Protection circuit for a power mosfet driving an inductive load

Abstract: A snubber circuit comprising two stages regulates the gate current of a MOSFET (12) in relation to the drain-to-source voltage at turnoff to clamp transient inductive voltages to a nondestructive level. At the onset of the turnoff, a current source (28) is activated to discharge the gate capacitance, and the snubber circuit controls the magnitude of such current in relation to the sensed drain-to-source voltage to stabilize the drain-to-source voltage at a nondestructive level. When the drain-to-source voltage approaches its limit value, a current injection circuit (76) supplies additional current to the gate (g) to sustain the MOSFET conduction, again in relation to the sensed drain-to-source voltage. When the inductive energy stored in the load (14) is substantially dissipated, the drain-to-source voltage falls; at such point, the current injection circuit (76) is disabled and the conduction of the current source is increased to complete the turnoff of the MOSFET.

Description: This invention relates to an inductive voltage protection or snubber circuit, for power metal oxide semiconductor field effect transistor (MOSFET), and more particularly to a protection circuit which may be conveniently fabricated as an integrated circuit (IC). Inductive voltage snubber circuits are generally employed in control applications where a transistor is required to interrupt the supply of current to an inductive load. The voltage transient V L at the instant of current interruption is given by the expression: V L = L * (di L / dt) (1) where L is the inductance of the load and (di L / dt) is the rate of change in the load current i L . Snubber circuits characterized by the circuit of this invention limit the transient voltage V L by controlling the turnoff rate of the transistor. A conventional circuit of this type is depicted in Figure 1 (described below), in the environment of an automotive fuel injector driver circuit. US-A-3949722 discloses an example of the prior art. An inductive voltage protection circuit in accordance with the present invention is characterised by the features specified in the characterising portion of claim 1. The present invention is directed to an inductive voltage snubber circuit which adequately protects the MOSFET from transient inductive voltages while avoiding drawbacks (detailed below) associated with conventional inductive voltage snubber circuits. As described below, the inductive voltage snubber circuit of this invention eliminates the requirement for a stabilizing capacitor and integrates with the driver circuits all but a pair of relatively low wattage resistors used to sense the drain-to-source voltage. According to the present invention, turnoff of the MOSFET is controlled in response to the sensed drain-to-source voltage by two circuit stages. At the onset of the turnoff, a current source is activated to discharge the gate capacitances, and the snubber circuit controls the magnitude of such current in relation to the sensed drain-to-source voltage to stabilize the drain-to-source voltage at a nondestructive level. When the drain-to-source voltage approaches its limit value, a current injection circuit supplies additional current to the gate to sustain the MOSFET conduction, again in relation to the sensed drain-to-source voltage. When the inductive energy stored in the load is substantially dissipated, the drain-to-source voltage falls; at such point, the current injection circuit is disabled and the conduction of the current source is increased to complete the turnoff of the MOSFET. Since the gate current of the MOSFET is continuously regulated in relation to the rise in drain-to-source voltage, the internal capacitance of the MOSFET is sufficient to ensure stability and no supplemental capacitance is required. The present invention will now be described, by way of example, with reference to the accompanying drawings, in which:- Figure 1 is a circuit diagram of a MOSFET driver for an inductive load, including a conventional inductive voltage snubber circuit for protecting the MOSFET when it attempts to interrupt the load current; and Figure 2 is a circuit diagram of a MOSFET driver for an inductive load, including the inductive voltage snubber circuit of this invention. In Figure 1, a circuit 10 is employed to bias a MOSFET 12 on and off to periodically energize a solenoid coil 14 of a conventional fuel injector. The automotive ignition voltage (IGN) is supplied to the unswitched terminal of the solenoid coil 14, and a second voltage (2V cc ) is provided for biasing the MOSFET 12 conductive when connected to its gate (g) via closure of a switch 18. Capacitors 32 and 34 are internal to the MOSFET 12 and represent the inherent gate-to-drain (C gd ) and gate-to-source (C gs ) capacitances, respectively. The remaining circuit elements -- diode 20, Zener diode 22, resistor 24, capacitor 26 and current source 28 -- form a voltage snubber. The snubber circuit is activated at turnoff of the MOSFET 12 by opening switch 18 and closing switch 30. When switch 18 is closed and switch 30 is open, the gate g is raised substantially to 2V cc and the MOSFET 12 is biased conductive in its drain-to-source circuit. In this mode, the diode 20 is reverse biased to block control signal loss through Zener diode 22, resistor 24 and the drain-to-source circuit of MOSFET 12. When switch 18 is opened and switch 30 is closed to interrupt the conduction of MOSFET 12, the current source 28 starts discharging the gate capacitance C gs , and the drain-to-source voltage V ds rises abruptly in relation to expression (1) (above) as the MOSFET 12 attempts to interrupt the coil current i L . The rise in the drain-to-source voltage V ds forward biases diode 20 and quickly exceeds the breakdown voltage of Zener diode 22. At such time, the Zener diode 22 conducts current to the gate g via the limiting resistor 24 to maintain a partial conduction of the MOSFET 12. This serves to reduce the drain voltage V d by reducing the rate of change of current through the sdolenoid coil 14. Eventually, the drain-to-source voltage V ds falls below the breakdown voltage of Zener diode 22, and the current source 28 completes the turnoff of MOSFET 12. The capacitor 26 is required to prevent rapid changes in the gate voltage V g and the instability which would otherwise occur. Although relatively simple, the inductive voltage snubber circuit described above has some drawbacks. Most significantly, the capacitance (capacitor 26) required for stability prolongs the turn-on and turn-off times. This limits the time response of the inductive voltage snubber circuit and is a serious drawback in an application such as a fuel injector driver, where fast speed of response is important. Moreover, the cost of the external components 20, 22, 24 and 26, as well as the circuit board space required for their assembly, is substantial. Referring to Figure 2 and the inductive voltage snubber circuit of this invention, the circuit elements described above in reference to Figure 1 have been assigned the same reference numerals. Thus, the switch 18 periodically opens and closes to forward bias the MOSFET 12 (power transistor), and the switch 30 opens and closes with switch 18 to enable the inductive voltage snubber circuit at the MOSFET turnoff. The reference numerals for solenoid coil 14 and internal MOSFET capacitances 32, 34 are also repeated. Likewise, the solenoid coil 14 is energized by the ignition voltage IGN and an auxiliary high source, 2V cc , is provided for forward biasing MOSFET 12. In contrast to the driver circuit shown in Figure 1, however, only two devices of the driver circuit shown in Figure 2 (resistors 88, 90) are external to the transistor control logic, which is preferably mechanized in the form of an integrated circuit. A current source generally corresponding to the current source 28 of Figure 1 is defined by a current mirror circuit (current sinking means) comprising the transistors 40, 42, 44 and 46 and the resistors 48, 50, 52 and 54. The reference current of the current mirror circuit is defined by the sum of the collector current i₁ of multi-collector transistor 46 and the current i₂ through the resistor 48. During conduction of the MOSFET 12, the switch 30 is closed to bias the transistors 40, 42 and 44 nonconductive and to sink the currents i₁ and i₂. The current i₂ is relatively small and is defined by resistor 48. The current i₁ is determined by the current in the reference collector 56 of transistor 46, which is connected to ground with the transistor base through the resistors 52 and 54. As indicated, the current i₁ is three times that in the reference collector 56. A transistor 58 is nonconductive and as described below, comes into effect during turnoff to increase the voltage at terminal 60 for controlling the reference current component i₁. A second current mirror circuit (current sourcing means) is defined by the transistors 64, 66 and 68 and the resistors 70, 52 and 54. The reference current i₃ is defined by resistor 70 and the transistor 64, which is forward biased through resistors 52 and 54. The reference current i₃ forward biases transistor 66, producing a mirrored current of approximately the same magnitude in the emitter-collector circuit of transistor 68. The collector of transistor 68 is connected directly to the base of transistor 72 and via a pull-up resistor 74 to an auxiliary supply voltage designated herein as V cc . Transistor 72, in turn, controls the operation of a current injection circuit, generally designated by the reference numeral 76. During conduction of the MOSFET 12, the