Dimmer - Random Phase Light Dimmer Using IGBT, Second Try
I previously mentioned that Jon wanted to try and simplify the design be getting rid of the photocoupler and the circuit that powers it. His reasoning was that it was just more stuff that could break. So, I moved the main power supply inside the bridge and referenced it to the 'bottom' of the bridge. This means we can drive the IGBT directly.
Here is the schematic for what Jon and I call the 'Inside Power Supply' design. Notice that this is a much simpler design.
The inspiration for this design can be found in this app note from ST Micro.
If I set the dimmer's duty cycle to 50%, then here is the first 300ms.
The green curve is the 6V power rail for the opamps. The red curve is the 3.3V power rail for the CPU (and it ADC). The yellow curve represents the current through the load. The purple curve represents a half cycle of the voltage. The idea is to take the yellow and purple curves and feed them into two ADC inputs on the CPU and do the power calculation in the CPU.
So, at 50% dimming with a 570W (25ohm) load, the power dissipation of the 'ramp' is 16.76mWs (= 8W * 4.19ms * 0.5 Estimating it to be a triangle). The power dissipation of the 'spike' is 420uWs (= 280W * 3us * 0.5 Esitimating it to be a triangle). Over the course of a half-cycle, that is an average power dissipation of 2.15W (= (16.7mWs + 0.42mWs) / 8ms).
I did this experiment with 50% dimming and a 1KW (14.4ohm) load, the power of the 'ramp' peaks at 18W and the 'spike' goes to 480W. At those numbers, the average power dissipation is 3.4W
Hmm... That seems like a lot of heat to dissipate.
***** EDIT 2010-06-15 *****
I have been messing around with this design and have decided to abandon it. The reason is that the power supply is too dependent on the load. If you remove the load (take the light out of the light socket), then the power supply stops working. I thought we had avoided this issue by making the power supply tap the neutral directly. However, since the return path is still through the load (when phase is high), the flow of current is blocked by no load. This makes the power supply 'freeze'. It works with very light loads. However, if you attach a 10K resistor or less to the output of the load, then it freezes (Spice refuses to converge).