System and Method for Driving a Transistor

The present disclosure relates generally to an electronic device, and more particularly to a system and method for driving a switching transistor.

High-voltage switching transistors, such as power MOSFETs, JFETs (Junction Field Effect Transistor) and Gallium Nitride High Electron Mobility Transistor (GaN-HEMT), are commonly used as semiconductor switches in high-voltage and high-power devices such as switched-mode power converters, motor controllers, and high-voltage and high-voltage devices high performance circuits used. Some of these devices, such as the GaN HEMT, have the ability to operate at very high voltages without device failure or damage.

Some devices, such as the JFET and GaN-HEMT, can be fabricated to have a negative threshold voltage, causing the device to conduct when zero voltage is present across the gate and source of these transistors. Such devices are appropriately referred to as "normally on" devices or transistors, since these devices are effectively on under zero-bias conditions. When such normally-on devices are used, provisions are generally made to ensure that a voltage is generated to ensure that the normally-on device can be turned off. For example, in a driver circuit used in a switched-mode power conversion, a negative voltage is generated or provided that is at a voltage sufficiently below the threshold voltage of the normally-on device to ensure that the device actually turns off as intended.

Out of<de-docref CY="DE" DNUM="102011087464" KI="A1">DE 10 2011 087 464 A1</de-docref> a driver circuit for driving a JFET transistor and a MOSFET transistor is known. The driver circuit includes a low voltage section and a high voltage section coupled via a Corless Transformer.

According to one embodiment, a circuit for driving a control terminal of a switching transistor includes: a driver having an output configured to be coupled to a control terminal of the switching transistor, a first power supply terminal configured to be coupled to the first terminal of a floating power supply, a second power supply terminal configured to have to be coupled to a second terminal of the floating power supply, and a switching input terminal, which is designed to receive a switching signal. The circuit further includes a bias circuit having an output terminal configured to be coupled to a common mode control terminal of the floating power supply, the bias circuit configured to provide a time dependent voltage.

For a more complete understanding of the present invention and the advantages thereof, reference is now made to the following descriptions taken in conjunction with the accompanying drawings, in which:<ul id="ul_0001" list-style="none"><li id="ul_0001_0001"><figref>1a-c</figref> illustrate a conventional switch drive system;</li><li id="ul_0001_0002"><figref>2</figref> illustrate a switch drive system according to an embodiment;</li><li id="ul_0001_0003"><figref>3</figref> 12 illustrates a switch drive system according to another embodiment;</li><li id="ul_0001_0004"><figref>4</figref> 12 illustrates a switch drive system according to another embodiment;</li><li id="ul_0001_0005"><figref>5</figref> 1 illustrates an embodiment of a transistor replica circuit; and</li><li id="ul_0001_0006"><figref>6</figref> FIG. 1 illustrates a flow diagram of a method according to an embodiment.</li></ul>

Corresponding numerals and symbols in different figures generally refer to corresponding parts unless otherwise noted. The figures are drawn to clearly illustrate the relevant aspects of the preferred embodiments and are not necessarily drawn to scale.

The making and using of the presently preferred embodiments are discussed in detail below. However, it should be understood that the present invention provides many applicable inventive concepts that can be embodied in a wide variety of specific contexts. The specific embodiments discussed are merely illustrative of specific ways to make and use the invention and do not limit the scope of the invention.

The present invention will be described in terms of preferred embodiments in a specific context, system and method for driving a normally-on switching transistor. Embodiments of the present invention can also be applied to various systems using normally-on transistors, such as switched-mode power converters, motor controllers, and other circuits. Embodiments can also relate to driving normally-off transistors.

In one embodiment of the present invention, a circuit is designed to drive a control terminal of a switching transistor. This switching transistor may have a negative threshold voltage, such as a JFET, a gallium nitride HEMT (GaN HEMT), a depletion mode MOSFET, or other transistor with a negative threshold voltage, or it may have a positive threshold voltage, such as an enhancement mode MOSFET . The driver includes power supply terminals that are coupled to a floating power supply. The floating power supply can generate a positive voltage and a negative voltage that are coupled to the power supply terminals of the driver. During operation, the driver applies the positive voltage of the floating power supply to turn on the switching transistor and applies the negative voltage of the floating power supply to turn off the switching transistor. A common mode biasing circuit is also provided to bias the common mode or intermediate voltage of the floating power supply. In some embodiments, this common-mode voltage can produce a time-varying voltage that can depend on various parameters.

