February_EDFA_Digital

edfas.org 27 ELECTRONIC DEVICE FAILURE ANALYSIS | VOLUME 21 NO. 1 last power-off. When start-up performances are tested, they need to start up by command many times and they always fail to work, or fuses burn out. The solution is generally to prolong the interval between power-off and start-up. If failures occur due to instant power-off of AC power, this can be solved by replacing the AC power with a UPS power supply. • Sudden change of load from low to heavy, and vice versa, while closed loop within the DC/DC converter performs well. When this second-order LC resonant circuit is excited, there is both a transient response and a steady-state response. The steady-state response is the desired drive signal. The transient response, which is parasitic within the steady-state response, is an exponentially decaying signal with its frequency f T dependant on L M, L W, and C. (Eq 1) where L M ismagnetizing inductance; L W iswire inductance; and C is the total capacitance in the drive circuit. In a direct-coupled gate drive circuit as shown in Fig. 1a, there is no L M . Therefore, L M =0. C ismainly speed-up capacitance C2. Frequency of parasitic ringing is approx- imately several hundred kHz. In Fig. 1b, input capaci- tance C iss and wire inductance L W can be neglected. C is the equivalent capacitance of the primary AC coupled capacitor C2 and secondary capacitor C3. As L M and C are respectively in the order of millihenry and microfarad, f T is in the order of kHz. The transient response ismixedwith the steady-state response. The total response can cause an overshoot of gate-to-source voltage beyond the speci- fied maximum rated voltage, resulting in a breakdown of gate-to-source. If the transient response amplitude is more than gate threshold voltage, a switching MOSFET will also respond to it. If on time is long enough—in other words, the frequency is low enough compared to the DC/DC converter’s switch frequency—the power trans- former will saturate, leading to primary high current. This primary high current is hazardous and could destroy switching transistors or cause input fuses to burn out. This hazardous parasitic LC ringing cannot be elimi- nated, but it can be damped by the series resistors, which include the PWM/driver output impedance R dri , external gate resistor R G in the drive circuit such as R1 in Fig. 1a, and the internal gate mesh resistor R int . R dri is decided by the PWM/driver and cannot be adjusted. Yet, R int is very small and therefore can be neglected. The only adjusted component is gate resistor R G . The series resistor, which can critically damp LC resonance, can be calculated by: (Eq 2) Rearranging equation (2) yields, (Eq 3) There is an urgent need to increase the resistance of R G to damp the ringing. But it will also result in a slow turn-on of the MOSFET, and therefore low efficiency of the DC/ DC converters. Consequently, it is difficult to decide R G . Typical resistance of R G ranges from 5.1 to 30Ω. A gate- to-source resistor such as R3 in Fig. 1b generally aims to protect the gate fromelectrostatic discharge breakdown. It can also be used to damp the parasitic ringing by sup- pressing the amplitude to the accepted limit, which is (continued on page 30) (b) (a) Fig. 1 Gate drive circuits adopted by some manufacturers. (a) Direct-coupled gate drive circuit. (b) Transformer- coupled unipolar gate drive circuit. (c) Transformer- coupled bipolar gate drive circuit. (c)