by Yi Zhang, Saed Sobhani, Rahul Chokhawala
International Rectifier Applications Engineering233 Kansas St., El Segundo, CA, 90245 USA
Abstract - Snubber circuits can be used to protect fastswitching IGBTs from turn-on and turn-off voltagetransients. Snubbers are available in variousconfigurations and a clear understanding of theiroperation is necessary to make the appropriateselection. This paper will discuss pros and cons ofthese circuits. Decoupling capacitors, RCD voltageclamp circuit, RCD charge-discharge snubbers areincluded in the discussion.
Introduction
When a power device is abruptly turned off, trappedenergy in the circuit stray inductance is dissipated inthe switching device, causing a voltage overshootacross the device. The magnitude of this transientvoltage is proportional to the amount of strayinductance and the rate of fall or turn-off current. Thesituation is at its worst for fast switching IGBTmodules. These devices switch at a high magnitude ofcurrrents in a short duration of time, giving rise topotentially destructive voltage transients. The highercurrent modules normally consist of several IGBT chipsin parallel. Each individual chip switches its share ofthe load currrent at a di/dt that is determined by thegate drive circuit. The total current and di/dt seen bythe external power circuit is the sum of currents anddi/dts through each IGBT chip. The di/dts producedcould easily be a few thousand A/us. Proper attentionneeds to be paid to protect these devices fromdestruction.
It is determined that the snubbers offer optimizedprotection against voltage transients during the normalturn-on and turn-off switching. Usage of suchprotection circuits allow faster yet safer operation bycontaining the operating loci within the boundaries ofthe rated Safe Operating Area. This paper discussesvarious snubber circuits. Pros and cons of decouplingcapacitors, PCD snubber clamp circuits, RCD charge-discharge snubbers are discussed. Circuit operationsare analyzed and the test results are illustrated.
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Note that the fault current transients are moreeffectively protected by considerably slowing the rateof fall of fault current. Fault current turn-off protectionthrough electronic gate control is convered in detailelsewhere[1].
Decoupling Capacitors
As mentioned earlier, the magnitude of transient voltagedepends on the trapped energy in the circuit strayinductance, also call “DC loop” inductance LS. As apreventive measure, steps should be taken to improvethe circuit layout. Usage of laminated copper plates,minimizing the size of the “DC loop” and choosingsource capacitance with inherently low self inductanceare ways to lower stray inductance [2]. Decouplingcapacitors, connected across the module’s busterminals, as shown in Fig.1a, are found to be usefulfor low/medium current applications. High frequencypolypropolene film capacitors available today aredesigned to fit IGBT terminal spacing for directmounting. The resultant internal inductances aredrastically lower than conventional leaded capacitors.
Figure 1a.
Decoupling capacitor
Figure 1b.
Discharge restricteddecoupling capacitor.
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