For example, MOSFET circuits and surge protection methods for tapped linear drivers

Abstract

When the source voltage exceeds a threshold level, for example in response to a voltage surge event between the source and drain, the MOSFET circuit (either directly or via a gate control circuit) clamps the MOSFET gate voltage. Specifically, the gate is maintained at a voltage relative to the source to disconnect the first MOSFET during such a surge event but not during normal operation. This provides automatic protection against unexpected increases in input voltage, particularly when the MOSFET is in its on-state during switching. The threshold circuit is connected between the gate (or gate control node) and a reference voltage. When the voltage at the source exceeds a voltage threshold level, it turns on a unidirectional circuit component (D18) between the source and the gate (or gate control node) with the threshold circuit.

Description

Technical Field

[0001] The present invention relates generally to MOSFET circuits, and is of particular interest to tapped linear driver circuits, for example for driving LED lighting loads. Background Art

[0002] Tapped linear drivers (TLDs) for LEDs are well known. Unlike switch-mode drivers, they do not contain high-frequency switching components that would degrade the electromagnetic interference (EMI) performance of the driver.

[0003] In a stacked arrangement, the LEDs to be driven are arranged in a series string, with sections of the string selectively bypassed by corresponding bypass switches. Each section typically comprises a series of individual LEDs. The switching operation is intended to match the mains voltage at different points in time during the mains cycle with the required LED forward voltage. By doing so, TLDs maintain high efficiencies comparable to switch-mode drivers.

[0004] Many topologies of TLDs are known. Typically, a rectifier is provided at the input to provide a fully rectified input to the TLD.

[0005] The basic stack topology uses a single current source with a bypass switch around the corresponding LED or group of LEDs.

[0006] Control of the bypass switch can be achieved using voltage-based control. This involves measuring the instantaneous input voltage and controlling the switch so that the input voltage and the LED voltage are matched. Alternatively, current-based switch control is based on the principle that the switch is normally closed and only opens when the current exceeds a certain preset level.

[0007] Problems arise if a voltage surge occurs at the input of the TLD, and surge events are common in MOSFET circuits. Surge events cause voltage increases at circuit nodes, and these increased voltages need to be withstood by circuit components (such as a MOSFET or multiple MOSFETs).

[0008] It is desirable to be able to use lower rated components to withstand surge events.

[0009] US20140265861A1 discloses a MOSFET and a surge control circuit, wherein the surge control circuit detects an input voltage on a BUS to disconnect the MOSFET. Summary of the invention

[0010] The concept of the present invention is to provide a MOSFET circuit that clamps the MOSFET gate voltage to disconnect it when the source voltage exceeds a threshold level (meaning that the MOSFET is turned on), such as in response to a voltage surge event between the source and the drain. Thus, the surge voltage is not applied to the MOSFET and the MOSFET is protected. In a specific example of an NMOS, the gate is maintained at a voltage lower than the voltage at the source to disconnect the MOSFET during such a surge event but not during normal operation. This provides automatic protection against unexpected increases in input voltage.

[0011] The invention is defined by the claims.

[0012] According to the present invention, there is provided a MOSFET circuit, comprising:

[0013] a first MOSFET including a drain connected to an input voltage, a gate, and a source;

[0014] a unidirectional circuit component connected between the source and a gate control node that controls switching of the first MOSFET using the gate; and

[0015] A threshold circuit connected between the gate control node and a reference voltage,

[0016] Wherein when the voltage at the source exceeds the voltage threshold level and forward conducts the unidirectional circuit component and the threshold circuit, the threshold circuit is adapted to clamp the voltage at the gate control node to the clamping voltage level, thereby maintaining the gate control node at a voltage such as to turn off the first MOSFET.

[0017] This MOSFET circuit provides protection against unexpected increases in input voltage. When the MOSFET is turned on, the source voltage increases in response to the increase in input voltage. To protect the MOSFET from these increased voltages, the MOSFET is automatically turned off. This occurs when the voltage at the source is high enough that the unidirectional circuit component turns on because the gate control node has reached the clamping voltage level of the threshold circuit. In the case of an n-channel MOSFET, for example, this turns off the MOSFET by holding the gate below the source voltage. Therefore, by clamping the gate voltage, the source voltage is clamped and the MOSFET is turned off.

[0018] In this way, the voltage (or voltage difference) seen by the MOSFET can be reduced. This means that lower voltage components can be used.

[0019] In an embodiment, the threshold circuit (D17, R4, D13) is decoupled from the input voltage. This embodiment does not include surge protection based on input voltage detection.

[0020] The unidirectional circuit component is, for example, connected between the source and the gate control node in its forward conduction direction, wherein when the voltage at the source exceeds a voltage threshold level, the threshold circuit is suitable for clamping the voltage at the gate control node to a clamping voltage level by enabling forward conduction through the unidirectional circuit component from the source to the gate control node.

[0021] Thus, when the source voltage increases above the maximum gate control node voltage imposed by the threshold circuit, the unidirectional circuit component provides a conductive path from the source to the gate control node.

