# Patent

## Title
Method and circuitry for controlling a depletion-mode transistor

## Abstract

In described examples, a first transistor has: a drain coupled to a source of a depletion-mode transistor; a source coupled to a first voltage node; and a gate coupled to a control node. A second transistor has: a drain coupled to a gate of the depletion-mode transistor; a source coupled to the first voltage node; and a gate coupled through at least one first logic device to an input node. A third transistor has: a drain coupled to the gate of the depletion-mode transistor; a source coupled to a second voltage node; and a gate coupled through at least one second logic device to the input node.

## Background
CROSS-REFERENCE TO RELATED APPLICATION
This application claims priority to U.S. Provisional Patent Application Ser. No. 61/904,777, filed Nov. 15, 2013, entitled Self-protection Mechanism for Depletion-mode Transistors, naming Michael Douglas Seeman et al. as inventors, which is hereby fully incorporated herein by reference for all purposes.


BACKGROUND
This relates in general to electronic circuitry, and in particular to a method and circuitry for controlling a depletion-mode transistor.
In many situations, depletion-mode (“d-mode”) transistors, such as gallium nitride (“GaN”) high-electron-mobility transistors (“HEMTs”) and silicon carbide (“SiC”) junction gate field-effect transistors (“JFETs”), have switching performance that is superior to enhancement-mode (“e-mode”) transistors. Nevertheless, in some power electronic circuit implementations, a normally “on” d-mode transistor (e.g., whose VGS=0V) may raise concerns about safety. By comparison, a normally “off” e-mode transistor may help to substantially prevent cross-conduction (such as short circuiting) in response to certain fault conditions.
SUMMARY
In described examples, a first transistor has: a drain coupled to a source of a depletion-mode transistor; a source coupled to a first voltage node; and a gate coupled to a control node. A second transistor has: a drain coupled to a gate of the depletion-mode transistor; a source coupled to the first voltage node; and a gate coupled through at least one first logic device to an input node. A third transistor has: a drain coupled to the gate of the depletion-mode transistor; a source coupled to a second voltage node; and a gate coupled through at least one second logic device to the input node.



BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a schematic electrical circuit diagram of circuitry of the example embodiments.



