A high frequency wide bandgap semiconductor driving circuit

Abstract

The present invention discloses a high-frequency wide-bandgap semiconductor driving circuit, belonging to the field of semiconductor switch technology. The driving circuit comprises a driving circuit and a wide-bandgap semiconductor S2, wherein the driving circuit is used to drive the wide-bandgap semiconductor S2; the driving circuit comprises an on-driving resistance unit, a pull-down resistance unit, a shut-off loop unit, a high-frequency low-impedance loop unit, and a negative voltage limiting unit; the KS pole of the wide-bandgap semiconductor S2 is connected to a ground line, thereby solving the technical problem that the driving circuit of an emerging wide-bandgap semiconductor switching device is easily interfered with, causing malfunction of the switch. In view of the extremely fast switching speed of the wide-bandgap semiconductor switching device and the susceptibility of the driving circuit to interference, the present invention proposes a driving circuit to reduce interference and improve the reliability of the wide-bandgap semiconductor switch. The present invention not only limits the positive spike caused by the action between the Drain pole and the Source pole of the wide-bandgap semiconductor switch, thereby preventing malfunction of the wide-bandgap semiconductor, but also limits the negative spike caused by the action, thereby preventing damage to the wide-bandgap semiconductor.

Description

Technical Field

[0001] The invention belongs to the technical field of semiconductor switches and relates to a high-frequency wide-bandgap semiconductor driving circuit. Background Art

[0002] Wide bandgap semiconductor materials, referred to as third-generation semiconductor materials, are materials with energy gaps greater than or equal to 2.3 eV. These materials primarily include diamond, silicon carbide, and gallium nitride. Compared to first- and second-generation semiconductor materials, third-generation semiconductor materials boast wide bandgap widths, high electron drift saturation velocities, low dielectric constants, and excellent electrical conductivity. They possess superior properties and hold enormous potential.

[0003] In existing chip technology, the bottom of the chip needs to be fixedly connected with a copper heat dissipation layer, an insulating layer, a copper heat dissipation layer, a welding connection layer and a copper heat dissipation layer in sequence. In order to increase the heat dissipation, a heat sink needs to be fixed at the end. These insulating layers and metal heat dissipation layers are indispensable heat dissipation structures in semiconductor devices. The insulating layer is provided to enable the positive and negative poles to be led out, and the multi-layer heat dissipation layer is to ensure the heat dissipation efficiency of the chip.

[0004] Wide bandgap semiconductors are an emerging class of semiconductor devices. Compared to traditional silicon-based semiconductor devices, they offer lower leakage current, higher breakdown voltage, lower on-resistance, greater temperature stability, and, most importantly, higher switching speeds. These advantages have led to their increasing popularity in high-frequency applications. Compared to traditional silicon-based semiconductor devices, wide bandgap switching frequencies can reach ten or even a hundred times higher. At such high switching frequencies, traditional silicon-based semiconductor drive circuits are no longer suitable for wide bandgap semiconductor applications.

[0005] The third-generation wide bandgap semiconductor materials are widely used in various fields, including power electronics, new energy vehicles, photovoltaics, locomotive traction, and microwave communication devices. Because they break through the development bottleneck of the first and second generation semiconductor materials, they have been favored by the industry.

[0006] Defects and shortcomings of existing technology:

[0007] Existing wide-bandgap semiconductor drive circuits are all based on silicon-based semiconductors. Because silicon's primary application frequency is relatively low, the impact of device and circuit parasitics is minimal. Furthermore, silicon-based semiconductors have a higher turn-on threshold than wide-bandgap semiconductors, allowing for application even in the presence of interference. However, wide-bandgap semiconductors typically have a turn-on threshold more than half that of silicon-based semiconductors, making interference from traditional drive schemes highly likely to cause them to mis-turn on. Furthermore, because wide-bandgap semiconductors are often used at higher frequencies and switch 5-10 times faster than traditional silicon-based semiconductors, oscillations in the power circuit are more easily coupled to the drive circuit, making them more susceptible to interference and malfunction.

