Jon found some examples of light dimmers constructed using FETs and IGBTs. He was intrigued by these dimmers because they apparently had no filter inductor like triac based dimmers. That inductor can be quite large for higher amperage and make small/compact layout difficult. Also, these dimmers could be triggered at any point in the AC cycle which means they can be switched at points where the EMI can be minimized.

Triac based dimmers work by 'squelching' the current through the load at the zero crossing point and then turning on the current later in the half-cycle. These so called 'trailing edge' dimmers have a problem however. Turning on the current near the peak of the voltage (say, for 50% dimming) leads to a lot of unwanted EMI because the current snaps to full in a very abrupt manner. To keep EMI from bleeding back into the power grid, designers have put large LC filters (100uH inductors and 470nF/250V capacitors) on the mains. These filters consume a lot of space and can be expensive (especially the inductors).

To avoid these problems (noise, space), designers have turned to IGBTs and FETs. These devices can be turned on at the zero-crossing point (a very quiet place because there is no energy to switch at that point) then later, turned off 'slowly' so as to squelch the current with minimal EMI. IGBTs are especially suited for this task because they can be made to turn off slowly.

Of course, FETs and IGBTs are unidirectional. However AC power to a light is bidirectional so you have to come up with a way of rectifying the current and making it pass through the FET or IGBT in only one direction. This is accomplished by using a full wave rectifying bridge. By placing the load on the 'outside' of the bridge and placing the switch (either a FET or an IGBT) on the 'inside', then current can be controlled while still maintaining positive and negative voltage swing around the load.

The full wave rectifier does present a problem, however. The switch referenced to the inside of the bridge. However, you want to construct a power supply for the rest of the dimmer that is outside the bridge so that it is unaffected by the load. Once you do this, the switch control logic is most likely referenced to something very different than the switch itself. To get around this problem, you have to opto-couple the switch and the switching logic. Easily done with an optocoupler and a few components.

Here is a schematic for what Jon and I call an 'outside power supply IGBT dimmer'. In this example, the power supply is just a capacitive dropper (transformerless power supply) with an impossibly large dropper capacitor (C1 = 10uF). This was done just to accomodate Spice. Notice that the power supply is driven directly from 'phase' and 'neutral'. This makes it independent of what is going on with the load and the switch. Also notice that the power supply is referenced to 'phase'. This is because we wanted to locate the shunt in the phase path to prevent requiring the installer from breaking the neutral connection. All the sensors (current, voltage, reference) are referenced to this ground making it possible to measure the voltage drop across the shunt resistor without having to get rid of high common mode voltage.

We are using the IRG4BC20S IGBT in this example. It is pretty big for our purposes (our maximum current is only 10A) but I had a good spice model for it already so I just used it for this simulation. Notice that the IGBT is driven from an optocoupler (U1). The power supply for the IGBT side of the optocoupler is fed directly from neutral so it is not effected by what is going on with the load.

Below is a picture of the simulation output. The green curve is the 6V power supply used to supply the sensor opamps. The red curve is the 3.3V power supply used to run the zero crossing detector(Q4 & Q5). The deep purple curve is the voltage reference used as an offset for the sensed AC voltage. The sensed AC voltage is the yellow curve. And finally, the light purple curve is the sensed current. Notice that the voltage is a full sine wave, but the sensed current is only half of a half AC cycle. This is because the dimmer (simulated with V2) is set to 50%. It is also due to the fact that the shunt is only active for half a cycle. Thus, the current in the circuit is actually double what you see in the light purple curve. If another shunt was placed on the neutral side, then you should see equal (but positive going) quarter cycles when the AC voltage goes positive.

***** EDIT 2010-06-15 *****
I have been messing around with this design some more. I decided to just tap the power coming into the bridge to power the photocoupler instead of making an independent tap to neutral. This means that the photocoupler, which triggers the IGBT, will be dead when there is no load. However, this should not be a problem since there is no load. :)

Here is the new schematic.

Below are some measurements of the voltage (red) and current (purple) across/thru the load. Also, the supplied voltage (green) is shown.

Result. Here are the first 500ms of operation. It takes about 100ms for the potocoupler power supply to get up to voltage. After that, you can see the voltage and current clipped. The duty cycle was set to 50%.
Result Detail. Here is a close-up of a couple of cycles.
FFT. Here is an FFT of the voltage and current across/thru the load and an FFT of the power source. As expected, there are a lot of harmonics since the switching frequency is so low.
FFT Detail. Here is a close-up of the first couple of harmonics.