0026-2714/$ - see front matter Ó 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.microrel.2005.07.100 Microelectronics Reliability 45 (2005) 1738–1741 www.elsevier.com/locate/microrel Non-destructive Testing Technique for MOSFET’s Characterisation during Soft-Switching ZVS Operations Francesco Iannuzzo DAEIMI – Università di Cassino – V. Di Biasio, 43 – 03043 CASSINO (FR) – ITALIA Phone: +39 0776 299 3741 – Fax: +39 0776 299 4325 – e-mail: iannuzzo@unicas.it Preferred session: poster Abstract. The use of the MOSFET’s embedded body diode in soft-switching applications often leads to the device failure also at reduced power. A non-destructive diagnosis equipment is proposed and employed to systematically investigate the breakdown, as a support to theoretical investigations. INTRODUCTION n the scenario of low- and medium-voltage power conversionapplications, MOSFET devices are largely considered the best choice for the switches of the converter. At the same time, special emphasis has been given in the last years to the study of soft-switching topologies, due to the advantages achievable in terms of losses reduction and switching frequency increase. In this class of converters, reactive components are placed in the circuitin orderto switchthe deviceat zero voltages or currents (ZVS or ZCS, respectively) and the freewheeling diodes, which are necessary to allow the inverse path for the load currents, are often employed to make the switches themselves to turn-on at zero voltage. Many research efforts have been done in the past 20 years to use the MOSFET’s intrinsic body diode as freewheeling diode, thus avoiding the usage of an external diode, but severe reliability limitations have been encountered. Some studies have been done on the instabilities occurring duringthe reverse-recovery of the intrinsic diode [1]-[3], and on the techniques oriented to improve the diode reliability in hard-switching conditions [4]-[8]. Even if the major problems to use the intrinsic MOSFET diode appear to be recently solved by means of lifetime-control techniques [4], [6], the soft-switching operation causes a new category of instability phenomena, apparently not ascribable to classic mechanisms, and requires further studies [9]-[12]. The aim of the present work is to propose a testing apparatus capable of reproducing the stresses conditions of the MOSFET’s intrinsic diode during soft-switching operations at controlled temperature, in presence of a protection circuitry capable of saving the device under test (DUT) by quickly removing the power from it. This objective is reached by means of a over-rated couple of switches which have been connected in series and in parallel to the DUT and are activated at increasing delays from the DUT commutation, till the failure instant. EXPERIMENTAL OBSERVATION Some experimental failurewaveforms at soft- switching conditions are brieflyintroduced and commented in the following. The experimental set-up is a classical Phase-Shift Bridge (PSB) topology using MOSFET devices (see Figure 1), in which the turn-on commutation of the four devices takes place during the diode conduction, i.e. in ZVS conditions. The optional capacitors provide the turn-off ZVS capability too. In our experiments, the devices are commercial new-generation N-channel MOSFETsrated at 500V - 0.08: - 45A@25°C. The dc-link is at 300V, the load current is 10A and the operating frequency is 100kHz. The waveforms of Figure 2 have been acquired by a Tektronix TDS784scope,and illustrate the breakdown during normal operations. The first two traces are the voltages over the MOSFETs of the same bridge leg, and the third is the load current (less important). The trigger of the scope has been set on the dc-link current (not reported), and has been tuned at a threshold of 50A which indicates a leg short-circuit condition. At the middle of the time axis, a failure occurs. In the inset of Figure 2 a 500ns/div detail is reported, showing that after the normal commutation, an instability characterized by a burst of high-frequency oscillations takes place in the MOSFET related to I Figure 1 – The analyzed Phase-Shift Bridge topology. Ó 2005 Elsevier Ltd. All rights reserved.