[0008] Existing driving silicon-based semiconductor driving circuits such as Figure 1 As shown, the circuit includes a resistor R1, a resistor R2, a resistor R3, a diode D1, and a semiconductor switch device S1. The input port A1 is connected to the gate of the semiconductor switch device S1 through the resistor R1. One end of the resistor R3 is connected to the gate of the semiconductor switch device S1 and the other end is connected to the output port B1. One end of the resistor R2 is connected to the input port A1 and the other end is connected to the cathode of the diode D1. The anode of the diode D1 is connected to the gate of the semiconductor switch device S1. The semiconductor switch device S1 is a traditional silicon-based semiconductor switch device. The resistor R1 is a turn-on drive resistor, the resistor R2 is a turn-off drive resistor, the diode D1 is a switching diode, which can make the turn-on and turn-off circuits different, facilitating the adjustment of the switching speed. The resistor R3 is a pull-down resistor. When the input port A1 and the input port B1 of the driving circuit are connected to a high-level drive signal, due to the unidirectional conductivity of the diode D1, the drive signal drives the semiconductor switch device S1 through the resistor R1. When the drive signal is low, the drive signal is connected in parallel with the resistor R1 and the resistor R2 to turn off the semiconductor switch device S1.

[0009] Due to the inherent properties of the materials and structural characteristics of the common silicon-based semiconductor devices, there are parasitic capacitances Cgd, Cgs, and Cds at the three ports of the semiconductor, such as Figure 2 As shown in the figure; Due to the existence of these parasitic parameters, when the semiconductor switch device S1 switches, the switching interference of the Drain and Source poles will be introduced into the Gate pole through the parasitic capacitance Cgd, causing driving interference and easily causing malfunction. The current introduced into the Gate pole is Cgd×dV÷dt, where dV÷dt is the voltage change rate of the Drain and Gate poles. When the frequency is higher and the switching speed is faster, the current introduced into the Gate pole will be larger, and it will be more likely to cause interference at the Gate pole. The interference generated at the Gate pole by the traditional drive circuit is as follows Figure 3As shown, Vgs1 is the voltage between the Gate and Source poles, and Vds1 is the voltage between the Drain and Source poles.

[0010] The turn-on threshold Vgsth of a typical wide bandgap semiconductor is generally less than 1.5V, and the damage threshold of negative voltage varies from manufacturer to manufacturer, usually ranging from -4V to -10V. Figure 3 As shown, the positive tip 1 may malfunction, and the negative tip 2 may risk damaging the wide bandgap semiconductor.

[0011] The parasitic capacitance structure of wide-bandgap semiconductors is consistent with that of traditional silicon-based semiconductors, so the above-mentioned interference will also exist. Moreover, due to the high switching frequency of wide-bandgap semiconductors, the switching action between the Drain level and the Source pole is more easily introduced into the Gate pole.

[0012] Wide bandgap semiconductors have taken this into consideration in their production structure and technology. Most wide bandgap semiconductors have introduced Kelvin Source poles, such as Figure 4 As shown, the KS terminal is connected to the Source terminal inside the component. The separate lead-out prevents connection to the Source terminal in the driver circuit, thus reducing interference. However, even with the KS terminal, interference cannot be avoided. Furthermore, because the turn-on threshold (Vgsth) of wide-bandgap semiconductors is much lower than that of traditional silicon-based semiconductors, they are prone to malfunction. Therefore, the typical power supply method is to provide a certain negative voltage. However, even with this negative voltage, the interference from the Gate terminal is still significant in high-frequency and high-speed applications, and the risk of malfunction still exists.

[0013] Therefore, a driving circuit is needed to suppress interference and improve system reliability. Summary of the Invention

[0014] The purpose of the present invention is to provide a high frequency wide bandgap semiconductor driving circuit, which solves the technical problem that the emerging wide bandgap semiconductor switch device driving circuit is easily disturbed and causes switch malfunction.

[0015] To achieve the above object, the present invention adopts the following technical solutions:

[0016] A high frequency wide bandgap semiconductor driving circuit, comprising a driving circuit and a wide bandgap semiconductor S2, wherein the driving circuit is used to drive the wide bandgap semiconductor S2;

[0017] The driving circuit includes an on-driving resistance unit, a pull-down resistance unit, a shut-off loop unit, a high-frequency low-impedance loop unit, and a negative pressure limiting unit;

[0018] The open drive resistance unit and the high-frequency low-impedance loop unit are both connected to the close loop unit, and the close loop unit, the pull-down resistance unit, and the negative voltage limiting unit are all connected to the Gate electrode of the wide bandgap semiconductor S2;

[0019] The KS electrode of the wide bandgap semiconductor S2 is connected to the ground line.