transistor 58 is nonconductive, and the current mirrored by the reference current i₃ is sufficiently large to bias the transistor 72 nonconductive, deactivating the current injection circuit 76. As described below, the transistor 58 becomes conductive during turnoff to increase the voltage at terminal 60; this reduces the reference current i₃ and allows the transistor 72 to become forward biased through pull-up resistor 74. The current injection circuit 76 comprises resistors 80 and 82, and a multi-collector transistor 78 connected between the high voltage source 2V cc and the gate g of MOSFET 12. When the transistor 72 is forward biased, the multi-collector transistor 78 also becomes forward biased and the current thereby injected into the gate g (control terminal) of MOSFET 12 is determined by the current in the reference collector 84. The drain-to-source voltage V ds of MOSFET 12 is detected by the voltage at junction 86 of a resistor divider comprising the external resistors 88 and 90 (detecting means). The junction 86 is connected to the base of transistor 58, which becomes forward biased only when the drain-to-source voltage reaches an upper threshold defined by the divider resistor 52, 54 and 88, 90. During conduction of MOSFET 12, V ds is very low and the transistor 58 is reverse biased. However, the drain-to-source voltage rises sufficiently during turnoff of the MOSFET 12 to forward bias the transistor 58 for the purpose of reducing the reference currents i₁ and i₃ as described below. When switch 18 is opened to remove the forward bias voltage from the gate g of MOSFET 12, the switch 30 opens and the transistor 40 is biased conductive by the reference currents i₁ and i₂. The emitter-collector current of transistor 40 biases the transistor pair 42, 44 conductive through the resistor 50, and the reference currents i₁ and i₂ combine to form the mirror reference current through the emitter-collector circuit of transistor 42. Due to the relative areas of the transistors 42 and 44, the transistor 44 conducts a current of approximately eight times that of the reference current (i₁ + i₂). This begins discharging the gate capacitance C gs of MOSFET 12 to interrupt the current in solenoid coil 14. At this point, the operation of the remaining circuit elements is essentially unchanged. As the gate capacitance C gs is discharged, the MOSFET 12 begins to turn off, and the drain-to-source voltage V ds rises abruptly due to the inductive energy stored in solenoid coil 14. When the potential at the (voltage divider) terminal 86 exceeds the voltage at terminal 60 by a diode voltage drop, the transistor 58 becomes forward biased. This supplies additional current through resistor 54 and increases the voltage at terminal 60. The increase in voltage at terminal 60 increases the base voltage of both transistors 46 and 64, thereby reducing the current conducted through their emitter-collector circuits. Regarding transistor 46, this reduces the reference current component i₁, decreasing the conduction of the current mirror transistors 40, 42 and 44. This reduces the rate of discharge of the MOSFET gate capacitance C gs and stabilizes the drain-to-source voltage V ds at a nondestructive level even without an external gate capacitor, such as the capacitor 26 of Figure 1. Even if transistor 46 is biased completely off, a limited current sinking ability is maintained due to the continued presence of reference current i₂. The decrease in conduction of the transistor 64 reduces the conduction of the current mirror transistors 66 and 68, allowing the base voltage of transistor 72 to rise. As the drain-to-source voltage approaches its limit value, the transistor 72 becomes forward biased through the resistor 74, allowing the current injection circuit 76 to supply additional current to the MOSFET gate g through the parallel connected collectors of multi-collector transistor 78. When the inductive energy of solenoid coil 14 is substantially dissipated, the drain-to-source voltage begins to fall, increasing the conduction of both transistors 46 and 64. The conduction increase of transistor 64 turns off the transistor 72, which in turn, turns off the current injection circuit 76. The conduction increase of transistor 46 increases the conduction of the current mirror transistors 40, 42 and 44 until the gate capacitance C gs is completely discharged, thereby completing the interruption of current in solenoid coil 14. As indicated above, the two stage operation of the inductive voltage snubber circuit of this invention adequately protects the MOSFET 12 while reducing the number of external snubber components and obviating the need for supplemental gate capacitance. This reduces the time required to change the conduction state of the MOSFET 12, and at the same time, reducing the overall cost of the driver circuit.