DETAILED DESCRIPTION
FIG. 1 is a schematic electrical circuit diagram of circuitry 100 of the example embodiments. As shown in FIG. 1, a high-voltage d-mode transistor 102, such as a GaN HEMT, is connected in series with a low-voltage e-mode NFET (“LV switch”) 104. In a first example, the LV switch 104 is discrete. In a second example, the LV switch 104 is integrated with another component (such as being integrated with driver circuitry 105).
A drain of the d-mode transistor 102 is connected to a voltage output node VOUT whose voltage may range up to 600 volts (or beyond). A source of the d-mode transistor 102 is connected to a drain of the LV switch 104. A source of the LV switch 104 is connected to a voltage reference node, such as a ground node GND whose voltage is 0 volts. In at least one example, the ground node GND is connected to a local ground instead of a global ground.
The LV switch 104: (a) turns on for normal operation, so that n-channel metal oxide semiconductor (“NMOS”) switching dynamics are substantially removed from overall switching dynamics of the circuitry 100 during normal operation; and (b) turns off for safety (such as device protection) in response to one or more detected fault conditions (such as during startup). Examples of such fault conditions are under-voltage, over-voltage, over-current, and over-temperature.
For example, in response to voltages at the +12 V, +5 V and −12 V nodes, under-voltage lockout (“UVLO”) circuitry 106 detects: (a) whether an under-voltage condition exists or is absent; and (b) whether an over-voltage condition exists or is absent. In response to such detection, the UVLO circuitry 106 outputs a signal on a PGOOD line to respective first inputs of AND gates 108 and 110. Accordingly, in response to the UVLO circuitry 106 detecting neither an under-voltage condition nor an over-voltage condition, the signal from the UVLO circuitry 106 on the PGOOD line has a binary logic 1 (“true”) state. Conversely, in response to the UVLO circuitry 106 detecting either an under-voltage condition or an over-voltage condition, the signal from the UVLO circuitry 106 on the PGOOD line has a binary logic 0 (“false”) state.
Similarly, in response to voltages at a gate of the LV switch 104 and at the drain of the LV switch 104, over-current protection (“OCP”) over-temperature protection (“OTP”) circuitry 112 detects: (a) whether an over-current condition exists or is absent; and (b) whether an over-temperature condition exists or is absent. In response to such detection, the OCP OTP circuitry 112 outputs a signal on a /FAULT line to respective second inputs of the AND gates 108 and 110. Accordingly, in response to the OCP OTP circuitry 112 detecting neither an over-current condition nor an over-temperature condition, the signal from the OCP OTP circuitry 112 on the /FAULT line has a binary logic 1 (“true”=no fault) state. Conversely, in response to the OCP OTP circuitry 112 detecting either an over-current condition or an over-temperature condition, the signal from the OCP OTP circuitry 112 on the /FAULT line has a binary logic 0 (“false”=fault) state. The OCP OTP circuitry 112 and the UVLO circuitry 106 are examples of fault detection circuitry.
An output of the AND gate 110 is coupled through a buffer 114 to a control node 115. The control node 115 is coupled to the gate of the LV switch 104. Accordingly, if the signal on the PGOOD line has the true state, and if the signal on the /FAULT line has the true state, then the output of the AND gate 110 has the true state, and the LV switch 104 turns on for normal operation. Conversely, if the signal on the PGOOD line has the false state, or if the signal on the /FAULT line has the false state, then the output of the AND gate 110 has the false state, and the LV switch 104 turns off for safety in response to one or more of those detected fault conditions.
Similarly, an output of the AND gate 108 is coupled through an inverter 116 to a gate of an n-channel field-effect transistor (“NFET”) 118. A source of the NFET 118 is connected to the ground node GND, and a drain of the NFET 118 is connected to a FAULT node. Accordingly, if the signal on the PGOOD line has the true state, and if the signal on the /FAULT line has the true state, then the output of the AND gate 108 has the true state, so the NFET 118 turns off. Conversely, if the signal on the PGOOD line has the false state, or if the signal on the /FAULT line has the false state, then the output of the AND gate 108 has the false state, thereby turning on the NFET 118. By turning on the NFET 118, the FAULT node is coupled through the NFET 118 to 0 volts, which thereby communicates (via the FAULT node) existence of one or more of those detected fault conditions.
Also, the output of the AND gate 108 is connected to a first input of an AND gate 120. An input node IN is coupled through a buffer 122 to a second input of the AND gate 120. Thus, if the input node IN has a binary logic 0 (“false”) state, then an output of the AND gate 120 has the false state.
For normal operation, the input node IN receives a pulse width modulated (“PWM”) signal (such as from a PWM controller), which alternates between a binary logic 1 (“true”) state and a binary logic 0 (“false”) state. Accordingly, during normal operation: (a) if the signal on the PGOOD line has the true state, and if the signal on the /FAULT line has the true state, then the logic state of the input node IN propagates through the AND gate 120, so the output of the AND gate 120 has the same logic (either true or false) state as the input node IN; and (b) conversely, if the signal on the PGOOD line has the false state, or if the signal on the /FAULT line has the false state, then the output of the AND gate 120 has the false state.
