Project Analog - April 2008 - (Page 7)

the gate charge value. Figure 1 shows the gate charge typical characteristic curve for the same MOSFET as it varies with gate voltage and drain voltage. Make sure the gate charge value you use for calculating power dissipation fits the conditions of your application. Taking a typical value from the graph in Figure 1 for VGS = 10V, we get a total gate charge of 98 nC (VDS = 400V). Using the relationship Q = C * V, we get a gate capacitance value of 9.8 nF, which is significantly higher than the 2.6 nF input capacitance that is specified in Table 1. This illustrates the fact that when a calculation calls for a gate capacitance value, the total gate capacitance value should be derived from the total gate charge value. gate-to-drain capacitance. A common mistake is to use the Input Capacitance rating of the MOSFET (CISS) as the total gate capacitance of the MOSFET. The proper method for determining gate capacitance is to look at the Total Gate Charge (QG) in the MOSFET data sheet. This information is typically shown in the Electrical Characteristics table and as a typical characteristics curve in any MOSFET data sheet. Table 1 shows a typical example of the data sheet representation of gate charge for a 500V, 14A, N-channel MOSFET. Note that the values given in the data sheet table have conditions associated with them: gate voltage and drain voltage. These conditions affect When using maximum values for gate charge from the Electrical Characteristics table for worst-case design, the values must be adjusted for the drain-to-source and gate-to-source voltages in your design. Using the MOSFET information provided above as an example, the power dissipation in a MOSFET driver due to the charging and discharging of the gate capacitance of this MOSFET with a VGS of 12V, a switching frequency of F=250 kHz and a drain-to-source voltage of 400V would be: EQuation 4 PC = CG x V2 x F PC = 9.5 x 10–9 x (12)2 x 250 x 103 PC = 342mW Contents Viewpoint Calculating Power Dissipation Accelerating Amplifier Design Driving Power MOSFETs Analog news taBlE 1 The proper method for determining gate capacitance is to look at the Total gate Charge (Qg) in the MOSFET data sheet. Microchip analog page MOSFET driver overview Sample center microchipDIRECT Reference designs/ app notes Technical training 7 · 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

Table of Contents Feed for the Digital Edition of Project Analog - April 2008

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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