2018.12.06 Si Power Device
This concludes the basic edition of Si power devices. As Si-based power devices, we have explained the basic characteristics of rectified diodes, Schottky barrier diodes, fast recovery diodes, MOSFETs, super-junction MOSFETs, PrestoMOS and Hybrid MOS devices. We have also explained methods and procedures for judging whether a selected transistor is appropriate under actual operating conditions, and have introduced application examples which exploit the features of these diodes and transistors. While focusing on the fundamentals of discrete components, these articles should have aided in understanding the features of the various devices. Below, the articles in this series and the key points of each are summarized.
・Understanding the features and characteristics of the latest Si power devices, to review and expand the coverage of Si power devices.
・Choosing between existing Si semiconductor devices and SiC power devices using new materials, according to the required specifications.
・Review of the most basic essentials.
・The types and categories of diodes used as Si power devices were described.
・It's important to know the differences in the general features of Si diodes for power applications.
・Si-SBD characteristics differ depending on the barrier metal.
・It should be recognized that IR values of Si-SBD are sufficiently large that they cannot be ignored.
・Thermal runaway remains a real possibility, so the thermal design should be verified adequately.
・Si FRD characteristics differ depending on the impurities diffused in the silicon.
・There is a tradeoff between the VF and Trr values of Si FRDs.
・Noise during reverse recovery has adverse effects in switching power supply applications, and so improved versions are being developed.
・In this chapter, we have focused on bipolar transistors, MOSFETs, and IGBTs as power transistors.
・The fundamental features of bipolar transistors, MOSFETs, and IGBTs should be reviewed.
・MOSFETs have parasitic capacitances, which are important parameters that have an effect on switching characteristics.
・Parasitic capacitances change hardly at all with temperature, and so temperature changes exert almost no effect on switching characteristics.
・MOSFET switching characteristics that are generally provided include the turn-on delay time, rise time, turn-off delay time, and fall time.
・Switching characteristics are greatly affected by the measurement conditions and measurement circuit, and so the suggested conditions should be confirmed.
・Switching characteristics are affected hardly at all by changes in temperature.
・The voltage at which a MOSFET turns on is called the gate threshold voltage.
・If VGS is constant, a rise in temperature will cause ID to increase, and so conditions of use must be considered carefully.
・Tj can be roughly estimated from a change in the value of VGS(th).
・Si-MOSFETs are positioned as elements capable of fast operation at low to intermediate power levels.
・The super-junction structure achieves a lower ON-resistance RDS(ON) and reduced gate charge Qg, while maintaining a high voltage.
・A super-junction MOSFET has a characteristic that irr is larger and trr is faster than for a planar MOSFET.
・There are different kinds of SJ-MOSFETs, depending on the device characteristics.
・SJ-MOSFETs are essentially faster and have a lower on-resistance than do planar MOSFETs, but efforts are being made to reduce noise and to further lower the on-resistance and increase speed.
・PrestoMOSs are ROHM SJ-MOSFETs that, in addition to high voltages, low on-resistances, and small total gate charges, also achieve fast reverse recovery times trr of the internal diode.
・By shortening the trr of the internal diode, inverter and motor driver circuits can be made smaller and more efficient.
・The Hybrid MOS is a new-structure MOSFET which combine the advantages of both SJ-MOSFET and IGBT.
・The Hybrid MOS brings together the high-speed operation and low losses in small-current ranges of SJ-MOSFETs and the low losses in large-current ranges of IGBTs.
・The Hybrid MOS is useful for raising the efficiency in low power ranges while still handing higher power levels, to satisfy demands for higher home appliance APFs.
＜Confirming the Suitability of a Transistor in Actual Operation＞
・In prototyping, it is essential to determine whether a selected transistor can be used in actual operation.
・For purposes of confirmation, the voltages and currents handled by the transistor are measured.
・Correctly understand the definition and purpose of absolute maximum ratings, and learn to judge whether a component can be used.
・The SOA (Safe Operating Area) is information used to confirm that a transistor is operating under safe conditions.
・In essence, the SOA indicates in a graph the safe area of operation with respect to the rated voltage and current and the allowable power dissipation (heat generation).
・The SOA conditions should be checked carefully, and differences with actual usage conditions should be considered, prior to device use.
・An SOA graph is for data at Ta=25°C, and so the SOA must be derated according to the temperature at which the transistor will actually be used.
・As the derating rate, the derating rate for allowable power dissipation is used.
・In the case of continuous-pulse (switching) operation, the average power consumption is determined, and it is confirmed that allowable power dissipation is within the rated value.
・A judgment will in the end depend on whether Tj exceeds the absolute maximum rating.
・It should be confirmed that Tj ultimately does not exceed the absolute maximum rating.
・Tj is the sum of Ta or Tc and the heat generation (the product of the thermal resistance and the power consumption).
＜Application Examples that Exploit Feature＞
・In PFC (power factor correction), the power factor is improved, bringing it closer to 1.
・PFC can use single or interleaved circuits; when using an interleaved circuit, losses can be distributed, so that thermal design is easier.
・In PFC, either a boundary current mode (BCM) or a continuous current mode (CCM) can be used; in general, CCM is used for large power levels.
・In boundary current mode PFC, the diode VF has a major effect on losses, whereas the effect of the diode trr is small.
・In PFC under boundary current mode control, by selecting a diode with a low VF, circuit efficiency can be improved.
・In continuous current mode (CCM) PFC, the diode trr has a major effect on losses, whereas the effect of the diode VF is small.
・In PFC under continuous current mode control, by selecting a diode with a low trr, circuit efficiency can be improved.
・Efficiency changes depending on differences in the characteristics of the MOSFETs used as switches in the PFC section and the DC/DC converter section, and so it is important to thoroughly study the MOSFET characteristics.
・In continuous current mode PFC, by replacing an IGBT + FRD with a Hybrid MOSFET + SiC SBD, efficiency can be improved
・In the example given, efficiency is improved due to the fast trr characteristic of the SiC SBD and the low on-resistance and fast switching of the Hybrid MOSFET.