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Posted on 01 October 2019

High Voltage SPT+ Diodes

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The Perfect Match

The newly developed SPT+ diode technology platform for 3.3kV, 4.5kV and 6.5kV diodes for next generation high power IGBT modules is described in this article. The new diode range offers low losses and soft recovery characteristics combined with a high reverse recovery safe operating area and superior surge current capability.

By A. Kopta, M. Rahimo and U. Schlapbach, ABB Switzerland Ltd, Semiconductors

 

The main challenge in the design of high voltage diodes for IGBT application is to ensure low losses combined with soft reverse recovery behaviour. The high stray inductances encountered in these applications, together with design restraints mainly given by the need for a high immunity against cosmic ray induced failures, have a strong impact on the diode performance. With the recent introduction of the next generation of high voltage SPT+ IGBTs comprising significantly reduced losses, the development of a new diode generation matching the performance of these IGBTs has become inevitable. Today, state of the art high voltage diode designs utilize technologies comprising either local lifetime control, or the usage of low concentration diffusion profiles to control the emitter efficiency of the anode and cathode emitters.

In this article, we present a newly developed technology utilizing a double local lifetime-control technique to optimize the on-state charge distribution in the diode. Thanks to the improved plasma distribution, the overall losses were reduced, while maintaining the soft recovery characteristics of the standard technology. This new diode technology is referred to as SPT+, where the abbreviation SPT stands for Soft Punch-Through, referring to the soft reverse recovery characteristics of the diode.

Figure 1 shows the maximum output current as a function of the switching frequency of the new 3.3kV/1500A HiPak module comprising 24 SPT+ IGBTs and 12 anti-parallel diodes using both SPT and SPT+ diodes. The figure shows the module output current in inverter mode (black curve) as well as in rectifier mode for the standard SPT diode (blue curve) and the new SPT+ diode (red curve). The standard SPT diode has too high total losses and would clearly limit the output current of the module in rectifier mode. At a switching frequency of 400Hz, the output current would only be 1250A as compared to the SPT+ IGBT capability in inverter mode of nearly 1500A. By using the new SPT+ diode with lower total losses, the output current in rectifier mode can be increased to match the inverter mode performance over the entire frequency range. Therefore, the main objective was to develop the new SPT+ diode technology with the required loss reduction to match the capability of the SPT+ IGBT. At the same time, the diode softness and ruggedness had to be at least as good as in the original technology to ensure that the new diode could be switched as fast as for the standard one. A lower di/dt capability would otherwise increase the IGBT turn-on losses, which would adversely decrease the output current in inverter mode and in this way limit the module performance.

Simulated output current as function of the switching

SPT+ Diode Technology

In Figure 2, a cross-section of the SPT+ diode and the corresponding carrier lifetime profile can be seen. The SPT+ diodes utilize the same silicon resistivity and thickness as well as anode and cathode diffusion profiles as the original SPT diodes. On the anode side, a high-doped P+ emitter is used. The emitter efficiency is adjusted with a first local lifetime peak placed inside the P+ diffusion profile as for the standard SPT diode. In order to control the plasma concentration in the N-base region and on the cathode side of the diode, the SPT+ technology utilizes a second local lifetime peak, placed deeply inside the N-base from the cathode side. In this way, a double local lifetime profile as shown in the right part of Figure 2 was achieved. With this approach, no additional homogenous lifetime control in the N-base as used in the SPT technology, is necessary. The new technology has been applied successfully for the full high voltage range including 3.3kV, 4.5kV and 6.5V.

SPT+ diode cross-section and carrier lifetime profile

In Figure 3, a comparison of the simulated on-state plasma distribution between the SPT+ and the standard SPT diode can be seen. Homogenous lifetime reduction in the N-base, as employed in the SPT diode, leads to a hammock shaped plasma distribution (red curve). The low plasma concentration in the middle part of the diode results in high conducting losses, whereas the high plasma concentration on the cathode side results in a long reverse recovery current tail and high recovery losses, without offering clear immunity against current snap-off under all conditions. In the SPT+ diode, the more advanced double local lifetime control scheme using Helium irradiation leads to an improved plasma distribution resulting in a shorter current tail and lower recovery losses. By controlling the depth and the concentration of the second local Helium lifetime peak, an optimum trade-off between losses and recovery softness can be achieved.

Simulated on-state hole density in high voltage SPT and SPT+ diodes

In Figure 4, the total reduction of the on-state losses as compared to the SPT technology for the entire voltage range can be seen.

