Power supply fails when PWM an halogen lamp
Let me start by saying that dimming halogen bulbs is not generally a good idea in terms of bulb lifetime. The halogen cycle that re-deposits evaporated tungsten back onto the filament is optimized for full-brightness operation, and is much less effective at lower temperatures.

Your 12V, 50W bulb draws 4.1A at full brightness. When cold, it will draw about 10&times; that amount, or about 40A. Note that this is an instantaneous value &mdash; if you are PWMing the voltage, the average current is the peak value multiplied by the PWM duty cycle. This means that at low duty cycle values, the average current drawn from your power supply ramps up linearly. If the bulb resistance didn't change, it would ramp all the way from 0A to 40A.

However, at high duty cycles, the bulb resistance does rise, reducing the instantaneous current as well as the average current, arriving at the final value of 4.1A at 100% duty cycle. Taken together, these effects mean that there is a maximum value of average current somewhere in between the two extremes, which apparently happens at 350/500 duty cycle in your setup. The average current at this setting is probably about 28A, based on extrapolating the ramp &mdash; well over the 23A rating of your power supply.