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High Power Switching Devices: Past, Present and Future

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High Power Switching Devices: Past, Present and Future RECENT ADVANCES in CIRCUITS, SYSTEMS, SIGNAL and TELECOMMUNICATIONS High Power Switching Devices: Past, Present and Future N. Y. A SHAMMAS, S.EIO, D. CHAMUMD Staffordshire University, Stafford, UK, [email protected], www.staffs.ac.uk Abstract Switching devices are key components in any power electronic circuit or system as they control and limit the flow of power from the source to the load. Their power level requirements (current & voltage) and switching frequency are continually increasing in the power electronic industry, and this demands larger and faster switching devices. This paper will focus on the development of high power switching devices and will present an up to date perspective of switching device technology and materials. The most important material has been and still is silicon (Si) for solid-state semiconductor devices. It dominates the world market at present, particularly in its crystalline form. However, silicon power device operation is generally limited to relatively low frequency and temperature. Silicon Carbide, Gallium Nitride and Diamond offer the potential to overcome the frequency, temperature and power management limitations of silicon. A large number of new concepts and materials are still in the research stage. At present, Silicon Carbide is considered to have the best trade-off between material properties and commercial maturity. Multilayer Silicon Carbide (SiC) power semiconductor devices being in development are promising devices for the near future, but long term reliability, crystal degradation and forward voltage drift problems need to be solved before commercialisation. Key-Words: - Vacuum Switches, Gas filled switches, Power Semiconductor Devices. ISSN: 1790-5117 192 ISBN: 978-960-474-152-6 RECENT ADVANCES in CIRCUITS, SYSTEMS, SIGNAL and TELECOMMUNICATIONS 1 Introduction main disadvantage is the high Power electronics is used to control the switching losses and the slow speed of flow of electric energy. Power recovery, which is due to minority electronic system generally involves a carrier injection during the conduction source of energy, which is released to state. the load by means of a switching device (Figure1). The limiting device The Insulated Gate Bipolar Transistor in high power electronic system is (IGBT) combines the advantages of often the switch, which limits the peak both. It has a simple gate drive circuit current, reverse blocking voltage, and like that of the MOSFET, with high the repetition rate. The switching current and low saturation voltage element is very special and falls into capability of bipolar transistor. The two basic categories: main problem remains with the relatively long tail turn-off current. To devices, reduce the turn-off time of the IGBT -state (semiconductor) switches and other bipolar devices, different lifetime control techniques and The conventional approach in very structural changes have been developed high power designs is to use a gas filled and used. switch such as a thyratron, ignitron or spark gap. However these devices have A general brief description of the limited lifetime, high cost, low above mentioned devices, and details repetition rate and high losses. On the of new techniques developed by using other hand high power semiconductor auxiliary electronic circuits with devices have under gone continued silicon-based semiconductor devices improvement in switching speed, for reducing the turn-off time and voltage and current rating and thus are increasing the switching speed of replacing the conventional gas filled bipolar devices are given in the devices in many applications. For very following sections and the availability high power applications such as high of these devices in Table 2. The voltage direct current (HVDC) operating temperature of silicon-based systems, series and parallel connection semiconductor devices is generally of these devices are used for high below 200 degree C. This imposes voltage and current respectively. Full restrictions in some application areas. details of methods used and problems To overcome this and other limitations encountered for IGBT connections as of silicon, wide band-gap materials an example are given in reference (1). such as silicon carbide and diamond have been investigated and used. The power level requirements and switching frequency are continually A brief review of progress made with increasing in the power electronic power devices using these materials industry, and this demands larger and are given in this paper. faster switching devices. As a result, both bipolar and unipolar semiconductor devices have undergone 2 Vacuum and gas-filled devices continued improvement in current and Two primary distinguishing features voltage ratings, and switching speed. can classify these types of switches: The main advantage of bipolar devices The source of free electrons within is their low conduction losses due to the device and conductivity modulation. But their ISSN: 1790-5117 193 