Pro/Forums - View Single Post - Fanless PSU build-up, first phase

Brians256 · 12-02-2002, 04:48 PM

This has got to be one of the best currently active threads, so keep up with such great informative posts!

I will add a bit of info to the MOSFET question, however. The heat generated by the MOSFET's is related mainly to two characteristics: on-resistance and switching speed.

The on resististance is shown as RDS(on) in the spec sheets, because it is the resistance between Drain and Source when the MOSFET is fully on. The lower this number is, the less power is wasted during the time when the MOSFET is supplying energy to the PSU output filters.

The switching time is usually specified as maximum frequency, and it is the time that it takes for the MOSFET to transition from fully off to fully on. A higher frequency means that the MOSFET can transition faster, and it spends less time in between the ON and OFF states. Why is this important? Because when the MOSFET is neither fully on or fully off, it is passing current at non-low resistance. At higher frequencies, the MOSFET could spend significant percentages of it's time in the transistion state, where it is wasting energy as heat. This speed/frequency is also related to the gate capacitance and the energy losses can be even worse if the driving circuitry cannot quickly overcome the gate capacitance.

A switching power supply attempts to keep the MOSFET either fully on (with low resistance) or fully off (with near infinite resistance). In either case, the power loss is low.

Another side benefit of faster MOSFET transitions is that the inductor becomes more efficient. Inductors store energy when the voltage changes. The faster the change, the more energy is quickly stored by the inductor.

There is indeed a whole science and art to this subject. If you want a very simple low power SMPS circuit to examine, look here for a cheap example. That is a single voltage low power circuit and I still don't fully understand it. I've been looking at it for the last two weeks and I still don't understand it completely. I don't consider myself slow, and so I think I am beginning to understand why these power supply engineers pull in a good salary. It's complicated.

12-02-2002, 04:48 PM	#43
Brians256 Pro/Staff Join Date: Oct 2001 Location: Klamath Falls, OR Posts: 1,439	This has got to be one of the best currently active threads, so keep up with such great informative posts! I will add a bit of info to the MOSFET question, however. The heat generated by the MOSFET's is related mainly to two characteristics: on-resistance and switching speed. The on resististance is shown as RDS(on) in the spec sheets, because it is the resistance between Drain and Source when the MOSFET is fully on. The lower this number is, the less power is wasted during the time when the MOSFET is supplying energy to the PSU output filters. The switching time is usually specified as maximum frequency, and it is the time that it takes for the MOSFET to transition from fully off to fully on. A higher frequency means that the MOSFET can transition faster, and it spends less time in between the ON and OFF states. Why is this important? Because when the MOSFET is neither fully on or fully off, it is passing current at non-low resistance. At higher frequencies, the MOSFET could spend significant percentages of it's time in the transistion state, where it is wasting energy as heat. This speed/frequency is also related to the gate capacitance and the energy losses can be even worse if the driving circuitry cannot quickly overcome the gate capacitance. A switching power supply attempts to keep the MOSFET either fully on (with low resistance) or fully off (with near infinite resistance). In either case, the power loss is low. Another side benefit of faster MOSFET transitions is that the inductor becomes more efficient. Inductors store energy when the voltage changes. The faster the change, the more energy is quickly stored by the inductor. There is indeed a whole science and art to this subject. If you want a very simple low power SMPS circuit to examine, look here for a cheap example. That is a single voltage low power circuit and I still don't fully understand it. I've been looking at it for the last two weeks and I still don't understand it completely. I don't consider myself slow, and so I think I am beginning to understand why these power supply engineers pull in a good salary. It's complicated.