[0020] Preferably, there are parasitic capacitances at the three ports of the wide bandgap semiconductor S2, and the parasitic capacitances include capacitance Cgd, capacitance Cgs and capacitance Cds. The capacitance Cgd is the parasitic capacitance between the Gate pole and the Drain pole of the wide bandgap semiconductor S2, the capacitance Cgs is the parasitic capacitance between the Gate pole and the Source pole of the wide bandgap semiconductor S2, and the capacitance Cds is the parasitic capacitance between the Drain pole and the Source pole of the wide bandgap semiconductor S2.

[0021] Preferably, the turn-on driving resistor unit includes a resistor R4 and a resistor R6, one end of the resistor R4 is a control signal input end, and the other end is connected to the gate electrode of the wide bandgap semiconductor S2 through the resistor R6.

[0022] Preferably, the pull-down resistor unit includes a resistor R8, one end of the resistor R8 is connected to the ground line, and the other end is connected to the Gate electrode of the wide bandgap semiconductor S2.

[0023] Preferably, the shutdown loop unit includes a field effect transistor Q1, a resistor R7 and a diode D2, the Gate electrode of the field effect transistor Q1 is connected to the connection node of the resistor R4 and the resistor R6, the Source electrode is connected to the Gate electrode of the wide bandgap semiconductor S2, the Drain electrode is connected to the positive electrode of the diode D2 through the resistor R7, and the negative electrode of the diode D2 is connected to the ground wire.

[0024] Preferably, the high-frequency low-impedance loop unit includes a capacitor C1 and a resistor R5, one end of the resistor R5 is connected to the control signal input end, and the other end is connected to the Source electrode of the field effect transistor Q1 through the capacitor C1.

[0025] Preferably, the negative voltage limiting unit includes a diode D3, a diode ZD1 and a capacitor C3, the positive electrode of the diode ZD1 is connected to the Gate electrode of the wide bandgap semiconductor S2, the negative electrode is connected to the negative electrode of the diode D3, the positive electrode of the diode D3 is connected to the ground wire, and the capacitor C3 is connected in parallel with the diode ZD1.

[0026] Preferably, a capacitor C2 is further included, one end of the capacitor C2 is connected to the Gate electrode of the wide bandgap semiconductor S2, and the other end is connected to the Source electrode of the wide bandgap semiconductor S2.

[0027] Preferably, the diode ZD1 is a Zener diode or a TVS diode.

[0028] The high-frequency wide-bandgap semiconductor drive circuit described in the present invention solves the technical problem that the drive circuits of emerging wide-bandgap semiconductor switching devices are easily interfered with, causing malfunction of the switches. In view of the extremely fast switching speed of wide-bandgap semiconductor switching devices and the susceptibility of the drive circuits to interference, the present invention proposes a drive circuit to reduce interference and improve the reliability of wide-bandgap semiconductor switches. The present invention not only limits the positive spike caused by the action between the Drain and Source poles of the wide-bandgap semiconductor switch, preventing malfunction of the wide-bandgap semiconductor, but also limits the negative spike caused by the action, preventing damage to the wide-bandgap semiconductor. BRIEF DESCRIPTION OF THE DRAWINGS

[0029] Figure 1 A circuit diagram of the present invention;

[0030] Figure 2 A circuit diagram of a conventional silicon-based semiconductor driving circuit in the background art;

[0031] Figure 3 An equivalent circuit diagram of parasitic capacitance generated by the material and structural characteristics of common silicon-based semiconductor devices in the background art;

[0032] Figure 4 This is a waveform diagram of the operation of an existing wide bandgap semiconductor in the background technology;

[0033] Figure 5 The equivalent circuit diagram of the parasitic capacitance of the wide bandgap semiconductor with a KS pole in the background art;

[0034] Figure 6 It is the working waveform diagram of the present invention;

[0035] In the picture: positive tip 1, negative tip 2. DETAILED DESCRIPTION

[0036] The following will clearly and completely describe the technical solutions in the embodiments of the present invention in conjunction with the accompanying drawings. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without making creative efforts are within the scope of protection of the present invention.

