V.A. Sankaran

Electrolytic capacitor life testing and prediction

The aluminum electrolytic capacitor is widely used in various power electronic circuits and systems such as 3-phase PWM inverters. Its functions include, bus voltage stabilisation, conduction of ripple current due to switching events, etc. In automotive applications, one of the big issues is the extreme and harsh temperatures they have to withstand, underhood, which during the summer months would reduce their life. Most of the capacitors have maximum temperature ratings of either 85 or 105/spl deg/C and a few are rated at 125/spl deg/C. According to literature found, one of the primary wear-out mechanisms in electrolytic capacitors is mainly due to the loss of electrolyte by vapor diffusion through the seals. Other reports suggest that the main wearout mechanism is deterioration of the electrolyte. Experiments conducted to date and the results from the same are compared against modeling results based on a model reported in literature.

Thermal analysis of high-power modules

A highly descriptive method for displaying heat flow in power modules is presented. Heat flow is studied for three different transistor-stack types: direct bond copper (DEC), thick-film printed substrate, and insulated metal substrate (IMS). DEC and thick film are thermally superior to IMS, but IMS shows potential. In addition, the effect of case-to-sink interface conductivity on heat flow is studied and shown to be of extreme importance in a proper thermal simulation.

Power cycling reliability of IGBT power modules

The goal of this study was to understand the power cycling reliability of IGBT power modules. These power modules are made up of multi-layer stacks and consist of multiple power dice in parallel. The interconnection schemes within the module include leadframes soldered to substrate, die attachment using solder and wirebonds. Thermal and power cycling fatigues material interfaces because of the CTE mismatch between dissimilar materials. In addition, wirebonds on the dice are prone to debonding because of the thermally induced stresses. Tests were designed to understand the power cycling reliability of these large transistor modules. Results from the tests are summarized in this paper.

Power electronics in electric vehicles: challenges and opportunities

The authors identify technical challenges in meeting customers expectations and possible opportunities for power electronics in EV (electric vehicle) development. It is emphasized that reliability, performance, safety, and cost are the major concerns of EV customers. Battery development is found to be the most challenging technical task. Development of other components should focus on reliability and efficiency. A systems approach is noted to be critical for success.<<ETX>>

Thermal analysis of high power modules

A highly descriptive method for displaying heat flow in power modules is presented. Heat flow is studied for three different transistor stack types: direct bond copper (DBC), thick film printed substrate, and insulated metal substrate (IMS). DBC and thick film are thermally superior to IMS, but IMS shows potential. In addition, the effect of case-to-sink interface conductivity on heat flow is studied.<<ETX>>

Role of the amplifying gate in the turn-on process of involute structure thyristors

The switching characteristics of involute thyristors with and without the amplifying gate structure are discussed. The effects of peak gate currents (10-100 A) on the anode current di/dt, switching delay, and energy loss in both types of devices are presented. The performance of the devices without the amplifying gate was far superior than that of the devices with the amplifying gate. A model is presented to explain this difference. Thyristors without the amplifying gate successfully switched anode currents on the order of 12.6 kA, at a di/dt of 100000 A/ mu s, from an anode voltage of 2 kV on a single-shot basis. >

High di/dt switching with thyristors

Two types of involute-structured SCRs, one with the amplifying gate shorted to the pilot gate (shorted devices), and the other with a normal gate arrangement (unshortened devices), were tested as closing switches in a PFN (pulse forming network). The shorted devices were able to switch larger anode currents at higher values of di/dt of the shorted devices. All energy losses in the shorted thyristors were less than the losses in the unshorted ones. The gate current amplitude had a strong direct relationship to the anode di/dt of the shorted devices. There seemed to be a very weak, if any, correlation between the gate current amplitude and the unshorted device di/dt. The gate pulse width has no measurable effect on the switching parameters. The results indicate that for high-current, narrow-pulse switching, the amplifying gate has a detrimental effect on the thyristor performance.<<ETX>>

High-energy pulse-switching characteristics of thyristors

Experiments were conducted to study the high energy, high di/dt pulse-switching characteristics of silicon controlled rectifiers (SCRs) with and without the amplifying gate. High di/dt, high-energy single-shot experiments were first done. Devices without the amplifying gate performed much better than the devices with the amplifying gate. A physical model is presented to describe the role of the amplifying gate in the turn-on process, thereby explaining the differences in the switching characteristics. The turn-on area for the failure of the devices was theoretically estimated and correlated with observations. This allowed calculation of the current density required for failure. Since the failure of these devices under high di/dt conditions was thermal in nature, a simulation using a finite-element method was performed to estimate the temperature rise in the devices. The results from this simulation showed that the temperature rise was significantly higher in the devices with the amplifying gate than in the devices without the amplifying gate. From these results, the safe operating frequencies for all the devices under high di/dt conditions was estimated. These estimates were confirmed by experimentally stressing the devices under high di/dt repetitive operation. >

Novel designs in power modules

This work is a brief comparison of the three different packaging technologies used in present-day power modules. Flux plots are used to diagnose the thermal problems of existing designs and to give credence to the superiority of the novel designs. The designs presented here make better thermal use of the materials in the power module stack to achieve better performance.

A comparison of power module transistor stacks

This work presents an analysis of the effects of substrate and baseplate thermal conductivity on thermal resistance for three power module technologies: direct bond copper (DBC); thick film printed substrate; and insulated metal substrate (IMS). The effects of fatigue induced solder voiding and thermal grease conductivity on thermal resistance are presented for DBC. Accurate modelling of the case-to-sink thermal grease interface is included. The effects of fatigue induced solder voiding are presented for DBC.<<ETX>>