Project Analog - August 2008 - (Page 9)

rating can be written as: dT = [dV x C] / I Where: dT = turn-on/turn-off time dV = gate voltage C = gate capacitance (from gate charge value) Knowing the relationship for gate charge is: Q=CxV The above equation can be rewritten as: dT = Q / I Where: Q = total gate charge The relationship shown in the equations above assumes that a constant current source is being used for the current (I). By using the peak drive current of the MOSFET driver, some error will be incurred. MOSFET drivers are rated by the driver output peak current drive capability. This peak current drive capability is generally given for one of two conditions. Either the MOSFET driver output is shorted to ground or the MOSFET driver output is at a particular voltage value (usually 4V, as this is the gate threshold voltage value which the MOSFET begins to turn on and the Miller effect comes into play). The peak current rating is also generally stated for the maximum bias voltage of the part. This means that if the MOSFET driver is being used with a lower bias voltage, the peak current drive capability of the MOSFET driver will be lower. deSIGn example Using the following design parameters, the MOSFET driver peak output current rating will be found: MOSFET gate charge = 20 nC (Q) MOSFET gate voltage = 12V (dV) Turn-on/turn-off time = 40 ns (dT) Using the equation referenced earlier: dT = Q / I I = Q / dT I = 20nC / 40ns I = 0.5A The equation has produced a peak drive current requirement of 0.5A. However, the gate drive voltage in the design parameters is 12V, and this must be taken into account when selecting the appropriate driver. For instance, if the driver you are selecting is rated for 0.5A at 18V, the peak output current at 12V will be less than 0.5A. For this reason, a driver with a 1.0A peak output current at 18V would be chosen for this particular application. Any external resistance between the MOSFET driver output and the gate of the power MOSFET will also need to be taken into account, as this will reduce the peak charging current supplied to the gate capacitance. concluSIon There are many parameters to consider when matching the appropriate MSOFET driver to the MOSFET in your application. As with any electronic device, no one device is appropriate for every application, which is why Microchip Technology supplies a variety of MOSFET driver current rating, output drive polarities and input logic configurations. mIcrochIp RESOURCES Motor Control Design Center http://www.microchip.com/ stellent/idcplg?IdcService=SS_ GET_GE&nodeId=2125¶m =en026178 Training Resources for Motor Control http://www.microchip.com/ stellent/idcplg?IdcService= SS_GET_E&nodeId=2125& param=en535758 Motor Control Design Solutions Brochure http://ww1.microchip.com/ downloads/en/DesignCenter/ 00896e.pdf AN799: Matching MOSFET Drivers to MOSFETs http://ww1.microchip.com/ downloads/en/AppNotes/ 00799b.pdf Contents Viewpoint Gate Drive Schemes for Motor Control Applications Considerations When Selecting the Proper MOSFET Driver Power Modules for Variable-Speed Motor Control Analog news Microchip analog page Motor Control Design Center Sample center microchipDIRECT Reference designs/ app notes Technical training 9 · PrOjECT ANAlOG · VOluME 2 / NuMbEr 5 http://www.microchip.com/stellent/idcplg?IdcService=SS_GET_PAGE&nodeId=2125¶m=en026178 http://www.microchip.com/stellent/idcplg?IdcService=SS_GET_PAGE&nodeId=2125¶m=en026178 http://www.microchip.com/stellent/idcplg?IdcService=SS_GET_PAGE&nodeId=2125¶m=en026178 http://www.microchip.com/stellent/idcplg?IdcService=SS_GET_PAGE&nodeId=2125¶m=en026178 http://www.microchip.com/stellent/idcplg?IdcService=SS_GET_PAGE&nodeId=2125¶m=en535758 http://www.microchip.com/stellent/idcplg?IdcService=SS_GET_PAGE&nodeId=2125¶m=en535758 http://www.microchip.com/stellent/idcplg?IdcService=SS_GET_PAGE&nodeId=2125¶m=en535758 http://www.microchip.com/stellent/idcplg?IdcService=SS_GET_PAGE&nodeId=2125¶m=en535758 http://ww1.microchip.com/downloads/en/DesignCenter/00896e.pdf http://www.microchip.com http://ww1.microchip.com/downloads/en/DesignCenter/00896e.pdf http://ww1.microchip.com/downloads/en/DesignCenter/00896e.pdf http://www.microchip.com/stellent/idcplg?IdcService=SS_GET_PAGE&nodeId=79&redirects=analog http://www.microchip.com/stellent/idcplg?IdcService=SS_GET_PAGE&nodeId=79&redirects=analog http://www.microchip.com/stellent/idcplg?IdcService=SS_GET_PAGE&nodeId=2125¶m=en026178 http://ww1.microchip.com/downloads/en/AppNotes/00799b.pdf http://www.microchip.com/stellent/idcplg?IdcService=SS_GET_PAGE&nodeId=2125¶m=en026178 http://ww1.microchip.com/downloads/en/AppNotes/00799b.pdf http://sample.microchip.com/Default.aspx?testCookies=true http://ww1.microchip.com/downloads/en/AppNotes/00799b.pdf 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 - August 2008

Project Analog - August 2008 Contents Viewpoint About Project Analog Sponsor Gate Drive Schemes for Motor Control Applications Considerations When Selecting the Proper MOSFET Driver for Your Application Power Modules for Variable-Speed Motor Control Efficient Motor/Controls Save Terrawatt-Hours/Year Analog News Contact Microchip Treelink Microchip Advanced Parts Selector (MAPS)

Project Analog - August 2008

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