May I make a suggestion? Use an optocoupler that is designed to drive MOSFET/IGBT gates.
The Avago HCPL3150 is an industry standard, I've used them many times: http://www.digikey.com/product-detail/en/HCPL-3150-500E/516-1584-1-ND/768375 or the very similar but cheaper VO3150 from Vishay: http://www.digikey.com/product-detail/en/VO3150A/VO3150A-ND/2096253 The opto-coupler/resistor arrangement you have works only for the smallest power devices, and only at relatively low voltages, currents, and switching speeds. FETs have significant gate to source capacitance, and if the driver doesn't charge or discharge that capacitance quickly, the result is very slow switching and a lot of switching loss due to the fact that the device spends a lot of time in an intermediate state between fully on and fully off. The devices also have a lot of gate to drain capacitance, which couples the large drain voltage back to the gate (Miller effect). That slows switching down even more, and under the wrong circumstances can lead to oscillation. The second set of scope waveforms is rather scary. The turn-off transition is not clean - the device switches on and off many times. At 40V and a few amps you can get away with it for a while. At higher power that could be instant death for the FET. Gate-driver optos have two major advantages. 1) They can source and sink much higher currents than the resistor arrangement. The 3150 drives up to a half amp into and out of the gate. (The slightly more expensive 3120 drives two amps.) This means much faster turn on and turn off. You control the actual speed by adding a single resistor between the driver and the gate. Unlike the pull-down resistor in the original circuit, there are no trade-offs dictating the resistor value. If you want slow switching, you can choose to use hundreds of ohms. If you want fast switching, you can go as low as twenty ohms or so (less than 10 ohms with the 3120). The ideal resistor depends on the size of the power device - bigger FETs need lower resistors. For the large (1700V, 450A) IGBTs I use at work the gate resistor is only a couple ohms - we have a booster stage that allows up to 10 amps of gate current. But a simple 3150 with a single resistor is typically all you need for 1200V IGBTs up to 50A or more. 2) They have undervoltage lockout. FETs and IGBTs don't like to be turned on half-way. What happens if your 15V power supply comes up slower or goes down faster than your 40V motor power supply? On a large drive where the motor supply is rectified line voltage, attempting to turn on the device when the control supply is too low results in an instant explosion. Gate-drive optos won't turn on until the supply on the isolated side is above a threshold. (See data sheet for exact value.) I've been designing motor drives for 20+ years. I can design a driver using discrete parts and conventional opto-couplers. But I haven't done that in years. The gate- drive optos do it at least as well as I can, in a fraction of the time and at a fraction of the cost. They let you ignore the nitty-gritty details of gate drive and move on to more interesting things. On Fri, Nov 28, 2014, at 12:29 PM, Kirk Wallace wrote: > Here is the start of my notes on playing with PWM and FETS. > > http://www.wallacecompany.com/machine_shop/LinuxCNC/pwm_fet/pwm_fet.html > > -- > Kirk Wallace > http://www.wallacecompany.com/machine_shop/ > http://www.wallacecompany.com/E45/ > > ------------------------------------------------------------------------------ > Download BIRT iHub F-Type - The Free Enterprise-Grade BIRT Server > from Actuate! Instantly Supercharge Your Business Reports and Dashboards > with Interactivity, Sharing, Native Excel Exports, App Integration & more > Get technology previously reserved for billion-dollar corporations, FREE > http://pubads.g.doubleclick.net/gampad/clk?id=157005751&iu=/4140/ostg.clktrk > _______________________________________________ > Emc-users mailing list > [email protected] > https://lists.sourceforge.net/lists/listinfo/emc-users -- John Kasunich [email protected] ------------------------------------------------------------------------------ Download BIRT iHub F-Type - The Free Enterprise-Grade BIRT Server from Actuate! Instantly Supercharge Your Business Reports and Dashboards with Interactivity, Sharing, Native Excel Exports, App Integration & more Get technology previously reserved for billion-dollar corporations, FREE http://pubads.g.doubleclick.net/gampad/clk?id=157005751&iu=/4140/ostg.clktrk _______________________________________________ Emc-users mailing list [email protected] https://lists.sourceforge.net/lists/listinfo/emc-users
