February_EDFA_Digital

edfas.org ELECTRONIC DEVICE FAILURE ANALYSIS | VOLUME 21 NO. 1 26 FUSE BURNOUT DUE TO GATE DRIVE CIRCUIT PARASITIC RINGING IN DC/DC CONVERTERS Guo Xianxin, Lanzhou Institute of Physics, China guoxianxin@126.com EDFAAO (2019) 1:26-31 1537-0755/$19.00 ©ASM International ® INTRODUCTION DC/DC converters sometimes suffer failures. Some of these failures are related to components or assemblies, such as burned MOSFETs, cracked packages, ferrite core cracks, and broken capacitors. These failures are com- monly attributed to overstress from electrical, thermal, or environmental sources. [1-3] Much attention is given to these reliability problems. [4] However, other failures are more subtle. One example is the burnout of a subsidiary input line fuse, resulting in no output from the DC/DC converter. This failure generally occurs at power-up and possibly more often when a converter is powered up a short time after the last power-off. If the failed fuse is repaired, the DC/DC converter returns to normal and can continue to work. The root cause of this failure is difficult to analyze—and the corresponding waveform is hard to capture—because it occurs in an instant, and the fuse will commonly protect DC/DC converters from further failures. This failure is most often associated with the design robustness of DC/DC converters. [5] Yet, thus far, failure analysts have paid little attention to this problem. This article presents a failure analysis of the gate drive circuit alongwith possible solutions. Experimental results validate the analysis and design. MOSFET GATE DRIVE CIRCUIT ANALYSIS Figure 1 shows typical MOSFET gate drive circuits of DC/DC converters from various manufacturers. Figure 1a shows a direct-coupled gate drive circuit, where C2 is a speed-up capacitor to promote efficiency. However, in some DC/DC converters, the pulse width modulator (PWM) is referenced to output ground; thus, an isolated drive transformer is needed to cross the input-output barrier. A drive transformer is also needed in buck regu- lators, double-ended forward converters, double-ended flyback converters, half-bridge converters, and full-bridge converters to drive the high-side switch transistor. Two transformer-coupled gate drive circuits are shown in Figs. 1b and 1c. In Fig. 1b, drive transformer T1, primary capacitor C2, and secondary capacitor C3 are necessary. A DC/DC converter made by a well-known manufacturer, which was applied in a burnout fuse DC power supply system, was disassembled. Analysis results revealed that the gate drive circuit matched the depiction in Fig. 1b. However, there is a subtlehazard in thesedrive circuits. In Fig. 1a, there is a parasitic ringing of wire inductance with capacitor C2 and theMOSFET’s input capacitor C iss . [6] Because the wire inductance is low, the frequency of parasitic ringing is generally high, and this ringing seldom induces a failure. In Figs. 1b and 1c, there are two types of storage elements: capacitors C2/C3/C4 and a magnetiz- ing inductor L M of the drive transformer along with a wire inductor L W . They form a second-order resonant circuit. The main component of inductance is the transformer’s magnetizing inductance, which is always very high. A tran- sient response in these drive circuits—an exponentially decaying, low-frequency parasitic ringing that can cause fatal failures—can be easily excited by a sudden change of duty cycle. [7] Some examples are as follows: • There is no soft-start circuit, or the soft-start time is very short, and the DC/DC converter starts up at step input voltage with a switch, such as a relay. This rarely occurs because there is generally a soft- start circuit with a working time on the order of a millisecond. • The DC/DC converter starts up a short time after the last power-off, such as instant power-off of input voltage. This is a common reason for the subtle failure of fuses or DC/DC converters. Some converters for military applications are controlled by a program control command or on-off command. The DC/DC converter may start up again a very short time after