Circuits containing high-power transistors

Abstract

Bipolar junction transistors (BJTs) operating at high voltages typically include a drift region to prevent breakdown. However, the drift region slows down the transistor's switching time, leading to inefficiency because the BJT consumes most of its power during switching. Another reason for inefficiency is the circuitry driving the BJT, which constantly draws current to keep the BJT on and off. A circuit with a controller is described here, adapted to operate the BJT's drive circuitry such that, during each "on-off" switching cycle of the BJT, the output of the drive circuitry is in a floating state for a longer period than it is in the "on" or "off" state. In the floating state, the drive circuitry does not consume power, thus improving power efficiency. The circuitry may include a bypass connection between the base and collector terminals of the BJT. When the transistor is in the "on" state, selecting an appropriate resistor for the bypass connection decays the charge in the drift region, reducing the BJT's switching time.

Description

Background Art

[0001] Bipolar junction transistors (BJTs) are used in high voltage applications, such as drive circuits for alternating current (AC) motors, and require relatively lightly doped collector and base regions to prevent breakdown. The problem caused by light doping is high collector resistance, which reduces the efficiency of the transistor. A known solution is to add a drift region to the transistor.

[0002] The drift region is a weakly doped region of the collector that is connected to the base region, providing one of the transistor's diode junctions. When the transistor is off, the drift region offers a high resistance to current flow; however, when the transistor is on, electrons saturate the drift region, which reduces the collector's resistance.

[0003] The negative effect of the presence of a drift region is that it slows down the switching time of the transistor. This is because to turn the transistor on, the drift region needs to be charged / saturated, and to turn the transistor off, the charge needs to be dissipated. Slower switching times introduce greater inefficiencies because a BJT consumes more power when switching than when it is either on or off.

[0004] Additionally, there are various applications that require fast switching. AC motors operate most efficiently when provided with an ideal sine wave; the motor speed is determined by the frequency of the sine wave driving it. Motor drives attempt to emulate a sine wave through the rapid switching of their transistors. Higher switching frequencies increase the accuracy of the sine wave approximation, especially as the motor speed increases.

[0005] Another source of inefficiency is that the drive circuit used to switch the BJT essentially constantly draws current to keep the BJT on and off. These inefficiencies can be avoided by using insulated gate bipolar transistors. GB 2 448 759A, US 4 725 741, US 4 239 989 and US 2014 / 355314 each describe a drive circuit for a transistor operating in a floating state. Summary of the invention

[0006] According to the first aspect of the present invention, a circuit is proposed, which includes: a bipolar junction transistor (BJT) having a collector drift region; a controller; a drive circuit having an input suitable for receiving a control signal from the controller, and an output connected to the base terminal of the BJT; the drive circuit is suitable for switching states at the output in response to the received control signal: an "on" state that switches the transistor to an "on" state; an "off" state that switches the transistor to an "off" state; and a floating state; and the controller is suitable for operating the drive circuit so that during the "on-off" switching cycle of the BJT, the output of the drive circuit is in a floating state for a longer time than it is in an "on" state and / or an "off" state.

[0007] Due to the drift region, the BJT will maintain its "on" state after switching from the "on" state to the floating state at the output of the driving circuit.

[0008] Therefore, the driver circuit only needs to provide short pulses to turn on the BJT. This reduces the time that current flows through the driver circuit during each "on-off" cycle (switching cycle), thereby reducing the power consumption of the driver circuit.

[0009] Additionally, when the driver circuit output is floating and the BJT is turned on, the charge in the drift region will slowly dissipate. This reduces the current required to turn the transistor off.

[0010] Typically, an "on-off" cycle will include the following transitions in the driver circuit output state: from "on" to "floating", from "floating" to "off" (when the BJT is off), from "off" to "floating", and from "floating" to "on" (when the BJT is on).

