Driving power switches

Abstract

In one implementation, a gate drive unit includes a high rail and a low rail configured to be coupled to input dc voltage; an intermediate rail configured to be coupled to a main terminal of a power switch; a linear voltage regulator coupled to either the high rail or the low rail, wherein the linear voltage regulator is configured to output a regulated voltage; a buffer capacitor coupled between the output of the linear voltage regulator and the intermediate rail; and a switch that is configured to couple the output of the linear voltage regulator to a control terminal of the power switch, thereby driving the power switch into or out of conduction.

Description

[0001] Attorney Docket No.: 36907-0039W01

[0002] DRIVING POWER SWITCHES

[0003] TECHNICAL FIELD

[0004] This invention relates to driving power switches, e.g., in power converters.

[0005] BACKGROUND

[0006] Power switches are switching devices designed to switch relatively high voltages and currents. Power switches can be implemented as insulated gate bipolar transistors (IGBTs), metal-oxide-semiconductor field-effect transistors (MOSFETs), high-electron- mobility transistors (HEMTs), and the like. Power switches are commonly made using semiconductor materials such as silicon, silicon carbide, gallium nitride, or other semiconductor materials.

[0007] In many power switches, the control terminal is a gate that is capacitively coupled to the main (i.e., emitter and collector or source and drain) terminals. To control switching of such power switches, a gate drive unit must appropriately bias the control terminal relative to the emitter or source. In enhancement mode devices, the gate must be positively biased relative to the emitter or source above a threshold voltage level for conduction between the main terminals. A zero or negative gate bias relative to the emitter or source effectively renders the power switch non-conductive. Switching the power switch between conductive and non-conductive states requires that the bias relative to the emitter or source be changed.

[0008] When the bias applied to the gate is changed, current is either provided to or drawn from the gate. The capacitance of the gate — which includes the capacitance between the gate and the main terminals as well as other components — must either be charged or discharged, depending on whether the bias is increased or decreased. In real- world devices, the current sources / sinks are often unable to charge or discharge the gate without undergoing voltage swings themselves.

[0009] To ensure consistent switching, unduly large swings in the voltage that is biasing the gate should be avoided. To do so, it is common practice to use buffer capacitors to source and sink current to / from the gate. Buffer capacitors can generally source and sink Attorney Docket No.: 36907-0039W01 current relatively more quickly than other current sources and can be used in combination with other sources such that a portion of the current is provided by the source and a portion is provided by the buffer capacitor. Buffer capacitors thus “buffer” the bias voltage that is applied to the gate, i.e., they reduce the amplitude of the voltage swings in the gate bias voltage.

[0010] However, buffer capacitors can be costly both in terms of money and space, e.g., footprint on a printed circuit board. A rule of thumb is that the ratio of gate charge in pC to buffer capacitance in pF should be 1:3. In commercial power switches, it is not unusual that between ~1 and ~30 pC of gate charge must be sourced / sunk from the gate to achieve nominal voltage swings of 30 Volts (e.g., between +15 and -15 Volts relative to the emitter). Application of the rule of thumb indicates that between ~3 and 90 pF of buffer capacitance are required for nominal voltage swing of 330 mV.

[0011] DESCRIPTION OF DRAWINGS

[0012] FIG. 1 is a schematic representation of a gate drive unit in which buffer capacitors source and sink current to / from the gate of an IGBT power switch

[0013] FIGS. 2, 3, 4 are schematic representations of portions of gate drive units which include linear voltage regulators.

[0014] FIG. 5 is a schematic representation of the switching pattern of a power switch and a graph of voltages in a power converter in which the voltage that is used to bias a power switch into conduction is synchronized with the switching pattern.

[0015] FIG. 6 is a schematic representation of the switching pattern of a power switch, a graph of voltages in a power converter in which the voltage between high and low rails is within an expected operating range, a standby (non-switching) pattern of a power switch when the voltage between high and low rails has fallen below the expected operating range, and a graph of voltages in the power converter in this standby (non- switching) state.

[0016] FIG. 7 is a schematic representation of an input stage to a gate drive unit.

[0017] As an aside, the power switches illustrated and described are all enhancement mode insulated gate bipolar transistors. This is not necessarily the case and other types of Attorney Docket No.: 36907-0039W01 power switches — including depletion mode devices — can be used with appropriate modification of the concepts described herein.

[0018] Like reference symbols in the various drawings indicate like elements. The details of one or more implementations are set forth in these drawings and the description below. Other features, objects, and advantages will be apparent from the description and drawings, and from the claims.

[0019] DETAILED DESCRIPTION

[0020] FIG. 1 is a schematic representation of a gate drive unit 100 in which buffer capacitors 105, 110 source and sink current to / from the gate of an IGBT power switch 102. As a result, buffer capacitors 105, 110 reduce the amplitude of voltage swings between a high rail 115 and a low rail 125 relative to an intermediate rail 120 when IGBT power switch 102 is switched into and out of conduction. Gate drive unit 100 and the other gate drive units described herein can be used to drive power switches in a variety of different contexts, including, e.g., switching power converters, inverters, and motor drives. While the power switch 102 is shown as an IGBT, it should be appreciated that other transistors may also be used, such as a metal-oxide field-effect transistors (MOSFETs) , high-electron-mobility transistors (HEMTs), etc.

