US20170222643A1 - Driver for the high side switch of the cascode switch - Google Patents
Driver for the high side switch of the cascode switch Download PDFInfo
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- US20170222643A1 US20170222643A1 US15/009,438 US201615009438A US2017222643A1 US 20170222643 A1 US20170222643 A1 US 20170222643A1 US 201615009438 A US201615009438 A US 201615009438A US 2017222643 A1 US2017222643 A1 US 2017222643A1
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- 230000004048 modification Effects 0.000 description 2
- 230000033228 biological regulation Effects 0.000 description 1
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- H—ELECTRICITY
- H03—ELECTRONIC CIRCUITRY
- H03K—PULSE TECHNIQUE
- H03K17/00—Electronic switching or gating, i.e. not by contact-making and –breaking
- H03K17/08—Modifications for protecting switching circuit against overcurrent or overvoltage
- H03K17/082—Modifications for protecting switching circuit against overcurrent or overvoltage by feedback from the output to the control circuit
- H03K17/0822—Modifications for protecting switching circuit against overcurrent or overvoltage by feedback from the output to the control circuit in field-effect transistor switches
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- H—ELECTRICITY
- H03—ELECTRONIC CIRCUITRY
- H03K—PULSE TECHNIQUE
- H03K17/00—Electronic switching or gating, i.e. not by contact-making and –breaking
- H03K17/51—Electronic switching or gating, i.e. not by contact-making and –breaking characterised by the components used
- H03K17/56—Electronic switching or gating, i.e. not by contact-making and –breaking characterised by the components used by the use, as active elements, of semiconductor devices
- H03K17/567—Circuits characterised by the use of more than one type of semiconductor device, e.g. BIMOS, composite devices such as IGBT
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- H—ELECTRICITY
- H03—ELECTRONIC CIRCUITRY
- H03K—PULSE TECHNIQUE
- H03K17/00—Electronic switching or gating, i.e. not by contact-making and –breaking
- H03K17/10—Modifications for increasing the maximum permissible switched voltage
- H03K17/102—Modifications for increasing the maximum permissible switched voltage in field-effect transistor switches
-
- H—ELECTRICITY
- H03—ELECTRONIC CIRCUITRY
- H03K—PULSE TECHNIQUE
- H03K17/00—Electronic switching or gating, i.e. not by contact-making and –breaking
- H03K17/22—Modifications for ensuring a predetermined initial state when the supply voltage has been applied
- H03K17/223—Modifications for ensuring a predetermined initial state when the supply voltage has been applied in field-effect transistor switches
-
- H—ELECTRICITY
- H03—ELECTRONIC CIRCUITRY
- H03K—PULSE TECHNIQUE
- H03K17/00—Electronic switching or gating, i.e. not by contact-making and –breaking
- H03K17/51—Electronic switching or gating, i.e. not by contact-making and –breaking characterised by the components used
- H03K17/56—Electronic switching or gating, i.e. not by contact-making and –breaking characterised by the components used by the use, as active elements, of semiconductor devices
- H03K17/687—Electronic switching or gating, i.e. not by contact-making and –breaking characterised by the components used by the use, as active elements, of semiconductor devices the devices being field-effect transistors
- H03K17/689—Electronic switching or gating, i.e. not by contact-making and –breaking characterised by the components used by the use, as active elements, of semiconductor devices the devices being field-effect transistors with galvanic isolation between the control circuit and the output circuit
- H03K17/691—Electronic switching or gating, i.e. not by contact-making and –breaking characterised by the components used by the use, as active elements, of semiconductor devices the devices being field-effect transistors with galvanic isolation between the control circuit and the output circuit using transformer coupling
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- H—ELECTRICITY
- H03—ELECTRONIC CIRCUITRY
- H03K—PULSE TECHNIQUE
- H03K17/00—Electronic switching or gating, i.e. not by contact-making and –breaking
- H03K17/51—Electronic switching or gating, i.e. not by contact-making and –breaking characterised by the components used
- H03K17/74—Electronic switching or gating, i.e. not by contact-making and –breaking characterised by the components used by the use, as active elements, of diodes
-
- H—ELECTRICITY
- H03—ELECTRONIC CIRCUITRY
- H03K—PULSE TECHNIQUE
- H03K7/00—Modulating pulses with a continuously-variable modulating signal
- H03K7/08—Duration or width modulation ; Duty cycle modulation
-
- H—ELECTRICITY
- H03—ELECTRONIC CIRCUITRY
- H03K—PULSE TECHNIQUE
- H03K17/00—Electronic switching or gating, i.e. not by contact-making and –breaking
- H03K17/22—Modifications for ensuring a predetermined initial state when the supply voltage has been applied
- H03K2017/226—Modifications for ensuring a predetermined initial state when the supply voltage has been applied in bipolar transistor switches
Definitions
- the present disclosure relates generally to switches, in particular cascode switches.
- Power supply systems are pervasive in many electronic applications from computers to automobiles.
- voltages within a power supply system are generated by performing a DC-DC, DC-AC, and/or AC-DC conversion by operating a switch loaded with an inductor or transformer.
- One class of such systems includes switched mode power supplies (SMPS).
- SMPS switched mode power supplies
- An SMPS is usually more efficient than other types of power conversion systems because power conversion is performed by controlled charging and discharging of the inductor or transformer and reduces energy lost due to power dissipation across resistive voltage drops.
- a SMPS usually includes at least one switch and an inductor or transformer. Some specific topologies include buck converters, boost converters, and flyback converters, among others.
- a control circuit is commonly used to open and close the switch to charge and discharge the inductor. In some applications, the current supplied to the load and/or the voltage supplied to the load is controlled via a feedback loop. In some typologies, the switches used in the SMPS are implemented using cascode switches.
- Cascode switches are typically designed with two or more MOSFETs (metal oxide semiconductor field effect transistors) or IGBTs (insulated gate bipolar transistors) connected in series.
- MOSFETs metal oxide semiconductor field effect transistors
- IGBTs insulated gate bipolar transistors
- the first transistor is coupled to the load and the second transistor is coupled in series between the first transistor and ground.
- the transistors are switched on and off in order to switch the load current as demanded or required.
- the load voltage is distributed across all of the series connected power transistors included in the cascode switch.
- two 800V rated MOSFETs may be connected in series for switching a 1000V or greater load.
- a circuit including a first switching transistor having a first source coupled to a first reference voltage terminal, and a second switching transistor having a second source coupled to a first drain of the first switching transistor, and a second drain configured to be coupled to a transformer.
- the circuit also includes a first diode coupled between the first drain of the first switching transistor and a first input terminal. The first diode is configured to clamp a voltage of the first drain of the first switching transistor to a voltage of the first input terminal.
- the circuit further includes a switching circuit coupled between the second switching transistor and the first input terminal. The switching circuit is configured to connect the second source of the second switching transistor to a second gate of the second switching transistor when a voltage of the second source of the second switching transistor exceeds the voltage of the first input terminal.