[0022] In a first set of examples, the first MOSFET is an n-channel and the gate control node is the gate, wherein the gate is held at a voltage lower than the voltage at the source by the conduction of the unidirectional circuit component and the conduction of the threshold circuit. Thus, the gate can be pulled low to turn off the n-channel MOSFET.

[0023] In a second set of examples, the first MOSFET is a p-channel, and wherein the first MOSFET has a gate drive circuit including an n-channel gate drive MOSFET, wherein the gate control node is the gate of the gate drive MOSFET, wherein the gate drive MOSFET is disconnected by conduction of a unidirectional circuit component and conduction of a threshold circuit, thereby disconnecting the first MOSFET.

[0024] Pulling down the gate of the p-channel MOSFET will turn on the transistor. Therefore, a gate drive circuit is used so that the pull-down voltage can be used to turn off the p-channel MOSFET. The gate drive MOSFET acts as an inverter.

[0025] The unidirectional circuit component may include a diode.

[0026] The diode provides a conduction path between the source and gate control nodes when the source voltage increases above the maximum gate control node voltage imposed by the threshold circuit.

[0027] The threshold circuit may be adapted to clamp the voltage at the gate control node in response to a surge event of the input voltage. Thus, the MOSFET does not need to withstand the voltage of a maximum allowed surge event.

[0028] The threshold circuit includes, for example, a Zener diode. The threshold circuit may also include a current limiting resistor connected in series with the Zener diode. The current limiting resistor protects components of the threshold circuit.

[0029] The circuit, for example, includes a series connection of multiple MOSFETs (including a first MOSFET), wherein the circuit includes multiple unidirectional circuit components, each connected from the source to the gate control node of a corresponding MOSFET in their forward conduction direction, wherein a threshold circuit is connected between the gate control node of each MOSFET and a reference voltage, and wherein when the voltage at the source of the corresponding MOSFET exceeds the corresponding voltage threshold level, the threshold circuit is suitable for clamping the voltage at the gate control node of each MOSFET to the clamping voltage level, thereby enabling forward conduction through the corresponding unidirectional circuit component and conduction of the threshold circuit, thereby disconnecting the corresponding MOSFET.

[0030] In this case, the threshold circuit enables the increase in input voltage to be shared among multiple MOSFETs, so that the voltage handling capability of each individual MOSFET can be further reduced.

[0031] The threshold circuit may comprise a shared Zener diode connected to the gate control node of each MOSFET via respective further unidirectional circuit components.

[0032] Other unidirectional circuit components (such as diodes) provide isolation between the gates so that the MOSFETs can be driven independently. However, the threshold circuit uses a shared Zener diode to set the clamping voltage. Therefore, additional circuit overhead is kept low.

[0033] The present invention also provides a tapped linear LED driver, comprising:

[0034] An LED string including a collection of LED segments connected in series;

[0035] The MOSFET circuit as defined above, wherein each LED segment is associated with a respective one of the MOSFETs, wherein each MOSFET provides a bypass current path for the associated LED segment; and

[0036] A controller is used to control the gate voltage of the MOSFET according to the instantaneous amplitude of the input voltage so that at least one of the bypassed LED sections presents a forward voltage of the non-bypassed LED section that matches the instantaneous amplitude of the input voltage.

[0037] A tapped linear driver (TLD), in particular a binary TLD, is one preferred use of a MOSFET circuit.

[0038] The tapped linear LED driver may also include a respective capacitor in parallel with each LED segment and a diode between the parallel circuit of the associated MOSFET and capacitor and the LED segment.

[0039] The tapped linear LED driver may further include a rectifier, wherein the rectifier includes an input for receiving an AC voltage, and an output of the rectifier output includes the input voltage of the MOSFET circuit.

[0040] The input voltage is therefore a rectified AC signal. It has a voltage that varies with time. The controller controls the MOSFET in a known manner according to the level of this input voltage corresponding to a particular phase of the pre-rectified AC input, thereby controlling the bypass function.

[0041] The tapped linear LED driver may further include a current source circuit connected in series with the LED string, wherein the controller is further configured to control the current source circuit. For example, a capacitor is arranged between the input voltage and ground.

[0042] Such a capacitor arrangement provides improved electromagnetic compatibility (EMC) by allowing transient surge currents to be absorbed and thereby diverted from the MOSFET.

[0043] The capacitor arrangement may include a first capacitor connected in parallel with the entire LED string and a second capacitor connected in parallel with the current source circuit.

[0044] The first capacitor provides improved electromagnetic compatibility (EMC) by allowing transient surge current to be absorbed and thus diverted from the MOSFET. The second capacitor acts as a voltage divider with the first capacitor, setting the voltage across the current source when a surge event occurs. The second capacitor can also improve EMC and reduce total harmonic distortion.

