A gate driver

Abstract

A gate driver includes inputs for receiving a control signal for controlling a state of a switch, and a current sense signal indicative of whether the switch is conducting current, and one or more out

Description

B ACKGROUND Power converters are utilised to convert electrical energy from one form to another. Power converters are often used in implementations such as battery charging, for example to convert AC electrical power to DC electrical power for charging a battery pack. SUMMARY A first aspect provides a gate driver comprising: inputs for receiving a control signal for controlling a state of a switch, and a current sense signal indicative of whether the switch is conducting current; and one or more outputs for outputting one or more gate signals for driving the state of the switch, wherein the switch has at least two states: ON in which the switch is conductive in both a first direction and a second direction, and DI in which the switch is conductive in the first direction and non-conductive in the second direction, and wherein, in response to a control signal to set the state of the switch to DI, the gate driver is operable to: output gate signals to drive the state of the switch to DI; monitor the current sense signal; and if the current sense signal indicates that the switch is conducting current in the first direction above a first direction current threshold value, output gate signals to drive the state of the switch to ON. As the gate driver is both able to receive a control signal for controlling a state of the switch, and able to monitor the current sense signal and to drive the state of the switch to ON when the current sense signal indicates that the switch is conducting current above the first direction current threshold value, the gate driver is both externally controlled by the control signal, and also locally and internally controlled in response to the current sense signal, with the local control overriding the external control signal. This may provide increased flexibility in control of the switch compared to, for example, a gate driver operable only in response to an external control signal, or a gate driver operable only in response to locally sensed current sense signals. In particular, this may allow for the switch to have both an active control period and a synchronous rectification period. Synchronous rectification is typically utilised to reduce losses that would otherwise occur when conducting through a diode. For example, a synchronous rectification (SR) switch comprising a MOSFET and a local gate driver can be utilised in place of a diode. The SR switch is arranged such that the intrinsic body diode of the MOSFET is in the same orientation as the diode that the SR switch replaces. The gate driver detects conduction through the body diode, and sets the MOSFET to its ON state, so that current flows through the MOSFET instead of through the body diode. This may reduce losses that would otherwise be associated with conduction through the body diode. An SR switch is typically a discrete, self-driven device that cannot be controlled externally. By contrast, the gate driver of the first aspect is capable of controlling a switch in response to an external control signal, whilst also providing synchronous rectification in response to a local current sense signal. A conventional controllable gate driver may receive a control signal from a microcontroller or the like and, in response, output a gate signal for controlling the state of a switch. However, such gate drivers do not perform synchronous rectification. In particular, the gate driver does not monitor whether the switch is conducting and, in response, change the state of the switch. The microcontroller may perform synchronous rectification. For example, the microcontroller may monitor current in the switch and, in response, change the control signal that is output to the gate driver. However, synchronous rectification requires precise timing to avoid the conduction of high reverse currents. As such, microcontrollers cannot usually perform synchronous rectification where high switching frequencies are required, such as in some power converters. The gate driver of the first aspect may mitigate for this, by both driving the switch in response to the externally received control signal, and by locally monitoring the current sense signal and driving the switch accordingly to override the externally received control signal when needed. The gate driver may be operable to: continue monitoring the current sense signal; and if the current sense signal indicates that the switch is conducting current in the first direction below the first direction current threshold value, is not conducting current, or is conducting current in the second direction, output gate signals to drive the state of the switch to DI. The gate driver is therefore capable of returning the switch to a diode state, DI, to prevent the conduction of high reverse currents. The switch may have at least a third state D2 in which the switch is conductive in the second direction and non-conductive in the first direction. In response to a control signal to set the state of the switch to D2, the gate driver may be operable to: output gate signals to drive the state of the switch to D2; monitor the current sense signal; and if the current sense signal indicates that the switch is conducting current in the second direction above a second direction current threshold value, output gate signals to drive the state of the switch to ON. The gate driver may therefore provide synchronous rectification irrespective of which direction current is flowing through the switch. Switches that have the switch state DI and the switch state D2 may be referred to as bidirectional switches. The gate driver may therefore be used to provide synchronous rectification in a controllable bidirectional switch. Such bidirectional switches, for example bidirectional Gallium Nitride (GaN) switches, also referred to as BiGaN switches, may comprise a fourth state, OFF, in which the switch is non-conductive in both the first direction and the second direction. These switches may be used in applications where inductive energy must be carefully managed when changing the states of the switches from ON to OFF. With the gate driver of the first aspect, bidirectional switches can be controlled in a way that safely and efficiently manages inductive energy within an application. The gate driver may be operable to: continue monitoring the current sense signal; and if the current sense signal indicates that the switch is conducting current in the second direction below the second direction current threshold value, is not conducting current, or is conducting current in the first direction, output gate signals to drive the state of the switch to D2. The gate driver is therefore capable of returning the switch to the second diode state, D2, to prevent the conduction of high reverse currents in the first direction. A second aspect provides a switch arrangement comprising the gate driver of the first aspect, and the switch. The switch arrangement may comprise a current sensor for sensing current in the switch and outputting the current sense signal. The switch may comprise a bidirectional switch. The switch may comprise a BiGaN switch. The switch may comprise a single device. The switch may comprise a plurality of devices that collectively operate as a switch. For example the switch may comprise two back-to-back MOSFETs. The switch arrangement may comprise a controller operable to output control signals for controlling the state of the switch. A third aspect provides a power converter comprising the gate driver of the first aspect, or the switch arrangement of the second aspect. Optional features of aspects may be equally applied to other aspects, where appropriate. BRIEF DESCRIPTION OF THE DRAWINGS Figure lisa diagram of a first example boost converter; Figure 2 is a table illustrating switch states of switches of the first example boost converter; and Figure 3 is a diagram of a second example boost converter. DETAILED DESCRIPTION A first example boost converter 10 is illustrated in Figure 1 connected to a power source 12. The first example boost converter 10 has an inductor LI, a first switch SW1, a second switch SW2, a capacitor Cl, first and second outputs 14a, 14b, a first gate driver 16, a second gate driver 18, and a controller 20. The inductor LI is connected in series with the second switch SW2 along a first line 