[0011] Advantageously, the circuit includes a bypass connection between the base terminal of the BJT and the emitter terminal of the BJT. The bypass connection includes a load having a resistance sized such that during the "on-off" cycle when the BJT is "on" and the driver signal is "floating", current will discharge from the base, causing the voltage at the base to approach the voltage at the emitter, at a sufficient rate to keep the BJT "on" until the drive circuit output switches "off". The presence of the bypass circuit accelerates the decay of charge in the drift region when the transistor is on and the drive circuit output is in a floating state. This allows a reduced current to turn the transistor off at a given switching speed. In order to minimize the current required to turn the transistor off, the resistor is selected to reduce the base voltage to a value such that the transistor is just turned on at the end of the "floating" period, although operation under such extreme conditions may not be preferred due to less than preferred overall power consumption characteristics.

[0012] Furthermore, because the bypass connection causes the base voltage to be closer to the emitter voltage than the collector voltage, when the BJT is "off" and the drive circuit output is "floating", it is ensured that there is enough voltage across the reverse biased base-collector diode junction to suppress the reduction of the depletion region at the drift region / base region junction. This means that the BJT can maintain its high breakdown voltage even when the output of the drive circuit is floating. This, in turn, means that the drive circuit can be controlled by the controller so that the BJT switches from the off state to the floating state as soon as it is turned off, further reducing the power consumption of the drive circuit. In other words, the drive circuit only needs to provide a short current pulse to turn off the BJT.

[0013] Therefore, the drive circuit only needs to draw current for a very short time in a complete "on-off" cycle, for example ≤50% of the time (≤50% duty cycle), but preferably ≤5% of the time (≤5% duty cycle), and remain in a floating state without drawing current for the rest of the time.

[0014] For example, the present invention can be used to implement a motor driver for a multi-phase motor. Thus, the motor driver may include: a switching circuit including a plurality of bipolar junction transistors (BJTs) for switching current through a multi-phase motor winding, each BJT including a collector drift region; a separate drive circuit associated with each BJT, each drive circuit including an input adapted to receive a control signal from a controller, and an output connected to a base terminal of its corresponding BJT; the drive circuit is adapted to switch states at its output in response to the received control signal: an "on" state switching the transistor to an "on" state; an "off" state switching the transistor to an "off" state; and a floating state; and the controller is adapted to operate the separate drive circuit so that within a complete "on-off" switching cycle of each BJT, the output of the drive circuit is in a floating state for a longer time than it is in an "on" state and / or an "off" state. BRIEF DESCRIPTION OF THE DRAWINGS

[0015] The present invention is described below by way of examples with reference to the following drawings, in which:

[0016] Figure 1A is a schematic diagram of a control circuit including a power bipolar junction transistor (BJT) for controlling current in an external circuit;

[0017] Figure 1B is a schematic diagram of a discrete power BJT including a drift region;

[0018] Figure 2 It is a timing diagram of the BJT state changing with the output state of the driving circuit;

[0019] Figure 3 is a circuit schematic diagram detailing an example drive circuit structure in simplified form;

[0020] Figure 4A It is shown based on Figure 3 The circuit schematic diagram of the actual implementation of the driving circuit;

[0021] Figure 4B It is shown in Figure 4A A timing diagram showing the modeled changes in voltage and current at various points in the circuit as it operates; and

[0022] Figure 5 It is a circuit schematic diagram for driving a multi-phase AC motor. DETAILED DESCRIPTION

[0023] Referring to FIG. 1 , there is shown a control circuit 1 for controlling current flowing through an external circuit 2 (shown conceptually).

[0024] The control circuit 1 includes a power bipolar junction transistor (BJT) 10 , a drive circuit 20 , a controller 30 , and a bypass resistor 40 .

[0025] The collector C and emitter E terminals of the BJT 10 are connected to the external circuit 2. The bypass resistor 40 is connected between the emitter E terminal and the base B terminal of the BJT 10, providing a bypass connection between the base B terminal and the emitter E terminal of the BJT 10.

[0026] The external circuit may operate at a high voltage, for example ≥600V, and the current flowing through the collector C may be ≥32A.

[0027] refer to Figure 1B , BJT 10 includes a collector region 11, a base region 12 and an emitter region 13; the base region 12 is interposed between the collector region 11 and the emitter region 13. In order to handle the high voltages experienced within the external circuit 2 without breakdown, BJT 10 includes a drift region 11A; a weakly doped region of semiconductor material that forms part of the collector region 11. The collector region 11 also includes a strongly doped region 11B of the same type connected to the collector terminal C.