[0021] In the illustrated implementation, the operating supply of the gate drive unit 100 is provided by a rectifier 20 and a transformer 25. In some implementations, transformer 25 is a step up transformer that is configured to be coupled at its input to a relatively low amplitude ac voltage (e.g., the output of an inverter) and output a higher amplitude ac signal to rectifier 20. However, implementations in which transformer 25 is a step down transformer are also possible. Rectifier 20 rectifies the ac signal received from transformer 25 and outputs a de signal across high rail 115 and low rail 125. In other implementations, the voltage across high rail 115 and low rail 125 can be provided in other ways, e.g., directly from de sources (e.g., a battery).

[0022] In addition to high rail 115 and low rail 125, gate drive unit 100 also includes a voltage regulator 130 and intermediate rail 120. Intermediate rail 120 is coupled to the emitter of IGBT power switch 102 and an output of voltage regulator 130. Voltage Attorney Docket No.: 36907-0039W01 regulator 130 is configured to regulate the voltage difference between intermediate rail 120 and either or both of low rail 125 and high rail 115. For example, in some applications, voltage regulator 130 regulates the voltage difference between high rail 115 and intermediate rail 120 to be substantially 15 volts (V) and the voltage difference between intermediate rail 120 and low rail 125 to be between substantially 8V. The regulated voltage difference between high rail 115 and intermediate rail 120 may have other values, e.g., values between 12V and 20V. The regulated voltage difference between intermediate rail 120 and low rail 125 may be, e.g., between 0 and 10V.

[0023] Gate drive unit 100 also includes gate driver circuitry 140. In the illustrated implementation, gate driver circuitry 140 is implemented as a totem pole that includes a pull-up transistor 150, a pull-down transistor 155, and a driver 145. The pull-up transistor 150 and pull-down transistor 155 are illustrated as n-type metal-oxide- semiconductor field-effect transistors (MOSFETs). Pull-up transistor 150 is coupled between high rail 115 and the gate of IGBT power switch 102 via a first, pull-up gate resistance. Pull-down transistor 155 is coupled between low rail 125 and the gate of IGBT power switch 102 via a second, pull-down gate resistance. Although the illustrated implementation includes separate pull up / pull down gate resistances, in other implementation, a single gate resistance can be used. In some implementations, driver 145 can be implemented as a voltage follower that buffers a control signal 160 that defines a switching pattern. For example, the switching pattern can coordinate the switching of power switch 102 with the switching of other power switches, e.g., in a bridge configuration to drive a motor.

[0024] In gate drive unit 100, buffer capacitor 105 is coupled between high rail 115 and intermediate rail 120. Buffer capacitor 110 is coupled between intermediate rail 120 and low rail 125. In the illustrated implementation, buffer capacitors 105, 110 are coupled to rails 115, 125 at intermediate positions along the length of rails 115, 125, namely, between pull-up / pull-down transistors 150, 155 and rectifier 20. As shown, one end of buffer capacitor 105 is coupled between rectifier 20 and pull-up transistor 150. One end of buffer capacitor 110 is coupled between rectifier 20 and pull-down transistor 155. In other implementations, buffer capacitors 105, 110 are coupled to rails 115, 125 at other Attorney Docket No.: 36907-0039W01 positions. For example, pull-up / pull-down transistors 150, 155 can be coupled to rails 115, 125 between rectifier 20 and buffer capacitors 105, 110.

[0025] In operation, buffer capacitors 105, 110 reduce the amplitude of voltage swings on high rail 115 and low rail 125 relative to the intermediate rail 120 when the bias applied to the gate of IGBT power switch 102 is changed and IGBT power switch 102 is switched into and out of conduction. For example, when IGBT power switch 102 is driven into the conductive ON state, driver 145 switches pull-down transistor 155 into the essentially non-conductive OFF state and pull-up transistor 150 into the ON state. This couples the gate of IGBT power switch 102 to high rail 115 and current is drawn from high rail 115 and through pull-up transistor 150 to positively bias the gate of IGBT power switch 102 relative to the emitter of IGBT power switch 102 and relative to intermediate rail 120. In order to positively bias the gate of IGBT power switch 102 relative to the emitter of IGBT power switch 102, the capacitance between the gate and the emitter — and between the gate and other components — should be charged. Charge that is stored on buffer capacitor 105 contributes to the charging current flow. If buffer capacitor 105 were infinitely large and there were no parasitic inductances, then the voltage difference between high rail 115 and intermediate rail 120 would remain unchanged. However, in real-world implementations, infinitely large capacitors do not exist, parasitic inductances do exist, and some changes in the voltage difference between high rail 115 and intermediate rail 120 occur. Provided that those changes are sufficiently small, they will not interfere with driving IGBT power switch 102 into conduction. The capacitance of buffer capacitor 105 is sized accordingly.

[0026] Correspondingly, when IGBT power switch 102 is driven into the OFF state, driver 145 switches pull-up transistor 150 into the non-conductive OFF state and pulldown transistor 155 into the ON state. This couples the gate of IGBT power switch 102 to low rail 125 and current is discharged from the gate of IGBT power switch 102, though pull-down transistor 155, and sunk to low rail 125. In general, this will negatively bias the gate of IGBT power switch 102 relative to the emitter of IGBT power switch 102 and relative to the intermediate rail 120. As an aside, implementations in which the gate and emitter of IGBT power switch 102 are at essentially the same potential are possible. In Attorney Docket No.: 36907-0039W01 either case, buffer capacitor 110 contributes to sinking the current from the gate of IGBT power switch 102. Once again, in real-world implementations, some changes in the voltage difference between intermediate rail 120 and low rail 125 are acceptable and the capacitance of buffer capacitor 110 is sized accordingly.