Reduction of the on-state voltage drop achieved by the SPT+ diode technology

In Figure 5, the technology curve of the 4.5kV SPT and SPT+ diodes can be seen. Both diodes have an active area of 0.8cm2 and were characterized using the SPT+ nominal current of 83A, which corresponds to a current density of 105A/cm2. Under these conditions, the original SPT diode has an on-state voltage drop of 3.2V and 255mJ recovery losses. The final SPT+ diode design has about 150mV higher on-state voltage drop but only 155mJ or 40% less recovery losses compared to the standard diode, which represents a significantly improved technology curve.

Technology curve of the 4.5kV SPT and SPT+ diodes under nominal conditions

In Figure 6, the on-state characteristics of the 4.5kV SPT+ diode are shown. The diode has a positive temperature coefficient of the onstate voltage drop (VF) already well below the nominal current level, which is necessary to ensure good parallel operation within the IGBT module. At rated current and 125°C, the diode has a typical on-state voltage drop of 3.4V, or 400mV higher than at room temperature. Under the same conditions, the standard SPT diode only has a difference of 200mV between the 125°C and room temperature voltage drops.

4.5kV SPT+ diode on-state characteristics at room temperature and 125°C

In Figure 7, the recovery waveforms measured under nominal conditions for both SPT and SPT+ diodes can be seen. The second local lifetime control peak used in the SPT+ diodes significantly reduces the plasma concentration on the cathode side, which reduces the recovery current tail and thereby the recovery losses. In spite of the fact that both diodes use the same Helium irradiation in the anode, due to the missing homogenous lifetime reduction, the SPT+ diodes have a higher plasma concentration on the anode-side of the N-base. This causes the SPT+ diode to have an increased peak current (IRR), which can have negative effects on diode softness and SOA. On the other hand, this also slows down the initial voltage rise during recovery (dV/dt), which reduces the stress on the electrical insulation in the driven motor. The plasma concentration in the middle of the Nbase can be controlled by the depth and irradiation dose of the second local lifetime Helium peak. Since the reduction of the cathodesided plasma concentration can be critical for the diode softness, the irradiation scheme in the SPT+ diodes had therefore to be thoroughly optimized in order to reach the desired characteristics. By choosing the parameters of the second Helium peak properly, the SPT+ diode can be made as soft as when utilizing the traditional irradiation scheme of the standard SPT technology. It was also concluded that a long current tail is not always necessary to achieve soft recovery behaviour. The shape of the current tail given by the shape of the remaining plasma is much more decisive for the softness then the actual tail length. In this way, the SPT+ diodes are designed to achieve the best trade-off between losses and softness.

4.5kV diode reverse recovery waveforms under nominal conditions

Reverse Recovery Ruggedness

The reverse recovery safe operating area (SOA) of the new SPT+ technology was extensively investigated and compared to the standard SPT technology. For the 6.5kV diodes, the SOA limit was measured using a high DC-link voltage and a high stray inductance (VDC = 4500V, Ls = 3uH). In Figure 8, the SOA reverse recovery waveforms of the SPT+ diode can be seen. The diode shows an extremely rugged performance with a peak-power of 600kW/cm2 under these extreme conditions. The high recovery robustness was achieved thanks to the combination of a highly doped anode emitter, which prevents any reach-through effects and a carefully designed charge shape.

6.5kV remove SPT+ diode SOA-waveforms at VDC = 4500V

In Figure 9, the surge current waveforms of the 4.5kV SPT+ diode are shown. The measurements were made on module level, which means that 12 diodes with a total active area of 9.5cm2 were tested in parallel. The pulse duration was 10ms in this test. The diodes reached a peak current of 12.4kA, corresponding to an I2t value of 830kA2s before failing. The achieved surge current capability is thereby very similar to the one of the standard SPT diodes. The different irradiation schemes do not have an influence on the capability. The high surge current capability is achieved thanks to the strongly doped and deeply diffused anode and cathode emitter profiles.

Surge current waveforms of the 4.5kV SPT+ diode on module level. The I2t value is 830kA2s

Conclusions

In this article, a newly developed high voltage diode technology for IGBT-modules was presented. The new diode technology uses a double-sided local lifetime control to adjust the on-state charge distribution. The new diodes offer significantly reduced total losses combined with soft reverse recovery behaviour and high ruggedness.

 

 

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