ISBN: 978-960-474-152-6 RECENT ADVANCES in CIRCUITS, SYSTEMS, SIGNAL and TELECOMMUNICATIONS The gaseous filling (or lack of it) ionized current paths that might form within the tube envelope. within other parts of the tube. The gas A vacuum tube is a device with a in a thyratron is typically at a fraction vacuum (very low pressure gas) filling. of the atmospheric pressure; 15 to 30 And a gas filled device is filled with milli bars is typical. gas that might be at a pressure Both hot and cold cathode versions are somewhat above or below atmospheric. encountered. A hot cathode is an The type of gas used is also an advantage, as ionization of the gas is important feature, particularly in made easier; thus, the tube‘s control switching tubes where a wide variety of electrode is more sensitive. Once fillings are encountered. The source of turned on, the thyratron will remain on the free conduction electrons in the (conducting) as long as there is a device may be either thermal such as a significant current flowing through it. heated filament – a hot cathode, or When the anode voltage or current falls alternatively a simple consequence of a to zero, the device switches off. Large high voltage gradient across the device, thyratrons are still manufactured, and resulting in auto-emission from the are capable of operation up to tens of cold cathode. A device employing this kilo amperes (kA) and tens of kilovolts latter method is known as a cold (kV). cathode device and is used in many Modern applications include pulse high-voltage switching circuits. drivers for pulsed radar equipment, high-energy gas lasers, radiotherapy 2.1 Thyratrons devices, and in Tesla coils. Thyratrons It is a type of gas filled tube used as a are also used in high-power UHF high energy switch. Triode, Tetrode television transmitters, to protect and Pentode variations of the thyratron inductive output tubes from internal have been manufactured in the past, shorts, by grounding the incoming though most are of the triode design. high-voltage supply during the time it Gases used include mercury vapor, takes for a circuit breaker to open and xenon, neon, and (in special high- reactive components to drain their voltage applications, or applications stored charges. This is commonly requiring very short switching times called a ―crowbar‖ circuit. Thyratrons hydrogen). Unlike a vacuum tube, a have been replaced in most low and thyratron cannot be used to amplify medium-power applications by signals linearly. corresponding semiconductor devices (Thyristors ). Thyratrons evolved in the 1920s from early vacuum tubes a typical hot- 2.2 Ingnitron cathode thyratron uses a heated It is a type of controlled rectifier dating filament cathode, contained within a from the 1930s. It is usually a large shield assembly with a control grid on steel container with a pool of mercury one open side, which faces the plate- acting as a cathode. A large graphite shaped anode. When positive voltage is cylinder, held above the pool by an applied to the anode and the control insulated electrical connection, serves electrode is kept at cathode potential, as the anode. An igniting electrode no current flows. When the control (ignitor) is briefly pulsed to create an electrode is made positive with respect electrically conductive mercury to the cathode, the gas between the plasma, triggering heavy conduction anode and cathode ionizes and between the cathode and anode. conducts current. The shield prevents ISSN: 1790-5117 194 ISBN: 978-960-474-152-6 RECENT ADVANCES in CIRCUITS, SYSTEMS, SIGNAL and TELECOMMUNICATIONS Ignitrons were long used as high- environments where high levels of current rectifiers in major industrial ionising radiation are present (because installations, where thousands of the radiation might cause the gas-filled amperes of AC current must be krytron to trigger inadvertently). converted to DC, such as aluminium smelters. 2.4 Over voltage spark-gap Because they are far more resistant to It is essentially just two electrodes with damage due to over current or back- a gap in between. When the voltage voltage, ignitrons are still between the two electrodes exceeds the manufactured and used in preference to breakdown voltage, the device arcs semiconductors in certain installations. over and a current is very rapidly They are often used to switch high established. The voltage at which energy capacitor banks for emergency arcing occurs is given by the Dynamic short-circuiting of high voltage power Breakdown Voltage for a fast rising sources (crowbar). impulse voltage. Note that this voltage may be as much as 1.5 times greater than the static breakdown voltage (breakdown voltage for a slowly rising 2.3 Krytron voltage.), a shorter rise time means a It is a cold-cathode gas filled tube higher breakdown voltage. intended for use as a very high-speed Commutation times for these devices switch. The krytron uses arc discharge are exceptionally low (sometimes less to handle very high voltages and than 1 nanosecond). currents (several kV and several kA), Overvoltage gaps are primarily used rather than the usual low-current glow for protection.
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