[0027] FIGS. 2, 3 are schematic representations of portions of gate drive units 200, 300. In gate drive units 200, 300, not only do buffer capacitors source and sink current to / from the gate of an IGBT power switch, but one or more linear voltage regulators further stabilize the voltage that is used to bias the gate of an IGBT power switch. The linear voltage regulator(s) thus allow the capacitances of these buffer capacitors to have smaller capacitances, saving cost and size without adversely impacting switching of the gate of IGBT power switch.

[0028] Gate drive units 200, 300 include components that correspond to components in gate drive unit 100. Corresponding components are designated with the same reference numbers. Some corresponding components are omitted for the sake of illustration but nevertheless included. For example, gate drive units 200, 300 also include transformer 25, gate driver circuitry 140, and IGBT power switch 102.

[0029] As with gate drive unit 100, rectifier 20 rectifies an ac signal and outputs a de signal across high rail 115 and low rail 125. However, gate drive units 200, 300 both include a linear voltage regulator 220 that is coupled to high rail 115 and low rail 125. Linear voltage regulator 220 is configured to use the output of rectifier 20 as a supply voltage and generate an output voltage on a rail 225 that is regulated with respect to either the voltage difference between the output voltage on a rail 225 and intermediate rail 120 or the voltage difference between intermediate rail 120 and low rail 125. In the illustrated implementation, linear voltage regulator 220 is shown as a low-dropout regulator (LDO). However, linear voltage regulators with an emitter follower topology are also possible.

[0030] In addition to linear voltage regulator 220, gate drive units 200, 300 both include a buffer capacitor 205 coupled between rail 225 and intermediate rail 120. Buffer capacitor 110 remains coupled between intermediate rail 120 and low rail 125. Attorney Docket No.: 36907-0039W01

[0031] In operation, linear voltage regulator 220 isolates rail 225 from voltage swings on high rail 115 that occur as the gate capacitance is charged. In more detail, regulator 130, buffer capacitor 205, and pull-up down transistor 150 are all coupled to rail 225. The regulated output voltage on rail 225 is used to bias the gate of IGBT power switch 102 relative to intermediate rail 120 and the emitter of IGBT power switch 102. To switch IGBT power switch 102 into conduction, buffer capacitor 205 quickly supplies most of the current to the gate of IGBT power switch 102. Linear voltage regulator 220 will generally respond more slowly. However, the current that is provided by linear voltage regulator 220 both recharges buffer capacitor 205 and stabilizes the voltage on buffer capacitor 205 by, e.g., reducing ripple. In some implementations, the voltage that is used to bias the gate of IGBT power switch 102 can be stabilized and does not undergo as large voltage swings as it would if linear voltage regulator 220 were not present. Examples of stabilized implementations include those in which the voltage that is used to bias a power switch into conduction is synchronized with the switching pattern, as discussed further below.

[0032] The voltage difference between the regulated voltage output from linear voltage regulator 220 and low rail 125 will be slightly lower than the voltage supplied by rectifier 20, i.e., slightly lower than the voltage difference between high rail 115 and low rail 125. This voltage difference is required to allow the linear voltage regulator 220 to regulate the output voltage on rail 225. The minimum difference depends on the implementation of the linear voltage regulator 220. Typical values are in the range of 400 mV. Concerning the maximum difference, practical values are in the range of a few volts (e.g. 4 V). This value may depend on the implementation of the linear voltage regulator 220 and desired maximum power dissipation of the linear voltage regulator 220. This not only provides headroom for linear voltage regulator 220, it also allows the size of the buffer capacitors 205 and 105 to be reduced in comparison to gate driver 100 for the same voltage swing. Return to the example discussed above in which 330 mV swings are acceptable based on a ratio of 1:3 between the gate charge and buffer capacitance. Assuming a headroom of the LDO of for instance 3 V, the ratio can be scaled to 3 V / 330 mV « 9, e.g., 1:9. Attorney Docket No.: 36907-0039W01

[0033] In both gate drive units 200, 300, the input voltage of linear voltage regulator 220 (effectively, the de output of rectifier 20) is also buffered using buffer capacitor(s). Although voltage regulator 220 can tolerate larger voltage swings on high rail 115 than are desirable when switching power switch 102, large voltage swings on high rail 115 can decrease the efficiency of voltage regulator 220. Gate drive units 200, 300 however differ in how high rail 115 is buffered. In particular, gate drive unit 200 includes buffer capacitor 210 coupled between high rail 115 and intermediate rail 120 and buffer capacitor 110 coupled between intermediate rail 120 and low rail 125. Buffer capacitors 210, 110 are effectively coupled in series and buffer high rail 115 and low rail 125. Gate drive unit 300 includes a buffer capacitor 310 that is coupled between high rail 115 and low rail 125. Buffer capacitors 210, 310 act as a source of charge for linear voltage regulator 220. Further, capacitors 110, 210, 310 reduce the voltage swings on high rail 115 relative to low rail 125.

[0034] FIG. 4 is a schematic representation of portions of a gate drive unit 400. Gate drive unit 400 includes a pair of linear voltage regulators 220, 420 and omits regulator 130. Gate drive unit 400 also includes transformer 25, gate driver circuitry 140, and IGBT power switch 102.

