Apparatus for driving switching devices and method of use thereof

Abstract

The present disclosure relates to an apparatus for driving a switching device and a method of using the same. The apparatus comprises a drive circuit (9) for applying a control signal to a control terminal (6) of a switching device (4) when activated. The switching device (4) is activatable to drive a load (7) in an operating mode when a control signal above a threshold voltage is applied to the control terminal (6). An activation circuit (20) is coupled to the control terminal (6) and comprises an energy store (11) and a switch (12) operable to discharge the energy store (11) to deliver an activation voltage above the threshold to the control terminal (6). In this way, the switching device (4) can be activated during a delay period before the drive circuit (9) is able to generate a control signal above the threshold voltage after being activated.

Description

TECHNICAL FIELD

[0001] The present invention relates to an apparatus and method for driving a switching device, in particular to a drive circuit for a MOSFET (Metal Oxide Semiconductor Field Effect Transistor) and more particularly to a power amplifier circuit. BACKGROUND

[0002] Switching devices such as high side N-type MOSFETs are commonly used in the automotive industry as part of a power amplifier circuit for providing high current power to drive various electronic systems using the DC battery of a vehicle. Figure 1 An example of such a high side N-type MOSFET switching circuit is shown. In use, current is provided from the battery 1 of a vehicle, the positive terminal being connected to the drain 3 of the MOSFET, the negative terminal being connected to the source 5 of the MOSFET through a load 7. The load can be, for example, the engine control unit (ECU) of a vehicle. The gate 6 of the MOSFET is driven by the output 8 from a gate driver 9. The gate driver 9 is connected to the source 5 of the MOSFET 4 through a driver terminal 10. An internal charge pump charges a driver capacitor 2 at a voltage higher than the voltage of the battery 1. When the output 8 of the gate driver applies a drive voltage to the gate 6 in excess of a threshold voltage, the MOSFET 4 is activated to supply power to the load 7 during its active mode.

[0003] A problem with this type of high current circuit is that the MOSFET 4 consumes current. Therefore, in order to save power during periods of inactivity of the load 7, the gate driver 9 is typically deactivated. However, once the gate driver 9 is turned off, any charge stored at the driver capacitor 2 is lost through leakage current. Therefore, when the gate drive circuit 9 is reactivated, the driver capacitor 2 needs to be recharged. In this way, before the MOSFET 4 turns on, the oscillation of the gate driver 9 must pump charge into the capacitor 2 to build up the voltage.

[0004] To further explain this, Figure 1 The concept of a gate capacitor 25 at the gate-source junction formed by the gate electrode 6 within the MOSFET 4 is shown schematically. It will be appreciated that a high current MOSFET 4 has a large parasitic input capacitance C gs. Therefore, before activating the MOSFET 4, the "gate capacitor 25" C gs must be charged by C in drv through the output 8 of the gate driver. The capacitance C in drv of the driver capacitor 2 therefore needs to be greater than C gs to cause this activation. As a result, the driver capacitor 2 takes time to charge.

[0005] For the above reasons, there is therefore typically a significant delay before the voltage generated by the drive circuit 9 rises above the MOSFET gate 6 threshold to activate the MOSFET 4. Thus, when the load is activated, there is a lag before the MOSFET 4 is activated to pass the high current required by the load for those operational modes of operation.

[0006] The above problems present particular problems in modern cars, where many loads have a "sleep mode" in which a low level of power is required even when the load is not working. For example, when the vehicle is switched off, certain loads such as the vehicle ECU switch to a standby or sleep mode in order to maintain certain critical systems. For example, operational software can be maintained to avoid having to restart the system when the vehicle is switched on. In the case of an ECU, the current requirement during the sleep mode can be in the range 5-100 microamperes (pA) whereas the requirement during normal operational mode can be much higher, for example in the range of milliampere (mA). Thus, it is common to provide an alternative low current power circuit for maintaining these operations in the sleep mode. When the current requirement from the load subsequently increases, the circuit subsequently switches to a high current source. However, during this switching step, there is often an interruption in the current supply due to the above-mentioned lag caused when the gate driver is activated. At the same time, in particular, the ECU will have a period of relatively high current consumption immediately after waking from the sleep mode when various diagnostic procedures are initiated. In this way, the above-mentioned power interruption and associated voltage rapid drop will typically interrupt the ECU during this critical initial wake-up phase. Thus, the ECU is typically unable to withstand a power interruption of more than 100 microseconds (ps) without the risk of logic function reset or requiring time to reload software.

