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This is my first question on stackexchange so if I do anything wrong, please let me know :-)

So my situation is the following:

I am working on a power supply module where I need 7 separated +28V DC outputs up to 25A. My current design incorporates of a small transformer-rectifier-regulator module with adjustable output that I want to use as a "pilot module". Then feed the output voltage of this "pilot module" into 7 "power modules" where on each of them it will steer a NPN transistor. The NPN transistor I'm thinking of using is the 2N3771 in TO3 package, rated at 50V, 30A, 150W.

Now I need to convert the 230V AC from my mains into a lower DC voltage for the power transistor emitter. Traditionally I would do that using a transformer, followed by a rectifier and smoothing cap. However, in this case I could see that getting very costly and hard to obtain, both using 7 different transformers or 1 big (huge) transformer.

So I was thinking about replacing the mains-transformer-rectifier-capacitor with mains-rectifier-capacitor-MOSFET and control the MOSFET with a pwm signal. Would this work? What would be the advantages and drawbacks of this approach? If I use a transformer, the output current will be lower then the input current while with a MOSFET this wouldn't happen. So could I double the load on the MOSFET given it will only be switched on half the time? Or would the peak current damage the MOSFET?

Thanks to everyone taking the time to read and answer this already :-)

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  • \$\begingroup\$ Interesting question. This approach might be followed for circuits which did not draw much current, and/or which needed a voltage close to that of mains. But without a transformer, it will otherwise be too inefficient. (Although PWM is often used in switch mode power supplies - but still with a transformer, just using high frequencies). Transformers add a level of safety since they isolate the supply from the mains. \$\endgroup\$ – CL22 Mar 30 '15 at 15:14
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I would not recommend that: it would result in enormous power wastage. The reason being, during the on part of the PWM, the circuitry (and cables, component leads etc) between the mains supply and the capacitor would be acting basically as a (very low value) resistor which would dissipate the heat. For example, the approximate average voltage across the MOSFET/cables/rectifier would be 200v. In other words, very inefficient. Lastly, There would have to be a current-limiting feature controlling the MOSFET, otherwise the surge in current would also blow fuses, because in essence it would just be one giant short circuit.

But yes, regarding the peak current through the MOSFET, the MOSFET will have graphs and other specs which will define separately, the peak current (for example over extremely short pulses) as well as a total power dissipation which if the MOSFET is pulsed on and off could essentially be doubled for your purposes. But calculations would have to take into account the effectiveness of what heatsink you have etc.

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  • \$\begingroup\$ Interesting! Would the peak current thing also work for other parts then MOSFETs, like resistors, bulbs etc.? \$\endgroup\$ – Daniël Mar 30 '15 at 16:57
  • \$\begingroup\$ Possibly, yes. Although unless stated in the specifications, there's a good chance it will shorten their life. And if you're not careful, yours!! Whatever you do, please just remember that even in this day and age, there are clever, careful, experienced people losing their lives to electricity (and associated risks e.g. fire). Don't be one of them. Health and safety rules and regulations are there for a very good reason. \$\endgroup\$ – CL22 Mar 30 '15 at 23:01
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Transformers provide for necessary isolation and safety. Those can be the big older style heavy iron core jobs that operate at the line frequency of say 50 or 60 Hz. More modern types of power supplies use transformers with ferrite types of materials in their cores that operate in the 10's to 100's of kHz range allowing for much smaller transformers.

You will be generally discouraged from trying to produce your +28V outputs directly from the rectified line voltage without going through a high frequency transformer to provide safety and isolation.

There are a few applications where this may be OK but you have provided no statement of what the application is and so one must guess that at 28V there is an expectation of low voltage providing a safe usage environment for users of the power source. If you rectify right off the line without a transformer isolation then a part of your device's output is referenced directly to the mains wiring and that could lead to a deadly situation for users.

One last thing of caution. The nature of the questions you are asking and the fact that you are already picking out specific transistor part numbers without a real understanding of what you are setting out to do leads to suggestion to slow down. Get a lot more familiar with primary to secondary power conversion techniques, requirements and agency approvals needed so that you can approach this problem in a fully informed way.

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  • \$\begingroup\$ I second these arguments. +1 \$\endgroup\$ – CL22 Mar 30 '15 at 15:02
  • \$\begingroup\$ I have never seen those other transformers yet I think? The only transformers I could find that would fit my requirements is 7.5kg (16.5 lbs). 7 of those would make my supply very bulky and heavy at once. Could you give me a hint where I could get those? \$\endgroup\$ – Daniël Mar 30 '15 at 17:05
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    \$\begingroup\$ @Daniël - Recommend that you start by researching what is inside of current technology high current output switching power supplies. You could, for example open up some PC type power supplies to get a look. But once again "Get a lot more familiar with primary to secondary power conversion techniques, requirements and agency approvals needed so that you can approach this problem in a fully informed way." Just focusing on specific transistor part numbers or weight of transformers is the totally wrong direction approach to this problem. \$\endgroup\$ – Michael Karas Mar 30 '15 at 19:23

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