- FIG. 1 illustrates a power converter with a cascode switch in some embodiments
- FIG. 2 illustrates a power converter circuit with a cascode switch in some other embodiments
- FIG. 3 illustrates a power converter circuit with a cascode switch in yet other embodiments.
- FIG. 4 illustrates a flow chart of a method for switching a power circuit in some embodiments.
- cascode switches that may be used in power converters (e.g., flyback converters) to switch on and off the load current.
- the invention may also be applied to other systems and applications having circuits that utilize cascode switches.
- Flyback converters generally include a switch coupled in series with a primary winding of a transformer and an input voltage of the flyback converter.
- the voltage across the switch may rise to a very high voltage that is much greater than then the input voltage, which may be on the order of a few hundred volts.
- the switch is constructed using a plurality of connected transistors that share the high voltage between them.
- a cascode switch is constructed using a low-side switching transistor and a high-side cascode transistor.
- the load-path voltage of the low-side switching transistor is clamped to input voltage of the flyback converter while the cascode transistor is permitted to conduct.
- the cascode transistor is turned-off. This way, the voltage across the low-side switching transistor limited to about the voltage of the power input bus of the flyback converter and the high-side cascode transistor is about limited to a voltage difference between the maximum voltage seen across the switch and the power input bus of the flyback converter.
- the maximum voltage across each device in the cascode switch is substantially independent of device geometry, device parameters, parasitic capacitance and device matching between the low-side switching transistor and the high-side cascode transistor.
- the low-side switching transistor and the high-side cascode transistor do not need to be over-specified to withstand the full voltage seen by the switch in some embodiments, yielding the ability to use smaller, less expensive transistors for the cascode switch.
- the turn-off time of the cascode switch is substantially independent of the load current of the cascode switch.
- FIG. 1 illustrates a switched-mode flyback converter 100 using a cascode switch that includes transistors Q 1 and Q 2 , in accordance with some embodiments.
- a flyback converter is illustrated in FIG. 1 an example.
- the use of a flyback converter in FIG. 1 is not intended to be limiting, as other suitable circuits and/or power converters may also be used with the cascode switch.
- the discussion below may refer to the power converter as a flyback converter, with the understanding that other types of power converter or circuits may also be used with the cascode switch disclosed herein.
- switching transistor Q 1 and Q 2 are coupled in series between transformer T 1 of flyback converter 100 and a reference voltage terminal G.
- the reference voltage terminal G is coupled to electrical ground, but may be coupled to other reference potentials in alternative embodiments.
- switching transistor Q 2 is coupled between the primary winding of transformer T 1 and switching transistor Q 1 , with the drain of switching transistor Q 2 coupled to terminal B of the primary winding of transformer T 1 , and the source of switching transistor Q 2 coupled to the drain of switching transistor Q 1 .
- the source of switching transistor Q 1 is coupled to reference voltage terminal G.
- the switching transistor e.g., transistor Q 1
- switching transistor e.g. transistor Q 2
- the high-side transistor the switching transistor closest to the transformer
- an input voltage V in is supplied to an input terminal C of circuit 100 .
- the input voltage V in is shown as a DC voltage source for the purpose of illustration, however, one skilled in the art will appreciate that other suitable input voltages, such as a rectified AC voltage, may also be used as the input voltage.
- Terminal A of the primary winding of transformer T 1 is coupled to input terminal C, thus input voltage V in is also applied to terminal A of the primary winding of transformer T 1 .
- the voltage at input terminal C and terminal A of the primary winding of transformer T 1 is sometimes referred to as the bus voltage.
- the secondary winding of transformer T 1 is coupled to a load represented by voltage V o via a rectifier diode D 1 , in some embodiments.
- FIG. 1 illustrates a driver circuit 110 , also referred to as a driver 110 , that is coupled to the gate of low-side transistor Q 1 .
- An input terminal of the driver 110 of low-side transistor Q 1 may be coupled to a pulsed voltage source, such as a pulse-width-modulator (PWM) 130 .
- PWM 130 may be coupled to and controlled by a controller 140 .
- Controller 140 may be a micro-controller unit (MCU), an application-specific integrated circuit (ASIC), a control circuit built using discrete components, or any other suitable controller. As shown in FIG.
- MCU micro-controller unit
- ASIC application-specific integrated circuit
- controller 140 may receive feedback signal(s) from feedback circuit 150 to monitor the operation and status of circuit 100 . Controller 140 may instruct PWM 130 to generate a pulse train (e.g., a series of voltage pulses with desired pulse width and pulse amplitude) to turn on and off low-side transistor Q 1 , which in turn causes high-side transistor Q 2 to turn on and off, as discussed in more detail hereinafter.
- the driver of low-side transistor Q 1 may comprises any suitable driver circuit.
- the gate of high-side transistor Q 2 is coupled to a high-side driver via resistor R 4 .
- the high-side driver is shown in another dashed box illustrated in FIG. 1 , and includes diodes D 2 , D 3 , D 4 , D 5 , resistor R 2 , and transistor Q 3 , in some embodiments.
- Transistor Q 3 is a bipolar junction transistor (BJT), in various embodiments. As illustrated in FIG.
- FIG. 1 further illustrates diode D 2 coupled between the drain of low-side transistor Q 1 and input terminal C, diode D 4 coupled between the base and the emitter of BJT transistor Q 3 , and diode D 3 coupled between diode D 4 and a voltage terminal E that is coupled to a voltage source V c .
- voltage source V c is a DC voltage source, although other suitable voltage source (e.g., a pulsed voltage source) may also be used.
- a first terminal of resistor R 3 is coupled to input terminal C, and a second terminal of resistor R 3 is coupled to the gate of high-side transistor Q 2 via resistor R 4 , in some embodiments.
- PWM 130 generates a voltage pulse (e.g., a voltage pulse of 10 volt with a pulse width of 10 ⁇ s) to turn on low-side transistor Q 1 .
- a voltage pulse e.g., a voltage pulse of 10 volt with a pulse width of 10 ⁇ s
- the source voltage of high-side transistor Q 2 is pulled down to a low voltage (e.g., close to electrical ground). Since the gate of high-side transistor Q 2 is coupled to voltage source V, (e.g., a DC voltage source of 15 volt), rectifier diode D 3 and D 4 begin to conduct.
- V e.g., a DC voltage source of 15 volt
- rectifier diode D 3 and D 4 charge high-side transistor Q 2 through resistor R 4 , thus turning on high-side transistor Q 2 .
- load current flows through the primary winding of transformer T 1 , low-side transistor Q 1 and high-side transistor Q 2 .
- BJT transistor Q 3 is off because the base-emitter voltage (V be ) of Q 3 is negative.
