As we understand now turning mosfetigbt on and off

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Unformatted text preview: quate power supply bypassing, layout and mismatch of driver to the driven MOSFET/IGBT. As we understand now, turning MOSFET/IGBT on and off amounts to charging and discharging large capacitive loads. Suppose we are trying to charge a capacitive load of 10,000 p from 0 to F 15 VDC (assuming we are turning on a MOSFET) in 25 ns, using a 14 amp ultra high speed driver. I = VxC / t Eq. 5.1 -12 -9 = 6 A I = (15-0)x10000x10 / 25x10 What this equation tells us is that current output from driver is directly proportional to voltage swing and/or load capacitance and inversely proportional to rise time. Actually the charging current would not be steady, but would peak around 9.6 Amps, well within the capability of 14 Amp driver. However, driver IC will have to draw this current from its power supply in just 25 ns. The best way to guarantee this is by putting a pair of by-pass capacitors (of at least 50 times the load capacitance) of complementary impedance curves in parallel, very close to the VCC pin of the driver IC. These capacitors should have the lowest possible ESR (Equivalent Series Resistance) and ESL (Equivalent Series Inductance). One good example of this is high quality surface mount type monolithic ceramic capacitors. Other preferred type is SMD Tantalum. One must keep the capacitor lead lengths to the bare minimum.. A smart way of accomplishing this is to solder the capacitor across the Vcc and ground pin of driver IC from the bottom (solder side). Another very crucial point is proper grounding. Drivers need a very low impedance path for current return to ground, avoiding loops. The three paths for returning current to ground are: 1. Between driver IC and the logic driving it; 2. between the driver IC and its own power supply; 3. between the driver IC and the source/emitter of MOSFET/IGBT being driven. All these paths should be extremely short in length to reduce inductance and be as wide as possible to reduce resistance. Also these ground paths should be kept distinctly separate to avoid re...
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