Diode design in IGBT with anti-parallel diode

Introduction
　　The correct design of antiparallel diodes requires consideration of various factors. Some of these are related to the technology itself, others are application related. However, the forward voltage drop Vf, the reverse recovery charge Qrr, and the heat dissipation capabilities of Rth and Zth will ultimately form a triangular relationship.

　　Since the size of the diode chip itself has been reduced to a very small size under current diode technology, diode designers are once again focusing on electrical performance (ignoring cost factors). This article will focus on diodes in drive applications, analyzing and thinking about the pros and cons. For all applications, the basic point to consider is the same: should a diode with lower static losses be used, or should the overall system (including IGBT) performance dictate a diode with slightly higher static losses but lower switching losses.

Diode Optimization
　　The relationship between the reverse recovery charge Qrr of the diode and the forward voltage drop Vf can represent the characteristics of the diode. This means that, in principle, every point on the curve can be realized, as shown in Figure 1. Therefore, it is possible to design a diode with low Qrr and high Vf, or a diode with low Vf and high Qrr. This curve can be achieved by changing the current density or lifetime suppression.

Figure 1 Qrr-Vf relationship curve of a diode.

　　Generally speaking, the larger the chip size, the forward voltage drop Vf will also decrease due to the reduced current density. This helps to improve the heat dissipation capacity of the chip, but at the same time the switching loss increases and the cost also increases. will be improved.

　　For a given current density and chip size, reduction of carrier lifetime by 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 the carrier lifetime can reduce the accumulated charge Qrr in the device, but reduces the conduction performance and increases the forward voltage drop Vf; extending the carrier lifetime can reduce the forward voltage drop Vf, but increases switching losses. Most practical diodes use one or more lifetime control methods, with the exception of rectifier diodes. Rectifier diodes have very low frequencies and high conduction loss requirements, so it is not always necessary to reduce the carrier lifetime.

Figure 2 Thermal resistance depending on chip size

　　For the diode technologies discussed in this article, changing either the current density or the chip size results in very similar curves. This article chose to change the current density and performed related calculations. This approach means smaller diode chips, allowing for higher chip yields per wafer, thus cutting the unit price of the chip.

　　On the other hand, smaller chips have a higher junction-to-case thermal resistance RthJC, so the first thing that comes to mind is the need for a larger heat sink. But 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 soldering thickness of the lead frame.

　　However, in order to arrive at a final evaluation, it is necessary to have a deeper understanding of the total losses and the distribution of losses between IGBTs and diodes.

Figure 3 Rectification process from diode to IGBT

　　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 portion of the collector waveform represents the reverse recovery characteristics of the diode and the additional charge created by discharging the parasitic output capacitance. But the output capacitance part can usually be ignored because the IGBT capacitance is very small, so it can be assumed that this area is completely caused by reverse recovery. It can be seen that, first, when the IGBT voltage is still at a high level, the reverse recovery current has begun to flow. Secondly, the diode current tails around 100ns. It is obvious that the reverse recovery performance of the diode plays a very important role in the switching losses in IGBT.

　　Observing the distribution of power loss, we can see that the main power loss usually comes from IGBT, so IGBT will cause the diode chip to heat up. This situation will only change if the diode itself has higher losses, and the heat generated by the diode itself is higher than the losses and heat generated by the IGBT. From a product perspective, it is advantageous to increase the temperature of the diode, which can reduce the overall losses and the IGBT junction temperature. Under rated conditions, optimal loss distribution is achieved when the IGBT junction temperature is equal to the diode junction temperature.

　　This means that although the optimized diode may achieve a higher RthJC due to the smaller chip size, this does not affect the performance of the IGBT combined with the diode because the overall power consumption is reduced. Compared with EmCon2 technology, the new anti-parallel diodes with EmCon3 technology have higher forward voltage drop, improved reverse recovery characteristics and lower switching losses.