[0035] As with gate drive units 100, 200, 300, rectifier 20 rectifies an ac signal and outputs a de signal across high rail 115 and low rail 125. Linear voltage regulator 220 is coupled to high rail 115 and outputs an output voltage on rail 225 that is regulated with respect to intermediate rail 120. Linear voltage regulator 420 is coupled to low rail 115 and outputs an output voltage on a rail 425 that is regulated with respect to intermediate rail 120. Linear voltage regulators 220, 420 use their respective outputs of rectifier 20 as a supply voltage and generate regulated output voltages.

[0036] Gate drive unit 400 also include buffer capacitors 410, 415. Buffer capacitor 410 is coupled between intermediate rail 120 and low rail 125. Buffer capacitor 415 is coupled between intermediate rail 120 and rail 425.

[0037] In operation, linear voltage regulator 220 stabilizes rail 225 relative to high rail 115 as the gate capacitance is charged, whereas linear voltage regulator 420 stabilizes rail 425 relative to low rail 125 as the gate capacitance is discharged. In particular, linear Attorney Docket No.: 36907-0039W01 voltage regulator 220 can charge buffer capacitor 205 during times when power switch 102 is in the OFF state whereas voltage regulator 420 can charge buffer capacitor 415 during times when power switch 102 is in the ON state.

[0038] The current that is sourced by linear voltage regulator 220 and sunk by linear voltage regulator 420 stabilizes the voltages that are used to bias the gate of IGBT power switch 102 into and out of conduction and allow the capacitance of buffer capacitors 205, 415 to be reduced.

[0039] In some implementations, the difference between the regulated voltages output from linear voltage regulators 220 / 420 will be slightly lower than the difference between the voltages on high rail 115 and low rail 125. For example, the difference between the regulated voltages output from linear voltage regulators 220 / 420 on rails 225, 425 may be between 4-8 volts, e.g., 6 volts lower than the difference between the voltages on high rail 115 and low rail 125.

[0040] For example, the difference between the voltage on rail 225 and intermediate rail 120 may be between 13 and 17 Volts, e.g., 15 Volts. The difference between the voltage on intermediate rail 120 and rail 425 and may be between 8 and 12 Volts, e.g., 10 Volts. Taking 25 Volts as the difference between the regulated voltages output from linear voltage regulators 220 / 420 on rails 225, 425, the difference between the voltages on high rail 115 and low rail 125 may be between 29 and 33, e.g., 31 Volts.

[0041] Further, gate drive unit 400 includes buffer capacitor 210 coupled between high rail 115 and intermediate rail 120 as well as a buffer capacitor 410 that coupled between intermediate rail 120 and low rail 125. Buffer capacitors 210, 410 act a source or sink of charge for linear voltage regulators 220, 420 and reduce the voltage swings on rails 115, 125 relative to intermediate rail 120 and relative to each other.

[0042] Other implementations of gate drive units that include a pair of linear voltage regulators coupled to high and low rails 115, 125 (akin to that gate drive unit 400) can include circuitry that defines the voltages output by those linear voltage regulators (voltages 225, 425 in gate drive unit 400) and the voltage on intermediate rail 120 relative to the voltages on high and low rails 115, 125. In some implementations, the circuitry is Attorney Docket No.: 36907-0039W01 passive circuitry. For example, a resistive voltage divider can be coupled between high rail 115, intermediate rail 120, and low rail 125 to define a relationship between the voltages on those rails. By defining this relationship, sufficient headroom for both of the linear voltage regulators can be ensured. As another example, active circuitry such as voltage regulator 130 can regulate the voltage difference between intermediate rail 120 and either or both of low rail 125 and high rail 115. Such active circuitry can also define a relationship between the voltages on those rails to ensure sufficient headroom for the linear voltage regulators.

[0043] In some implementations, linear voltage regulators 220 / 420 are configured to output a constant regulated output voltage. In other implementations, linear voltage regulators 220 / 420 are configured to output an adjustable, but regulated, output voltage. For example, in some implementations, linear voltage regulators 220 / 420 can be programmable to synchronously adjust the magnitude of the voltages on rails 225, 425 with the switching pattern of that power switch. Similarly, referring to gate drive units 200, 300 (FIGS. 2, 3), linear voltage regulator 220 and voltage regulator 130 can be configured to output adjustable, but regulated, output voltages that are synchronously adjusted with the switching pattern of that power switch.

[0044] FIG. 5 is a schematic representation of the switching pattern 500 of a power switch and a graph 505 of voltages in a gate drive unit in which the voltage on rail 225 is synchronized with the switching pattern 500. Graph 505 most accurately represents voltages in the context of gate drive unit 300 and, at least as far as the target level of the voltage on rail 225 is concerned, in gate drive units 200, 400 as well (assuming that linear voltage regulator 420 is configured to output a constant regulated output voltage). However, details regarding the voltages on the high and low rail will differ as a consequence of buffer capacitors 210, 410.

[0045] In all cases, switching pattern 500 defines an alternating series of ON times 510, 511 and OFF times 515, 516. Continuing to use an enhancement mode power switch as an example, ON times 510, 511 are represented as logic high periods of time and OFF times 515, 516 are represented as logic low periods of time. Attorney Docket No.: 36907-0039W01

[0046] Graph 505 includes an x-axis 520 and a y-axis 525. Position along x-axis 520 denotes time. Position along y-axis 525 denotes voltage. In graph 505, y-axis 525 extends from approximately 20 volts positive relative to a reference voltage to approximately 10 volts negative.