[0045] The present invention also provides a surge protection method for a MOSFET circuit, wherein the MOSFET circuit comprises:

[0046] a first MOSFET including a drain connected to an input voltage, a gate, and a source;

[0047] a unidirectional circuit component connected between the source and a gate control node that controls switching of the first MOSFET using the gate; and

[0048] A threshold circuit connected between the gate control node and a reference voltage,

[0049] The method includes:

[0050] When the voltage at the source exceeds the threshold voltage level and a unidirectional circuit component is forward conducted between the source and the gate control node and the threshold circuit, a threshold circuit is used to perform voltage clamping of the voltage at the gate control node to a clamping voltage level, thereby maintaining the gate control node at a voltage such as to disconnect the first MOSFET.

[0051] This is the method implemented by the MOSFET circuit defined above.

[0052] Surge protection may be applicable to a tapped linear LED driver, wherein the tapped linear driver comprises an LED string comprising a set of LED segments, wherein each LED segment is associated with a respective MOSFET of a MOSFET circuit, and wherein the method comprises performing voltage clamping of a voltage at a gate control node of each MOSFET to a clamping voltage level when, in the event of a surge event of an input voltage, a voltage at a source of the respective MOSFET exceeds a respective threshold voltage level.

[0053] These and other aspects of the invention will be apparent from and elucidated with reference to the embodiment(s) described hereinafter. BRIEF DESCRIPTION OF THE DRAWINGS

[0054] Examples of the present invention will now be described in detail with reference to the accompanying drawings, in which:

[0055] Figure 1 A known tapped linear driver architecture is shown in simplified form;

[0056] Figure 2 Shown in more detail Figure 1 implementation of the circuit;

[0057] Figure 3 By way of example, one of the output voltages of the controller before and after a surge event is shown;

[0058] Figure 4 An example of an embodiment of the present invention and based on an n-channel MOSFET is shown. Figure 2 Modification of the circuit;

[0059] Figure 5 Shown with Figure 3 The same curve for Figure 4 Circuit;

[0060] Figure 6 Shows Figure 2 The circuit and Figure 4 The voltage across the current source of the circuit.

[0061] Figure 7 An example of the present invention is shown. Figure 2 A modification of a p-channel version of the circuit; and

[0062] Figure 8 A half-bridge based resonant converter circuit is shown to which the present invention is applied. DETAILED DESCRIPTION

[0063] The present invention will be described with reference to the accompanying drawings.

[0064] It should be understood that, in indicating exemplary embodiments of the apparatus, system and method, the detailed description and specific examples are intended for illustrative purposes and are not intended to limit the scope of the invention. These and other features, aspects and advantages of the apparatus, system and method of the present invention will become better understood through the following description, the appended claims and the accompanying drawings. It should be understood that the drawings are schematic only and are not drawn to scale. It should also be understood that the same reference numerals are used throughout the drawings to indicate the same or similar parts.

[0065] The present invention provides a MOSFET circuit that clamps the MOSFET gate voltage (directly or via a gate control circuit) when the source voltage exceeds a threshold level, for example in response to a voltage surge event between the source and the drain. Specifically, the gate is maintained at a voltage relative to the source to disconnect the first MOSFET during such a surge event but not during normal operation. This provides automatic protection against unexpected increases in input voltage, especially when the MOSFET is in its on state during switching. The threshold circuit is connected between the gate (or gate control node) and a reference voltage. When the voltage at the source exceeds the voltage threshold level, conduction is allowed between the source and the gate (or gate control node), and conduction of the threshold circuit is also allowed.

[0066] Figure 1 The basic stacked topology of a tapped linear driver is shown in simplified schematic form to explain the general operating principle. The linear driver circuit receives a full-wave rectified mains signal as input. The rectifier is not shown, but typically comprises a diode bridge circuit. A single current source 10 draws current through a string of LEDs having segments D1, D2, D3, D4. Each segment comprises one or more LEDs connected in series and may also include a resistor.

[0067] The number of LEDs in each segment can be the same or different than other segments, and they can be sets of LEDs of the same color (different colors for different segments). Alternatively, all LEDs can be the same color, or the segments can have LEDs of different colors.

[0068] As shown, all of the segments D1 to the last segment D4 have corresponding parallel bypass switches S1, S2, S3, S4 (although in versions where D1 is always connected, the parallel bypass switch for D1 may be omitted).

[0069] These switches are operated to match the voltage across the LED string (i.e. those sections that are not bypassed) to the mains voltage at specific points in time during each rectified half-cycle of the mains voltage cycle. A current source 10 provides the LEDs with a required current (preferably a low total harmonic distortion sine wave) corresponding to the power to be delivered to the LEDs.

[0070] The difference between the instantaneous rectified mains voltage and the LED voltage of the connected segment is dropped across the current source 10. In this way, energy is dissipated across the current source 10.

[0071] One method for controlling switches S1 to S4 is to provide voltage-based control of the switches using controller 12. The instantaneous input voltage is measured by controller 12, and the switches are switched to match the input voltage and the summed LED segment voltage. By doing this, the losses in linear current source 10 are minimized.

[0072] Another method for controlling switches S1 to S4 is to use current-based control, with the bypass switches normally closed. When the current exceeds a certain preset level, they open. In this way, no voltage information is required, and switching is done based on the current flowing through the switch.

