Proceedings of the 13th WSEAS International Conference on CIRCUITS Electronic Circuits for Switching-Time Reduction of Bipolar Semiconductor Devices N. Y. A SHAMMAS, S.EIO, D. CHAMUMD Staffordshire University, Stafford, UK, [email protected], www.staffs.ac.uk Abstract: Bipolar semiconductor devices are often used as switches in very high power electronic circuits and systems. They have replaced the old conventional gas filled tubes and vacuum devices in many applications. This is mainly due to the fact that solid-state devices are more efficient, smaller in size, cheaper and more reliable. In addition, solid-state devices are considered environmental friendly, since they do not contain nasty gases and toxic materials used in old devices. The power level requirements and switching frequency are continually increasing in the power electronic industry, and this demands larger and faster switching devices. As a result, both bipolar and unipolar semiconductor devices have undergone continued improvement in current and voltage ratings, and switching speed. The main advantage of bipolar devices is their low conduction losses due to conductivity modulation, but their main disadvantage is the high switching losses which is due to minority carrier injection. The Insulated Gate Bipolar Transistor (IGBT) combines the advantages of both unipolar and bipolar devices. It has a simple gate drive circuit like that of the MOSFET, with high current and low saturation voltage capability of bipolar transistor. The main problem remains with the relatively long tail turn-off current. To reduce the turn-off time of the IGBT and other bipolar devices, different lifetime control techniques and structural changes have been developed and used. Details of these and new techniques developed by using auxiliary electronic circuits for reducing the turn-off time and increasing the switching speed of bipolar semiconductor devices are presented in this paper. Key-Words: - power semiconductor diodes, thyristors, Insulated gate bipolar transistors, ISSN: 1790-5117 28 ISBN: 978-960-474-096-3 Proceedings of the 13th WSEAS International Conference on CIRCUITS 1 Introduction recovery, which is due to minority Power electronic system generally carrier injection during the conduction involves a source of energy, which is state. released to the load by means of a switching device (Figure1). The The Insulated Gate Bipolar Transistor limiting device in high power (IGBT) combines the advantages of electronic system is often the switch, both. It has a simple gate drive circuit which limits the peak current, voltage like that of the MOSFET, with high and 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 Vacuum and Gas filled switching relatively long tail turn-off current. To tubes, reduce the turn-off time of the IGBT Solid-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 semiconductor devices for reducing the improvement in switching speed, turn-off time and increasing the voltage and current rating and thus are switching speed of bipolar devices are replacing the conventional gas filled given in the following sections. devices in many applications. For very high power applications such as high voltage direct current (HVDC) 2 Vacuum and gas-filled devices systems, series and parallel connection Two primary distinguishing features of these devices are used for high can classify these types of switches: voltage and current respectively. Full the source of free electrons within details of methods used and problems the device and encountered for IGBT connections as the gaseous filling (or lack of it) an example are given in reference (1). within the tube envelope. A vacuum tube is a device with a The power level requirements and vacuum (very low pressure gas) filling. switching frequency are continually And a gas filled device is filled with increasing in the power electronic gas that might be at a pressure industry, and this demands larger and somewhat above or below atmospheric. faster switching devices. As a result, The type of gas used is also an both bipolar and unipolar important feature, particularly in semiconductor devices have undergone switching tubes where a wide variety of continued improvement in current and fillings are encountered. The source of voltage ratings, and switching speed. the free conduction electrons in the The main advantage of bipolar devices device may be either thermal such as a is their low conduction losses due to heated filament – a hot cathode, or conductivity modulation. But their alternatively a simple consequence of a main disadvantage is the high high voltage gradient across the device, switching losses and the slow speed of resulting in auto-emission from the ISSN: 1790-5117 29 ISBN: 978-960-474-096-3 Proceedings of the 13th WSEAS International Conference on CIRCUITS cathode. A device employing this latter kiloamperes (kA) and tens of kilovolts method is known as a cold cathode (kV). device and is used in many high- Modern applications include pulse 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 Ignitrons were long used as high- ionized current paths that might form current rectifiers in major industrial within other parts of the tube. The gas installations where thousands of in a thyratron is typically at a fraction amperes of AC current must be of the atmospheric pressure ; 15 to 30 converted to DC, such as aluminium milli bars is typical. smelters. Both hot and cold cathode versions are Because they are far more resistant to encountered. A hot cathode is an damage due to over current or back- advantage, as ionization of the gas is voltage, ignitrons are still made easier; thus, the tube’s control manufactured and used in preference to electrode is more sensitive. Once semiconductors in certain installations. turned on, the thyratron will remain on They are often used to switch high (conducting) as long as there is a energy capacitor banks for emergency significant current flowing through it. short-circuiting of high voltage power When the anode voltage or current falls sources (crowbar). to zero, the device switches off. Large thyratrons are still manufactured, and are capable of operation up to tens of ISSN: 1790-5117 30 ISBN: 978-960-474-096-3 Proceedings of the 13th WSEAS International Conference on CIRCUITS 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. But in combination with discharge. There are four electrodes in the other devices mentioned here they a krytron. Two are conventional anode are commonly used to sharpen the and cathode. One is a keep-alive output pulses (decrease the rise times) electrode, arranged to be close to the of very high current pulses form cathode. The keep-alive has a low triggered switching devices e.g. positive voltage applied, which causes Thyratrons. The size of these devices is a small area of gas to ionize near the almost entirely dependent upon how cathode. High voltage is applied to the much current/voltage they are intended anode, but primary conduction does not to switch, There is no limit as to the occur until a positive pulse is applied to size of these devices.