[0047] Graph 505 also includes four traces 527, 530, 535, 540. Trace 527 represents the voltage of intermediate rail 120, i.e., the voltage VE of the emitter of IGBT power switch 102. Trace 527 is coincident with x-axis 520 and the voltage VE of the emitter of IGBT power switch 102 acts as a reference against which other voltages are shown.

[0048] Trace 530 represents the voltage on high rail 115. Trace 540 represents the voltage on low rail 125. The voltage difference 545 between high rail 115 and low rail 125 reflects, e.g., the output voltage of rectifier 20, the voltage generated by a battery that biases high rail 115 and low rail 125, or other source.

[0049] Trace 550 represents the voltage on rail 225, i.e., the voltage that is output from linear voltage regulator 220 and used to bias power switch 102 into conduction. As shown, during OFF times 515, 516, linear voltage regulator 220 is configured to increase the voltage on rail 225 by an amount 555 above a target level 560. During the ON times 510, 511 of the power switch, trace 550 approaches target level 560 and remains at target level 560 due to the regulation by the linear voltage regulator 220. Target level 560 is selected relative to voltage VE of the emitter of IGBT power switch 102 to ensure consistent, safe, and fast switching of IGBT power switch 102.

[0050] During the increase of the voltage on rail 225, additional charge accumulates on buffer capacitor 205. When power switch 102 is switched into conduction (e.g., the transition between OFF time 515 and ON time 511), buffer capacitor 205 shares the charge with the gate of power switch 102 and the voltage on rail 225 drops to target level 560. However, because the voltage on rail 225 is starting from a higher value relative to the emitter voltage VE, it remains at a level that ensures consistent and fast switching of IGBT power switch 102.

[0051] Graph 505 also illustrates a point discussed above, namely,

[0052] -the voltage difference between the regulated voltage output from linear voltage regulator 220 and low rail 125 — i.e., the distance between traces 550, 540 — Attorney Docket No.: 36907-0039W01 is slightly lower than

[0053] -the voltage difference between high rail 115 and low rail 125 — i.e., the distance between traces 530, 540. This provides some headroom for linear voltage regulator 220 and allows linear voltage regulator 220 to operate more efficiently. However, when linear voltage regulator 220 increases the voltage on rail 225 during OFF times 516, 516, this headroom decreases and linear voltage regulator 220 become less efficient. This decrease in efficiency is however compensated by the stable voltage on rail 225 (trace 550) during ON times 510, 511.

[0054] In the illustrated implementation, the voltage difference between high rail 115 and low rail 125 is approximately 30 V However, the voltage difference between target level 560 and low rail 125 is approximately 25 V. This 5 volts difference is sufficient headroom for a low-dropout regulator 220 to operate efficiently. However, during the OFF times 515, 516, linear voltage regulator 220 increases the voltage on rail 225 by amount 555 of approximately 4 V The headroom for low-dropout regulator 220 has decreased to 1 volt.

[0055] In some implementations, the value of amount 555 is determined by empirically (e.g., by successive approximation) in a particular application. For example, amount 555 can be adjusted until the voltage on rail 225 (trace 550) has an acceptable value after the power switch is switched into the ON state. The voltage on rail 225 (trace 550) can be measured at a settable delay after the power switch is switched into the ON state.

[0056] For the sake of completeness, please note that trace 540 schematically represents changes in the voltage that is sinking charge from the gate during transitions from ON times 510, 511 to OFF times 515, 516. In particular, trace 540 rises toward the trace 527, i.e., the voltage on low rail 125 rises toward the voltage VE of intermediate rail 120 and the emitter of IGBT power switch 102. This results in a decrease 565 in the difference between the voltage on low rail 125 (trace 540) and the voltage VE of intermediate rail 120 (i.e., x-axis 520). If decrease 565 were to be too large, then turn-off of IGBT power switch 102 could be impaired.

[0057] FIG. 6 is a schematic representation of the switching pattern 600 of a power switch, a graph 605 of voltages in a gate drive unit in which the voltage between high and Attorney Docket No.: 36907-0039W01 low rails 115, 125 is within an expected operating range, a standby (non-switching) pattern 602 of a power switch when the voltage between high and low rails 115, 125 has fallen below the expected operating range, and a graph 607 of voltages in the gate drive unit in this standby (non-switching) state. As an aside, please note that, in graph 605, the voltage on rail 225 is not synchronized with switching pattern 600. However, in other implementations, voltage on rail 225 is synchronized with switching pattern 600. The prioritization schemes described below can be used in either case when the voltage between high and low rails 115, 125 has fallen below the expected operating range.

[0058] In any case, the voltage between high and low rails 115 may be below the expected operating range for any of a number of different reasons. For example, the voltage that is input into transformer 25 may fall. As another example, in implementations where the gate drive unit is powered by a battery (rather than rectifier 20 and transformer 25), the battery may be partially discharged. Graphs 605, 607 can represent the voltages in the context of any of gate drive units 100, 200, 300, 400.

[0059] Switching pattern 600 defines an alternating series of ON times 610, 611 and OFF times 615, 616. ON times 610, 611 are represented as logic high periods of time and OFF times 615, 616 are represented as logic low periods of time. In contrast, in standby (non-switching) pattern 602, the power switch is maintained in the OFF state during the standby period 617.