In a specific example, this common mode voltage may depend on the threshold voltage of the driven transistor or a replica device thereof. In such an embodiment, the common-mode voltage is set to or about the threshold voltage of the switching transistor. Embodiments of the present invention may be applied to driving switching transistors having negative threshold voltages, positive threshold voltages, and/or zero threshold voltages.

<figref>1a</figref> 10 illustrates a conventional switch drive system 100 having a voltage source 106, a driver circuit 104 and a normally-off transistor 102. As shown, the driver 104 is supplied with a fixed voltage VP from the voltage source 106, which is related to the source potential S of the transistor 102. The threshold voltage Vth of transistor 102 is the gate-source voltage level that defines the transition between the "on" and "off" states. In the case of a positive threshold voltage Vth present in "normally-off" transistor 102, the illustrated supply scheme uses a single positive voltage source 106. If the voltage VP is greater than the threshold voltage Vth of the normally-off transistor 102, the normally-off transistor 102 is turned on. Likewise, if the voltage at the negative terminal of voltage source 106 is applied to the gate electrode G of normally-off transistor 102, the normally-off transistor is turned off.

<figref>1b</figref> 12 illustrates a conventional switch drive system 120 having a voltage source 126, a driver circuit 124 and a normally on transistor 122. As shown, the driver circuit 124 is supplied with a fixed voltage VN from the voltage source 126 which is related to the source potential S of the transistor 122. Similar to the in<figref>1b</figref> In the normally-off transistor 102 shown, the threshold voltage Vth of transistor 122 is the gate-source voltage level that defines the transition between the "on" and "off" states. In the case of normally-on transistor 122, however, this threshold voltage Vth is a negative voltage, which means that the voltage of the gate electrode G of the normally-on transistor 122 is brought to a voltage potential that is below the voltage of the source node S in order to turn the normally-on transistor 122 off. Accordingly, if the voltage potential VN of voltage source 126 exceeds the negative threshold voltage Vth of normally-on transistor 122, the normally-on transistor can be turned off by applying the voltage to the negative terminal of voltage source 126. However, if the gate-source voltage of normally-on transistor 122 is a voltage is zero, normally on transistor 122 becomes conductive.

In cases where the threshold voltage Vth of the transistor being driven is a low positive or low negative voltage close to ground, a bipolar drive scheme can be used to ensure that the transistor turns on and off.<figref>1c</figref> 13 illustrates a conventional bipolar switch drive system 130 having a positive voltage source 134, a negative voltage source 136, a driver circuit 138 and a transistor 132. When the transistor 132 is on, the voltage VP generated by the positive voltage source 134 is applied to the gate terminal G of the transistor 132 . Likewise, when transistor 132 is off, voltage VN generated by negative voltage source 136 is applied to gate terminal G of transistor 132 . Using such a bipolar drive scheme can improve switching performance when the threshold voltage Vth is a low voltage and can provide margin to ensure that transistor 132 switches properly. For example, if the threshold voltage Vth of transistor 132 is approximately 1V, driving transistor 132 with a positive 12V power supply may provide an asymmetric drive voltage. In such cases, using a negative voltage source 136 helps ensure that transistor 132 is turned off with sufficient overdrive.

Because the threshold voltage Vth of a transistor can change over temperature variation, process variation, statistical variations, drift effects, and other causes, such changes are often taken into account during the design of conventional driver circuits. For example, if the threshold voltage Vth of a normally-on transistor is subject to a variation between -5V and -9V, a negative supply voltage in a conventional system provides a negative voltage designed to provide sufficient overdrive to turn off the transistor. In this example, a negative voltage of -11V provides a 2V overdrive to turn off the transistor in the worst case scenario when the Vth threshold voltage of the normally-on transistor is at -9V.

In one embodiment, supply voltages for a gate driver are related to a gate potential that corresponds to a switching threshold, for example VG=VS+Vth. In other words, the drive levels are related to VS + Vth instead of directly to VS. Thus, a positive (“on”) gate drive level can be expressed as Vth + VP (positive overdrive), and the negative (“off”) level can be expressed as Vth - VN. However, under real working conditions, the instantaneous threshold voltage Vth may change over time due to changes in temperature and other drift effects.