- PWM 130 during turn off of the cascode switch, PWM 130 generates a corresponding voltage pulse (e.g., a voltage pulse with zero volt and pulse width of 4 ⁇ s) to turn off low-side Q 1 , which in turn causes the drain-to-source voltage (V ds ) of Q 1 to begin rising.
- V ds of low-side transistor Q 1 reaches a threshold voltage, e.g., a diode's drop above the bus voltage
- rectifier diode D 2 conducts and load current is redirected through D 2 . Once conducting, diode D 2 clamps the drain-to-source voltage of low-side transistor Q 1 to the bus voltage.
- clamp to the bus voltage means that the drain-to-source voltage V ds of low-side transistor Q 1 is limited to a voltage near the bus voltage, e.g., a diode drop above the bus voltage. Since the drain-to-source voltage V ds of low-side transistor Q 1 is clamped at bus voltage, the gate voltage of high-side transistor Q 2 is above bus voltage. As a result, diodes D 4 becomes reverse biased, diode D 5 becomes forward biased, and BJT transistor Q 3 conducts.
- BJT transistor Q 3 when BJT transistor Q 3 conducts (e.g., when the source of high-side transistor Q 2 is above the bus voltage), its emitter and base are forward biased and its collector and base are reverse biased, current flows through the emitter and collector, therefore BJT transistor Q 3 connects the gate and the source of high-side transistor Q 2 when conducting, in accordance with some embodiments.
- the BJT transistor Q 3 , resistor R 2 and diode D 5 may be collectively referred to as a switching circuit.
- the low-side transistor Q 1 and high-side transistor Q 2 turn off successively, in some embodiments.
- Diode D 1 of the secondary winding of transformer T 1 is reverse biased when the cascode switch is on, therefore, no current flows through the secondary winding, and the load current flows through the primary winding and stores energy as magnetic field in the transformer.
- Diode D 1 may be replaced by a switch that is used as a synchronous rectifier, as skilled artisans will appreciate.
- the voltage rating which defines the maximum voltage under which a transistor can safely operate, may be different for low-side transistor Q 1 and high-side transistor Q 2 .
- low-side transistor Q 1 may have a first voltage rating
- high-side transistor Q 2 may have a second voltage rating different from the first voltage rating.
- the first and the second voltage ratings may be pre-determined based on various factors such as the bus voltage, winding ratio of the transformer, and the output voltage across the secondary winding. In other embodiments, the first voltage rating and the second voltage rating are the same.
- the embodiment cascode switch in, e.g., FIG. 1 has pre-determined voltage ratings for switching transistor Q 1 and Q 2 that are independent of parasitic capacitances of transistors Q 1 and Q 2 .
- This allows greater flexibility in the selection of transistors. For example, not only can transistors of the same type manufactured in different batches be used as switching transistor Q 1 and Q 2 , but also can transistors of different types be used (see FIG. 5 and the discussion therewith below).
- the voltage ratings for switching transistors Q 1 and Q 2 do not have to accommodate the worse scenario parasitic capacitance variations, thus lower voltage ratings, hence cheaper transistors, can be used in the cascode switches disclosed in the present disclosure.
- start-up cell 120 could be coupled to the source of high-side transistor Q 2 . Since the drain of low-side transistor Q 1 is coupled to the source of high-side transistor Q 2 , start-up cell 120 is also coupled to the drain of low-side transistor Q 1 , in various embodiments. In some embodiments, start-up cell 120 provides a stable supply voltage V in to module(s) of circuit 100 (e.g., the power supply controller), before input voltage V in at the input terminal C is fully applied and stabilized, so that modules such as the power supply controller can perform the necessary regulation. Once the input voltage stabilizes, start-up cell 120 becomes inactive unless power is interrupted.
- module(s) of circuit 100 e.g., the power supply controller
- a start-up cell normally has an internal switch, e.g., a depletion mode MOSFET, and a driver stage.
- Start-up cell 120 might include circuits that limit the charge current below a pre-determined maximum value.
- a charge current flows through the internal switch to charge a capacitor coupled to a V CC terminal before the stabilization of the input voltage.
- high-side transistor Q 2 could be used to charge the capacitor C vcc coupled to the V cc terminal (see FIG. 1 ) and be used a part of the start-up cell.
- driver 110 for low-side transistor Q 1 , PWM 130 , controller 140 , start-up cell 120 and other related circuits form the power supply controller of circuit 100 , which is illustrated by the dashed box in FIG. 1 .
- FIG. 2 illustrates another embodiment circuit 200 having a power converter with a cascode switch.
- Circuit 200 is similar to circuit 100 illustrated in FIG. 1 , with similar numbers denote similar components.
- a resistor R 1 is coupled to the gate of low-side transistor Q 2 .
- a difference between FIG. 1 and FIG. 2 is that both low-side transistor Q 1 and high-side transistor Q 2 in FIG. 2 are coupled to the same voltage terminal E, which is coupled to voltage source V d , in accordance with some embodiments.
- voltage source V d is a PWM 230 .
- PWM 230 in FIG. 2 may be controlled by a controller in a similar fashion as illustrated in FIG. 1 .
- FIG. 2 may be controlled by a controller in a similar fashion as illustrated in FIG. 1 .
- a start-up cell similar to the one shown in FIG. 1 could be coupled to the drain of low-side transistor Q 1 .
- no start-up cell is coupled to the drain of low-side transistor Q 1 , therefore resistor R 3 in FIG. 2 becomes optional and could be omitted from circuit 200 .
- Operation of the cascode switch in FIG. 2 is similar to that of FIG. 1 discussed above, as one skilled in the art will readily appreciate.
- FIG. 3 illustrates yet another embodiment circuit 300 having a power converter with a cascode switch.
- circuit 300 comprises a high-side transistor Q 3 , which may be a first type of transistor, e.g., a MOSFET.
- High-side transistor Q 3 may have a first voltage rating.
- the gate of high-side transistor Q 3 is coupled to a high-side driver, which may be similar to the high-side driver illustrated in FIG. 1 .
- circuit 300 also includes a low-side transistor Q 1 , which may be a second type of transistor different from high-side transistor Q 3 .
- low-side transistor may be a Gallium Nitride field-effect transistor (GaN FET).
- GaN FET Gallium Nitride field-effect transistor
- the low-side transistor Q 1 has a second voltage rating, which may be different from the first voltage rating of high-side transistor Q 3 .
- high-side transistor Q 3 may have a voltage rating of 800 volt, while low-side transistor Q 1 may have a voltage rating of 500 volt.
- high-side transistor Q 3 may have a voltage rating of 500 volt, while low-side transistor Q 1 may have a voltage rating of 800 volt.
- the ability to choose transistors of different types and/or different voltage ratings is another advantage of the present disclosure, as discussed above with reference to FIG. 1 .
- the high-side transistor Q 3 and low-side transistor Q 1 are the same type of transistors and/or have similar or the same voltage ratings.
