Patent Publication Number: US-5157351-A

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

Description:
BACKGROUND OF THE INVENTION 
     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. 
     SUMMARY OF THE INVENTION 
     The present invention provides a slew-rate control circuit in combination with a drive circuit which includes an insulating gate enhancement mode field effector transistor. Such a transistor has an insulated gate, a source and a drain. The drive circuit also includes an inductive load connected in circuit with the drain of the field effect transistor. 
     The improvement of the present invention comprises providing 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. 
     Preferably, 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. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     For a better understanding of the invention, reference may be had to the accompanying drawings exemplary of the invention, in which: 
     FIG. 1 is a schematic diagram of the present invention; 
     FIG. 2 is a schematic diagram showing an equivalent circuit of the current integrator; and 
     FIG. 3 is a detailed schematic diagram of a preferred embodiment of the present invention. 
    
    
     BRIEF DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring to FIG. 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 FIG. 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 30b of the capacitor 28. A 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. FIG. 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: ##EQU1## The amplifier 36 shown in FIG. 2 represents the generic gain, -K, of the buffer amplifier 34 together with the gain of the field effect transistor 12, shown in FIG. 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 &#34;high speed&#34; buffer amplifier. 
     Referring to FIG. 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 &#34;high speed&#34; unity gain buffer amplifier 34. The term &#34;high speed&#34; 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, ##EQU2## 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 FIG. 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 FIGS. 1, 2 and 3. 
     
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TABLE OF COMPONENTS                                                       
Component  Manufacturer                                                   
                       Model        Value                                 
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Transistors (44a,      2N2222                                             
44b)                                                                      
Transistor (46)        2N2907                                             
Resistor (46)                       2.2 Kohm                              
Diode (48)             2N2222 Collector                                   
                       Base Connected                                     
Diode (50)             2N2907 Collector                                   
                       Base Connected                                     
Field Effect                                                              
           SGS-Thomson SGS P358                                           
Transistor (12)                                                           
Capacitor (28)                      25 pF                                 
DC Voltage                           5 volts                              
source (22)                                                               
Inductor (20)                        1 mH,                                
                                     5 ohm                                
                                    resistance                            
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     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 ##EQU3## 
     The supply voltage 54 may be set at 12 volts, for example. Using the buffer amplifier shown in FIG. 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.