<figref>2</figref> 12 illustrates a switch drive system 200 that includes a positive voltage source 206 and a negative voltage source 208, a drive circuit 204, and a transistor 202. FIG. Transistor 202 can be implemented using different types of transistors. For example, the transistor may include a power MOSFET transistor, a GaN HEMT, a JFET, an enhancement mode MOSFET, a depletion mode MOSFET, or a bipolar junction transistor (BJT), among others.

In some embodiments, the combination of positive voltage source 206 and negative voltage source 208 forms a floating power supply whose common-mode voltage can be adjusted by applying a voltage to a common node 220 coupled between positive voltage source 206 and negative voltage source 208. By coupling the voltage source 210 between the common node 220 and the source node S of the transistor 202, the common mode output of the driver circuit 204 can be set according to the time-varying threshold voltage V_th(t) of transistor 202 or any other time dependent voltage. In some embodiments in which the outputs of voltage sources 206 and 208 track the threshold voltage of transistor 202, the voltage VP of the positive voltage source 206 and the voltage VN of the negative voltage source 208 can be selected without a variation in the The threshold voltage of the transistor 202 must be taken into account. In such embodiments, the minimum values ​​of VP and VN can be chosen according to switching dynamics.

For example, in one embodiment, voltage sources 206 and 208 are each set to generate approximately 3V and voltage source 210 is configured to provide a voltage that approximates the threshold voltage of transistor 202 . Thus, if the approximation of the threshold voltage of transistor 202 is about -5V, voltage source 210 provides about -5V, the positive terminal of voltage source 206 provides about -2V and the negative terminal of voltage source 208 provides about -8V. In alternatives In other embodiments, other voltage levels may be used depending on the particular system and its specifications.

Voltage sources 206 and 208 can be implemented using power circuits known in the art. For example, switched mode power supplies, voltage regulators, batteries, and other power supply circuits and systems may be used to implement voltage sources 206 and 208 . For example, voltage source 210 may be implemented using various biasing circuits and/or power supply circuits known in the art. In some embodiments, voltage source 210 may be implemented using a replica of transistor 202 to generate a voltage that approximates the threshold voltage Vth of transistor 202 . Driver circuit 204 may be implemented using driver circuits known in the art, such as an Infineon EiceDRIVER family driver or a Texas Instruments UCC27x series driver.

<figref>3</figref> FIG. 3 illustrates a switch drive system 300 according to an embodiment of the present invention. As shown, the transistor copy circuit 306 generates a time dependent voltage V(t) that is coupled to the common mode terminal VCM of the floating current source 302 . In one embodiment, floating power supply 302 generates a voltage at terminal VP that is at a voltage potential that is greater than the voltage at terminal VCM and generates a voltage at terminal VN that is at a voltage potential that is below the voltage at the terminal VCM lies. Effectively, the voltages at terminals VP and VN track the voltage V(t) generated by transistor copy circuit 306.

In one embodiment, transistor copy circuitry 306 generates a voltage V(t) that approximates and/or is related to the threshold voltage of transistor 202 by using a transistor having a similar structure and/or device geometry as transistor 202 is used. Voltage V(t) may vary over time to track changes in threshold voltage with respect to temperature, drift effects, and changes in other parameters that may affect the threshold voltage of the transistor and replica device in transistor replica circuit 306 . In alternative embodiments, voltage V(t) may be generated by other types of circuitry besides transistor copy circuitry 306 .

Driver circuit 304 has supply terminals coupled to floating power supply 302 terminals VP and VN. In one embodiment, the driver circuit alternately applies the voltages at terminals VP and VN of floating power supply 302 to the gate terminal G of transistor 202 according to a switching signal at input VSW of driver circuit 304 . For example, in one embodiment, driver circuit 304 applies the voltage to terminal VP of floating power supply 302 when the signal at input VSW is a logic high, and applies the voltage to terminal VN of floating power supply 302 when the signal at input VSW is on is logical L. Alternatively, the relationship between the logic sense of the VSW input and the voltage applied to the gate electrode of transistor 202 can be reversed. In some embodiments, the output of driver circuit 304 may introduce a voltage drop between its power supply connections and its output terminal.