[0037] like Figure 1 and Figure 6 A high frequency wide bandgap semiconductor driving circuit shown includes a driving circuit and a wide bandgap semiconductor S2, wherein the driving circuit is used to drive the wide bandgap semiconductor S2;

[0038] The driving circuit includes an on-driving resistance unit, a pull-down resistance unit, a shut-off loop unit, a high-frequency low-impedance loop unit, and a negative pressure limiting unit;

[0039] The open drive resistance unit and the high-frequency low-impedance loop unit are both connected to the close loop unit, and the close loop unit, the pull-down resistance unit, and the negative voltage limiting unit are all connected to the Gate electrode of the wide bandgap semiconductor S2;

[0040] The KS electrode of the wide bandgap semiconductor S2 is connected to the ground line.

[0041] Preferably, there are parasitic capacitances at the three ports of the wide bandgap semiconductor S2, and the parasitic capacitances include capacitance Cgd, capacitance Cgs and capacitance Cds. The capacitance Cgd is the parasitic capacitance between the Gate pole and the Drain pole of the wide bandgap semiconductor S2, the capacitance Cgs is the parasitic capacitance between the Gate pole and the Source pole of the wide bandgap semiconductor S2, and the capacitance Cds is the parasitic capacitance between the Drain pole and the Source pole of the wide bandgap semiconductor S2.

[0042] Preferably, the turn-on driving resistor unit includes a resistor R4 and a resistor R6, one end of the resistor R4 is a control signal input end, and the other end is connected to the gate electrode of the wide bandgap semiconductor S2 through the resistor R6.

[0043] Preferably, the pull-down resistor unit includes a resistor R8, one end of the resistor R8 is connected to the ground line, and the other end is connected to the Gate electrode of the wide bandgap semiconductor S2.

[0044] Preferably, the shutdown loop unit includes a field effect transistor Q1, a resistor R7 and a diode D2, the Gate electrode of the field effect transistor Q1 is connected to the connection node of the resistor R4 and the resistor R6, the Source electrode is connected to the Gate electrode of the wide bandgap semiconductor S2, the Drain electrode is connected to the positive electrode of the diode D2 through the resistor R7, and the negative electrode of the diode D2 is connected to the ground wire.

[0045] Preferably, the high-frequency low-impedance loop unit includes a capacitor C1 and a resistor R5, one end of the resistor R5 is connected to the control signal input end, and the other end is connected to the Source electrode of the field effect transistor Q1 through the capacitor C1.

[0046] Preferably, the negative voltage limiting unit includes a diode D3, a diode ZD1 and a capacitor C3, the positive electrode of the diode ZD1 is connected to the Gate electrode of the wide bandgap semiconductor S2, the negative electrode is connected to the negative electrode of the diode D3, the positive electrode of the diode D3 is connected to the ground wire, and the capacitor C3 is connected in parallel with the diode ZD1.

[0047] Preferably, a capacitor C2 is further included, one end of the capacitor C2 is connected to the Gate electrode of the wide bandgap semiconductor S2, and the other end is connected to the Source electrode of the wide bandgap semiconductor S2.

[0048] Preferably, the diode ZD1 is a Zener diode or a TVS diode.

[0049] In this embodiment, as shown in FIG1 , the resistor R4 and the resistor R6 are turn-on driving resistors, and their resistance values determine the turn-on speed. The user can set the values of the resistor R4 and the resistor R6 according to their own needs to control the turn-on speed.

[0050] Resistor R8 is a pull-down resistor. Due to the presence of resistor R8, the impedance of the input end of the wide bandgap semiconductor S2 is reduced, that is, the impedance of the Gate pole of the wide bandgap semiconductor S2 is reduced, thereby improving the anti-interference capability. Field effect transistor Q1, resistor R7 and diode D2 constitute a shutdown loop, which greatly shortens the overall path and improves the anti-interference capability. When the voltage between the Drain pole and the Source pole of the wide bandgap semiconductor S2 changes from low to high, the parasitic capacitor Cgd will be charged, causing Vgs1 to increase. The function of capacitor C2 is to be connected in parallel with the parasitic capacitor Cgs to reduce the high-frequency impedance between the Gate pole and the Source pole of the wide bandgap semiconductor S2 and reduce the voltage of Vgs1. Capacitor C1 and resistor R5 provide another high-frequency low-impedance loop to further reduce the voltage of Vgs1. In this embodiment, capacitor C1 is generally larger than capacitor C2, and the impedance of resistor R5 is very small.

