For my internship, I am tasked to design a power supply whose main purpose will be to power a 90 Vdc motor. The motor is used for a short amount of time (roughly 30 seconds), pulling about 1 A during most of this time, but towards the end of its use as torque increases it can pull up to about 3.1 A for a few seconds. Since the operation time is very short and the motor will not be used very frequently, efficiency is not a key issue. Additionally, since the load is a motor not much regulation is required and quite a bit of ripple is tolerated.
To accomplish this, I have decided to use a simple series pass transistor with a 91 V zener diode as a reference. Input will come from mains 120 Vac, 60 Hz, with a rated output of 90 Vdc, 4 A. The design will also be transformerless in order to reduce cost and size. My design right now uses a full bridge rectifier to convert 120 Vac to 170 Vdc, with a capacitor to reduce the amount of ripple. I am wanting to use a npn Darlington configuration for the series pass transistor but this is where I have questions.
When looking at transistor power ratings, is this average power or instantaneous power? Going from 170 V to 90 V requires a Vce of 80 V, so at rated current instantaneous power will be over 300 W. Due to the amount of ripple, average power will be much less than this. Either way, the Darlington will have to have a very high power rating and a large heat sink. The highest power rating Darlington that I have been able to find is the MJ11032 (300W) but I do not know if this is suitable for this application. Are there any other transistors out there that are better suited for this or do you have any suggestions on other ways to go about this?
Lastly, I know there will be concerns about safety, but all of my designing right now is on the computer and when I get to the prototyping stage this will be done with the help of an experienced engineer. Also the power supply will be completely enclosed in the final product, and all interface will be done via a separate 24V side that has already been designed.