Claims: An inductive voltage protection circuit for a supply circuit in which a control terminal (g) of a power transistor (12) is periodically supplied with a conduction producing control signal which causes such power transistor (12) to supply current to an inductive load (14), for interrupting the load current and protecting the power transistor (12) from damage due to the generation of transient inductive voltages when the conduction producing control signal is removed, characterised by detecting means (88,90) for detecting the magnitude of the transient inductive voltage seen by the power transistor (12); current sinking means (28) activated when the conduction producing control signal is removed for sinking current from the control terminal (g) of the power transistor (12), the magnitude of such sinking current being regulated in relation to the detected transient inductive voltage to thereby stabilize the rate of increase of the transient inductive voltage seen by the power transistor (12); and current sourcing means (64-76) activated as the detected transient inductive voltage approaches a limit value for supplying supplemental current to the control terminal (g) of the power transistor (12) to sustain its conduction until the energy stored in the inductive load (14) is substantially dissipated, whereafter the current sinking means (28) completes the turn off of the power transistor (12). An inductive voltage protection circuit as claimed in Claim 1, wherein the current sinking means comprises a current mirror circuit (40-44) which sinks current from the control terminal (g) of the power transistor (12) in relation to a reference current supplied thereto; and reference current means (46) for supplying a reference current to the current mirror circuit which is relatively large when the conduction producing control signal is first removed, and which thereafter decreases in substantial relation to the consequent increase in the detected transient inductive voltage. An inductive voltage protection circuit as claimed in Claim 1, wherein the current sinking means comprises a current mirror circuit (40-44) which sinks current from the control terminal (g) of the power transistor (12) in relation to a reference current supplied thereto; reference current means (46) for supplying a reference current to the current mirror circuit which is relatively large when the current sinking means is activated; and voltage responsive means (58) effective when the detected transient inductive voltage exceeds a threshold voltage for reducing the current supplied by the reference current means in relation to further increases in the detected transient inductive voltage. An inductive voltage protection circuit as claimed in Claim 1, wherein the current sinking means includes a first current mirror (40-44) for sinking current from the control terminal (g) of the power transistor (12) in relation to the magnitude of a first reference current which is relatively large at the time the conduction producing control signal is removed; wherein the current sourcing means includes a second current mirror (64-68) for supplying supplemental current to the control terminal (g) of the power transistor (12) in relation to the magnitude of a second reference current which is relatively small at the time the conduction producing control signal is removed; and comprising voltage responsive means (58) effective when the detected transient inductive voltage exceeds a threshold voltage for (1) reducing the first reference current in relation to further increases in the detected transient inductive voltage, to thereby stabilize the rate of increase of the transient inductive voltage seen by the power transistor (12), and (2) increasing the second reference current in relation to further increases in the detected transient inductive voltage, to thereby sustain the conduction of the power transistor until the energy stored in the inductive load (14) is substantially dissipated.

Inventors: OSBORN DOUGLAS BRUCE, OSBORN, DOUGLAS BRUCE
Applicants: DELCO ELECTRONICS CORP, DELCO ELECTRONICS CORPORATION
Application year: 1989
Main classification: H02H9
IPC codes: H03K17, H02H7, H01L29, H01L27