Figure 4 Optimized loss balance of diode (RthHS = 4.2 K/W, TA = 50℃, cosΦ= 0.7)

　　This conclusion is consistent with most people’s understanding that diodes used in driving applications must be optimized for low conduction losses. contradiction. Especially in home appliance drives, such as inverter washing machines, low switching losses are also crucial. Because in those applications, the switching frequency can reach 15 kHz or higher. In this case, switching losses will constitute a large part of the overall losses in the drive and must not be ignored. This optimization opens the door to a variety of applications – not only in the drive market, but also in the so-called “high-speed” sector.

Figure 5 Switching losses of TrenchStop-IGBT using Vf optimized diodes (left bar graph) and final design diodes (right bar graph)

Benchmark of EMCON3 and EMCON2 technologies
　　The balance of power loss per unit ampere of two IGBTs with diodes is shown in Figure 4. The bar graph on the left shows the results of combining the latest EmCon3 technology with TrenchStop-IGBT (IGBT3 technology). As mentioned above, EmCon3 technology is optimized for lower switching losses and slightly higher forward voltage drop. The right bar graph shows the results of EmCon2 technology combined with TrenchStop-IGBT. The EmCon2 diodes used in this benchmark are anti-parallel diodes from Infineon's Fast-IGBT series. This diode is optimized for low forward voltage drop. In Figure 4, IGP10N60T is used, a heat sink with thermal resistance RthHS =4.2 K/W, and ambient temperature TA = 50°C, which raises the junction temperature to about 125°C. The switching frequency fP is 16 kHz, demonstrating the performance of the combination of IGP10N60T and EmCon3 technology. As can be seen from Figure 5, as expected, the IGBT conduction losses are not affected by the diode at all. The increase in Qrr of the Vf optimized diode has a great impact on the dynamic loss PvsI of the IGBT and the dynamic loss PvsD of the diode. Two effects combined: the increase in the dynamic losses of the diode itself and its effect on the IGBT overwhelm the advantages of the Vf-optimized diode during conduction. This characteristic is already very obvious when the switching frequency is about 5 kHz, and the higher the switching frequency, the greater the impact.

Figure 6 Thermal equivalent circuit for temperature calculation

　　Of course, determining the various parts of the loss balance for a specific hardware circuit design is not easy. Typically, engineers measure temperature on the enclosure or lead frame. The thermal resistance RthJC of the two diodes is considered to be the same. The thermal equivalent circuit of the combined system is shown in Figure 6. The constant ambient temperature creates a common case temperature TC, which is determined by the heat sink thermal resistance and the total losses of the IGBT and diode. Therefore, the different junction-to-case thermal resistances RthJCD and RthJCI of 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. With a junction temperature of nearly 125°C, the IGP10N60T combined with a Qrr-optimized EmCon3 diode achieves a lower junction temperature than the IGP10N60T combined with a Vf-optimized EmCon2 diode. In the bar graph on the left, the diode and IGBT temperatures are 4K lower, and the power loss is 0.7 W lower for the IGBT and 0.2 W lower for the diode. Due to the lower RthJC of IGBTs, the greater loss reduction of IGBTs has a smaller impact on the junction temperature than the relatively smaller loss reductions of diodes. So the temperature difference is the same.

Figure 7 Junction temperature formed by two combination systems

　　Of course, the loss reduction is also sacrificed by the smaller RthJC. However, calculations show that when the ambient temperature TA is 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 junction temperature of the IGBT is also 4°C lower. Therefore, the system overall benefits from the chosen diode optimization approach. If the same junction temperature as the final diode is reached, higher current can be drawn from the inverter, resulting in higher power output, as shown in Figure 8. On the other hand, for a given output current, the size of the heat sink can even be reduced, thereby reducing the cost of the drive unit. No matter which approach the designer uses, the system will achieve greater efficiency.

Figure 8 Output RMS current of a half-bridge in the inverter

Conclusion:
　　It is not enough to only consider forward voltage drop for diode optimization. IGBT technology and application conditions must be considered. In this article, the diode connected in parallel with TrenchStop-IGBT is designed according to IGBT technology and application conditions. These diode chips are smaller but can achieve lower junction temperatures than larger Vf optimized chips. This allows engineers to make greater use of IGBTs and diodes. It can reduce the size of the heat sink or increase the power output of a given system, cutting system costs.