- circuit 300 includes a low-side driver coupled between the gate of low-side transistor Q 1 and a voltage source, which voltage source may be a PWM.
- the low-side driver comprises a transistor Q 2 , capacitor C 1 , diode D 1 , and resistors R 1 , R 2 , R 3 , R 4 and R 5 , in some embodiments.
- the transistor Q 2 may be a BJT transistor, for example. Operation of the cascode switch in FIG. 3 is similar to that of FIG. 1 discussed above, as one skilled in the art will readily appreciate.
- FIG. 4 illustrates a flow chart of a method of switching a power circuit, in accordance with some embodiments. It should be understood that the embodiment method shown in FIG. 4 is an example of many possible embodiment methods. One of ordinary skill in the art would recognize many variations, alternatives, and modifications. For example, various steps as illustrated in FIG. 4 may be added, removed, replaced, rearranged and repeated.
- a first switching transistor is turned off.
- the first switching transistor is coupled in series with a second switching transistor between a reference voltage and a primary winding of a transformer.
- the second switching transistor is coupled between the primary winding and the first switching transistor.
- a voltage at a drain of the first switching transistor is clamped to a bus voltage provided to the primary winding.
- second switching transistor is turned off after the voltage at the drain of the first switching transistor is above a pre-determined threshold.
- One general aspect includes a circuit including a first switching transistor having a first source coupled to a first reference voltage terminal, and a second switching transistor having a second source coupled to a first drain of the first switching transistor, and a second drain configured to be coupled to a transformer.
- the circuit also includes a first diode coupled between the first drain of the first switching transistor and a first input terminal. The first diode is configured to clamp a voltage of the first drain of the first switching transistor to a voltage of the first input terminal.
- the circuit further includes a switching circuit coupled between the second switching transistor and the first input terminal. The switching circuit is configured to connect the second source of the second switching transistor to a second gate of the second switching transistor when a voltage of the second source of the second switching transistor exceeds the voltage of the first input terminal.
- the switching transistor further includes a third transistor having a first terminal coupled to the second gate of the second switching transistor, and having a second terminal coupled to the second source of the second switching transistor.
- the switching transistor may also include a first resistor and a second diode coupled in series between a third terminal of the third transistor and the first input terminal.
- the third transistor is a bipolar junction transistor (BJT), the first terminal is an emitter of the BJT, the second terminal is a collector of the BJT, and the third terminal is a base of the BJT.
- BJT bipolar junction transistor
- the circuit further includes a third diode coupled between the base and the emitter of the BJT, and a fourth diode coupled between the third diode and a first voltage terminal.
- the circuit may further include a DC voltage source coupled to the first voltage terminal.
- the circuit may further include a pulse width modulator coupled to the first voltage terminal.
- the circuit further includes a first driver circuit coupled to a first gate of the first switching transistor.
- the first driver circuit may comprise a resistor.
- the circuit further includes a pulse width modulator coupled to the first driver circuit.
- the circuit includes a second resistor coupled between the second gate of the second switching transistor and the emitter of the BJT.
- the circuit may further include a third resistor coupled between the emitter of the BJT and the first input terminal.
- the circuit may further include a start-up cell coupled to the second source of the second switching transistor.
- the first switching transistor and the second switching transistor are different types of transistors.
- a first one of the first and the second switching transistors is a metal-oxide-semiconductor field-effect transistor (MOSFET), and a second one of the first and the second switching transistors is a Gallium Nitride field-effect transistor (GaN FET).
- the circuit includes the transformer.
- Another general aspect includes a circuit including a first switching transistor and a second switching transistor, where a drain of the first switching transistor is coupled to a source of the second switching transistor, a source of the first switching transistor is coupled to a reference voltage terminal, and a drain of the second switching transistor is configured to be coupled to a first terminal of a primary winding of a transformer.
- the circuit also includes a first diode coupled between the drain of the first switching transistor and a second terminal of the primary winding, where the first diode is configured to limit a voltage at the drain of the first switching transistor to a pre-determined voltage.
- the circuit further includes a switching circuit coupled between the second switching transistor and the second terminal of the primary winding, where the switching circuit is configured to discharge a gate-source capacitor of the second switching transistor when the voltage at the drain of the first switching transistor is at the pre-determined voltage.
- the switching circuit may include a bipolar junction transistor (BJT) comprising an emitter coupled to a gate of the second switching transistor, and a collector coupled to the source of the second switching transistor, and a first resistor and a second diode coupled in series between a base of the BJT and the second terminal of the primary winding.
- the circuit may further include a third diode; and a fourth diode, wherein the fourth diode and the third diode are coupled in series between the emitter of the BJT and a first voltage terminal.
- the first voltage may be coupled to a DC voltage source.
- the first voltage terminal may be coupled to a pulse width modulator.
- the circuit may include a driver circuit coupled between a gate of the first switching transistor and a pulse width modulator.
- the circuit also includes a second resistor coupled between the emitter of the BJT and the gate of the second switching transistor.
- the circuit may further include a third resistor coupled between the emitter of the BJT and the second terminal of the primary winding.
- the circuit may further include a start-up cell coupled to the source of the second switching transistor.
- the first switching transistor and the second switching transistor have different voltage ratings.
- the circuit further includes the transformer.
- a further general aspect includes a method of switching a power circuit including
- Implementations may include one or more of the following features.
- the clamping is performed by a diode coupled between the drain of the first switching transistor and the primary winding.
- the turning off the second switching transistor may include turning on a third transistor, and discharging a gate-source capacitor of the second switching transistor through a conductive path including the third transistor, a resistor, and a second diode.
- the third transistor is a bipolar junction transistor (BJT).
- Advantages of embodiments of the present invention include a turn off time for high-side transistors that is independent of load current, the ability to use transistors of different types and/or different voltage ratings for the high-side transistor and the low-side transistor.
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Abstract
Description
- The present disclosure relates generally to switches, in particular cascode switches.
- Power supply systems are pervasive in many electronic applications from computers to automobiles. Generally, voltages within a power supply system are generated by performing a DC-DC, DC-AC, and/or AC-DC conversion by operating a switch loaded with an inductor or transformer. One class of such systems includes switched mode power supplies (SMPS). An SMPS is usually more efficient than other types of power conversion systems because power conversion is performed by controlled charging and discharging of the inductor or transformer and reduces energy lost due to power dissipation across resistive voltage drops.
- A SMPS usually includes at least one switch and an inductor or transformer. Some specific topologies include buck converters, boost converters, and flyback converters, among others. A control circuit is commonly used to open and close the switch to charge and discharge the inductor. In some applications, the current supplied to the load and/or the voltage supplied to the load is controlled via a feedback loop. In some typologies, the switches used in the SMPS are implemented using cascode switches.
