System and method for generating an auxiliary voltage

The present disclosure relates generally to an electronic assembly, and more particularly to a system and method for generating an auxiliary voltage.

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 supplies, 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 on 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 supply, 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 is actually turned off as intended.

Another problem associated with the use of normally-on devices is the possibility of high current transients during start-up of a system using normally-on devices. For example, if a normally-on device is coupled between the terminals of a high-voltage power supply, large currents can result when power is applied to the system. To avoid these high start-up currents, a normally-off device such as an enhancement MOSFET can be coupled in series with the normally-on device. After the power supplies configured to provide the turn-off voltage to the normally-on device have reached a voltage sufficient to turn off the normally-on device, the normally-off device can then be turned on.

In some cases, however, the system's sequencer during start-up uses various circuits to control the timing and behavior of the system when power is applied. In these cases, an auxiliary power supply may be used to power the start-up circuitry.

the<de-docref CY="DE" DNUM="102011087368" KI="A1">DE 10 2011 087 368 A1</de-docref> deals with an arrangement and a method for bootstrapping a switch driver. It discloses a circuit in which the load paths of a JFET and a PMOS transistor are connected in series and whose gates are driven by a driver. The driver can be supplied via an auxiliary supply that can be activated.

According to one embodiment, a circuit includes a first normally-on transistor having a drain coupled to a first switching output node, a normally-off transistor having a drain coupled to a source of the first normally-on transistor, a driver circuit configured to receive a switching signal, the driver circuit has an output coupled to a gate of the first normally on transistor, and a second normally-on transistor having a drain coupled to a supply node, a gate coupled to the output of the driver circuit, and a source configured to supply an auxiliary 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-1c</figref> illustrate conventional switch drive systems and auxiliary voltage generators;</li><li id="ul_0001_0002"><figref>2</figref> illustrate a switch drive system with an auxiliary voltage generator according to an embodiment;</li><li id="ul_0001_0003"><figref>3</figref> illustrates an embodiment of a switching power supply; and</li><li id="ul_0001_0004"><figref>4</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 indicated. 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, a system and method for providing an auxiliary supply voltage in a switching circuit. For example, embodiments of the present invention can also be applied to various systems that use auxiliary power supplies associated with switching circuits, such as switching power supplies, motor controller circuits, high-voltage systems, and other circuits and systems.

In one embodiment, an auxiliary voltage is generated using a normally on transistor having a drain coupled to a main supply voltage and a gate coupled to a switching signal. The auxiliary voltage is output at the source of the normally on transistor and can be filtered with a capacitor. In some embodiments, the gate of the normally-on transistor is further coupled to a gate of a second normally-on transistor used for a switching function in a circuit. For example, this second normally-on transistor can be coupled to a coil in a switched-mode power supply and configured to magnetize and demagnetize the coil.

Certain types of power transistors, such as JFETs (Junction Field Effect Transistors) and Gallium Nitride High Electron Mobility Transistors (GaN HEMTs) behave like “normally on” devices because they are in a conducting state when between the gate and zero volts are applied to the source of the transistor. To turn these transistors off, a sufficiently high negative gate voltage is applied between the gate and source of the transistor. For example, in the case of a GaN HEMT, this reverse voltage can range from about -5V to about -8V, but turn-off voltages outside of this range can also occur in some devices. Accordingly, in some systems, such as those that use charge pumps to generate negative bias voltages, there is a risk that the normally-on device may cause a short circuit between the device's power supply rails before the charge pump has had sufficient time to generate a sufficiently high negative voltage to turn off the self-conducting component. In addition, there is also a risk of short circuits during various fault conditions.

One way this problem has been addressed is by coupling the normally-on device in series with a normally-off device such as an enhancement-mode MOSFET device in a cascode configuration, as in FIG<figref>1a</figref> shown. As shown, the source of normally-on transistor 102 is coupled to the drain of normally-off transistor 104 at node Vx and the gate G of normally-off transistor 104 is driven by driver circuit 106 . Diode 110 represents the body diode of normally-off transistor 104.

Here, the gate G of normally-off transistor 104 acts as the control terminal driven by driver circuit 106 , whereas the gate of normally-on transistor 102 is coupled to the source S of normally-on transistor 104 . During startup, the series combination of the normally-on device and the normally-off device is non-conductive. When the required supply voltage becomes available, the normally-off device can be driven with an input signal so that the normally-on device functions as a cascode device. If the current flow through the switch is forced in the reverse direction, the body diode BD of the normally-off transistor 104 will be forward-biased.