[0028] In the example shown, the BJT 10 has an NPN structure, so the drift region 11A and the heavily doped region 11B are both N-type. The drift region is directly connected to the base region 12 , providing one of the diode junctions 14 of the BJT 10 .

[0029] For ease of explanation, Figure 1B BJT 10 is shown as a discrete component. However, it should be understood that transistor 10 may be formed in a semiconductor die (e.g., as a vertical transistor) with circuitry providing drive circuit 20 and / or controller 30 and / or circuitry for controlling one or more other BJTs for other external circuits or circuit segments, for example.

[0030] Refer to Figure 1A The controller 30 includes an output 30A. The drive circuit 20 includes an input 20A connected to the controller output 30A and an output 20A connected to the base terminal B of the BJT 10.

[0031] The controller 30 is adapted to generate a first control signal at an output 30A for receipt at an input 20A of the driver circuit 20. The driver circuit 20 is adapted to respond to the received control signal by causing a tri-state driver signal at its output 20B to change between an "on" state (e.g., high), an "off" state (e.g., low (grounded), and a "floating" state (high impedance) to control the current between the emitter E and collector C terminals of the BJT 10.

[0032] The resistance value of the bypass resistor 40 is as follows:

[0033] When the driver signal is "on", the current flowing through the bypass resistor 40 is less than the current at the output 20B of the driver 20, thereby not inhibiting the BJT 10 from turning on; and

[0034] When BJT 10 is "on" and the driver signal is "floating", current will discharge from the base, causing the base voltage to approach the emitter voltage, at a rate slow enough to keep the BJT

[0035] "ON" until the driver signal switches to "OFF".

[0036] The controller 30 can be implemented by an external microcontroller. Alternatively, the circuit of the controller 30 can be integrated in the same semiconductor die that forms the BJT 10 and / or the driver circuit 20. In the latter case, the circuit may include one or more counters, subtractors, and comparators. For example, the subtractor can calculate the time required for each of the three states output by the driver 20, and the comparator can be used to compare the target time with the current time period. These can be implemented by digital or analog circuits.

[0037] In a preferred embodiment, the entire control circuit 1 can be provided by a single discrete semiconductor device. In this arrangement, the transistor 10, the driver 20, the controller 30 and the bypass resistor will all be integrated on a single semiconductor die. A description of how to achieve this can be found in WO2019 / 155239A1, which is incorporated herein by reference in its entirety.

[0038] In one embodiment, controller 30 may be configured to generate a tri-state logic signal at its output 30A, for example in one of three states: high (Vcc), low (ground), and high impedance (floating).

[0039] The controller 30 may be adapted to receive a binary signal or a periodic pulse when the BJT 10 needs to be turned on or off.

[0040] Figure 2 is a timing diagram showing two complete “on-off” switching cycles of the BJT 10 , showing how the drive circuit 20 clocks the tri-state control signal to switch the BJT 10 “on” and “off” under the control of the controller 30 .

[0041] The top line shows the changing state of the BJT 10 between "ON" and "OFF" during a switching cycle. The bottom line shows the changing state of the driver signal between "ON", "OFF" and "Floating".

[0042] At the beginning of the cycle (W), the driver signal switches from "floating" to "on", switching the BJT 10 "on" to allow current to flow through the external circuit 2. Shortly after, the driver signal switches back to "floating" (X). However, since the drift region 11A is saturated, the BJT 10 remains "on".

[0043] The charge in the drift region 11A is slowly depleted through the base B and the bypass resistor 40. At (Y), the driver signal switches to "off", causing the BJT 10 to switch to "off". Because the charge in the drift region 11A has been depleted between (X) and (Y), the time required to establish a depletion region around the diode junction 14 is reduced.