<figref>4</figref> FIG. 4 illustrates a switch drive system 400 in accordance with another embodiment of the present invention in which the floating power supply is implemented using windings 416 and 418 of center-tapped transformer 414. FIG. In one embodiment, windings 416 and 418 may be secondary and/or auxiliary windings of a transformer of a switched mode power converter, such as a flyback converter. In some embodiments, the transformer 414 may include a primary winding 430 coupled to primary-side switched-mode power supply circuitry, which is not shown to simplify the illustration. The implementation and operation of such a primary-side switched-mode power supply circuit can be accomplished using circuits and methods known in the art.

The switch drive system 400 comprises the transistor 202 and the drive circuit 402 which is designed to apply a switching signal VSW to the gate electrode of the transistor 202 . Transistor 202 may be any type of transistor that has a positive threshold voltage or a negative threshold voltage, for example. The positive power supply terminal 422 of the driver circuit 402 is coupled to the winding 416 of the transformer 414 via a diode 410 and the negative power supply terminal 424 of the driver circuit 402 is coupled to the winding 418 of the transformer 414 via the diode 412 . Diodes 410 and 412 rectify the current in windings 416 and 418 of transformer 414 . In some embodiments, diodes 410 and 412 may be implemented using switching transistors that operate as synchronous rectifiers. Capacitors 406 and 408 coupled to diodes 410 and 412 provide filtering and dampen supply ripple.

A unity gain buffer amplifier 404 is configured to buffer the voltage V(t) to the center tap 432 of the transformer 414 . For example, voltage V(t) may represent a voltage that approximates the threshold voltage of transistor 202 . Alternatively, the voltage V(t) can be any time-varying voltage. In many embodiments, the time variation of V(t) is slow compared to the switching transients. Then capacitor C, coupled between the source node S of transistor 202 and the output of unity gain buffer amplifier 404, blocks the common mode supply component V(t) but provides a low impedance path for the fast switching transients.

Amplifier 404, shown in a unity gain feedback configuration, may be implemented using a transconductance amplifier, an operational amplifier, or any other type of amplifier known in the art. In alternative embodiments, other amplifier configurations besides a unity configuration can be used. For example, an amplifier with a power factor less than one or greater than one can be used. In some embodiments, amplifier 404 may be omitted. It is understood that the system 400 is just one of many embodiments that can be used to implement an embodiment of transistor drive circuits and systems. In alternate embodiments, other circuit architectures and topologies may be used.

<figref>5</figref> FIG. 12 illustrates one embodiment of a circuit for generating a time dependent voltage V(t) based on a replica of the switching transistor to be driven. As shown, the copy transistor 508 is diode-connected with its drain electrode coupled to its gate electrode. Voltage source 506 may be coupled between the drain and gate of copy transistor 508 to account for negative threshold voltages for normally-on devices. Current source 504 is coupled to copy transistor 508 and provides a bias current. According to various embodiments, the source node S of the copy transistor 508 is coupled to the same node as the source electrode of the switching transistor that is being driven (e.g Transistor 202 in<figref>3</figref> and<figref>4</figref>), or is coupled to a node with an equal or similar voltage as the source electrode of the switching transistor.

In one embodiment, copy transistor 508 has a similar structure as the driven switching transistor. For example, if the switching transistor is a GaN HEMT, then the copy transistor 508 is also a GaN HEMT. In some embodiments, the geometry of the copy transistor can also correspond to the geometry of the switching transistor. For example, the switching transistor can be constructed using n unit devices, while the copy transistor can be realized using one or two of the unit devices. In such embodiments, the current of the current source 504 need only be on the order of 1/n of the current of the switching transistor for V(t) to track the threshold voltage of the switching transistor. In some embodiments, the unitary components of the copy transistor 508 may be in the same location as the switching transistor to improve matched performance. By placing the copy transistor 508 in the same location as the main switching transistor, changes in temperature at the switching transistor are applied to the copy transistor 508.

It is understood that the circuit of<figref>5</figref> is just one of many examples of circuits that can be used to generate an approximation of a threshold voltage of a switching transistor. In alternative embodiments, circuits and systems described in co-pending application co-pending with the current<de-docref CY="US" DNUM="14473377" KI="">US number 14/473,377</de-docref> to be discribed.