[0051] When the voltage between the Drain and Source poles of the wide bandgap semiconductor S2 changes from high to low, the parasitic capacitance Cgd discharges, and the parasitic capacitance Cgs discharges, causing Vgs1 to decrease. Since the negative voltage threshold of the wide bandgap semiconductor is relatively high, the decrease in Vgs1 can easily trigger the threshold, causing damage to the wide bandgap semiconductor. In this embodiment, diode D3, diode ZD1 and capacitor C3 are used to limit the negative voltage. Capacitor C3 limits the rate of change of the negative voltage. When the negative voltage exceeds the clamping limit of diode ZD1, diode ZD1 operates to clamp the negative voltage and protect the wide bandgap semiconductor. Diode ZD1 can be a zener, that is, a Zener diode, or a TVS.

[0052] like Figure 6 As shown, it can be seen from the waveform diagram of the present invention that the positive tip 1 and the negative tip 2 of Vgs2 are greatly reduced, which not only limits the positive tip to prevent malfunction, but also limits the negative tip to protect the wide bandgap semiconductor from damage.

[0053] The high-frequency wide-bandgap semiconductor drive circuit described in the present invention solves the technical problem that the drive circuits of emerging wide-bandgap semiconductor switching devices are easily interfered with, causing malfunction of the switches. In view of the extremely fast switching speed of wide-bandgap semiconductor switching devices and the susceptibility of the drive circuits to interference, the present invention proposes a drive circuit to reduce interference and improve the reliability of wide-bandgap semiconductor switches. The present invention not only limits the positive spike caused by the action between the Drain and Source poles of the wide-bandgap semiconductor switch, preventing malfunction of the wide-bandgap semiconductor, but also limits the negative spike caused by the action, preventing damage to the wide-bandgap semiconductor.

[0054] In the present invention, any process or method description in a flowchart or otherwise described herein may be understood to represent a module, fragment or portion of code comprising one or more executable instructions for implementing the steps of a custom logical function or process, and the scope of the preferred embodiments of the present invention includes additional implementations in which functions may be performed in a different order than shown or discussed, including performing functions in a substantially simultaneous manner or in a reverse order depending on the functions involved, which should be understood by those skilled in the art to which the embodiments of the present invention belong.

[0055] The logic and / or steps represented in the flowcharts or otherwise described herein, for example, can be considered as a sequenced list of executable instructions for implementing the logical functions, and can be embodied in any computer-readable medium for use by, or in conjunction with, an instruction execution system, apparatus, or device (e.g., a computer-based system, a system including a processor, or other system that can fetch and execute instructions from an instruction execution system, apparatus, or device). For purposes of this specification, a "computer-readable medium" can be any device that can contain, store, communicate, propagate, or transport a program for use by, or in conjunction with, an instruction execution system, apparatus, or device. More specific examples (a non-exhaustive list) of computer-readable media include the following: an electrical connection with one or more wires (electronic devices), a portable computer disk cartridge (magnetic device), random access memory (RAM), read-only memory (ROM), erasable and programmable read-only memory (EPROM or flash memory), fiber optic devices, and a portable compact disc read-only memory (CDROM). Furthermore, the computer-readable medium may even be paper or other suitable medium on which the program is printed, since the program may be obtained electronically, for example, by optically scanning the paper or other medium and then editing, interpreting or processing it in another suitable manner if necessary, and then storing it in a computer memory.

[0056] It should be understood that various parts of the present invention can be implemented using hardware, software, firmware, or a combination thereof. In the above-described embodiments, multiple steps or methods can be implemented using software or firmware stored in a memory and executed by a suitable instruction execution system. For example, if implemented using hardware, as in another embodiment, any one of the following technologies known in the art or a combination thereof can be used: a discrete logic circuit having a logic gate circuit for implementing a logic function on a data signal, an application-specific integrated circuit having a suitable combination of logic gate circuits, a programmable gate array (PGA), a field programmable gate array (FPGA), etc.