[0073] Similarly, for a switched mode power supply with power switching components such as inductors or capacitors, a switch or switch arrangement is turned on and off at high frequency to control power communication. This will be explained by referring to Figure 8 Describe in more detail.

[0074] In summary, there are times when the switch is on. When a surge occurs at this time, the surge will be applied to the switch, and this may damage the switch.

[0075] Figure 2 Shown in more detail Figure 1 An embodiment of the circuit and having voltage-based bypass switch control.

[0076] Each LED segment D1 to D4 has an associated parallel bypass switch, implemented in this example as an n-channel MOSFET M1 to M4.

[0077] A problem with TLD circuits (especially stacked TLDs) is light flickering due to feeding a pulsating DC current to the LEDs. The frequency of this pulsation is twice the frequency of the mains voltage, i.e. 100 Hz or 120 Hz. This pulsation is seen as light flickering.

[0078] Electrolytic capacitors C1 to C42 are added to address this issue, with one in parallel with each LED segment D1 to D4. By doing this, high power factor and low ripple of the LEDs are maintained.

[0079] The parallel capacitors C1 to C4 are decoupled by diodes D5 to D8. These are connected in series with the main light emitting diode sections D1 to D4 so that there is a decoupling diode at the connection between two adjacent LED sections. For example, decoupling diode D6 is connected in series between LED sections D1 and D2.

[0080] When the associated switch is not conducting, the drive current drawn by the current source charges the capacitor in parallel with the LED section, and when the switch is conducting, the capacitor discharges the LED section. In this way, the pulsation of the current is reduced, and therefore the light flicker is reduced.

[0081] Decoupling diodes D5 to D8 in series with each switch prevent the capacitor from discharging via switches M1 to M4 when the switch is turned on and the LED segments are bypassed.

[0082] The circuit comprises a series connection of MOSFETs M1 to M4. Each MOSFET comprises a source, a drain and a gate. The drain of the first MOSFET M1 is connected to an input voltage BUS. The source of the last MOSFET M4 is connected to a current source 10, which is implemented as a further MOSFET M5, whose gate voltage is controlled by a controller 12.

[0083] The gate of each MOSFET controls its conduction state. More generally, there is a gate control node to which a control signal is applied. Figure 2 In the case of MOSFETs, the gate control node is the actual gate, but as will be explained below, there may be a circuit for each MOSFET that interfaces between the command signal used to control the MOSFET and the actual gate.

[0084] Each bypass switch MOSFET M1 to M4 has a unidirectional circuit component, in particular a diode, connected from the source to the gate (more generally between the source and gate control nodes) in its forward conducting direction. These diodes are shown as D18 to D21. They can be added circuit components or part of the MOSFET package.

[0085] Figure 2 Additionally shown is an input rectifier formed as a full diode bridge rectifier of diodes D9 to D12, receiving live and neutral mains inputs and delivering a rectified bus voltage BUS to the driver circuit.

[0086] Voltage divider R2, R3 and filter capacitor C5 provide sensing of the instantaneous input voltage, which is used by controller 12 to time the operation of bypass switches M1 to M4. Current sense resistor R1 is used to generate a feedback voltage that is provided to controller 12 to enable control of current source M5 to deliver the desired current.

[0087] One problem with this circuit is that the components, specifically the bypass switches M1 to M4 , need to be designed to take into account possible surge events.

[0088] For outdoor 10kV surge applications, surge protection devices are usually provided in the luminaire. This limits the residual voltage input to the driver to 2kV residual voltage input. In known driver designs, the residual voltage is used in the bus Figure 2 The varistor VDR shown is clamped to 794V.

[0089] In this topology, MOSFET M1 to M4 are turned on and off according to a binary rule. During the switching cycle, the on state of MOSFET M1 to M4 can be defined as state '1', and the off state can be defined as state '0'. As the input mains voltage phase changes from 0° to 90° (maximum), then from 90° to 180°, MOSFET M1 to M4 switches from binary state '0000' to '1111', and then switches back to '0000' from '1111'. Due to the rectification of the mains input, the same process occurs in the mains voltage phase from 180° to 360°.

[0090] The drain-to-source voltages of the MOSFETs M1 to M4 when turned on may be denoted as VM1on to VM4on. The drain-to-source voltage of M5 is denoted as VM5.

[0091] The residual bus voltage during a surge event may be denoted as Vbus.

[0092] The diode forward voltage may be denoted as VD5 to VD8 for the decoupling diodes and as VD18 to VD21 for the source-gate diodes.

[0093] The voltages on the output control pins of the controller 12 are Vout1 to Vout4 , and these voltages are applied to the gates of the MOSFETs M1 to M4 .

[0094] A surge event can occur in any state of MOSFETs M1 to M4. The worst state is "1111", where each of M1 to M4 is on. In this case, the voltage in the circuit can be:

[0095] Vout1=Vbus-VM1on-VD18≈Vbus

[0096] Vout2=Vbus-VM1on-VM2on-VD19≈Vbus

[0097] Vout3=Vbus-VM1on-VM2on-VM3on-VD20≈Vbus

[0098] Vout4=Vbus-VM1on-VM2on-VM3on-VM4on-VD21≈Vbus

[0099] Vm5=Vbus-(VM1on+…+VM4on)-VR1≈Vbus

[0100] Therefore, the voltage of current source MOSFET M5 follows Vbus from 0V (mains at zero phase angle) to close to the surge limit, which is 794V in this example.

