Quote:
Originally Posted by Jetijs
Another interesting observation. If I put the rotor so that one phase is in the firing position and then just pulse the coil with high frequencies so that the rotor can't move because of too small current, the transistor of that phase gets almost freezing cold. I found this very interesting. This is the first time I actually observed this radiant cold effect.
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Are you sure that MOSFETs are getting cold? Could it be possible that they're simply don't heat up as much as you observed before? If the latter is the case it would be consistent with the high frequency short duty cycle because of the very small current consumption resulting in extremely small power dissipation on MOSFETs.
Please do measurements to verify two important parameters of this configuration you're using. First of all measure current consumption of phase you're firing (put shunt resistor between Source of MOSFET and minus) and turn on RMS measurement on your DSO. RMS current measurement should give you pretty good estimation of the real current levels you're dealing with and you can easily calculate power dissipation on MOSFET for the given case. When dealing with frequencies over 1kHz and noisy, distorted signals most multimeters will give you false reading so you would really want to check that out with oscilloscope as I suggested in order to avoid false readings. A simple rule of thumb- when dealing with pulsed, distorted, noisy, high frequency signals trust only DSO measurement or if you have them, the mesurements of RF thermal voltmeter and ampermeter (or thermal wattmeter).
Also, when everything is turned off for some time in order to equalize with ambient temperature, dismount heat sink from MOSFET and press thermometer to it (body temperature thermometer will do fine for now). Now turn on circuit in the configuration that you suspect you're getting cooling effect and leave it running for at least 5 minutes. Thermometer should be pressed firmly to the MOSFET and if any cooling is occurring you should be able to see at least some temperature variation. During the measurement take care that you ambient temperature don't change and that thermometer is pressed to MOSFET with constant force and with some mechanical device in order not to transfer your body heat to the thermal connection.
When you get your IR thermometer you can make precise measurements but the measurements I suggested should give you at least vague idea if you got something tangible in your circuit or if it's but a combination of various factors playing tricks on you.