[0057] Those skilled in the art will understand that all or part of the steps in the method of the above embodiment can be completed by instructing related hardware through a program, and the program can be stored in a computer-readable storage medium. When the program is executed, it includes one or a combination of the steps of the method embodiment.

[0058] In addition, the functional units in the various embodiments of the present invention may be integrated into a single processing module, or each unit may exist physically separately, or two or more units may be integrated into a single module. The aforementioned integrated modules may be implemented in the form of hardware or in the form of software functional modules. If the integrated modules are implemented in the form of software functional modules and sold or used as independent products, they may also be stored in a computer-readable storage medium.

[0059] Although the embodiments of the present invention have been shown and described above, it will be understood that the above embodiments are illustrative and are not to be construed as limitations on the present invention. A person skilled in the art may change, modify, replace and modify the above embodiments within the scope of the present invention.

Claims

1. A high-frequency wide-bandgap semiconductor driving circuit, characterized in that: It includes a driving circuit and a wide bandgap semiconductor S2, wherein the driving circuit is used to drive the wide bandgap semiconductor S2; The driving circuit includes an on-driving resistance unit, a pull-down resistance unit, a shut-off loop unit, a high-frequency low-impedance loop unit, and a negative pressure limiting unit; The open drive resistance unit and the high-frequency low-impedance loop unit are both connected to the close loop unit, and the close loop unit, the pull-down resistance unit, and the negative voltage limiting unit are all connected to the Gate electrode of the wide bandgap semiconductor S2; The KS electrode of the wide bandgap semiconductor S2 is connected to the ground wire; The shutdown loop unit includes a field effect transistor Q1, a resistor R7 and a diode D2, wherein the Gate electrode of the field effect transistor Q1 is connected to the connection node of the resistor R4 and the resistor R6, the Source electrode is connected to the Gate electrode of the wide bandgap semiconductor S2, the Drain electrode is connected to the positive electrode of the diode D2 through the resistor R7, and the negative electrode of the diode D2 is connected to the ground wire; The negative voltage limiting unit includes a diode D3, a diode ZD1 and a capacitor C3, wherein the anode of the diode ZD1 is connected to the gate of the wide bandgap semiconductor S2, and the cathode is connected to the cathode of the diode D3, the anode of the diode D3 is connected to the ground, and the capacitor C3 is connected in parallel with the diode ZD1; The diode ZD1 is a Zener diode or a TVS tube.

2. The high-frequency wide-bandgap semiconductor driving circuit according to claim 1, wherein: There are parasitic capacitances at the three ports of the wide bandgap semiconductor S2, which include capacitance Cgd, capacitance Cgs and capacitance Cds. Capacitor Cgd is the parasitic capacitance between the Gate pole and the Drain pole of the wide bandgap semiconductor S2, capacitance Cgs is the parasitic capacitance between the Gate pole and the Source pole of the wide bandgap semiconductor S2, and capacitance Cds is the parasitic capacitance between the Drain pole and the Source pole of the wide bandgap semiconductor S2.

3. The high-frequency wide-bandgap semiconductor driving circuit according to claim 1, wherein: The turn-on driving resistor unit includes a resistor R4 and a resistor R6 , wherein one end of the resistor R4 is a control signal input end, and the other end is connected to the gate electrode of the wide bandgap semiconductor S2 through the resistor R6 .

4. A high-frequency wide-bandgap semiconductor driving circuit according to claim 3, characterized in that: The pull-down resistor unit includes a resistor R8 , one end of which is connected to the ground line, and the other end of which is connected to the Gate electrode of the wide bandgap semiconductor S2 .

5. The high-frequency wide-bandgap semiconductor driving circuit according to claim 4, characterized in that: The high-frequency low-impedance loop unit includes a capacitor C1 and a resistor R5 , one end of the resistor R5 is connected to the control signal input end, and the other end is connected to the Source electrode of the field effect transistor Q1 through the capacitor C1 .

6. The high-frequency wide-bandgap semiconductor driving circuit according to claim 1, wherein: It also includes a capacitor C2, one end of which is connected to the Gate electrode of the wide bandgap semiconductor S2, and the other end of which is connected to the Source electrode of the wide bandgap semiconductor S2.

7. The high-frequency wide-bandgap semiconductor driving circuit according to claim 6, characterized in that: The diode ZD1 is a Zener diode or a TVS tube.

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