[0101] Since the drain to ground connection of M5 usually has a large parasitic capacitor, and according to i(t)=C*(dv / dt), the bypass MOSFETs M1 to M4 will have very large instantaneous surge currents.

[0102] Therefore, it can be seen that the voltage of the IC pin and the MOSFET is close to the residual surge voltage Vbus, and there is a very large instantaneous impact current flowing through the transistors M1 to M4.

[0103] A known approach to this problem is to use a controller and MOSFETs that can withstand the residual surge voltage, and select MOSFETs that can withstand the surge current. This increases cost and requires more space.

[0104] Figure 3 The voltage Vout1 before and after a surge event is shown by way of example.

[0105] It can be seen that the surge event will generate a residual surge voltage (eg, 794V) at pin Vout1.

[0106] Figure 4 An example of the present invention is shown. Figure 2 The circuit is modified. Figure 2 The same components are given the same reference numerals, and the description is not repeated.

[0107] and Figure 2 A modification compared to EMI1.1 is to add a threshold circuit between the gate of each bypass MOSFET M1 to M4 (and thus more generally the gate control node explained above) and the reference voltage Vref.

[0108] The threshold circuit includes a Zener diode D13 connected to a reference voltage and a current limiting resistor R4 connected in series with the Zener diode. For example, D13 has a threshold voltage of 350V. R4 has a resistance lower than 10kΩ and is a surge resistor. Node N1 is defined at the terminal of resistor R4 opposite to Zener diode D13. The node N1 is connected to the gate of each bypass MOSFET M1 to M4 through corresponding additional unidirectional circuit components (e.g., diodes D14 to D17). In the following description, these will be referred to as clamping diodes.

[0109] Thus, the entire threshold circuit is connected between the gate of each bypass MOSFET M1 to M4 and the reference voltage Vref. The threshold circuit clamps the voltage at the gate of each MOSFET to a clamping voltage level (which depends primarily on the threshold of the Zener diode D13, i.e., ignoring the effect of R4). This clamping occurs when the voltage at the source of the corresponding MOSFET exceeds the corresponding voltage threshold level. This threshold voltage level is sufficient to make the associated source-gate diode D18 to D21 become conductive in the direction from source to gate, and to make the associated clamping diode D14 to D17 become conductive. Thus, each source is connected to the node N1 through two diodes (e.g., D18 and D17) in series with the same polarity, i.e., their forward conduction direction is from the source to the node N1.

[0110] The threshold circuit is specifically adapted to clamp the voltage at the gate in response to a surge event of the input voltage. However, the normal operating voltage of the circuit does not produce a source voltage sufficient to turn on the clamping diodes D14 to D17.

[0111] Forward conduction is enabled by the conduction of the corresponding unidirectional circuit components (D18 to D21) and the threshold circuit, in particular the corresponding clamping diodes (D14 to D17) and the shared Zener diode D13. This turns off the corresponding MOSFET because the voltage at the gate is lower than the voltage at the source (corresponding to the amount of the voltage drop across the diodes D18 to D21).

[0112] Any MOSFET that was conducting when the surge event occurred will switch to a non-conducting state. Therefore, regardless of the initial combination of switching states, all MOSFETs will turn off in response to the surge event.

[0113] The threshold circuit enables the increase in input voltage to be shared among multiple MOSFETs (including the current source MOSFET M5), so that the voltage handling capability of each individual MOSFET can be reduced. It provides a low-cost, space-saving solution and can also improve EMI (electromagnetic interference) performance, THD (total harmonic distortion) and high-voltage performance.

[0114] Figure 4 Also shown is a capacitor arrangement between the input voltage BUS and ground. The capacitor arrangement comprises a first capacitor C6 connected in parallel with the entire LED string and a second capacitor C7 connected in parallel with the current source circuit M5.

[0115] Capacitor C6 is a bypass and EMC capacitor. When a surge event is generated, C6 can bypass the current from MOSFET M1 to M4 to reduce the instantaneous impact current. During normal operation, C6 is charged and discharged to improve EMC.

[0116] C7 is used to divide the input voltage with C6. When a surge event is generated, the voltage division means that VM5 can be close to Vbus-VD1-VD2-VD3-VD4, and it also reduces the current through M1 to M4. The maximum Vbus voltage is known, and based on this, the appropriate capacitance values ​​of C6 and C7 can be set.

[0117] C7 also improves EMC and THD. The turning on and off of MOSFETs M1 to M4 generates pulse voltage and current, so C7 acts as a snubber capacitor to limit the rate of change of current and voltage.