[0060] Graphs 605, 607 each includes a respective x-axis 620 and a y-axis 625. Position along x-axis 620 denotes time. Position along y-axis 625 denotes voltage. In graphs 605, 607, y-axis 625 extends from approximately 20 volts positive relative to a reference voltage to approximately 10 volts negative.

[0061] Graphs 605, 607 each includes four traces 627, 630, 640, 650. Traces 627 represent the voltage of intermediate rail 120, i.e., the voltage VE of the emitter of IGBT power switch 102. Traces 627 are coincident with x-axes 620 and the voltage VE of the emitter of IGBT power switch 102 acts as a reference against which other voltages are shown.

[0062] Traces 630 represent the voltage on high rail 115. Traces 640 represent the voltage on low rail 125. The voltage differences 645 between high rail 115 and low rail Attorney Docket No.: 36907-0039W01

[0063] 125 reflect, e.g., the output voltage of rectifier 20, the voltage generated by a battery that biases high rail 115 and low rail 125, or other source. Trace 650 represents the voltage on rail 225, i.e., the voltage that is output from linear voltage regulator 220 and used to bias power switch 102 into conduction.

[0064] In graph 605, voltage difference 645 between high rail 115 and low rail 125 is within the expected operating range. In the illustrated implementation, voltage difference 645 is approximately 30 V. The voltage on rail 225 and represented in trace 650 is prioritized in that a voltage difference 647 between rail 225 and intermediate rail 120 is maintained at the expense of other voltage. To achieve this prioritization, regulator 130 can maintain the voltage difference between rail 225 (trace 650) and intermediate rail 120 (trace 627) even if the voltage on high rail 115 (trace 630) falls. Alternatively, linear voltage regulator 220 can receive a signal representative of voltage difference 647 and regulate the voltage difference between high rail 115 (trace 630) and rail 225 (trace 650) to a desired value. In the illustrated implementation, voltage difference 647 is approximately 17 V. In turn, the linear voltage regulator 220 or voltage regulator 130 that is not responsible for prioritizing the voltage on rail 225 and represented in trace 650 maintains a voltage difference 649 between intermediate rail 120 and either low rail 125 or rail 425. In the illustrated implementation, voltage difference 649 is approximately 10 V

[0065] In graph 607, voltage difference 645 between high rail 115 and low rail 125 has fallen below the expected operating range. In the illustrated circumstance, voltage difference 645 is approximately 22 V and the voltage on high rail 115 (trace 630) is approximately equal to the voltage on rail 225 (trace 650). Regulation of the voltage difference 647 between rail 225 and intermediate rail 120 is deprioritized relative to regulation of the voltage difference 649 between intermediate rail 120 and low rail 125.

[0066] In the context of gate drive units 200, 300, regulator 130 responds to an undervoltage signal indicating that the voltage difference 645 between high rail 115 and low rail 125 has fallen below the expected operating range by sourcing current into intermediate rail 120 until the voltage difference 649 between intermediate rail 120 and Attorney Docket No.: 36907-0039W01 low rail 125 is at a desired value. However, one consequence of this is that voltage difference 647 between high rail 125 and intermediate rail 120 falls.

[0067] Further, the voltage difference 645 between high rail 115 and low rail 125 acts as the input voltage of regulator 220. As voltage difference 645 falls, so does the input voltage-to-output voltage differential of regulator 220. As regulator 220 enters its dropout region, regulator 220 stops regulating the voltage difference 647 between rail 225 and intermediate rail 120. As a result, voltage difference 649 between intermediate rail 120 and low rail 125 remains stable but voltage difference 647 between rail 225 and intermediate rail 120 falls. With voltage difference 649 between intermediate rail 120 and low rail 125 stable, it is assured that a sufficiently large voltage to switch the power switch into the OFF state is present. In the illustrated implementation, voltage difference 647 is approximately 12 V whereas voltage difference 649 remains approximately 10 V

[0068] In the context of gate drive unit 400, deprioritization of the regulation of the voltage difference 647 between rail 225 and intermediate rail 120 relative to regulation of the voltage difference 649 between intermediate rail 120 and low rail 125 can be implemented by control of linear voltage regulators 220, 420. For example, the reference voltage to which voltage regulator 220 regulates voltage difference 647 between rail 225 and intermediate rail 120 can be lowered in response to voltage difference 645 between high rail 115 and low rail 125 falling below the expected operating range. In some implementations, the reference voltage of voltage regulator 220 can be a filtered and scaled version of voltage difference 645 itself. As another example, voltage regulator 220 can be turned off in response to, e.g., voltage difference 645 between high rail 115 and low rail 125 falling below the expected operating range or voltage difference 649 between intermediate rail 120 and low rail 125 falling below a threshold level.

[0069] In any case, by changing the priority of the voltage regulation, it is assured that the power switch can be switched into the OFF state, e.g., even in the face of voltage transients. Further, when the voltage between high and low rails 115, 125 returns to the expected operating range, the priority can again be shifted and switching can resume.

[0070] FIG. 7 schematically represents a prioritization scheme that is responsive to voltage difference 645 between high rail 115 and low rail 125 falling below the expected Attorney Docket No.: 36907-0039W01 operating range. The illustrated prioritization scheme includes a negative threshold voltage 705 for the voltage on low rail 125. For ease of illustration, only traces 617, 630, 640 are shown and they are illustrated without any deviations from nominal set points. However, linear voltage regulator 220 outputs a voltage, notwithstanding the omission of trace 650, and traces 630, 640 may deviate from their nominal set points as they source or sink current during ON / OFF transitions.

