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Motor Control question: I have a few FET concerns. My concern is that on the falling edge, the gate signal remains at 5V for some microseconds. The time it hangs there is a function of the value of R22. If R22 is increased, the hang time will also increase. I would assume that the 4N35 optoisolator parameter is involved. The other area is how to understand and calculate the temperature rise. I currently see a max temperature of 180 F for the FET under a 1.5 amp load after 1 minute. I believe that it is the turn off time because the turn on time is done at the zero cross and there should be no switching lost there. I would like to have the FET Temperature as low as possible for 1.5 amp draw for 2 minutes. On page 5, the motor voltage appears to flat-top at 150VDC, I understand that this is an inductive load and this may because it is an inductive load but I would like your opinion, I don't see the same flat-topping if a ;light-blub; dummy load is driven at same current level.On page 6, the drain voltage is at 5 because of the O'scope but really is above 100VDC. My concern here is that VDS begins to increase from 0 to 2 volts over 4 mS. This tells me the Rds is changing. The Rds is spec'ed at 0.55 ohms but if only 2 amps is drawn then I would expect this to be around 1 volt? On page 7, the plot provides the no-load VDS of the motor.
I would try the following changes to your circuit: replace D5 with a shorting wire; remove R22, remove T7, change the value of R24 to 500 ohms or less. This is what I believe is happening: Transistor T7 is not able to discharge the gate of the fet effectively due to its Vce(on). I believe you can leave out T7 and its associated components, replace D5 with a piece of wire (it is really redundant because the opto-isolator has a bjt output), and use a lower value for R24 to discharge the gate capacitance. R24 should be as low a resistance as the opto isolator can effectively drive as a load. Also, I would recommend that you try using the MTP16N25E mosfet (lower rds(on) and still rated at a high enough voltage for your rectified ac line motor drive voltage). (You can order samples via www.onsemi.com webpage.) Another way to effectively drive the gate is to use one of ON Semiconductor's gate driver ICs, which provide a totem pole (source ; sink) output to actively drive the gate both high and low.
Answered by: ON Semiconductor
2011-06-15 17:57:18.0