- Cascode switches are typically designed with two or more MOSFETs (metal oxide semiconductor field effect transistors) or IGBTs (insulated gate bipolar transistors) connected in series. For example in a two transistor cascode switch, the first transistor is coupled to the load and the second transistor is coupled in series between the first transistor and ground. The transistors are switched on and off in order to switch the load current as demanded or required. The load voltage is distributed across all of the series connected power transistors included in the cascode switch. For example, two 800V rated MOSFETs may be connected in series for switching a 1000V or greater load.
- In accordance with an embodiment, a circuit including a first switching transistor having a first source coupled to a first reference voltage terminal, and a second switching transistor having a second source coupled to a first drain of the first switching transistor, and a second drain configured to be coupled to a transformer. The circuit also includes a first diode coupled between the first drain of the first switching transistor and a first input terminal. The first diode is configured to clamp a voltage of the first drain of the first switching transistor to a voltage of the first input terminal. The circuit further includes a switching circuit coupled between the second switching transistor and the first input terminal. The switching circuit is configured to connect the second source of the second switching transistor to a second gate of the second switching transistor when a voltage of the second source of the second switching transistor exceeds the voltage of the first input terminal.
- For a more complete understanding of the present invention, and the advantages thereof, reference is now made to the following descriptions taken in conjunction with the accompanying drawings, in which:
-
FIG. 1 illustrates a power converter with a cascode switch in some embodiments ; -
FIG. 2 illustrates a power converter circuit with a cascode switch in some other embodiments; -
FIG. 3 illustrates a power converter circuit with a cascode switch in yet other embodiments; and -
FIG. 4 illustrates a flow chart of a method for switching a power circuit in some embodiments. - Corresponding numerals and symbols in different figures generally refer to corresponding parts unless otherwise indicated. The figures are drawn to clearly illustrate the relevant aspects of the preferred embodiments and are not necessarily drawn to scale. To more clearly illustrate certain embodiments, a letter indicating variations of the same structure, material, or process step may follow a figure number.
- The making and using of the presently preferred embodiments are discussed in detail below. It should be appreciated, however, that the present invention provides many applicable inventive concepts that can be embodied in a wide variety of specific contexts. The specific embodiments discussed are merely illustrative of specific ways to make and use the invention, and do not limit the scope of the invention.
- The present invention will be described with respect to preferred embodiments in a specific context, a system and method for cascode switches that may be used in power converters (e.g., flyback converters) to switch on and off the load current. The invention may also be applied to other systems and applications having circuits that utilize cascode switches.
- Flyback converters generally include a switch coupled in series with a primary winding of a transformer and an input voltage of the flyback converter. During operation of the flyback converter when the switch is turned off, the voltage across the switch may rise to a very high voltage that is much greater than then the input voltage, which may be on the order of a few hundred volts. In order to help the switch withstand these high voltages, the switch is constructed using a plurality of connected transistors that share the high voltage between them. According to embodiments of the present invention, a cascode switch is constructed using a low-side switching transistor and a high-side cascode transistor. When the cascode switch is turned off, the load-path voltage of the low-side switching transistor is clamped to input voltage of the flyback converter while the cascode transistor is permitted to conduct. Once the drain of the low-side is clamped to the input voltage of the flyback converter, the cascode transistor is turned-off. This way, the voltage across the low-side switching transistor limited to about the voltage of the power input bus of the flyback converter and the high-side cascode transistor is about limited to a voltage difference between the maximum voltage seen across the switch and the power input bus of the flyback converter. In some embodiments, the maximum voltage across each device in the cascode switch is substantially independent of device geometry, device parameters, parasitic capacitance and device matching between the low-side switching transistor and the high-side cascode transistor. Advantageously, in such embodiments, the low-side switching transistor and the high-side cascode transistor do not need to be over-specified to withstand the full voltage seen by the switch in some embodiments, yielding the ability to use smaller, less expensive transistors for the cascode switch. In addition, in some embodiments, the turn-off time of the cascode switch is substantially independent of the load current of the cascode switch.
-
FIG. 1 illustrates a switched-mode flyback converter 100 using a cascode switch that includes transistors Q1 and Q2, in accordance with some embodiments. A flyback converter is illustrated inFIG. 1 an example. The use of a flyback converter inFIG. 1 is not intended to be limiting, as other suitable circuits and/or power converters may also be used with the cascode switch. The discussion below may refer to the power converter as a flyback converter, with the understanding that other types of power converter or circuits may also be used with the cascode switch disclosed herein. - Referring to
FIG. 1 , two switching transistors Q1 and Q2 are coupled in series between transformer T1 offlyback converter 100 and a reference voltage terminal G. The reference voltage terminal G is coupled to electrical ground, but may be coupled to other reference potentials in alternative embodiments. As illustrated inFIG. 1 , switching transistor Q2 is coupled between the primary winding of transformer T1 and switching transistor Q1, with the drain of switching transistor Q2 coupled to terminal B of the primary winding of transformer T1, and the source of switching transistor Q2 coupled to the drain of switching transistor Q1. The source of switching transistor Q1 is coupled to reference voltage terminal G. In the discussion hereinafter, the switching transistor (e.g., transistor Q1) closest to reference voltage terminal G may be referred to as the low-side transistor and switching transistor (e.g. transistor Q2) closest to the transformer may be referred to as the high-side transistor. - As shown in
