Project Analog - April 2008 - (Page 8)

loss in the driver. For some MOSFETs, driving the gate voltage above 8V to 10V does not result in any further decrease in MOSFET resistance (RDS-ON). Using the same MOSFET as above as an example, a 10V gate drive results in the following power dissipation: EQuation 6 PC = QG x V x F PC = 98 x 10–9 x 10 x 250 x 103 PC = 245mW fig 1 fig 2 Contents Viewpoint Calculating Power Dissipation Accelerating Amplifier Design Driving Power MOSFETs Analog news The 16% reduction in gate voltage (going from 12V to 10V) resulted in a 28% reduction in power dissipation due to gate drive. Further savings will also be seen in the cross-conduction losses due to gate drive voltage reduction. Equation 3 represents the power dissipation due to MOSFET driver cross-conduction, or what is commonly referred to as shoot-through. This is a result of the P-channel and N-channel FETs in the output drive stage being on at the same time as they transition between the on and off states. Cross-conduction characteristics are shown in the MOSFET driver data sheet as a typical characteristic curve and as “Crossover Energy vs. Supply Voltage”. An example of this is shown in Figure 2. The units for the crossover constant are typically shown as Amp-Seconds (A*sec). Multiplying this number by The value for CG is arrived at by using the graph in Figure 1 and finding the value for QG at 12V. QG is then divided by 12V to get the CG value. Knowing that QG is equal to CG * VG the equation for PC could be rewritten as: EQuation 5 PC = QG x V x F A note of importance is that the voltage in this equation is squared. Therefore, a reduction in the gate drive voltage can result in a significant reduction in power Microchip analog page MOSFET driver overview Sample center microchipDIRECT Reference designs/ app notes Technical training 8 · prOjECT ANAlOg · Apr 08 http://www.microchip.com http://www.microchip.com/analog http://www.microchip.com/analog http://www.microchip.com/mosfetdriver http://www.microchip.com/mosfetdriver http://sample.microchip.com/Default.aspx?testCookies=true http://www.microchipdirect.com/catalogselection.aspx?returnURL=default.aspx http://www.microchip.com/stellent/idcplg?IdcService=SS_GET_PAGE&nodeId=1469&filter1=function&redirects=appnotes http://www.microchip.com/stellent/idcplg?IdcService=SS_GET_PAGE&nodeId=1469&filter1=function&redirects=appnotes http://www.microchip.com/stellent/idcplg?IdcService=SS_GET_PAGE&nodeId=1423

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Project Analog - April 2008 Contents Viewpoint About Project Analog Sponsor Calculating Power Dissipation in a MOSFET Driver Accelerating Design of >50-W Class D Amplifiers Driving Power MOSFETs in Switch Mode Power Supplies Synergistic MOSFET Solutions Trends in MOSFET Gate Drivers Analog News—Analog news from multiple sources Enter to win an iPhone Contact Project Analog Sponsor Treelink Microchip Advanced Parts Selector (MAPS)

Project Analog - April 2008

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