[0007] The present invention therefore aims to address these problems. SUMMARY

[0008] According to a first aspect, there is provided an apparatus comprising: a drive circuit for applying a control signal to a control terminal of a switching device when activated, the switching device being activatable to drive a load in an operational mode when a control signal above a threshold voltage is applied to the control terminal; and a start-up circuit coupled to the control terminal and comprising an energy store and a switch operable to discharge the energy store for passing a start-up voltage above the threshold voltage to the control terminal for activating the switching device during a delay period before the drive circuit generates a control signal above the threshold voltage after being activated.

[0009] In this way, when the load is transitioned from the sleep mode to the active mode, a delay during activation of the switching device is avoided, which can be caused by the time taken by the drive circuit to establish the required threshold voltage after switching on. That is, in embodiments, the energy store can be discharged quickly (on the order of a tenth of a nanosecond) to quickly activate the switching device. This thereby fills the lag time taken by the control signal generated by the drive circuit to establish above the threshold voltage, at which point the drive circuit will control the switching device. In power amplifier devices, this provides for a quick transfer of high current for powering the load in its active mode. In embodiments where the load has a sleep mode requiring low current to maintain standby functionality, a low power circuit can further be provided. In such embodiments, the device can provide a quick response to changes in current demand from the load when the load is woken up, and thus can provide for an uninterrupted transition from low current supply to high current supply.

[0010] In embodiments, the apparatus further comprises a controller for actuating the switch to discharge the energy store in response to an increased current demand of the load. In this way, the apparatus can automatically respond to a detected increase in current demand. In this way, the apparatus can respond to the demand when the load is woken up from its respective sleep mode and draws more current.

[0011] In embodiments, the energy store comprises a capacitor. In this way, the stored charge can be discharged quickly to quickly apply the control signal above the threshold voltage. At the same time, in embodiments where the voltage is applied to the capacitor during the sleep mode, the capacitor will charge to a predetermined level based on its capacitance and then not consume more current. In this way, the capacitor can remain charged ready to be discharged upon activation of the active mode without compromising efficiency. In embodiments, the capacitor can be a ceramic capacitor.

[0012] In embodiments, the energy store comprises a charging connection for connecting the energy store to a power source to charge the energy store. In this way, the capacitor can be charged by a power source, such as a battery, and remain in a charged state.

[0013] In embodiments, the switch comprises a bypass connection for bypassing the charging connection when the switch is operated. In this way, the charge applied to the energy store can be discharged by bypassing the connection of the energy store to the power source and thus removing the potential difference applied thereby.

[0014] In embodiments, the charging connection comprises a first diode for directing current discharged from the energy store to the control terminal.

[0015] In an embodiment, the device further comprises an activation connection for connecting the energy storage to the control terminal of the switching device, wherein the activation connection comprises a second diode for preventing a current flow from the drive circuit to the energy storage. Thus, the second diode can prevent a reverse current from the drive circuit to charge the energy storage.

[0016] In an embodiment, the activation connection further comprises a resistor connected between the anode of the second diode and the negative terminal of the power supply. Thereby, the operation of the second diode can be maintained.

[0017] In an embodiment, the drive circuit comprises an output connection for connecting the drive circuit to the control terminal, wherein the output connection comprises a blocking component for preventing a reverse current to flow to the drive circuit.

[0018] In an embodiment, the blocking component comprises at least one of a diode and a bypass switch. In an embodiment, the diode can allow a current to flow only from the drive circuit to the control terminal. In other embodiments, the bypass switch can be provided in addition to the diode.

[0019] In an embodiment, the controller activates the switch for recharging the energy storage after a predetermined time interval after actuation to discharge the energy storage. In this way, the time taken to activate the switching device can be predicted based on the estimated discharge characteristics of the energy storage. Thus, after triggering the activation sequence to activate the switching device, the activation circuit can be reset to recharge the energy storage when required again.

