Introduction The correct design of an anti-parallel diode requires consideration of various factors. Some are related to the technology itself, while others are application-related. However, the forward voltage drop Vf, reverse recovery charge Qrr, and the heat dissipation capabilities of Rth and Zth ultimately form a triangular relationship. Due to the current state of diode technology, the size of the diode chip itself has been reduced to a very small size, so diode designers are once again focusing on electrical performance (ignoring cost factors). This article will focus on diodes in driver applications, analyzing their advantages and disadvantages. For all applications, the basic consideration is the same: should a diode with lower static losses be used, or should a diode with slightly higher static losses but lower switching losses be used considering the overall system performance (including IGBTs)? Diode Optimization The relationship between the reverse recovery charge Qrr and the forward voltage drop Vf of a diode can represent its characteristics. This means that, in principle, every point on this curve can be achieved, as shown in Figure 1. Therefore, diodes with low Qrr and high Vf, or low Vf and high Qrr, can be designed. This curve can be achieved by changing the current density or lifetime suppression. Figure 1. Qrr-Vf relationship curve of a diode. Generally, the larger the chip size, the lower the forward voltage drop Vf due to the reduced current density. This helps improve the chip's heat dissipation, but at the same time, switching losses increase, and costs also rise. For a given current density and chip size, reducing carrier lifetime through local (e.g., helium ion irradiation) or global (electron irradiation or doping with recombination centers, such as gold or platinum) methods has a similar effect. Shortening carrier lifetime reduces the accumulated charge Qrr in the device, but reduces conduction performance and increases the forward voltage drop Vf; extending carrier lifetime reduces the forward voltage drop Vf, but increases switching losses. Most practical diodes employ one or more lifetime control methods, except for rectifier diodes. Rectifier diodes operate at very low frequencies and have high requirements for conduction losses, so it is not always necessary to reduce carrier lifetime. Figure 2. Thermal resistance depending on chip size. For the diode technology discussed in this paper, changing the current density or chip size can lead to very similar curves. This paper chose to change the current density and performed the relevant calculations. This approach implies smaller diode chips, leading to higher chip yields per wafer and thus reducing the unit price of the chip. On the other hand, smaller chips have higher junction-to-case thermal resistance (RthJC), so the first thought is that a larger heatsink is needed. However, this conclusion is premature. The relationship between chip size and thermal resistance (RthJC) is shown in Figure 2. It can be seen that the hyperbolic value is approximately determined by the wafer mounting, the chip itself, and the solder thickness of the lead frame. However, to arrive at a final evaluation, it is necessary to understand the total losses and the loss distribution between the IGBT and the diode more deeply. Figure 3 shows the rectification process from diode to IGBT. The analysis of the rectification process shows that the current generated by the reverse recovery charge of the diode is not only added to the diode itself but also flows through the rectified IGBT, as shown in Figure 3. The shaded area in the collector waveform represents the reverse recovery characteristics of the diode and the additional charge generated by the discharge of the parasitic output capacitance. However, the output capacitance can usually be ignored because the IGBT capacitance is very small; therefore, it can be assumed that this region is entirely caused by reverse recovery. It can be seen that, firstly, when the IGBT voltage is still at a high level, the reverse recovery current has already begun to flow. Secondly, the diode current tail is around 100ns. Clearly, the reverse recovery performance of the diode plays a crucial role in the switching losses of the IGBT. Observing the power loss distribution, it can be seen that the main power loss usually comes from the IGBT, thus causing the diode chip to heat up. This situation only changes if the diode itself has higher losses, and the heat generated by the diode itself exceeds the heat generated by the IGBT losses. From a product perspective, increasing the diode temperature is beneficial, as it reduces overall losses and the IGBT junction temperature. Under rated conditions, the optimal loss distribution is achieved when the IGBT junction temperature equals the diode junction temperature. This means that although an optimized diode may achieve a higher RthJC due to its smaller chip size, this does not affect the performance of the IGBT-diode combination because the overall power consumption is reduced. Compared to EmCon2 technology, the new anti-parallel diode using EmCon3 technology has a higher forward voltage drop, improved