FIG. 1 , an input voltage Vin is supplied to an input terminal C ofcircuit 100. InFIG. 1 , the input voltage Vin is shown as a DC voltage source for the purpose of illustration, however, one skilled in the art will appreciate that other suitable input voltages, such as a rectified AC voltage, may also be used as the input voltage. Terminal A of the primary winding of transformer T1 is coupled to input terminal C, thus input voltage Vin is also applied to terminal A of the primary winding of transformer T1. The voltage at input terminal C and terminal A of the primary winding of transformer T1 is sometimes referred to as the bus voltage. The secondary winding of transformer T1 is coupled to a load represented by voltage Vo via a rectifier diode D1, in some embodiments. -
FIG. 1 illustrates a driver circuit 110, also referred to as a driver 110, that is coupled to the gate of low-side transistor Q1. An input terminal of the driver 110 of low-side transistor Q1 may be coupled to a pulsed voltage source, such as a pulse-width-modulator (PWM) 130. In the discussion below, a PWM is also used to refer to a pulsed voltage source and may be used interchangeably with a pulsed voltage source. PWM 130 may be coupled to and controlled by a controller 140. Controller 140 may be a micro-controller unit (MCU), an application-specific integrated circuit (ASIC), a control circuit built using discrete components, or any other suitable controller. As shown inFIG. 1 , controller 140 may receive feedback signal(s) fromfeedback circuit 150 to monitor the operation and status ofcircuit 100. Controller 140 may instruct PWM 130 to generate a pulse train (e.g., a series of voltage pulses with desired pulse width and pulse amplitude) to turn on and off low-side transistor Q1, which in turn causes high-side transistor Q2 to turn on and off, as discussed in more detail hereinafter. The driver of low-side transistor Q1 may comprises any suitable driver circuit. - Still referring to
FIG. 1 , the gate of high-side transistor Q2 is coupled to a high-side driver via resistor R4. The high-side driver is shown in another dashed box illustrated inFIG. 1 , and includes diodes D2, D3, D4, D5, resistor R2, and transistor Q3, in some embodiments. Transistor Q3 is a bipolar junction transistor (BJT), in various embodiments. As illustrated inFIG. 1 , the emitter of BJT transistor Q3 is coupled to the gate of high-side transistor Q2 via resistor R4, the collector of BJT transistor Q3 is coupled to the source of high-side transistor Q2, and the base of BJT transistor Q3 is coupled to the input terminal C via diode D5 and resistor R2, with resistor R2 coupled between diode D5 and transistor Q3. In addition,FIG. 1 further illustrates diode D2 coupled between the drain of low-side transistor Q1 and input terminal C, diode D4 coupled between the base and the emitter of BJT transistor Q3, and diode D3 coupled between diode D4 and a voltage terminal E that is coupled to a voltage source Vc. In the example ofFIG. 1 , voltage source Vc is a DC voltage source, although other suitable voltage source (e.g., a pulsed voltage source) may also be used. Furthermore, as illustrated inFIG. 1 , a first terminal of resistor R3 is coupled to input terminal C, and a second terminal of resistor R3 is coupled to the gate of high-side transistor Q2 via resistor R4, in some embodiments. - Operation of the cascode switch is described now with reference to
FIG. 1 . Referring toFIG. 1 , during turn on of the cascode switch, PWM 130 generates a voltage pulse (e.g., a voltage pulse of 10 volt with a pulse width of 10 μs) to turn on low-side transistor Q1. Once low-side transistor Q1 is turned on, the source voltage of high-side transistor Q2 is pulled down to a low voltage (e.g., close to electrical ground). Since the gate of high-side transistor Q2 is coupled to voltage source V, (e.g., a DC voltage source of 15 volt), rectifier diode D3 and D4 begin to conduct. In the conducting state, rectifier diode D3 and D4 charge high-side transistor Q2 through resistor R4, thus turning on high-side transistor Q2. Once low-side transistor Q1 and high-side transistor Q2 are turned on, load current flows through the primary winding of transformer T1, low-side transistor Q1 and high-side transistor Q2. During the turn on process, BJT transistor Q3 is off because the base-emitter voltage (Vbe) of Q3 is negative. - Still referring to
FIG. 1 , during turn off of the cascode switch, PWM 130 generates a corresponding voltage pulse (e.g., a voltage pulse with zero volt and pulse width of 4 μs) to turn off low-side Q1, which in turn causes the drain-to-source voltage (Vds) of Q1 to begin rising. When Vds of low-side transistor Q1 reaches a threshold voltage, e.g., a diode's drop above the bus voltage, rectifier diode D2 conducts and load current is redirected through D2. Once conducting, diode D2 clamps the drain-to-source voltage of low-side transistor Q1 to the bus voltage. One skilled in the art will appreciate that “clamp to the bus voltage” means that the drain-to-source voltage Vds of low-side transistor Q1 is limited to a voltage near the bus voltage, e.g., a diode drop above the bus voltage. Since the drain-to-source voltage Vds of low-side transistor Q1 is clamped at bus voltage, the gate voltage of high-side transistor Q2 is above bus voltage. As a result, diodes D4 becomes reverse biased, diode D5 becomes forward biased, and BJT transistor Q3 conducts. Once BJT transistor Q3 conducts, load current flows through resistor R4, BJT transistor Q3, resistor R2 and diode D5 to discharges the gate-source capacitor Cgs of high-side transistor Q2, turning off Q2. Therefore, the primary winding, high-side transistor Q2, resistor R4, BJT transistor Q3, resistor R2 and diode D5 form a conductive loop to discharge the gate-source capacitor Cgs, in some embodiments. The high-side transistor Q2 turns off when Cgs is discharged. Note that when BJT transistor Q3 conducts (e.g., when the source of high-side transistor Q2 is above the bus voltage), its emitter and base are forward biased and its collector and base are reverse biased, current flows through the emitter and collector, therefore BJT transistor Q3 connects the gate and the source of high-side transistor Q2 when conducting, in accordance with some embodiments. In the discussion hereinafter, the BJT transistor Q3, resistor R2 and diode D5 may be collectively referred to as a switching circuit. As discussed above, the low-side transistor Q1 and high-side transistor Q2 turn off successively, in some embodiments. - Since high-side transistor Q2 is actively turned off by BJT transistor Q3 independently of load current, the turn off time of high-side transistor Q2 does not depend on the load current, meaning that compared with the case of a single transistor, the circuit topology does not result in any additional dependency of turn off time on load current. This illustrates an advantage of the present disclosure.