Voltage source 108 having a voltage VP provides power to driver circuit 106. Thus, when the output of driver circuit 106 is high, a voltage of about VP can be provided to the gate of normally-off transistor 104. On the other hand, when the output of driver circuit 106 is low, zero volts are applied between the gate and source of transistor 104, causing normally-off transistor 104 to turn off.

<figref>1b</figref> FIG. 13 illustrates a switching circuit 130 in which the source node Vx of the normally-on transistor 102 is coupled to the diode D and the capacitor C. FIG. During operation, the source node Vx of normally-on transistor 102 alternates between ground potential and a peak voltage Vpk that is higher than -Vth, the inverse of the threshold voltage of normally-on transistor 102. For example, if the threshold voltage of the normally-on transistor 102 is -5 V, the peak voltage Vpk of the source node Vx is above +5 V. The actual value of Vx depends on the switching dynamics and can be significantly above -Vth (e.g. 20 V ). Diode D rectifies the voltage at the source node Vx and capacitor C filters the voltage ripple at the output of diode D. Diode D also prevents capacitor C from discharging when normally-off transistor 104 is turned on. Accordingly, an auxiliary voltage is provided at the Vaux output, which can be used to power driver circuit 106 during circuit start-up and/or during periods of time when voltage source 108 is unable to provide an adequate voltage potential for switching normally-off transistor 104 deliver.

The auxiliary voltage Vaux depends on a number of factors, including the threshold voltage of transistor 102, switching dynamics, the diode voltage of diode D, circuit parasitic effects, and other factors.

<figref>1c</figref> FIG. 15 illustrates a switching circuit 150 in which the auxiliary voltage is taken from a source of a second normally-on transistor 152 that shares a drain node and a gate node with the normally-on transistor 102. FIG. Transistor 152 is typically implemented by disconnecting the source connection of a small portion of large power transistor 102 and making it electrically accessible. Transistors 102 and 152 may be referred to as a "split transistor cell." Circuit 130 has less variation than circuit 103 of FIG<figref>1b</figref>, because the source potential of transistor 152 is not switched and remains close to -Vth.

<figref>2</figref> FIG. 2 illustrates a switching circuit 200 in accordance with an embodiment of the present invention in which a normally-on transistor 222 generates an auxiliary voltage Vaux directly at its source again. Circuit 200 includes a voltage source 208, a driver circuit 206, a normally-off transistor 204, and a normally-on transistor 202 with its gate coupled to the driver circuit 206 output. Thus, the power transistor 202 operates in a "directly driven" configuration, whereas in the circuits of FIG<figref>1</figref> the gate of the power transistor 102 is at a fixed potential, so they operate in a "cascode" configuration. Diode 210 represents the body diode of normally-off transistor 204. In addition, normally-on transistor 222 has its gate coupled to the output of driver circuit 206 and its source connected to capacitor C to generate the auxiliary voltage Vaux. As shown in the illustrated embodiment, the drains of normally-on transistors 202 and 222 are connected to separate nodes: the drain of normally-on transistor 202 is connected to switch 226 and the drain of normally-on transistor 222 is connected to voltage source 224 . Switch 226 is representative of various circuitry such as switches that may be coupled to the drain of normally on transistor 202 . In some embodiments, inductors, capacitors, resistors, diodes, transistors, and other components may be coupled to the drain of normally-on transistor 202 . For example, the circuitry coupled to the drain of the normally-on transistor 202 may be circuitry used to implement a switched-mode power supply or a motor controller.

By coupling the drain of normally-on transistor 202 to various circuits within the application circuit while the drain of normally-on transistor 222 is coupled to voltage source 224, the auxiliary voltage Vaux can be kept relatively constant and approximately equal to the negative threshold voltage -Vth of normally-on transistor 222 without that the need for a diode as in<figref>1</figref> consists. For example, in the present embodiment, Vaux may be used to power the gate driver of transistor 204, which is on during normal operation. Controlling its gate consumes very little power.

For example, in one embodiment, Vaux may be used to provide power to startup circuitry and/or to provide a reference voltage based on a threshold voltage of the normally-on transistor. For example, embodiments of the present invention may be used to provide a voltage related to the threshold voltage of the normally-on transistor. This voltage can be used to bias a switching signal having a common mode voltage approximately equal to the threshold voltage of normally-on transistor 202 . Such a system is described, for example, in co-pending US application Serial No. 14/473,300.