[0118] When a surge event occurs, the circuit operates to turn off the transistors. To analyze the circuit operation, in addition to the above definitions, the drain-source voltage when the bypass MOSFETs M1 to M4 are turned off can be defined as VM1off to VM4off, and the voltage across the capacitors C1 to C4 is defined as VC1 to VC4. This gives:

[0119] VM1off=VD5+VC1≈VD1

[0120] VM2off=VD6+VC2≈VD2

[0121] VM3off=VD7+VC3≈VD3

[0122] VM4off=VD8+VC4≈VD4

[0123] When the driver operates normally, the voltage at node N1 is less than 350V, so no current flows through D13.

[0124] When a surge event is generated, if M1 is turned on at this time, the source voltage of M1 will quickly rise to more than 350V. The gate voltage of M1 will quickly rise to more than 350V, so the ground path of D18, D17, R4, and D13 will be connected, so that there is a current path from the gate of M1 to the ground. Once this current flows, the gate voltage will drop below the source voltage, causing MOSFET M1 to turn off.

[0125] In the same way:

[0126] D16, R4, and D13 provide a disconnect path for M2;

[0127] D15, R4, and D13 provide a disconnect path for M3; and

[0128] D14, R4, and D13 provide a disconnect path for M4.

[0129] In each case, the disconnect path is active when the corresponding MOSFET source voltage rises above a threshold voltage, which is, for example, two diode forward voltages above the threshold voltage of Zener diode D13, ie, 350V+1.4V.

[0130] The resistor limits the ground current, thereby ensuring that the components in the loop are not damaged and that the gate-to-source voltage of the MOSFETs M1 to M4 does not exceed the breakdown voltage.

[0131] Therefore, when Vout1-Vout4 (corresponding to the gate voltage of M1 to M4) exceeds the clamping voltage (exceeding the forward threshold of diodes D14 to D17), D14 to D17 turn on and current flows to ground through R4 and D13, and MOSFETs M1 to M4 turn off.

[0132] When disconnected, the following voltages are generated:

[0133] Vout1=Vbus-VM1off-VD18≈Vbus-VD1

[0134] Vout2=Vbus-VM1off-VM2off-VD19≈Vbus-VD1-VD2

[0135] Vout3=Vbus-VM1off-VM2off-VM3off-VD20≈Vbus-VD1-VD2-VD3

[0136] Vout4=Vbus-VM1off-VM2off-VM3off-VM4off-VD21≈Vbus-VD1-VD2-VD3-VD4

[0137] Vout5=Vbus-VM1off-VM2off-VM3off-VM4off≈Vbus-VD1-VD2-VD3-VD4

[0138] Assuming the input voltage is 230VAC, and the surge residual voltage Vbus is 794V, this gives:

[0139] VD1≈144V

[0140] VD2≈72V

[0141] VD3≈36V

[0142] VD4≈18V

[0143] therefore:

[0144] Vout1≈Vbus-VD1=794V-144V=650V

[0145] Vout2≈Vbus-VD1-VD2=794V-144V-72V=578V

[0146] Vout3≈Vbus-VD1-VD2-VD3=794V-144V-72V-36V=542V

[0147] Vout4≈Vbus-VD1-VD2-VD3-VD4=794V-144V-72V-36V-18V=524V

[0148] VM5≈Vbus-VD1-VD2-VD3-VD4=794V-144V-72V-36V-18V=524V

[0149] Figure 5 Shown with Figure 3 The curve is the same as curve 50. Curve 52 is the curve using Figure 4 Vout1 of the circuit.

[0150] Figure 6 Shows Figure 2 The curve 60 of the voltage VM5 of the circuit and Figure 4 Curve 62 of the voltage VM5 of the circuit.

[0151] The present invention avoids the need to reduce the clamping voltage provided by the varistor VDR. This is undesirable because the normal mains voltage may cause potential component breakdown. The lower the clamping voltage, the more components are required.

[0152] From the above analysis, it can be seen that different MOSFETs may have different rated voltages.

[0153] For example, MOSFET M1 is clamped (in parallel) with D5, C1, and D1, M2 is clamped with D6, C2, and D2, etc. Diodes D1 to D4 have different voltages.

[0154] By way of example, for a 230VAC input voltage, the source-drain voltage ratings are 200V for M1 , 100V for M2 , 60V for M3 , and 30V for M4 .

[0155] The above examples use a tapped linear driver which switches the LEDs depending on the instantaneous amplitude of the input voltage to describe the invention. It is noted that the invention can also be applied to simple linear drivers where the LEDs are not switched but fixed.

[0156] The above examples are based on n-channel MOSFET circuits. However, the same concepts can also be applied to p-channel MOSFET circuits, such as Figure 7 shown.

[0157] and Figure 4 Like components are given like reference numerals and the description is not repeated. MOSFETs M1 to M4 are now p-channel devices.

[0158] This means that if the gate is pulled down to a low reference, the p-channel MOSFET will turn on, which is the opposite of the desired function. Therefore, each MOSFET M1 to M4 has a gate control circuit connected to the gate of the MOSFET. The gate control circuit includes a small n-channel MOSFET, Q1 to Q4, and pull-up resistors R5 to R8. The pull-up resistor is located between the drain and the gate of the corresponding p-channel MOSFET. The input of the gate control circuit is the gate control node, and the threshold circuit diodes D14 to D17 are connected between the gate control node and the reference voltage. Diodes D18 to D21 are connected between the source and the corresponding gate control node.