In response to a 12-volt input voltage at a node (“+12 V node”), a low-dropout (“LDO”) regulator 124 generates a 5-volt voltage at a node (“+5 V node”). The +12 V node is connected to a source of a p-channel field-effect transistor (“PFET”) 126. An inverting buck-boost controller 128 is connected to a gate of the PFET 126 and to a gate of an NFET 130. A source of the NFET 130 is connected to a line 132. A switch node SW is connected to a drain of the PFET 126 and to a drain of the NFET 130. In at least one example, an inductor (not shown for clarity) is connected between the switch node SW and the ground node GND whose voltage is 0 volts. Accordingly, in response to signals (such as voltage signals) at a feedback node FB, the controller 128 controls switching (between on and off) of the PFET 126 and NFET 130 to regulate a voltage of −12 volts on the line 132. In another example, the controller 128 is replaced by an inverting charge pump to regulate the voltage of −12 volts on the line 132 (“−12 V node”).
A gate of the d-mode transistor 102 is connected to a drain of a PFET 134 and to a drain of an NFET 136. A source of the PFET 134 is connected to the ground node GND whose voltage is 0 volts, and a source of the NFET 136 is connected to the line 132 whose voltage is −12 volts. A body diode 138 of the PFET 134 is connected from the drain of the PFET 134 to the source of the PFET 134.
For an inverter 140, an OR gate 142 and a buffer 144, a binary logic 0 (“false”) state is represented by −5 volts, and a binary logic 1 (“true”) state is represented by 0 volts. For an inverter 146, an AND gate 148 and a buffer 150, a binary logic 0 (“false”) state is represented by −12 volts, and a binary logic 1 (“true”) state is represented by −7 volts.
A level shifter (L/S) 152: (a) receives the output of the AND gate 120; and (b) converts such output to corresponding signals that are suitable for the inverters 140 and 146. Accordingly, in response to the output of the AND gate 120 having the false state, the L/S 152 outputs: (a) to an input of the inverter 140, a signal whose voltage is −5 volts; and (b) to an input of the inverter 146, a signal whose voltage is −12 volts. Conversely, in response to the output of the AND gate 120 having the true state, the L/S 152 outputs: (a) to the input of the inverter 140, a signal whose voltage is 0 volts; and (b) to the input of the inverter 146, a signal whose voltage is −7 volts.
An output of the inverter 140 is connected to a first input of the OR gate 142. An output of the OR gate 142 is connected to an input of the buffer 144. An output of the buffer 144 is connected to a gate of the PFET 134.
An output of the inverter 146 is connected to a first input of the AND gate 148. An output of the AND gate 148 is connected to an input of the buffer 150. An output of the buffer 150 is connected to a gate of the NFET 136.
A level shifter (L/S) 154: (a) receives the output of the AND gate 148; and (b) converts such output to a corresponding signal that is suitable for the OR gate 142. Accordingly: (a) in response to the output of the AND gate 148 having the false state (−12 volts), the L/S 154 outputs (to a second input of the OR gate 142) a signal whose voltage is −5 volts; and (b) conversely, in response to the output of the AND gate 148 having the true state (−7 volts), the L/S 154 outputs (to the second input of the OR gate 142) a signal whose voltage is 0 volts.
Similarly, the level shifter (L/S) 154: (a) receives the output of the OR gate 142; and (b) converts such output to a corresponding signal that is suitable for the AND gate 148. Accordingly: (a) in response to the output of the OR gate 142 having the false state (−5 volts), the L/S 154 outputs (to a second input of the AND gate 148) a signal whose voltage is −12 volts; and (b) conversely, in response to the output of the OR gate 142 having the true state (0 volts), the L/S 154 outputs (to the second input of the AND gate 148) a signal whose voltage is −7 volts.
In that manner, the respective outputs of the inverters 140 and 146 have the same binary logic state as one another, and such logic state is latched by the respective outputs of the OR gate 142 and the AND gate 148.
In at least one embodiment, a threshold voltage (VT) of the d-mode transistor 102 is −10 volts, so the gate of the d-mode transistor 102 operates from a negative potential relative to the source of the LV switch 104. For example, during normal operation, the circuitry 100 is operable to actively switch the gate of the d-mode transistor 102 between 0 volts and −12 volts. Accordingly, the circuitry 100 achieves a native d-mode device's superior switching performance and maintains a controllable edge rate, while preserving a cascode arrangement's inherent normally-off capability.
For turning off the d-mode transistor 102, the input node IN is cleared to the false state, so the output of the AND gate 120 has the false state, thereby turning off the PFET 134 and turning on the NFET 136. Likewise, in response to one or more of the detected fault conditions (irrespective of whether the input node IN is cleared to the false state or set to the true state), the output of the AND gate 120 has the false state, thereby turning off the PFET 134 and turning on the NFET 136. By turning on the NFET 136 in that manner, the gate of the d-mode transistor 102 is coupled through the NFET 136 to the line 132 whose voltage is −12 volts, so the d-mode transistor 102 is turned off.
For turning on the d-mode transistor 102, the input node IN is set to the true state, so the output of the AND gate 120 has the true state (but only while the output of the AND gate 108 likewise has the true state), thereby turning on the PFET 134 and turning off the NFET 136. By turning on the PFET 134 in that manner:

    
    
        (a) the gate of the d-mode transistor 102 is coupled through the PFET 134 to the ground node GND (and likewise to the source of the LV switch 104) whose voltage is 0 volts, so VGS of the d-mode transistor 102 is approximately equal to (but opposite polarity from) VDS of the LV switch 104; and
        (b) accordingly, if the LV switch 104 is turned on, then VDS of the LV switch 104 is relatively small, and VGS of the d-mode transistor 102 is relatively small, so the d-mode transistor 102 is turned on.
    
    


If the driver circuitry 105 is unpowered, then the LV switch 104 is turned off, and the gate of the d-mode transistor 102 is coupled to near 0 volts (of the ground node GND) through the diode 138. Or, if the driver circuitry 105 has power, yet any one or more of the +12 V, +5 V or −12 V nodes is not at its suitable voltage level, then the signal from the UVLO circuitry 106 on the PGOOD line has a binary logic 0 (“false”) state, so the LV switch 104 is turned off. If the LV switch 104 is turned off (such as for safety in response to one or more of the detected fault conditions), then VDS of the LV switch 104 increases, which eventually causes VGS of the d-mode transistor 102 to reach (and continue beyond) its threshold voltage (VT), so the d-mode transistor 102 begins (and continues) turning off, even if the line 132 is not at its suitable voltage level of −12 volts.
Modifications are possible in the described embodiments, and other embodiments are possible, within the scope of the claims.

## Claims

1. Circuitry for controlling a depletion-mode transistor, the circuitry comprising:
a first transistor having: a drain coupled to a source of the depletion-mode transistor; a source coupled to a first voltage node; and a gate coupled to a control node;
a second transistor having: a drain coupled to a gate of the depletion-mode transistor; a source coupled to the first voltage node; and a gate coupled through at least one first logic device to an input node; and
a third transistor having: a drain coupled to the gate of the depletion-mode transistor; a source coupled to a second voltage node; and a gate coupled through at least one second logic device to the input node.

2. The circuitry of claim 1, wherein the depletion-mode transistor is a gallium nitride high-electron-mobility transistor.

3. The circuitry of claim 1, wherein the first transistor is an enhancement-mode NFET.

4. The circuitry of claim 1, wherein the second transistor is a PFET, and the third transistor is an NFET.

5. The circuitry of claim 1, wherein: the at least one first logic device is adapted to turn on the second transistor in response to the input node having a first logic state, and is adapted to turn off the second transistor in response to the input node having a second logic state; and the at least one second logic device is adapted to turn off the third transistor in response to the input node having the first logic state, and is adapted to turn on the third transistor in response to the input node having the second logic state.

6. The circuitry of claim 1, wherein a voltage of the second voltage node is for turning off the depletion-mode transistor.

7. The circuitry of claim 1, and comprising: fault detection circuitry adapted to detect a fault condition.

8. The circuitry of claim 7, wherein the fault detection circuitry is coupled to the control node and is adapted to turn off the first transistor in response to existence of the fault condition, and wherein turning off the first transistor is for turning off the depletion-mode transistor.

9. The circuitry of claim 7, wherein the fault condition includes at least one of: an under-voltage condition; an over-voltage condition; an over-current condition; and an over-temperature condition.