Normally-on transistors 202 and 222 may be implemented using GaN HEMT transistors, for example. GaN HEMT transistors are basically structures with lateral current flow. Therefore, both source and drain connections are available at the semiconductor surface. A split transistor cell with separate source and drain connections can thus be easily formed within a single die. This is in contrast to conventional power MOSFET structures that use vertical current flow, where the drain electrode is formed by the chip substrate and does not allow for a split transistor structure with a separate drain. In embodiments using GaN HEMT transistors, the value of Vaux may range according to the variation in the threshold of the particular GaN transistor used. For example, in one embodiment, the value of Vaux may be in the voltage range between about 5V and about 9V, which corresponds to a range of threshold voltage values ​​of between about -5V and about -9V. Alternatively, values ​​outside of this range can also be achieved depending on the respective semiconductor process and the respective device geometry. In other embodiments, other types of transistors may be used, such as a power MOSFET transistor, a GaN HEMT, a JFET, an enhancement mode MOSFET, a depletion mode MOSFET, a bipolar junction transistor (BJT), or another type of transistor.

<figref>3</figref> 12 illustrates a switched mode power converter 300 configured to convert an AC input signal, such as a 50 Hz to 60 Hz line voltage, at input port Vin to a DC output voltage at output port Vout, according to an embodiment of the present invention. For example, in one embodiment, switched mode power converter 300 is configured to convert a 120 VAC or 240 VAC, 50 Hz to 60 Hz line input voltage to a DC output voltage of approximately 400V. Alternatively, other input voltages operating on the same or different frequency ranges can be used.

Switched mode power converter 300 utilizes at least one embodiment of a compound switch driver 306 including a normally on transistor 202, a normally off transistor 204, a normally on transistor 222, and associated driver and control circuitry. The normally on transistor is coupled to a positive output node and generates an auxiliary voltage Vaux according to the embodiments described above. As shown, the Vaux voltage is used to provide power to the startup circuitry 326 .

Switched mode power converter 300 includes an H-bridge implemented using an embodiment of a composite switch and driver 302 and 306 that includes a normally-on transistor 202 coupled in series with a normally-off transistor 204, along with associated drivers and a Control circuit included. Switching circuits 304 and 308, which include MOSFET switching transistors, operate as synchronous rectifiers that provide a reverse current path and may also be implemented using diodes in some embodiments. This H-bridge is coupled to the input port Vin via an inductor 312 . During operation of switched mode power converter 300, switch and driver assemblies 302 and 306 and switching circuits 304 and 308 magnetize and demagnetize inductor 312 such that power is transferred from input port Vin to output port Vout.

Each combination of switch and driver 302 and 306 can be operated as cascode devices, in which the normally-off transistor 204 is switched according to the input switching signal Vin, and/or can be operated as direct drivers, in which the normally-off transistor 204 is turned on and the switching signal is applied to the gate of normally on transistor 202 . The switch and driver assembly 306 includes the normally-on transistor 202 as well as the normally-on transistor 222 used to provide the auxiliary voltage Vaux as described in embodiments above. The control of the series of normally-on transistors and normally-off transistors in the combination of switches and drivers 302 and 306, switching circuits 304 and 308, and associated circuitry may be configured and operated as described in co-pending US application Serial No. 14/ 473,300. In alternative embodiments, the composite switch and drivers 302 may be implemented using a single switching transistor and/or normally off transistor 204 .

The capacitor 314 represents the input capacitance of the switched-mode power converter 300, and the capacitor 310 represents the load capacitance of the switched-mode power converter 300. In one embodiment, a controller 318 provides switching signals to an embodiment of the combination of switches and drivers 302 and 306, switching circuits 304 and 308 , whose timing is configured to convert an AC input signal at input port Vin to a DC output signal at port Vout. In some embodiments, signals S1 and S2 are non-overlapping switching signals to avoid cross current. The controller 318 may be implemented using an H-bridge based switch mode power converter known in the art. In some embodiments, the controller 318 may utilize current and/or voltage feedback from various nodes and power branches of the switched mode power converter 300 to provide feedback control of the output voltage, output current, and/or input currents. For example, the switching of signals S1 and S2 respectively coupled to switch and driver assembly 302 and 306 may be configured to provide a regulated output voltage and input current such that power factor correction is achieved.

Startup circuitry 326 may be used to control the state of the normally-off transistor during startup via a START signal. For example, normally-off transistor 204 may be kept off during startup until a bias generator that provides the turn-off voltage for normally-on transistors 202 has reached a voltage that can safely turn off normally-on transistors 202. The startup circuitry 326 may include logic circuitry and supply sense circuitry known in the art and used to implement, for example, under-voltage shutdown functions. In other embodiments, Vaux may be used to provide power to other circuit functions.

