February_EDFA_Digital

edfas.org ELECTRONIC DEVICE FAILURE ANALYSIS | VOLUME 21 NO. 1 30 less than the 2V minimum gate threshold voltage. Saber software simulation typically shows not more than 2kΩ. However, this resistance is related to inductance, capaci- tance, and series damping resistance, as previously stated. For robust design, it is important to eliminate the speed-up capacitors such as C3 and C4 in Fig. 1c and (a) FUSE BURNOUT DUE TO GATE DRIVE CIRCUIT PARASITIC RINGING (continued from page 27) (b) Fig. 2 Advised gate drive circuits. (a) Direct-coupled gate drive circuit. (b) Transformer-coupled unipolar gate drive circuit. (c) Transformer-coupled bipolar gate drive circuit. (a) (b) (c) Fig. 3 Failure analysis graph of burnt fuse. (a) X-ray image. (b) Microscope image. (c) Zoomed-in microscope image. reduce coupling capacitance as much as possible. For low-power DC/DC converters, C3 and C4 make little con- tribution to high efficiency and can be eliminated. Ad- vised gate drive circuits are shown in Fig. 2. What is more important is their parameter value. Compared to Fig. 1a, there is no speed-up capacitor C2 in Fig. 2a; in Fig. 2b, there are two additional resistors, R1 and R2, to damp the parasitic ringing compared to Fig. 1b; in Fig. 3c, there are no capacitors C3 and C4, and there is an additional damp resistor R1 compared to Fig. 1c. EXPERIMENTAL RESULTS A 100V input, 1.2V, 15A output DC/DC converter with a 3A fuse located in a positive input line was applied in a Chinese satellite project. The converter with 130 kHz switch frequency adopts double-ended forward topology and MOSFETs as its main switching tran- sistors. The gate drive circuit is as shown in Fig. 1b, but without Q1 or D4. R3=10kΩ. When the converter started up a very short time after last power-off, the fuseburnedout. Three failure analysis graphs of the burned out fuse are shown in Fig. 3. Failure analysis revealed that the fuseburnedout due toexcessively high current. When the fuse was shorted, the con- verter continued to work. After starting up the converter again a very short time after last power-off, a MOSFET failed. Fig. 4 shows a gate drive waveformat this start-up. Approximately 6 kHz of parasitic ringing is evident in the normal 130 kHz drive signal. During the first period of parasitic ringing, the total amplitude of drive signal voltage is almost 20V, which is the specified maximum rating for gate-to-source voltage. The amplitude of parasitic ringing is about 10V, more than 4V (the maximum gate threshold voltage). MOSFETs respond to the parasitic ringing and are switched on. The power transformer satu- rates and consequently high input current occurs as shown in Fig. 5. The parasitic ringing’s amplitude (c)