[0071] The prioritization scheme is schematically represented in four different states 710, 715, 7120, 725. In state 710, voltage difference 645 between high rail 115 and low rail 125 is within the expected operating range. Further, in state 710, the voltage on low rail 125 represented by trace 640 is negative with respect to negative threshold voltage 705. Negative threshold voltage 705 is a negative voltage at which the switching of the power switch into the OFF state can be assured.

[0072] In state 715, voltage difference 645 between high rail 115 and low rail 125 is still within the expected operating range but has begun to decrease. The decrease in voltage difference 645 is reflected by trace 640 — which represents the voltage on low rail 125 — rising toward the negative threshold voltage 705. Regulation of the voltage on high rail 115 is prioritized and the voltage difference between high rail 115 and intermediate rail 120 (as represented by the distance between traces 630, 627) remains more stable.

[0073] In state 720, voltage difference 645 between high rail 115 and low rail 125 has decreased still further. The voltage on low rail 125 has risen to the level of negative threshold voltage 705 and then stopped. In response to the voltage on low rail 125 rising to the level of negative threshold voltage 705, regulation of the voltage on low rail 125 is prioritized such that voltage on low rail 125 is maintained at the level of negative threshold voltage 705. The switching of the power switch into the OFF state using the voltage on low rail 125 can be assured.

[0074] Also, regulation of the voltage on high rail 115 is deprioritized and the voltage difference between high rail 115 and intermediate rail 120 (as represented by the distance between traces 630, 627) begins to decrease. Depending on the extent of the decrease, voltage difference 645 between high and low rails 115, 125 may or may not be within the Attorney Docket No.: 36907-0039W01 operating range and the power switch may or may not be switched in the illustrated state 720.

[0075] In state 725, voltage difference 645 between high rail 115 and low rail 125 has decreased still further. Neither the voltage on high rail 115 nor the voltage on low rail 125 can be effectively regulated and even the prioritization of the voltage on low rail 125 fails to maintain that voltage negative with respect to negative threshold voltage 705. Voltage difference 645 between high and low rails 115, 125 is well outside the operating range and the power switch is not switched in state 725.

[0076] FIG. 8 is a schematic representation of an input stage 800 to a gate drive unit, input stage 800 can couple to transformer 25 of any of gate drive units 100, 200, 300, 400 and provide power to the gate drive unit.

[0077] Input stage 800 includes an inverter 705 and control electronics 810. Inverter 805 is configured to receive a de input on rails 815, 820 and generate an ac output. For example, inverter 705 can be either a half-bridge inverter or full-bridge inverter. The ac output is coupled to the input winding of transformer 25. In some implementations, input stage 800, transformer 25, and rectifier 20 interoperate as an LLC converter, with input stage 800 including power switches and a resonant tank. Input stage 800 can be activated or deactivated by a signal on an input 825. For example, in implementations where input stage 800 includes power switches that are switched to invert the de input across rails 815, 820, the switching of the power switches can be activated by a signal on input 825.

[0078] Control electronics 810 is also coupled to rails 815, 820 and configured to determine whether the voltage across rails 815, 820 is sufficiently high for operation. For example, the voltage across rails 815, 820 may be unduly low at start up or after a reset upon occurrence of a fault. In the illustrated implementation, control electronics 810 includes a comparator 830 that is configured to compare the voltage across rails 815, 820 to a threshold level. In response to the voltage across rails 815, 820 exceeding the threshold level, control electronics 810 outputs a signal on input 825 activating inverter 805 and / or other components that are powered by inverter 805. In response to the voltage across rails 815, 820 being below the threshold level, control electronics 810 can either Attorney Docket No.: 36907-0039W01 refrain from activating inverter 805 and / or the other components or actively de-activate inverter 805 and / or the other components.

[0079] The logic implemented by control electronics 810 can be much more complex than indicated. For example, in some implementations, control electronics 810 can compare the voltage across rails 815, 820 with two thresholds and introduce hysteresis into the activation / deactivation. For example, at start up or after reset, control electronics 810 can compare the voltage across rails 815, 820 to a relatively higher threshold and latch inverter 805 and / or the other components in the active state. However, once inverter 805 and / or the other components are already active, control electronics 810 can compare the voltage across rails 815, 820 to a relatively lower threshold and reset inverter 805 and / or the other components in the inactive state in response to the voltage across rails 815, 820 falling below that relatively lower threshold.

[0080] As another example, in some implementations, control electronics 810 can output more than one activation signal and activate input stage 800 and other components in series, over time. For example, control electronics 810 may output a first signal to activate inverter 805 when the voltage across rails 815, 820 has risen above a first threshold. Anticipating that the voltage across rails 815, 820 may rise more slowly or even fall after inverter 805 is first activated, control electronics 810 may later output a second signal to activate other components (e.g., rectifier 20, gate drive units 100, 200, 300, 400) after a suitable delay or after the voltage across rails 815, 820 (re)crosses a threshold. Control electronics 810 can thus define the operational range of the voltage difference between high rail 115 and low rail 125 and trigger responses thereto.