<figref>4</figref> 4 illustrates a flow chart of an embodiment of a method 400 for operating a switching circuit, including: a first normally on transistor having a drain coupled to a first switch output node, a normally off transistor having a drain coupled to a source of the first normally on transistor, a driver circuit having an output coupled to a gate of the first normally on transistor, a second normally on transistor having a drain coupled to a supply node and a gate coupled to the output of the driver circuit. This method may be used, for example, in connection with various illustrated embodiments disclosed herein. In one embodiment, a switching signal is received at an input of the driver circuit at step 402 . The first normally on transistor and the second normally on transistor are driven according to the received switching signal in step 404 and an auxiliary voltage is generated at a source terminal of the second normally on transistor in step 406 .

According to one embodiment, a circuit includes a first normally-on transistor having a drain coupled to a first switching output node, a normally-off transistor having a drain coupled to a source of the first normally-on transistor, a driver circuit configured to receive a switching signal , the driver circuit having an output coupled to a gate of the first normally on transistor, and a second normally-on transistor having a drain coupled to a supply node, a gate coupled to the output of the driver circuit, and a source configured to supply an auxiliary voltage. The circuit may further include a capacitor coupled between the source of the second normally-on transistor and a source of the normally-off transistor.

In one embodiment, the driver circuit further includes a first power supply terminal coupled to a source of the first normally-on transistor and a second power supply terminal configured to couple to a driver reference voltage node. The circuit may also include a power supply circuit coupled between the first driver circuit power supply terminal and the second driver circuit power supply terminal. In some embodiments, the normally-off transistor is configured to couple to ground.

In one embodiment, the circuit further includes a switch coupled between the supply node and the switch output node, wherein the supply node is configured to have a voltage of at least 100V, and/or a normally on transistor is implemented using a gallium nitride high electron -Mobility-Transistors (GaN-HEMT) implemented. In some embodiments, the normally-off transistor is implemented using an enhancement-mode MOSFET device, the first normally-on transistor and the second normally-on transistor are disposed on a same semiconductor substrate, and/or the auxiliary voltage is coupled to startup circuitry.

According to another embodiment, a switching circuit includes a first normally-on transistor having a drain coupled to a first switching output node, a normally-off transistor having a drain coupled to a source of the first normally-on transistor, a driver circuit having an output coupled to a gate terminal of the first normally on transistor is coupled, a second normally-on transistor having a drain coupled to a supply node and a gate coupled to the output of the driver circuit. A method for operating this circuit includes: receiving a switching signal at an input of the driver circuit, driving the first normally on transistor and the second normally on transistor with the driver circuit according to the received switching signal and generating an auxiliary voltage at a source connection of the second normally on transistor.

The method may further include low-pass filtering the auxiliary voltage using a capacitor coupled between the source of the second normally-on transistor and a source of the normally-off transistor. The method may also include turning on the normally-off transistor and may include turning on and off a switch coupled between the first switching output node and the supply node. In some embodiments, the normally-on transistor includes a gallium nitride high electron mobility transistor (GaN HEMT). The method may further include providing the auxiliary voltage to a startup circuit.

According to another embodiment, a power supply system includes a first circuit having a driver and a switch such that the driver of the first circuit is coupled to a first power supply. The power supply system also includes a second circuit coupled in series with the first circuit. The first circuit includes a first normally-on transistor having a drain coupled to a first switching output node, a normally-off transistor having a drain coupled to a source of the first normally-on transistor, a driver circuit configured to receive a switching signal and having an output that coupled to a gate of the first normally on transistor, and a second normally-on transistor having a drain coupled to a supply node, a gate coupled to the output of the driver circuit, and a source coupled to an auxiliary voltage node. The power supply system also includes a power-up circuit having a power supply coupled to the auxiliary voltage node. The power-up circuit is configured to provide power to the driver of the first circuit when a voltage of the first power supply is below a first threshold.

In one embodiment, the power supply system also includes an inductor coupled between a first AC input terminal and a first input node between the first circuit and the second circuit. In some embodiments, the normally-on transistor includes a gallium nitride high electron mobility transistor (GaN HEMT).

Advantages of some embodiments include the ability to generate a stable auxiliary voltage that can be used to provide power to a startup circuit. This auxiliary voltage can be used, for example, to generate a local supply for start-up circuits and other circuitry to provide a stable voltage based on a voltage threshold of a normally-on device.

Another advantage of embodiments includes the ability to integrate supply generation for GaN HEMTs within the power transistor.