[pbsurf]

The Dart and nearly all other wall-plug AC to DC converters use a rectify-invert-rectify method. They first rectify the 50/60Hz mains AC to DC, then convert it back to AC at much higher frequencies in the kHz - MHz range, or in the case of the Dart, hundreds of MHz, and finally rectify it again to produce the final DC output. For a given amount of power output, at higher frequency less energy has to be stored in the circuit per cycle (i.e. stored in reactive elements like capacitors), which enables more compact components to be used.

[pd0wm]

However, there is also a downside to a higher switching frequency. In the inverter transistors are used to generate the high frequency AC voltage.

A transistor generates no loss if it is in the full on or full off state (saturated). But every time a transistor switches from the on to off state (or back) it goes through its linear region. While in the linear region the transistor acts as a resistor and generates heat. If you increase the switching frequency the transistor switches more often and thus generates more heat.

The solution to this problem is to use more efficient transistors or decrease the switching time (the time it takes to switch from high to low, or back).

Of course higher switching frequencies also have lots of other problems such as radiation, skin effect, etc.

[msandford]

> A transistor generates no loss if it is in the full on or full off state (saturated)

Not entirely true. They make a lot LESS loss when fully on than when linear, but there's still some loss.

Even with a highly efficient transistor you can still get losses while in the linear region if your gate drive circuit can't push enough current. When designing a switching power supply you don't just hook the microcontroller output to the gate of the transistor. To do it right you might need one or two or three intermediate stages of power amplification so that you can switch the main transistor's gate very quickly.

[jjoonathan]

How does switching current behave in the limit of power transistors? I know that it's significant in high-performance ASICs but I could certainly imagine the junction capacitance of a power-MOSFET being small enough that kHz or even low MHz switching would cost a miniscule fraction of the power being switched. I mean, I've been able to switch the floating gates of sizable MOSFETs by waving my hand at them, I'd be surprised if the power usage at tens to hundreds of kHz was more than a few mW.

[zAy0LfpBZLC8mAC]

There are at least three different ways switching a transistor consumes energy, the power for charging the gate is only one of them and tends to not be the limiting factor in small power bricks. The other two are current spikes in the channel during the switch in logic circuits (because you normally cannot let the output float, which means that during the switch you have to switch on both transistors at the same time, but that's not a problem when driving some inductor/transformer), and energy loss due to voltage drop while the transistor is moving between on and off states (that's where all the heat tends to come from - apart from the heat from the rectifying/free-wheeling diodes).

[jjoonathan]

Ah, ok. That makes sense.

[dskhatri]

Another solution is to use soft-switching (zero-current or zero-voltage switching) methods.

[hackcasual]

It's not AC because the flow of current doesn't reverse direction. It's pulsed on/off. The last stage isn't rectification, it's an LC circuit to smooth out the resulting voltage. The reason higher frequency switching is for smaller value capacitors and inductors.

[davepage]

If we're talking off-line power supplies, it most definitely _is_ AC for the simple reason non-ideal transformers will pass no DC. And, further, inductors and capacitors (LC) are all merely losses to DC signals. These components only have a meaningful function for AC waveforms (even though, in the case of a buck converter, a DC signal is superimposed).