[0020] In an embodiment, the controller activates the switch for recharging the energy storage in response to detecting that the switching device is activated. In this way, once it is determined that the switching device has been reactivated, the controller can reset the activation circuit to recharge the energy storage.

[0021] In an embodiment, the switching device is a MOSFET. In an embodiment, the switching device can be a high-side N-type MOSFET. In an embodiment, the switching device and the gate driver can provide a power amplifier circuit.

[0022] In an embodiment, the power supply is a battery. In this way, the activation circuit, the drive circuit and the switching device can be powered by direct current.

[0023] According to a second aspect, there is provided a method of activating a switching device for driving a load in an operating mode, the switching device being activatable in response to a control signal higher than a threshold voltage being applied to a control terminal, the method comprising:

[0024] A drive circuit is provided for applying a control signal to a control terminal of a switching device when activated; a start-up circuit is provided, coupled to the control terminal and including an energy storage device for storing a charge above a threshold voltage and a switch for discharging the energy storage device; and the switch is operated to discharge the energy storage device to deliver a start-up voltage to the control terminal for activating the switching device during a delay period before the drive circuit generates a control signal above the threshold voltage after activation. Attached Figure Description

[0025] An illustrative embodiment will now be described with reference to the accompanying drawings, in which:

[0026] Figure 1 This illustrates a conventional switching device and gate drive circuit;

[0027] Figure 2 A first embodiment of a device for providing rapid start-up of a switching mechanism is shown;

[0028] Figure 3 A second embodiment of a device for providing rapid start-up of a switching mechanism is shown;

[0029] Figure 4 A third embodiment of a device for providing rapid start-up of a switching mechanism is shown; and

[0030] Figure 5 An exemplary gate driver is shown. Detailed Implementation

[0031] Figure 2 A first embodiment of a device for providing rapid start-up of a switching device 4 is shown. In this embodiment, the switching device 4 is configured as a high-side N-type MOSFET having a drain terminal 3, a source terminal 5, and a gate terminal 6.

[0032] and Figure 1 As with the conventional circuit shown, gate terminal 6 serves as a control terminal controlled by output connection 8 from drive circuit 9. Drive circuit 9 itself can be implemented according to various different drive circuit designs known in the art, such as charge pumps, inductor boost converters, or other such power amplifier devices. Figure 5 An illustrative example of a gate drive circuit is shown. As mentioned above, this drive circuit 9 is known to have a delay after activation before it can establish a sufficient voltage at the gate 6 to activate the MOSFET 4.

[0033] Figure 2The illustrated embodiment also has a start-up circuit 20 for delivering a start-up voltage to the MOSFET gate 6. This start-up circuit 20 comprises an energy store or reservoir, which is arranged as a capacitor 11, the anode of which is connected to the positive terminal of the battery 1 via a first diode 13. The cathode of the capacitor 11 is connected to the negative terminal of the battery 1 via a first resistor 16, which serves to limit the current flowing through the capacitor 11. By this arrangement, the capacitor 11 is charged by the battery 1 up to the voltage V_bat at the power input side of the system, and, once charged, the capacitor 11 will remain charged without consuming further current or only a very small further current. In this regard, in embodiments, the capacitor 11 can be a ceramic capacitor which provides a very low current leakage. The voltage V_bat is higher than the threshold voltage of the gate 6.

[0034] The anode of the capacitor 11 is also connected to the gate 6 of the MOSFET via a start-up connection 14 comprising a second diode 15. The second diode 15 is configured to allow current to flow to the gate 6 when the start-up circuit 20 is activated, and to block reverse current from the gate driver 6 during activation of the gate driver 6.

[0035] The start-up circuit 20 is also provided with a first switch 12, which is connected as a bypass around the capacitor 11 and is actuatable for discharging the capacitor 11.

[0036] A controller 24 is provided to control this first switch 11. The controller 24 may, for example, comprise a current load detection sensor for sensing when the current drawn from the load 7 increases, and an actuator for actuating the first switch 12 and a logic circuit for controlling the actuator based on the sensed current demand.

