Project Analog - August 2008 - (Page 6)

fiG 2 SiNGlE-ENDED GaTE DrivE TraNSformEr time balancing. With the solution shown in Figure 2, the capacitor, in series with the winding, is charged during the on time of the drive signal and then provides the negative bias/drive voltage to the transformer during the off time. This acts as the reset mechanism for the transformer and also the mechanism to provide the negative gate drive voltage to the power-switching element, which is often very useful and needed, if an IGBT is being used. If a MOSFET is being used as the switching element and the negative drive is not desired (negative drive often increases delay times), a few additional components can be added to the circuit to fix this issue. As shown Figure 5. With the addition of the diode and N-channel FET (low voltage, small signal type FET), the main N-channel MOSFET still sees the same positive level drive signal as before (minus a diode drop), but is clamped to zero volts during the off time. The diode blocks the negative bias that now turns on the small signal FET that clamps the gate-to-source voltage to zero. The second gate drive transformer drive configuration shown in Figure 3 is a double-ended type drive, meaning that the transformer is driven in both directions. This type of drive is often used for half-bridge and full-bridge topologies. The bidirectional drive, coupled with the dot polarity of the transformer, drives Q1 on and Q2 off and vice versa. If the duty cycles of the MOSFETs are modulated differently, additional gate drive circuitry may be required to balance the volt-time of the transformer. The same negative bias-blocking circuitry shown in Figure 5 can also be used in the double-ended drive scheme. The other feature of the gate drive transformer is that it can be driven from the secondary side with ground referenced circuitry. This means that it can provide a high voltage isolation boundary and allow the drive circuitry (PWM and MOSFET driver) to be ground-referenced and near the control circuitry, which is typically on the secondary side. This makes interfacing between the small signal-sensing circuitry (temperaturesensing, feedback loops, and shutdown circuits) and the PWM very easy. With the drive circuitry now ground-referenced, low-side MOSFET drivers can be used. This expands the selection of available devices and will reduce the cost of the driver. The circuit shown in Figure 4 is often used in very high power applications where IGBT/MOSFET modules are being used. In these applications, the IGBT modules are often located a slight distance from all of the control circuitry. This makes it difficult to bus the gate drive signal to the module as the inductance in the wires will cause ringing at the gate of the module. For this reason, the isolated bias circuit is often built on a separate PC card 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 fiG 3 DouBlE-ENDED GaTE DrivE TraNSformEr 6 · PrOjECT ANAlOG · VOluME 2 / NuMbEr 5 http://www.microchip.com 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://www.microchip.com/stellent/idcplg?IdcService=SS_GET_PAGE&nodeId=2125¶m=en026178 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 - 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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