for example. The following is a table of components for the circuits shown in Figures 1, 2 and 3. TABLE OF COMPONENTS Component Manufacturer Model Value Transistors (44a, 44b) 2N2222 Transistor (46) 2N2907 Resistor (46) 2.2 Kohm Diode (48) 2N2222 Collector Base Connected Diode (50) 2N2907 Collector Base Connected Field Effect Transistor (12) SGS-Thomson SGS P358 Capacitor (28) 25 pF DC Voltage source (22) 5 volts Inductor (20) 1 mH, 5 ohm resistance Biasing current source 52 may be set at 40 µA, for example. The constant current source 32 may be set at 5 µA. In this example, assuming a base current of transistor 44a is negligible, the voltage slew-rate is a function of the dv dt = I in C = 5µA 25pF = 0.20 v µsec. The supply voltage 54 may be set at 12 volts, for example. Using the buffer amplifier shown in Figure 3, buffer stage 38 is a class AB-type amplifier which imposes a biasing current on the output stage of the buffer which is beneficial to maintain low output impedance levels during operation.

Claims: An electronic circuit comprising an insulated gate enhancement mode field effect transistor means (12), including a gate (14), a source (16) and a drain (18), and an inductive load means (20) connected in circuit with the drain of the field effect transistor means (12), wherein said circuit includes load voltage slew rate control means (28,32,34) connected to said gate (14) and said drain (18) of said field effect transistor means, cooperating with said field effect transistor means to form a current integrator receiving an input current and providing a voltage output at said drain defined by the time integral of said input current, the slew rate control means having capacitor means (28) with one end connected in circuit between said load (20) and said drain of said field effect transistor means, the other end of said capacitor means being connected 1. to a current source circuit (32) and 2. to the input of a unity gain buffer amplifier means (34) the output of which is connected to said gate of said field effect transistor means (12) whereby local feedback at said gate (14) of said field effect transistor means due to gate-drain capacitance is effectively annihilated. A circuit in accordance with claim 1, wherein said unity gain buffer amplifier means (34) has a predetermined operating frequency range. A circuit in accordance with claim 2, wherein said current source circuit (32) is a constant current source. A circuit in accordance with claim 2 or claim 3, wherein said unity gain buffer amplifier means (38) comprises NPN-type transistors (44a, 44b, 46). A method of controlling load voltage slew-rate at an inductive load using an enhancement mode field effect transistor (12) comprising: providing a capacitor (28) having a first terminal in circuit between said load and the drain (18) of said field effect transistor, and a second terminal connected to a current source; providing a unity gain buffer amplifier (34) in circuit between said second terminal and the gate (14) of the field effect transistor, to form a current integrator for integrating the current from the current source; and driving said gate (14) by said buffer amplifier (34) such that voltage variations at said drain are coupled via the capacitor (28) and buffer amplifier (34) to said gate to annihilate the effect of gate-drain capacitance (40).

Inventors: CAROBOLANTE FRANCESCO, CAROBOLANTE, FRANCESCO
Applicants: SGS THOMSON MICROELECTRONICS, SGS-THOMSON MICROELECTRONICS, INC.
Application year: 1992
Main classification: H03K17
IPC codes: H03F3, H03K17, H01F7