reverse recovery characteristics, and lower switching losses. Figure 4. Loss Balance for Diode Optimization (RthHS = 4.2 K/W, TA = 50℃, cosΦ = 0.7) This conclusion contradicts the common understanding that diodes used in drive applications must be optimized for low conduction losses. Low switching losses are equally crucial, especially in home appliance drives such as inverter washing machines, where switching frequencies can reach 15 kHz or higher. In such applications, switching losses constitute a significant portion of the overall drive losses and cannot be ignored. This optimization opens doors to a variety of applications—not only in the drive market but also in the so-called "high-speed" domain. Figure 5. Switching Losses of TrenchStop-IGBT with Vf-Optimized Diode (Left Bar) and with Final Design Diode (Right Bar) EMCON3 vs. EMCON2 Technology Benchmarks Figure 4 shows the loss balance per ampere for the two diode-equipped IGBTs. The left bar shows the result of combining the newly introduced EmCon3 technology with the TrenchStop-IGBT (IGBT3 technology). As mentioned above, EmCon3 technology is optimized for lower switching losses and a slightly higher forward voltage drop. The right bar chart shows the results of combining EmCon2 technology with TrenchStop-IGBT. The EmCon2 diode used in this benchmark is an anti-parallel diode from Infineon's Fast-IGBT series. This diode is optimized for low forward voltage drop. The IGP10N60T used in Figure 4 has a heatsink with a thermal resistance RthHS = 4.2 K/W and an ambient temperature TA = 50°C, raising the junction temperature to approximately 125°C. The switching frequency fP is 16 kHz, demonstrating the performance of the IGP10N60T and EmCon3 technology combination. As can be seen from Figure 5, as expected, the IGBT conduction losses are not affected by the diode at all. The Qrr improvement of the Vf-optimized diode has a significant impact on the IGBT's dynamic loss PvsI and the diode's dynamic loss PvsD. The two effects combined—the increase in the diode's own dynamic loss and its impact on the IGBT—outweigh the advantages of the Vf-optimized diode during conduction. This characteristic is already very noticeable at a switching frequency of around 5 kHz, and the higher the switching frequency, the greater the impact. Figure 6 shows the thermal equivalent circuit for temperature calculation. Of course, determining the loss balance of each part for a specific hardware circuit design is not easy. Typically, engineers measure the temperature on the case or lead frame. The thermal resistances RthJC of the two diodes are assumed to be the same. The thermal equivalent circuit of the combined system is shown in Figure 6. A constant ambient temperature results in a common case temperature TC, which is determined by the heatsink thermal resistance and the total losses of the IGBT and diodes. Therefore, different junction-to-case thermal resistances RthJCD and RthJCI of the diodes and IGBTs can lead to different junction temperatures TJD and TJI. The junction temperatures formed by the two combined systems are shown in Figure 7. The junction temperature is close to 125°C. Compared to the combination of the IGP10N60T and the Vf-optimized EmCon2 diode, the combination of the IGP10N60T and the Qrr-optimized EmCon3 diode achieves a lower junction temperature. In the left-hand bar chart, the diode and IGBT temperatures are 4K lower, with the IGBT having 0.7 W less power loss and the diode 0.2 W less. Because the IGBT has a lower RthJC, the greater loss reduction of the IGBT has a smaller impact on the junction temperature than the relatively smaller loss reduction of the diode. Therefore, the temperature difference is the same. Figure 7 shows the junction temperatures formed by the two combined systems. Of course, the loss reduction is also partially sacrificed by the smaller RthJC. However, calculations show that at an ambient temperature TA of 50°C, when combined with the 10A-IGBT IGP10N60T, the final diode junction temperature is approximately 4°C lower. It can also be seen that the IGBT junction temperature is also 4°C lower. Therefore, the system generally benefits from the chosen diode optimization method. If the same junction temperature as the final diode is achieved, higher current can be obtained from the inverter, resulting in higher power output, as shown in Figure 8. On the other hand, for a given output current, even the heatsink size can be reduced, thereby lowering the cost of the drive unit. Regardless of the method used by the designer, the system will achieve higher efficiency. Figure 8. Output RMS Current of a Half-Bridge in an Inverter. Conclusion Diode optimization cannot solely consider forward voltage drop; it must also take into account IGBT technology and application conditions. In this paper, the diodes connected in parallel with the TrenchStop-IGBT are designed based on IGBT technology and application conditions. These diode chips are smaller but achieve lower junction temperatures than larger Vf-optimized chips. This allows engineers to make greater use of IGBTs and diodes. It can reduce heatsink size or increase the output power of a given system, reducing system cost.