- Still referring to
FIG. 1 , as mentioned above, when the cascode switch is turned on, load current flows through the primary winding of transformer T1 Diode D1 of the secondary winding of transformer T1 is reverse biased when the cascode switch is on, therefore, no current flows through the secondary winding, and the load current flows through the primary winding and stores energy as magnetic field in the transformer. Diode D1 may be replaced by a switch that is used as a synchronous rectifier, as skilled artisans will appreciate. After the cascode switch is turned off (e.g., after high-side transistor Q2 is turned off), the stored magnetic field collapses, and energy is transferred to the output of the transformer (e.g., the secondary winding), generating an output voltage across the secondary winding and a current in the secondary winding. Due to the electromagnetic coupling between the primary winding and the secondary winding, the output voltage across the secondary winding is reflected back to the primary winding, thus the cascode switch may be subject to a high load voltage. In some embodiments, the high load voltage is equal to a sum of the bus voltage and a reflected voltage, which reflected voltage is determined, at least in part, by the output voltage across the secondary winding and the winding ratio between the primary winding and the second winding. As discussed above, diode D2 clamps the drain-to-source voltage Vds of low-side transistor Q1 to the bus voltage, thus high-side transistor Q2 is subject to the rest of the load voltage. Therefore, the voltage rating, which defines the maximum voltage under which a transistor can safely operate, may be different for low-side transistor Q1 and high-side transistor Q2. For example, low-side transistor Q1 may have a first voltage rating, and high-side transistor Q2 may have a second voltage rating different from the first voltage rating. The first and the second voltage ratings may be pre-determined based on various factors such as the bus voltage, winding ratio of the transformer, and the output voltage across the secondary winding. In other embodiments, the first voltage rating and the second voltage rating are the same. - Advantages of the different voltage ratings for switching transistors Q1 and Q2 include the flexibility in choosing the transistors for use as switching transistor Q1 and Q2, and cost saving due to the ability to use transistors with lower voltage rating (e.g., cheaper transistors). The embodiment cascode switch in, e.g.,
FIG. 1 , has pre-determined voltage ratings for switching transistor Q1 and Q2 that are independent of parasitic capacitances of transistors Q1 and Q2. This allows greater flexibility in the selection of transistors. For example, not only can transistors of the same type manufactured in different batches be used as switching transistor Q1 and Q2, but also can transistors of different types be used (seeFIG. 5 and the discussion therewith below). In some embodiments, the voltage ratings for switching transistors Q1 and Q2 do not have to accommodate the worse scenario parasitic capacitance variations, thus lower voltage ratings, hence cheaper transistors, can be used in the cascode switches disclosed in the present disclosure. - By adding resistor R3 coupled between input terminal C and the gate of high-side transistor Q2 as shown in
FIG. 1 , a start-upcell 120 could be coupled to the source of high-side transistor Q2. Since the drain of low-side transistor Q1 is coupled to the source of high-side transistor Q2, start-upcell 120 is also coupled to the drain of low-side transistor Q1, in various embodiments. In some embodiments, start-upcell 120 provides a stable supply voltage Vin to module(s) of circuit 100 (e.g., the power supply controller), before input voltage Vin at the input terminal C is fully applied and stabilized, so that modules such as the power supply controller can perform the necessary regulation. Once the input voltage stabilizes, start-upcell 120 becomes inactive unless power is interrupted. A start-up cell normally has an internal switch, e.g., a depletion mode MOSFET, and a driver stage. Start-upcell 120 might include circuits that limit the charge current below a pre-determined maximum value. During the start-up process, a charge current flows through the internal switch to charge a capacitor coupled to a VCC terminal before the stabilization of the input voltage. As shown below, high-side transistor Q2 could be used to charge the capacitor Cvcc coupled to the Vcc terminal (seeFIG. 1 ) and be used a part of the start-up cell. - Referring to
FIG. 1 , during start up ofcircuit 100, input voltage Vin applied at input terminal C turns on high-side transistor Q2 via resistors R3 and R4, thus allowing a charging current to flow through high-side transistor Q2 and start-upcell 120 to charge capacitor Cvcc at the Vcc terminal. This allows start-upcell 120 to generate voltage Vcc used by other modules (e.g., the power supply controller) ofcircuit 100 for proper operation ofcircuit 100 during the start-up process. Therefore, by adding resistor R3 incircuit 100, high-side transistor Q2 could be used as a part of the start-up cell, this illustrates another advantage of the current disclosure. In some embodiments, driver 110 for low-side transistor Q1, PWM 130, controller 140, start-upcell 120 and other related circuits (not shown) form the power supply controller ofcircuit 100, which is illustrated by the dashed box inFIG. 1 . -
FIG. 2 illustrates anotherembodiment circuit 200 having a power converter with a cascode switch.Circuit 200 is similar tocircuit 100 illustrated inFIG. 1 , with similar numbers denote similar components. InFIG. 2 , a resistor R1 is coupled to the gate of low-side transistor Q2. A difference betweenFIG. 1 andFIG. 2 is that both low-side transistor Q1 and high-side transistor Q2 inFIG. 2 are coupled to the same voltage terminal E, which is coupled to voltage source Vd , in accordance with some embodiments. In various embodiments, voltage source Vd is aPWM 230. Although not shown,PWM 230 inFIG. 2 may be controlled by a controller in a similar fashion as illustrated inFIG. 1 . In addition, although not shown inFIG. 2 , a start-up cell similar to the one shown inFIG. 1 could be coupled to the drain of low-side transistor Q1. InFIG. 2 , no start-up cell is coupled to the drain of low-side transistor Q1, therefore resistor R3 inFIG. 2 becomes optional and could be omitted fromcircuit 200. Operation of the cascode switch inFIG. 2 is similar to that ofFIG. 1 discussed above, as one skilled in the art will readily appreciate. -
FIG. 3 illustrates yet another embodiment circuit 300 having a power converter with a cascode switch. InFIG. 3 , circuit 300 comprises a high-side transistor Q3, which may be a first type of transistor, e.g., a MOSFET. High-side transistor Q3 may have a first voltage rating. The gate of high-side transistor Q3 is coupled to a high-side driver, which may be similar to the high-side driver illustrated inFIG. 1 . As illustrated inFIG. 3 , circuit 300 also includes a low-side transistor Q1, which may be a second type of transistor different from high-side transistor Q3. For example, low-side transistor may be a Gallium Nitride field-effect transistor (GaN FET). In some embodiments, the low-side transistor Q1 has a second voltage rating, which may be different from the first voltage rating of high-side transistor Q3. For example, high-side transistor Q3 may have a voltage rating of 800 volt, while low-side transistor Q1 may have a voltage rating of 500 volt. As another example, high-side transistor Q3 may have a voltage rating of 500 volt, while low-side transistor Q1 may have a voltage rating of 800 volt. The ability to choose transistors of different types and/or different voltage ratings is another advantage of the present disclosure, as discussed above with reference toFIG. 1 . In other embodiments, the high-side transistor Q3 and low-side transistor Q1 are the same type of transistors and/or have similar or the same voltage ratings. - As illustrated in
FIG. 3 , circuit 300 includes a low-side driver coupled between the gate of low-side transistor Q1 and a voltage source, which voltage source may be a PWM. The low-side driver comprises a transistor Q2, capacitor C1, diode D1, and resistors R1, R2, R3, R4 and R5, in some embodiments. The transistor Q2 may be a BJT transistor, for example. Operation of the cascode switch inFIG. 3 is similar to that ofFIG. 1 discussed above, as one skilled in the art will readily appreciate. -
FIG. 4 illustrates a flow chart of a method of switching a power circuit, in accordance with some embodiments. It should be understood that the embodiment method shown inFIG. 4 is an example of many possible embodiment methods. One of ordinary skill in the art would recognize many variations, alternatives, and modifications. For example, various steps as illustrated inFIG. 4 may be added, removed, replaced, rearranged and repeated. - Referring to
FIG. 4 , at step 1010, a first switching transistor is turned off. The first switching transistor is coupled in series with a second switching transistor between a reference voltage and a primary winding of a transformer. The second switching transistor is coupled between the primary winding and the first switching transistor. At step 1020, a voltage at a drain of the first switching transistor is clamped to a bus voltage provided to the primary winding. Atstep 1030, second switching transistor is turned off after the voltage at the drain of the first switching transistor is above a pre-determined threshold. - Embodiments of the present invention are summarized here. Other embodiments can also be understood form the entirety of the specification and the claims filed herein. One general aspect includes a circuit including a first switching transistor having a first source coupled to a first reference voltage terminal, and a second switching transistor having a second source coupled to a first drain of the first switching transistor, and a second drain configured to be coupled to a transformer. The circuit also includes a first diode coupled between the first drain of the first switching transistor and a first input terminal. The first diode is configured to clamp a voltage of the first drain of the first switching transistor to a voltage of the first input terminal. The circuit further includes a switching circuit coupled between the second switching transistor and the first input terminal. The switching circuit is configured to connect the second source of the second switching transistor to a second gate of the second switching transistor when a voltage of the second source of the second switching transistor exceeds the voltage of the first input terminal.