<figref>6</figref> FIG. 6 illustrates a flow diagram of one embodiment of a method 600 for driving a switching transistor. This method may be used, for example, in connection with various illustrated embodiments disclosed herein. In one embodiment, a common mode control terminal of a floating power supply is biased with a voltage based on a threshold voltage of the switching transistor in step 602 . In step 604 the switching transistor is turned on by driving a control terminal of the switching transistor with a first floating power supply voltage, and in step 606 the switching transistor is turned off by driving the control terminal of the switching transistor with a second floating power supply voltage.

According to one embodiment, a circuit for driving a control terminal of a switching transistor includes: a driver having an output configured to be coupled to a control terminal of the switching transistor, a first power supply terminal configured to be coupled to a first terminal of a floating power supply, a second power supply terminal configured to have to be coupled to a second terminal of the floating power supply, and a switching input terminal, which is designed to receive a switching signal. The circuit further includes a bias circuit having an output terminal configured to be coupled to a common mode control terminal of the floating power supply, the bias circuit configured to provide a time dependent voltage. In some embodiments, the circuit further includes the floating power supply.

In one embodiment, the floating power supply includes a first coil, a first diode coupled between the first terminal of the floating power supply and the first coil, a second coil coupled to the first coil at the common mode control terminal, and to the second terminal of the floating power supply is coupled, and a second diode, coupled between the second coil and the second terminal of the floating power supply. The second coil is magnetically coupled to the first coil.

In one embodiment, the switching transistor includes a normally-on transistor, which may be part of the circuit. The normally on transistor can be implemented using a GaN HEMT device and the control terminal of the normally on transistor can be a gate electrode of the GaN HEMT. In some embodiments, the time-varying voltage is a voltage based on a threshold voltage of the switching transistor. This voltage may be based on the threshold voltage of the switching transistor and may be a voltage substantially equal to the threshold voltage of the switching transistor. In one embodiment, the bias circuit has a copy of the switching transistor.

In one embodiment, the driver is configured to turn on the switching transistor by coupling a voltage of the first power supply terminal to the control terminal of the transistor and to turn off the switching transistor by coupling a voltage of the second power supply terminal to the control terminal of the switching transistor.

According to another embodiment, a method for controlling a switching transistor includes: Turning on the switching transistor by driving a control terminal of the switching transistor with a first floating power supply voltage, turning off the switching transistor by driving a control terminal of the switching transistor with a second floating power supply voltage, and biasing a common mode control terminal of the floating power supply with a voltage based on a threshold voltage of the switching transistor , which can be substantially equal to the threshold voltage of the switching transistor. Biasing the common mode control terminal of the floating power supply may include providing a turn-on voltage to a copy of the switching transistor.

In one embodiment, the turning on and turning off steps are performed according to a switching signal. The switching transistor may include a normally-on transistor, and the switching transistor includes a GaN HEMT device such that the control terminal of the switching transistor is a gate electrode of the GaN HEMT.

According to another embodiment, a circuit includes a floating power supply having a positive terminal, a negative terminal, and a common mode terminal. The circuit further includes a driver circuit having a first power supply terminal coupled to the positive terminal of the power supply, a second power supply terminal coupled to the negative terminal of the power supply, and an output terminal configured to be coupled to a control terminal of a switching transistor to become. The circuit further includes a common mode bias circuit having an output coupled to the common mode terminal of the floating power supply, such that the common mode bias circuit is configured to output a voltage based on a threshold voltage of the switching transistor at the output of the common mode bias - Provide tension. In some embodiments, the switching circuit includes the switching transistor.

In one embodiment, the switching transistor includes a normally-on transistor, and the normally-on transistor includes a GaN HEMT device, such that the control terminal of the switching transistor includes a gate electrode of the GaN HEMT. The common mode bias circuit may include a voltage buffer amplifier having an output coupled to the common mode terminal of the floating power supply.

The switching circuit may include a capacitor coupled between a load path terminal of the switching transistor and the output of the voltage buffer amplifier, and may also include a copy transistor coupled to an input of the voltage buffer amplifier. In some embodiments, the floating power supply includes a transformer, and the common mode terminal of the floating power supply includes a center-tapped terminal of the transformer.

Advantages of some embodiments include power savings due to using lower supply voltages to deliver power to switching drivers. Another advantage includes the ability to track threshold voltage variation over time and the ability to provide symmetrical drive voltages to switching transistors.

Other advantages of the embodiments include the ability to vary gate drive levels not only in terms of transistor threshold voltage, but also to account for other aspects such as transistor operating mode (switch/diode), load current variation, and switching speed.