15 igbt ps 12vce ic fsw t ont off eq16 main

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Unformatted text preview: .1.5 IGBT: Ps = 1/2*VCE * IC * fsw *(t on+t off ) Eq.1.6 Main emphasis in modern Power Electronics is on reducing total losses dissipated in devices and subsystems for achieving higher operating efficiency and more compact designs, reducing volume and weight of resultant systems. Thus, operation at higher and higher switching frequencies is now a necessity, and as a result, switching losses predominate in power-lossbudget in semiconductor switches. Reducing switching losses then becomes the single most crucial goal. Keeping this goal in mind, drive circuits should be so designed as to yield ultra fast rise (t r) and fall times (t f ). Assuming sum of tr and tf be no more than 2% of the PWM period in hard switching applications, a Table:1 is prepared of 1000 Volts rated MOSFETs of three types: standard, Low Gate Charge types and FClass, which are optimised for megahertz switching. Table: 1 shows, for a given fsw, peak current required to drive it, recommended external gate resistor, total power loss in driver+gate resistor and the driver IC used - All for SMPS designs of 500W, 1KW and 2 KW ratings. While standard MOSFETs are optimally usable up to about 400 kHz, Low Gate Charge MOSFETs give adequate performance up to 800 kHz in hard switching SMPS applications. Designing above 800 kHz in hard switching mode, the levels of RFI/EMI noise and switching losses are excessive and hence soft switching (resonant mode, in which sum of tr and tf should be no more than 10% of the total one cycle PWM period) is preferred. The third section of rows in Table:1, shows how F-Class MOSFETs perform with the same driver ICs at 1MHz, 2 MHz and 4 MHz in resonant switching mode. Both, in hard switching and in resonant switching modes, trying to achieve values of tr and tf less that what are assumed tend to generate higher electrical noise and oscillations in drain current, while increasing them will tend to reduce overall efficiency. One can appreciate that energy lost in gate drive system, say, @ fsw =100 kHz for a 500 W SMPS, is only 0....
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