[0159] When the source is pulled high, there is again a conduction path through two diodes, for example from the source of M1, through diode D18 and diode D17. This pulls the gate of Q1 low, turning it off. The pull-up resistor R8 then pulls the gate of MOSFET M1 up to the drain voltage, turning off the p-channel MOSFET M1.

[0160] The circuit functions otherwise as described above. It can thus be seen that the concepts of the present invention can be applied to both n-channel MOSFET circuits and p-channel MOSFET circuits.

[0161] The invention has been described above in conjunction with n-channel TLD circuits and p-channel TLD circuits. However, the invention is more generally applicable to MOSFET circuits that may be exposed to voltage surges, particularly when there is a series connection of MOSFETs. The invention then enables the residual voltage appearing at the circuit nodes to be reduced and / or allows the residual voltage to be shared between multiple MOSFETs.

[0162] Figure 8 An example of a half-bridge based switch mode power supply to which the concept of the present invention is applied is shown. The circuit includes a half-bridge including a pull-up n-channel MOSFET M100 and a pull-down n-channel MOSFET M200. The two MOSFETs are connected in series between the input, BUS and ground. The connection between them is connected to an LC circuit of a capacitor C300 and an inductor L100. The inductor L100 is the primary side of an isolation transformer. The secondary side is connected to a load, not shown.

[0163] The circuitry for generating the normal control signals applied to the gates of the MOSFETs is not shown. These are high frequency switching signals. Figure 8 Only the circuit components added for surge protection are shown. Basically, two MOSFETs are switched on / off alternatively.

[0164] When MOSFET M100 is turned on and a surge comes, the surge may damage it. Pull-up MOSFET M100 has the same threshold circuit as described above, including unidirectional circuit components, such as diode D300, connected between the source and the gate (the gate itself is the gate control node in this circuit). The threshold circuit of Zener diode D400 and resistor R100 is connected between the gate and ground, acting as a reference voltage. The pull-up MOSFET is disconnected in response to the surge event in the same manner as explained above by the conduction of D300 and D400.

[0165] The pull-down MOSFET M200 is also disconnected by the surge event, whether the MOSFET M100 is in the on state or the off state. Since the source is connected to a fixed reference, i.e., ground, a different shutdown circuit is used. The gate of the pull-down MOSFET M200 is controlled by the operational amplifier U100. The inverting input provides a scaled version of the input voltage BUS by the resistor divider R200, R300. When the input BUS is high, the inverting input rises to a reference voltage higher than the non-inverting input, so the output of the operational amplifier U100 is pulled low and disconnects the pull-down MOSFET M200.

[0166] In case of a surge event, the input voltage BUS is divided into two parts, so the drain-source voltage of the two MOSFETs will be limited to half of the residual surge voltage, so the lower voltage components can be used.

[0167] The MOSFETs have parallel connected diodes and capacitors D100, C100, D200, C200, respectively, to provide a conduction path during a surge event.

[0168] Figure 8 This is just a simplified example of a switch mode power supply circuit as an example of a MOSFET circuit that is not a tapped linear driver circuit. This shows that the invention is generally applicable to MOSFET circuits that are designed to withstand surge events.

[0169] Variations of the disclosed embodiments may be understood and implemented by those skilled in the art in practicing the claimed invention by studying the drawings, the disclosure and the appended claims. In the claims, the word "comprising" does not exclude other elements or steps, and the indefinite article "a" or "an" does not exclude a plurality. A single processor or other unit may implement the functions of several items recited in the claims. The fact that certain measures are recited in mutually different dependent claims does not indicate that a combination of these measures cannot be used to advantage. If the term "suitable for" is used in a claim or description, it is to be noted that the term "suitable for" is intended to be equivalent to the term "configured to". Any figure marks in the claims should not be interpreted as limiting the scope.

Claims

1. A MOSFET circuit, comprising: a first MOSFET (M1) comprising a drain, a gate and a source connected to an input voltage (BUS); a unidirectional circuit component (D18) connected between the source and a gate control node, the gate control node controlling switching of the first MOSFET (M1) using the gate; as well as A threshold circuit (D17, R4, D13) connected between the gate control node and a reference voltage, When the voltage at the source exceeds a voltage threshold level and causes the unidirectional circuit component (D18) to become conductive in the direction from the source to the gate control node and allows conduction of the threshold circuit (D17, R4, D13), the threshold circuit (D17, R4, D13) is adapted to clamp the voltage at the gate control node to a clamping voltage level, thereby maintaining the gate control node at a voltage to turn off the first MOSFET (M1).

2. A MOSFET circuit according to claim 1, wherein the unidirectional circuit component (D18) is connected between the source and the gate control node in its forward conduction direction, wherein the threshold circuit is adapted to clamp the voltage at the gate control node to a clamping voltage level by enabling forward conduction through the unidirectional circuit component from the source to the gate control node when the voltage at the source exceeds a voltage threshold level, and The threshold circuit (D17, R4, D13) is decoupled from the input voltage (BUS).