[0037] In use, when the load 7 is in a sleep mode, the gate driver 9 will be inactive, so the gate 6 will discharge and the MOSFET 4 will not be conductive. In this way, in this circuit, the load 7 is not powered by the illustrated circuit. However, it will be appreciated that in other embodiments, an alternative power circuit can be provided for providing a lower current to the load 7 to maintain sleep mode operation. At the same time, the first switch 12 is open, so the capacitor 11 is charged by the battery 1 to the voltage V_bat and remains in this charged state.

[0038] When the rising current demand from the load 7 is detected by the controller 24, the controller 24 actuates the first switch 12 to close. This connects the cathode of the capacitor 11 to the battery output voltage and thus causes the anode of the capacitor 11 to have a voltage in the range of twice the battery voltage V_bat. Thus, the start-up capacitor 11 begins to discharge from V_bat. The first diode 13 acts to block the discharge current, thus forcing the current to flow through the start-up connection 14 and the diode 15 and onto the gate 6. Thus, as the start-up capacitor 11 discharges V_bat, the voltage at the gate 6 rises rapidly, causing the gate voltage to exceed the threshold. In embodiments, this discharge can be in a few tens of nanoseconds. Thus, the MOSFET 4 is activated extremely quickly.

[0039] At the same time, the rising current demand also causes activation of the gate driver 9, which begins to pump charge to its driver capacitor 2. In this way, the voltage generated by the gate driver 9 settles until the threshold voltage of the gate is reached, at which point the gate driver is able to control the gate 6. Thus, by the time the capacitor 11 has fully discharged, or even before, the output voltage from the gate driver 9 is high enough to control the MOSFET 4 to continue driving the load 7. That is, when the gate driver 9 is still building up its output voltage, the charge from the capacitor 11 is used to activate the MOSFET 4 during the lag period.

[0040] Once the capacitor 11 has started the MOSFET 4, the controller 24 can open the first switch 12 to reset the start-up circuit and recharge the capacitor 11 ready for the next start-up operation. In embodiments, this is achieved under the control of a timer, which is set to a predetermined estimated discharge time of the capacitor 11 based on known characteristics of the capacitor 11. In other embodiments, the controller 24 can detect whether the MOSFET 4 has activated before opening the first switch 12.

[0041] Figure 3 A second embodiment is shown, which operates in substantially the same way as the first embodiment, but includes additional circuit components. In particular, the start-up circuit 20 further includes a second switch 17 and a second resistor 18 connected along the start-up connection 14 either side of the second diode 15. The second switch 17 enhances the ability of the circuit to turn off the MOSFET 4 by breaking the connection path between the gate 6 and the positive battery terminal via the start-up connection 14. The start-up circuit 20 further includes a third resistor 19 and a third diode 21 along the output connection 8. A third switch 22 is also provided for bypassing the third diode 21. The third diode 21 and the third switch 22 act to protect the gate driver 9 from reverse current.

[0042] In use, as with the first embodiment, when the load is in sleep mode or off, the first switch 12 is open and thus the capacitor 11 is charged to the voltage V_bat by the battery 1. At the same time, the second switch 17 is open to prevent current leakage through the start connection 14. The third switch 22 is also closed.

[0043] When the load is activated, the increased current demand is detected by the controller 24 and the third switch 22 is opened to release the MOSFET 4 from the gate driver 9. The first switch 12 and the second switch 17 are then operated to close, causing the capacitor 11 to discharge. The second resistor 18 regulates the current flow through the start connection 14 by clipping the peak current. The third diode 21 prevents current from the capacitor 11 from passing through the gate driver 9, thereby maximising the voltage applied to the gate 6. Furthermore, the voltage at the gate 6 rises rapidly to activate the MOSFET 4 extremely quickly.

[0044] Once the capacitor 11 has activated the MOSFET 4, the controller 24 can then open the first switch 12 and the second switch 17 to reset the capacitor 11 for recharging. At the same time or later, the third switch 22 can be closed to allow the output 8 of the gate driver to control the gate 6 to continue in normal operating mode.