[0044] After a short while, the driver signal switches back to "floating" (Z); the BTJ 10 remains "off". The bypass resistor 40 provides a path for leakage current to flow from the collector 11 out of the base 12, ensuring that the breakdown voltage Vce of the BJT 10 is maintained. Without the bypass resistor 40, the voltage at the base would rise toward the voltage at the collector, thereby reducing the voltage across the collector and base, and therefore reducing the size of the depletion region around the reverse biased diode junction 14.

[0045] The controller 30 is adapted to control the timing of the switching between the three states so that the duration WX is shorter than the duration XY, and the duration YZ is shorter than the duration ZW. Thus, the duration within the switching cycle that the output of the driver circuit 20 is "floating" is longer than the combined time that the output is "on" and "off". WX may be less than 5% of the duration XY, and YZ may be less than 5% of the duration ZW. The sum of WX and YZ, i.e., the total duration that the driver output is "on" or "off", is less than 5% of the complete "on-off" cycle WW of the transistor 10 (<5% duty cycle). In this way, the losses within the driver circuit 20 are significantly reduced.

[0046] Figure 3 An example drive circuit 20 arrangement is shown. The drive circuit 20 comprises complementary first and second Darlington pairs 201 , 202 connected to base terminals of respective high-side transistors 203 and low-side transistors 204 .

[0047] The controller 30 is adapted to provide at its output 30A a three-state control signal “ON”, “OFF” or “FLOATING”, which control signal is visible at the input 20A of the driver circuit 20 .

[0048] When the control signal is "ON", the first Darlington pair 201 is turned on, thereby turning on the high-side transistor 203. Current flows into the base B of the BJT 10, switching the BJT 10 "ON". The second Darlington pair 202 and the low-side transistor 204 remain turned off.

[0049] When the control signal is "off", the first Darlington pair 201 is turned off, thereby turning off the high-side transistor 203. The second Darlington pair 202 is switched "on", thereby turning on the low-side transistor 204. The current flowing into the base B stops. Any remaining charge in the collector 11A flows out of the base terminal B, and the BJT 10 is switched "off".

[0050] When the control signal is "floating", both high-side and low-side transistors 203 204 are turned off, leaving output 20B in a floating (high impedance) state.

[0051] The Darlington pair 201 , 202 is used to increase the current through the base of the BJT 10 to compensate for the low gain of the BJT 10 - this is a result of the relatively large and low doped base region 12 of the BJT 10 , which, in combination with the collector drift region 11A, provides the BJT 10 with a high breakdown voltage.

[0052] The first Darlington pair 201 includes an NPN transistor and drives a PNP high-side transistor 204, while the second Darlington pair 202 includes a PNP transistor and drives an NPN low-side transistor 204. This arrangement balances the gain between the two complementary sides.

[0053] Figure 4A shows a driving circuit 20', which is Figure 3 The modeling of the driving circuit 20 is actually realized.

[0054] Figure 4B shows that during approximately four and a half switching cycles of the BJT 10', Figure 4A Model voltage and current at various points in a circuit.

[0055] The first top line shows the voltage variation at the output 20B' of the driving circuit 20'. The second top line shows the current variation through the base terminal of the BJT 10'. The second bottom line shows the voltage Vce variation between the collector and emitter of the BJT 10', and the first bottom line shows the current variation through the collector C of the BJT 10'.

[0056] Lines W', X', Y' and Z' show Figure 2 . The second line from the top shows a short pulse of current into the base terminal B of the BJT 10' after the high-side transistor 203' is turned on, but before both the high-side and low-side transistors 203', 204' are turned off at X'. Subsequently, when the low-side transistor 204' is turned on, the current through the collector C gradually decreases, and Vce increases accordingly, until Y'. There is then a small reversal in the current flowing out of the base terminal B of the BJT 10' due to the discharge of the collector drift region 11A of the BJT 10'.

[0057] Figure 5 is a simplified diagram showing application of the above control circuit in a motor driver 1000 for a multi-phase AC motor 500 .