- Implementations may include one or more of the following features. The switching transistor further includes a third transistor having a first terminal coupled to the second gate of the second switching transistor, and having a second terminal coupled to the second source of the second switching transistor. The switching transistor may also include a first resistor and a second diode coupled in series between a third terminal of the third transistor and the first input terminal. In some embodiments, the third transistor is a bipolar junction transistor (BJT), the first terminal is an emitter of the BJT, the second terminal is a collector of the BJT, and the third terminal is a base of the BJT. In some embodiments, the circuit further includes a third diode coupled between the base and the emitter of the BJT, and a fourth diode coupled between the third diode and a first voltage terminal. The circuit may further include a DC voltage source coupled to the first voltage terminal. The circuit may further include a pulse width modulator coupled to the first voltage terminal.
- In some embodiments, the circuit further includes a first driver circuit coupled to a first gate of the first switching transistor. The first driver circuit may comprise a resistor. In some embodiments, the circuit further includes a pulse width modulator coupled to the first driver circuit.
- In various embodiments, the circuit includes a second resistor coupled between the second gate of the second switching transistor and the emitter of the BJT. The circuit may further include a third resistor coupled between the emitter of the BJT and the first input terminal. The circuit may further include a start-up cell coupled to the second source of the second switching transistor.
- In accordance with some embodiments, the first switching transistor and the second switching transistor are different types of transistors. In other embodiments, a first one of the first and the second switching transistors is a metal-oxide-semiconductor field-effect transistor (MOSFET), and a second one of the first and the second switching transistors is a Gallium Nitride field-effect transistor (GaN FET). In yet other embodiments, the circuit includes the transformer.
- Another general aspect includes a circuit including a first switching transistor and a second switching transistor, where a drain of the first switching transistor is coupled to a source of the second switching transistor, a source of the first switching transistor is coupled to a reference voltage terminal, and a drain of the second switching transistor is configured to be coupled to a first terminal of a primary winding of a transformer. The circuit also includes a first diode coupled between the drain of the first switching transistor and a second terminal of the primary winding, where the first diode is configured to limit a voltage at the drain of the first switching transistor to a pre-determined voltage. The circuit further includes a switching circuit coupled between the second switching transistor and the second terminal of the primary winding, where the switching circuit is configured to discharge a gate-source capacitor of the second switching transistor when the voltage at the drain of the first switching transistor is at the pre-determined voltage.
- Implementations may include one or more of the following features. The switching circuit may include a bipolar junction transistor (BJT) comprising an emitter coupled to a gate of the second switching transistor, and a collector coupled to the source of the second switching transistor, and a first resistor and a second diode coupled in series between a base of the BJT and the second terminal of the primary winding. The circuit may further include a third diode; and a fourth diode, wherein the fourth diode and the third diode are coupled in series between the emitter of the BJT and a first voltage terminal. The first voltage may be coupled to a DC voltage source. The first voltage terminal may be coupled to a pulse width modulator. The circuit may include a driver circuit coupled between a gate of the first switching transistor and a pulse width modulator.
- In some embodiments, the circuit also includes a second resistor coupled between the emitter of the BJT and the gate of the second switching transistor. The circuit may further include a third resistor coupled between the emitter of the BJT and the second terminal of the primary winding. The circuit may further include a start-up cell coupled to the source of the second switching transistor.
- In various embodiments, the first switching transistor and the second switching transistor have different voltage ratings. In other embodiments, the circuit further includes the transformer.
- A further general aspect includes a method of switching a power circuit including
-
- turning off a first switching transistor, where the first switching transistor is coupled in series with a second switching transistor between a reference voltage and a primary winding of a transformer, where the second switching transistor is coupled between the primary winding and the first switching transistor. The method also include clamping a voltage at a drain of the first switching transistor to a bus voltage provided to the primary winding, and turning off the second switching transistor after the voltage at the drain of the first switching transistor is above a pre-determined threshold.
- Implementations may include one or more of the following features. In some embodiments, the clamping is performed by a diode coupled between the drain of the first switching transistor and the primary winding. The turning off the second switching transistor may include turning on a third transistor, and discharging a gate-source capacitor of the second switching transistor through a conductive path including the third transistor, a resistor, and a second diode. In an embodiment, the third transistor is a bipolar junction transistor (BJT).
- Advantages of embodiments of the present invention include a turn off time for high-side transistors that is independent of load current, the ability to use transistors of different types and/or different voltage ratings for the high-side transistor and the low-side transistor. In addition, it is possible to use the high-side transistor as a part of a start-up cell. If quasi-resonant mode of operation is used, both the low-side transistor and the high-side transistor can be turned off quickly, thus minimizing turn-off loss.
- While this invention has been described with reference to illustrative embodiments, this description is not intended to be construed in a limiting sense. Various modifications and combinations of the illustrative embodiments, as well as other embodiments of the invention, will be apparent to persons skilled in the art upon reference to the description.
Claims (30)
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US15/009,438 US9722599B1 (en) | 2016-01-28 | 2016-01-28 | Driver for the high side switch of the cascode switch |
DE102017101272.9A DE102017101272A1 (en) | 2016-01-28 | 2017-01-24 | Driver for a high-side switch of a cascode switch |
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US15/009,438 US9722599B1 (en) | 2016-01-28 | 2016-01-28 | Driver for the high side switch of the cascode switch |
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US10243551B1 (en) | 2017-09-06 | 2019-03-26 | Alpha And Omega Semiconductor (Cayman) Ltd. | Over voltage protection for cascode switching power device |
TWI716980B (en) * | 2018-08-28 | 2021-01-21 | 美商高效電源轉換公司 | GaN DRIVER USING ACTIVE PRE-DRIVER WITH FEEDBACK |
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US5313109A (en) * | 1992-04-28 | 1994-05-17 | Astec International, Ltd. | Circuit for the fast turn off of a field effect transistor |
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US20090140791A1 (en) * | 2007-11-29 | 2009-06-04 | Young Paul D | Switching Element Control |
US8653881B2 (en) * | 2012-01-31 | 2014-02-18 | Infineon Technologies Austria Ag | Half bridge flyback and forward |
US8779841B2 (en) | 2012-01-31 | 2014-07-15 | Infineon Technologies Austria Ag | Cascode switch with robust turn on and turn off |
US20140035627A1 (en) * | 2012-08-06 | 2014-02-06 | Fairchild Semiconductor Corporation | SiC Proportional Bias Switch Driver Circuit with Current Transformer |
US9041433B2 (en) * | 2013-06-21 | 2015-05-26 | Infineon Technologies Austria Ag | System and method for driving transistors |
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