[0045] Figure 3 The concept of a gate capacitor 25 formed by the gate electrode 6 within the MOSFET 4 is also shown schematically. It will be appreciated that once the MOSFET is activated, the gate-source capacitance C_gs of the gate capacitor 25 serves to maintain the voltage at the gate 6 while the gate driver 9 controls the MOSFET 4.

[0046] Figure 4 A third embodiment is shown which operates in substantially the same way as the second embodiment but includes additional circuit components. In particular, this embodiment also includes a fourth resistor 26 located between the start connection 14 and the negative battery terminal which serves to enhance the blocking effect of the second diode 15. Furthermore, a Zener diode 23 is provided at the junction of the gate 6 to the source 5 to protect the gate 6.

[0047] With the above embodiments, the start-up circuit 20 can operate independently of the output of the gate driver 9. In this way, the switching device 4 can be activated quickly in response to an increased current demand, with the capacitor 11 of the start-up circuit acting as a charging capacitor for quickly activating the switching device 4. Delays caused by the period required for the drive circuit to establish its output voltage are thereby avoided. In the case where a low current is provided to a load in sleep mode, a seamless transition to a high current supply can be provided once the operating mode of the load is activated. That is, the embodiments can provide an uninterruptible power supply with an automatic wake-up in response to detecting an increased current demand from the load.

[0048] It will be appreciated that the above-described implementations are merely illustrative of the application. Indeed, implementations can be applied to many different configurations, the details of which are readily apparent to those skilled in the art.

Claims

1. An apparatus, comprising: a drive circuit that, when activated, applies a control signal to a control terminal of a switching device that is activatable to drive a load in an operating mode when a control signal above a threshold voltage is applied to the control terminal; and a start-up circuit coupled to the control terminal and including an energy store and a switch operable to discharge the energy store to deliver a start-up voltage above the threshold voltage to the control terminal to activate the switching device during a delay period before the drive circuit is activated to generate a control signal above the threshold voltage, wherein the energy store includes a charging connection connecting the energy store to a power source to charge the energy store, and wherein the switch includes a bypass connection that bypasses the charging connection when the switch is operated.

2. The apparatus of claim 1, further comprising a controller that actuates the switch to discharge the energy store in response to an increased current demand of the load.

3. The apparatus of claim 1 or 2, wherein, The energy store includes a capacitor.

4. The apparatus of claim 1, wherein, The charging connection includes a first diode that directs current discharged from the energy store to the control terminal.

5. The apparatus of claim 1, further comprising an activation connection connecting the energy store to the control terminal of the switching device, wherein, The start-up connection includes a second diode that prevents current flow from the drive circuit to the energy store.

6. The apparatus of claim 5, wherein, The start-up connection further includes a resistor connected between an anode of the second diode and a negative terminal of the power source.

7. The apparatus of claim 1, wherein, The drive circuit includes an output connection connecting the drive circuit to the control terminal, wherein the output connection includes a blocking component that prevents reverse current flow to the drive circuit.

8. The apparatus of claim 7, wherein, The blocking component includes at least one of a diode and a bypass switch.

9. The apparatus of claim 2, wherein, The controller actuates the switch to recharge the energy store after a predetermined time interval after actuating the energy store to discharge.

10. The apparatus of claim 2, wherein, The controller actuates the switch to recharge the energy store in response to detecting that the switching device is activated.

11. The apparatus of claim 1, wherein, The switching device is a MOSFET.

12. The apparatus of claim 1, wherein, The power source is a battery.

13. A method of activating a switching device that drives a load in an operating mode, the switching device being activatable in response to a control signal above a threshold voltage being applied to a control terminal, the method comprising the steps of: providing a drive circuit that, when activated, applies a control signal to the control terminal of the switching device; providing a start-up circuit coupled to the control terminal and including an energy store that stores a charge above the threshold voltage and a switch that discharges the energy store; and operating the switch to discharge the energy store to deliver a start-up voltage to the control terminal to activate the switching device during a delay period before the drive circuit is activated to generate a control signal above the threshold voltage, wherein the energy store includes a charging connection connecting the energy store to a power source to charge the energy store, and operating the switch includes bypassing the charging connection.

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