[0058] The driver 1000 controls the switching of current through a plurality of windings 500A (six in this example) of the motor 500. The motor driver 1000 includes a switching circuit 400 including a separate BJT 100 associated with each winding 500A (only three are shown in the figure) to switch the current through it. Each BJT 100 is controlled by a separate drive circuit 200 (again only three are shown in the figure). Each drive circuit 200 has an input arranged to receive a different three-state control signal from the controller 300. Each drive circuit 200 uses the control signal to control the state of their respective BJT 100. Although not shown, a separate bypass resistor is connected between the base terminal and the emitter terminal of each BJT 100 in the manner described above.

[0059] Therefore, the circuit offers an alternative to IGBTs and SiC MOSFETs in applications where high voltages need to be switched.

[0060] exist Figure 5 In a variation of , a separate controller 300 may be associated with each transistor 100. In this case, the transistor 100 with the corresponding driver 200 and controller 300 may be integrated in a single semiconductor die and may therefore be provided by a four-pin discrete semiconductor device. Two pins of the device are assigned to the emitter and collector of the BJT 100, the third pin is assigned to the base of the BJT 10 via the driver 200, and the fourth pin is assigned to the input of the controller 300 to receive a selectable two-state input signal for indicating when to turn the BJT 100 on and off.

[0061] The above control circuit may comprise a PNP type BJT (mutatis mutandis) instead of an NPN BJT.

[0062] Although less preferred because it would compromise the breakdown voltage of the transistor, the bypass resistor 40 may be omitted.

Claims

1. A circuit (1), comprising: A bipolar junction transistor BJT (10) having a collector drift region (11A); Controller (30); A drive circuit (20) having an input (20A) adapted to receive a control signal from a controller (30), and an output (20B) connected to a base terminal (B) of the BJT (10); the drive circuit (20) being adapted to switch states at the output (20B) in response to the received control signal: Switching the transistor (10) to an “on” state; Switching the transistor (10) to an "off" state; and floating state; And it is characterized in that the controller (30) is suitable for operating the driving circuit (20) so that during the "on-off" switching cycle of the BJT (10), the output of the driving circuit (20) is in a floating state for a longer time than it is in the "on" state and the "off" state.

2. A circuit (1) according to claim 1, comprising a bypass connection between the base terminal (B) of the BJT (10) and the emitter terminal (E) of the BJT (10), the bypass connection comprising a load (40) of sufficiently large resistance so that, in operation, the current through the bypass connection when the BJT (10) is "off" is less than the current through the base terminal (B) when the BJT (10) is "on".

3. The circuit (1) according to claim 1 or 2, wherein the controller (30) is adapted to operate the drive circuit (20) so that the time for which the output (20B) of the drive circuit (20) is in a floating state is equal to or greater than 50% of a complete "on-off" cycle of the BJT (10).

4. The circuit (1) according to claim 3, wherein the controller (30) is adapted to operate the driver circuit (20) so that the output (20B) of the driver circuit (20) is in a floating state for a time equal to or greater than 95% of a complete "on-off" cycle of the BJT (10).

5. A circuit (1) according to any one of claims 1-2 and 4, wherein the "on-off" switching cycle includes the following transitions in the state of the output (20B) of the driver circuit: from "on" to "floating", from "floating" to "off", from "off" to "floating" and from "floating" to "on".

6. A circuit (1) according to claim 3, wherein the "on-off" switching cycle includes the following transitions in the state of the output (20B) of the driver circuit: from "on" to "floating", from "floating" to "off", from "off" to "floating" and from "floating" to "on".

7. A motor driver (1000) for a multi-phase motor (500), comprising: A circuit according to any one of claims 1 to 5, wherein the circuit comprises a plurality of bipolar junction transistors (BJTs) (100) for switching current flowing through windings (500A) of a multi-phase motor (500); A separate driver circuit (200) associated with each BJT (100), each driver circuit (200) comprising an input adapted to receive a control signal from the controller (300), and an output connected to the base terminal of its respective BJT (100); each driver circuit (200) being adapted to switch state at its output in response to the received control signal: Switch the transistor to the "on" state; Switch the transistor to the "off" state; and floating state; And the controller (300) is suitable for operating the individual drive circuits (200) so that within a complete "on-off" switching cycle of each BJT (100), the output of each drive circuit (200) is in a floating state for a longer time than it is in the "on" state and the "off" state.

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