22 between the power source and the first output 14a. A second line 24 extends between the power source and the second output 14b. The first switch SW1 is connected between the first line 22, at a location between the inductor LI and the second switch SW2, and the second 24 line. The capacitor Cl is connected between the first 22 line, at a location between the second switch SW2 and the first output 14a, and the second 24 line. The first switch SW1 is a bidirectional gallium nitride (BiGaN) switch having a first gate G1 and a second gate G2. States of the first switch SW1 are illustrated in Figure 2. As can be seen, the first switch SW1 has a DI state in which the first switch SW1 can conduct in a first direction, but not in a second direction. The first switch SW1 has a D2 state in which the first switch SW1 can conduct in the second direction, but not in the first direction. The first switch SW 1 has an OFF state, in which the first switch SW 1 cannot conduct in either the first or the second direction. The first switch SW 1 has an ON state in which the first switch SW 1 can conduct in either the first direction or the second direction. The second switch SW2 is a bidirectional gallium nitride (BiGaN) switch having a first gate G3 and a second gate G4. States of the second switch SW2 are the same as those illustrated in Figure 2 for the first switch SW1. In particular, the second switch SW2 has a DI state in which the second switch SW2 can conduct in a first direction, but not in a second direction. The second switch SW2 has a D2 state in which the second switch SW2 can conduct in the second direction, but not in the first direction. The second switch SW2 has an OFF state, in which the second switch SW2 cannot conduct in either the first or the second direction. The second switch SW2 has an ON state in which second switch SW2 can conduct in either the first direction or the second direction. The second switch SW2 thereby has the same states as the first switch SW 1. The first gate driver 16 has a first control input CI1 for receiving a first control signal CS1 from the controller 20, and first DRV1 and second DRV2 gate outputs. The first DRV1 and second DRV2 gate outputs control a state of the respective first G1 and second G2 gates of the first switch SW 1. The second gate driver 18 has a second control input CI2 for receiving a second control signal CS2 from the controller 20, monitoring inputs VI, V2, and first DRV3 and second DRV4 gate outputs. The monitoring inputs VI,V2 are connected either side of the second switch SW2, such that the second gate driver 18 can receive signals indicative of a voltage across the second switch SW2. The first DRV3 and second DRV4 gate outputs control a state of the respective first G3 and second G4 gates of the second switch SW2. Collectively, the second switch SW2 and the second gate driver 18 can be thought of as a switch arrangement. The controller 20 is configured to send the first control signal CS1 to the first control input CI1 when it is desired to actively control the first switch SW1, and to send the second control signal CS2 to the second control input CI2 when it is desired to actively control the second switch SW2. In operation of the first example boost converter 10, the controller 20 sends the first control signal CS1 to the first control input CI1 of the first gate driver 16, and in response the first gate driver 16 outputs a gate drive signal over the first gate output DRV1 and the second gate output DRV2 to place the first switch SWI in its ON state. At the same time, the controller 20, either through active provision of a particular form of the second control signal CS2, or through the absence of the second control signal CS2, controls the second gate driver 18 to place the second switch SW2 in its DI state. With such a switch configuration, and as the voltage on the capacitor Cl is a higher amplitude than that of the power source 12, current from the power source 12 is able to flow in a loop through the inductor LI and the first switch SWI, without passing through the second switch SW2. The controller 20 then, either through active provision of a particular form of the first control signal CS1, or through the absence of the first control signal CS1, controls the first gate driver 16 to place the first switch SWI in its DI state. In such a switch configuration, with the second switch SW2 also in its DI state, current flows through the second switch SW2, such that current is provided at the first 14a and second 14b outputs. The second gate driver 18 senses a voltage across the second switch SW2 through the monitoring inputs VI,V2. Where the sensed voltage is above a threshold value, the second gate driver 18, via its first DRV3 and second DRV4 gate outputs, controls the second switch SW2 to be in its ON state, irrespective of how the controller 20 is currently controlling the second switch SW2. Current then flows through the second switch SW2 in its ON state, rather than in its DI state, in a manner similar to conventional synchronous rectification. There may, however, be circumstances in which active control of the state of the second switch SW2 is desired when synchronous rectification is not taking place. For example, BiGaN switches be used in applications where inductive energy must be carefully managed when changing the states of the switches from ON to OFF, and in some circumstances such BiGaN switches may only be switched to OFF once current in a power converter has reduced to zero. The second gate driver 18 having both the second control input CI2 and the monitoring inputs VI, V2 enables flexibility in control, allowing for both control of the second switch SW2 by the controller 20, for example where a desired switching sequence is required, and control of the second switch SW2 by the second gate driver 18, for example to achieve synchronous rectification. In the described operation above, when the second gate driver 18 either senses the voltage across the second switch SW2 dropping below the threshold value, or no longer senses a voltage across the second switch SW2, the second gate driver 18 controls the second switch SW2 to return to its DI state. It will of course be appreciated that, if necessary, current can also flow through the second switch SW2 in the opposite direction when the second switch is in its D2 state, with the second gate driver 18 moving the second switch SW2 to its ON state when the second gate driver 18 senses a voltage across the second switch SW2. It will further be appreciated that examples in which the first gate driver 16 is also able to monitor voltage across the first switch SW1, and cause synchronous rectification to occur, are also envisaged, as are examples where current flow through either the second switch SW2 or the first switch SW1 is monitored by means other than measuring a voltage across the relevant switch. A second example boost converter 100 is illustrated in Figure 3. The second example boost converter 100 is substantially the same as the first example boost converter 10, save that each of the first switch SW1 and the second switch SW2 of the second example boost converter 100 are formed as back-to-back MOSFET devices, instead of the BiGaN switches of the first example boost converter 10. Operation of the second example boost converter 100 is the same as operation of the first example boost converter 10. Although in the second example boost converter 100 of Figure 3 the first switch SW1 and the second switch SW2 are formed as back-to-back MOSFET devices, it will also be appreciated that there are certain scenarios in which a it may be desirable to have a gate driver that is able to both locally control a single MOSFET device based on whether or not a current is flowing through the MOSFET device, and to control the MOSFET device based on a signal received from a separate controller Thus, in each of the power converters described above, a gate driver is able to both locally control a switch based on whether or not a current is flowing through the switch, and to control the switch based on a signal received from a separate controller. It will further be appreciated that although described above in the context of a boost converter, the teachings may be equally applied to a buck converter, or other types of power converter. Whilst particular examples have been described, it should be understood that these are illustrative examples only and that various modifications may be made without departing from the scope of the claims.