[0081] In other implementations, activation functionality and the definition of the operational range of the voltage difference between high rail 115 and low rail 125 is distributed amongst control electronics on both sides of transformer 25. For example, in some implementations, control electronics 810 is responsible for activating circuitry that is coupled to the primary side of transformer 25 (e.g., inverter 805), whereas additional control electronics are responsible for activating circuitry on the secondary side of transformer 25. For example, in some implementations, secondary-side control electronics can compare voltages on the input of rectifier 20 to determine whether to Attorney Docket No.: 36907-0039W01 activate switching of power switches in rectifier 20. As another example, in some implementations, secondary-side control electronics can compare voltage on the output of rectifier 20 (i.e., the voltage across rails 115, 125) to determine whether to activate regulator 130, linear voltage regulator 220, and / or linear voltage regulator 420. In the illustrated implementation, an optional dc / dc converter 850 provides the voltage across rails 815, 820. For example, in some implementations, dc / dc converter 850 may be a buck converter.

[0082] A number of implementations have been described. Nevertheless, it will be understood that various modifications may be made. For example, a linear voltage regulator on low rail 125 can be used in conjunction with regulator 130. As another example, the output voltage of linear voltage regulator 420 can also be synchronized with a switching pattern. Enhancement mode power switches can be used. The power switches can be external to the gate drive unit, e.g., the power switches can be separately packaged as stand-alone devices. As another example, in some implementations, the reference voltage for linear voltage regulator 220 can be can be provided by intermediate rail 120 rather than low rail 125. Accordingly, other implementations are within the scope of the following claims.

Claims

Attorney Docket No.: 36907-0039W01WHAT IS CLAIMED IS:

1. A gate drive unit comprising: a high rail and a low rail configured to be coupled to input de voltage; an intermediate rail configured to be coupled to a main terminal of a power switch; a linear voltage regulator coupled to either the high rail or the low rail, wherein the linear voltage regulator is configured to output a regulated voltage; a buffer capacitor coupled between the output of the linear voltage regulator and the intermediate rail; and a switch that is configured to couple the output of the linear voltage regulator to a control terminal of the power switch, thereby driving the power switch into or out of conduction.

2. The gate drive unit of claim 1, wherein the linear voltage regulator is programmable to synchronously adjust a magnitude of the regulated output voltage with a switching pattern of the power switch.

3. The gate drive unit of any preceding claim, wherein the linear voltage regulator is a low dropout regulator with an open collector or open drain topology.

4. The gate drive unit of any preceding claim, further comprising a voltage regulator coupled to regulate a voltage difference between the intermediate rail and at least one of either the high rail or the low rail.

5. The gate drive unit of claim 4, wherein the voltage regulator is programmable to synchronously adjust the regulated voltage difference between the intermediate rail and at least one of either the high rail or the low rail with a switching pattern of the power switch.

6. The gate drive unit of any preceding claim, further comprising: a second linear voltage regulator coupled to the other of the high rail or the low rail, wherein the second linear voltage regulator is configured to output a voltage that is regulated with respect to the voltage on the intermediate rail; andAttorney Docket No.: 36907-0039W01 a second buffer capacitor coupled between the output of the second linear voltage regulator and the intermediate rail; and a second switch that is configured to couple the output of the second linear voltage regulator to the control terminal of the power switch.

7. The gate drive unit of any preceding claim, further comprising: a rectifier having outputs coupled to the high rail and to the low rail; and a transformer coupled to inputs of the rectifier.

8. The gate drive unit of claim 7, further comprising: an inverter coupled to the transformer, wherein the inverter, the transformer, and the rectifier are configured to interoperate as an LLC converter.

9. The gate drive unit of claim 8, further comprising: a controller coupled to an input of the LLC converter and configured to compare a voltage on the input with a threshold, wherein the controller is configured to activate the LLC converter in response to the voltage on the input exceeding the threshold.

10. The gate drive unit of any preceding claim, further comprising a voltage divider coupled to the high rail, the intermediate rail, and the low rail.

11. The gate drive unit of any preceding claim, further comprising a third buffer capacitor coupled between the low rail and the intermediate rail.

12. A power converter comprising: the gate drive unit of any preceding claim; and the power switch, wherein the main terminal of the power switch is coupled to the intermediate rail of the gate drive unit.

13. A method of driving a power switch, the method comprising: supplying a power switch controller with a de supply voltage across a high rail and a low rail; coupling a control terminal of the power switch to another rail of the power switch controller to bias a control terminal of the power switch relative to a main terminal of the power switch and thereby switch the power switch either into or out of conduction,Attorney Docket No.: 36907-0039W01 wherein the other rail is coupled to the main terminal of the power switch by a buffer capacitor and a linear voltage regulator is coupled to either the high rail or the low rail and configured to output a regulated voltage on the other rail, wherein the regulated voltage is intermediate between the voltage on the high rail and the voltage on the low rail.

14. The method of claim 13, further comprising synchronously adjusting a magnitude of the regulated voltage with a switching pattern of the power switch.

15. The method of claim 13 or 14, wherein the linear voltage regulator is a low dropout regulator with an open collector or open drain topology.

16. The method of any one of claims 13 to 15, further comprising regulating a voltage difference between the main terminal of the power switch and at least one of either the high rail or the low rail.

17. The method of any one of claims 13 to 16, further comprising coupling the control terminal of the power switch to a fourth rail to bias the control terminal of the power switch relative to the main terminal and thereby switch the power switch either into or out of conduction, wherein the fourth rail is coupled to the main terminal of the power switch by a second buffer capacitor and a second linear voltage regulator is coupled to either the high rail or the low rail and configured to output a second regulated voltage on the fourth rail, wherein the regulated voltage is intermediate between the voltage on the high rail and the voltage on the low rail.

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