10. The circuitry of claim 7, wherein: the at least one first logic device is coupled to the fault detection circuitry, and is adapted to turn on the second transistor in response to absence of the fault condition, and is adapted to turn off the second transistor in response to existence of the fault condition; and the at least one second logic device is coupled to the fault detection circuitry, and is adapted to turn off the third transistor in response to absence of the fault condition, and is adapted to turn on the third transistor in response to existence of the fault condition.

11. A method of controlling a depletion-mode transistor, the method comprising:
in response to absence of a fault condition, turning on a first transistor having: a drain coupled to a source of the depletion-mode transistor; a source coupled to a first voltage node; and a gate coupled to a control node;
in response to an input node having a first logic state, turning on a second transistor having: a drain coupled to a gate of the depletion-mode transistor; a source coupled to the first voltage node; and a gate coupled through at least one first logic device to the input node;
in response to the input node having the first logic state, turning off a third transistor having: a drain coupled to the gate of the depletion-mode transistor; a source coupled to a second voltage node; and a gate coupled through at least one second logic device to the input node; and
in response to the input node having a second logic state, turning off the second transistor, and turning on the third transistor.

12. The method of claim 11, wherein the depletion-mode transistor is a gallium nitride high-electron-mobility transistor.

13. The method of claim 11, wherein the first transistor is an enhancement-mode NFET.

14. The method of claim 11, wherein the second transistor is a PFET, and the third transistor is an NFET.

15. The method of claim 11, wherein a voltage of the second voltage node is for turning off the depletion-mode transistor.

16. The method of claim 11, and comprising: in response to existence of the fault condition, turning off the first transistor, wherein turning off the first transistor is for turning off the depletion-mode transistor.

17. The method of claim 11, wherein the fault condition includes at least one of: an under-voltage condition; an over-voltage condition; an over-current condition; and an over-temperature condition.

18. The method of claim 11, and comprising:
in response to absence of the fault condition, turning on the second transistor, and turning off the third transistor; and
in response to existence of the fault condition, turning off the second transistor, and turning on the third transistor.

19. Circuitry for controlling a depletion-mode transistor, the circuitry comprising:
a first transistor having: a drain coupled to a source of the depletion-mode transistor; a source coupled to a first voltage node; and a gate coupled to a control node; wherein the first transistor is an enhancement-mode NFET;
a second transistor having: a drain coupled to a gate of the depletion-mode transistor; a source coupled to the first voltage node; and a gate coupled through at least one first logic device to an input node; wherein the second transistor is a PFET;
a third transistor having: a drain coupled to the gate of the depletion-mode transistor; a source coupled to a second voltage node; and a gate coupled through at least one second logic device to the input node; wherein the third transistor is an NFET; and
fault detection circuitry adapted to detect a fault condition, wherein the fault detection circuitry is coupled to the control node and is adapted to turn off the first transistor in response to existence of the fault condition, wherein turning off the first transistor is for turning off the depletion-mode transistor, and wherein the fault condition includes at least one of: an under-voltage condition; an over-voltage condition; an over-current condition; and an over-temperature condition;
wherein: the at least one first logic device is adapted to turn on the second transistor in response to the input node having a first logic state, and is adapted to turn off the second transistor in response to the input node having a second logic state; and the at least one second logic device is adapted to turn off the third transistor in response to the input node having the first logic state, and is adapted to turn on the third transistor in response to the input node having the second logic state; and
wherein: the at least one first logic device is coupled to the fault detection circuitry, and is adapted to turn on the second transistor in response to absence of the fault condition, and is adapted to turn off the second transistor in response to existence of the fault condition; and the at least one second logic device is coupled to the fault detection circuitry, and is adapted to turn off the third transistor in response to absence of the fault condition, and is adapted to turn on the third transistor in response to existence of the fault condition.

20. The circuitry of claim 19, wherein the depletion-mode transistor is a gallium nitride high-electron-mobility transistor.

21. The circuitry of claim 19, wherein a voltage of the second voltage node is for turning off the depletion-mode transistor.