3. A MOSFET circuit according to claim 1 or 2, wherein the first MOSFET (M1) is an n-channel and the gate control node is the gate, wherein the gate is maintained at a voltage lower than the voltage at the source by the conduction of the unidirectional circuit component (D18) and the conduction of the threshold circuit.

4. The MOSFET circuit according to claim 1 or 2, wherein the first MOSFET (M1) is a p-channel, and wherein the first MOSFET (M1) has a gate drive circuit including an n-channel gate drive MOSFET, wherein the gate control node is the gate of the gate drive MOSFET, wherein the gate drive MOSFET is turned off by turning on the unidirectional circuit component (D18) and the threshold circuit, thereby turning off the first MOSFET (M1).

5. The MOSFET circuit according to any one of claims 1 to 4, wherein: The unidirectional circuit component (D18) comprises a diode; and / or The threshold circuit includes a Zener diode (D13).

6. A MOSFET circuit according to any one of claims 1 to 5, comprising a series connection of a plurality of MOSFETs (M1-M4), the plurality of MOSFETs including the first MOSFET (M1), wherein the circuit comprises a plurality of unidirectional circuit components (D18-D21), each unidirectional circuit component being connected between the source and the gate control node of a corresponding one of the MOSFETs (M1-M4), wherein the threshold circuit (D17, R4, D13) is connected between the gate control node of each MOSFET (M1-M4) and the reference voltage, And wherein when the voltage at the source of the corresponding MOSFET exceeds the corresponding voltage threshold level, the threshold circuit (D17, R4, D13) is suitable for clamping the voltage at the gate control node of each MOSFET to the clamping voltage level, thereby enabling forward conduction through the corresponding unidirectional circuit component (D18-D21) and conduction of the threshold circuit (D17, R4, D13), thereby disconnecting the corresponding MOSFET.

7. A MOSFET circuit according to claim 6, wherein the threshold circuit comprises a shared Zener diode (D13) connected to the gate control node of each MOSFET via respective further unidirectional circuit components (D14-D17).

8. A tapped linear LED driver, comprising: An LED string, comprising a set of LED segments (D1, D2, D3, D4) connected in series; A MOSFET circuit according to claim 6 or 7, wherein each LED segment is associated with a respective one of the plurality of MOSFETs, wherein each MOSFET provides a bypass current path for the associated LED segment; as well as A controller (12) is used to control the gate voltage of the MOSFET (M1 to M4) according to the instantaneous amplitude of the input voltage so as to bypass at least one of the LED sections to present a forward voltage of the non-bypassed LED section that matches the instantaneous amplitude of the input voltage.

9. The tapped linear LED driver of claim 8, further comprising a respective capacitor (C1-C4) connected in parallel with each LED segment and a diode (D5-D8) between the associated MOSFET and the parallel circuit of the capacitor and LED segment.

10. A tapped linear LED driver according to claim 8 or 9, further comprising a rectifier, wherein the rectifier (D9-D12) comprises an input for receiving an AC voltage and an output of the rectifier output comprises the input voltage for the MOSFET circuit.

11. The tapped linear LED driver according to any one of claims 8 to 10, further comprising a current source circuit (M5) connected in series with the LED string, wherein the controller is further configured to control the current source circuit.

12. A tapped linear LED driver according to any one of claims 8 to 11, further comprising a capacitor arrangement between the input voltage (BUS) and ground.

13. The tapped linear LED driver according to claim 12, wherein the capacitor arrangement comprises a first capacitor (C6) connected in parallel with the entire LED string and a second capacitor (C7) connected in parallel with the current source circuit.

14. A surge protection method for a MOSFET circuit, wherein the MOSFET circuit comprises: a first MOSFET (M1) comprising a drain, a gate and a source connected to an input voltage (BUS); a unidirectional circuit component (D18) connected between the source and a gate control node, the gate control node controlling switching of the first MOSFET (M1) using the gate; as well as a threshold circuit (D17, R4, D13) connected between the gate control node and a reference voltage and decoupled from the input voltage (BUS), The method comprises: When the voltage at the source exceeds the threshold voltage level and causes the unidirectional circuit component (D18) to become conductive in the direction from the source to the gate control node and allow conduction of the threshold circuit, the threshold circuit is used to perform voltage clamping of the voltage at the gate control node to a clamping voltage level, thereby maintaining the gate control node at a voltage so as to disconnect the first MOSFET.

15. The surge protection method according to claim 14, applied to a tapped linear LED driver, wherein the tapped linear driver comprises an LED string, the LED string comprising a set of LED segments (D1, D2, D3, D4), wherein each LED segment is associated with a respective MOSFET of the MOSFET circuit, and wherein the method comprises: Voltage clamping of the voltage at the gate control node of each MOSFET to the clamping voltage level is performed when the voltage at the source of the corresponding MOSFET exceeds a corresponding threshold voltage level in the event of a surge event of the input voltage.

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