Claims

1. A gate driver comprising:inputs for receiving a control signal for controlling a state of a switch, and a current sense signal indicative of whether the switch is conducting current; andone or more outputs for outputting one or more gate signals for driving the state of the switch,wherein the switch has at least two states: ON in which the switch is conductive in both a first direction and a second direction, and DI in which the switch is conductive in the first direction and non-conductive in the second direction, andwherein, in response to a control signal to set the state of the switch to DI, the gate driver is operable to:output gate signals to drive the state of the switch to DI;monitor the current sense signal; andif the current sense signal indicates that the switch is conducting current in the first direction above a first direction current threshold value, output gate signals to drive the state of the switch to ON.

2. The gate driver of claim 1, wherein the gate driver is operable to:continue monitoring the current sense signal; andif the current sense signal indicates that the switch is conducting current in the first direction below the first direction current threshold value, is not conducting current, or is conducting current in the second direction, output gate signals to drive the state of the switch to D1.

3. The gate driver of claim 1 or claim 2, wherein:the switch has at least a third state D2 in which the switch is conductive in the second direction and non-conductive in the first direction; andin response to a control signal to set the state of the switch to D2, the gate driver is operable to:output gate signals to drive the state of the switch to D2;monitor the current sense signal; andif the current sense signal indicates that the switch is conducting current in the second direction above a second direction current threshold value, output gate signals to drive the state of the switch to ON.

4. The gate driver of claim 3, wherein the gate driver is operable to:continue monitoring the current sense signal; andif the current sense signal indicates that the switch is conducting current in the second direction below the second direction current threshold value, is not conducting current, or is conducting current in the first direction, output gate signals to drive the state of the switch to D2.

5. A switch arrangement comprising the gate driver of any one of the preceding claims, and the switch.

6. The switch arrangement of claim 5, wherein the switch comprises a bidirectional switch.

7. The switch arrangement of claim 5 or claim 6, wherein the switch arrangement comprises a controller operable to output control signals for controlling the state of the switch.

8. A power converter comprising the gate driver of any one of claims 1 to 4, or the switch arrangement of any one of claims 5 to 7.

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