Constant Power Supply That Can Tolerate a Phase Control Dimmed Signal
I have an application for a 5V 1-2W power supply that would be used on a lighting fixture. Because it is on a lighting fixture, it may be put on a phase control dimmer (120V/60Hz). I don't want the power supply to be affected by the dimmed signal through as much of the dimmed range as possible. What kind of design might be best for this application?
Thank you in advance,
Torrey Bievenour
Comments
Yes, you are correct, this design does not need to include PFC.
However, I think you may be slightly misinterpreting my application. This circuit will be running in parallel with another load. That other load will be responsible for providing the required load to keep the triac operating properly. This load may be incandescent, CFL, or LED, but it is a separate design from what I am talking about here.
The circuit in question will actually be powering a small set of electronics that drive a specialized cooling system (Synjet system by Nuventix). These require the constant voltage 5V supply @ 1-2W. I want to make sure that this will continue to operate down to a very low dimming level (say 5-10%).
Thanks,
Torrey
Now I understand, take a look at DI-152 on our website. This power supply has a very wide input voltage range that allows it to operate down to 18 VAC.
Z. Cochrane
I'll take a look at that design, thanks.
There are several issues to overcome for compatibility with triac dimmers. I am assuming for you low power application that you do not require power factor correction. I will address the issues for non-power factor corrected ballasts. The first issue is that most inexpensive dimmers use a phase delay pulse to turn on the triac. The triac need a minimum current to keep itself conducting (hold current). If the gate pulse occurs before the bridge rectifier begins to conduct, the triac will turn off for the rest of the half line cycle. After several skipped line cycles, the LED ballast's bulk capacitor will discharge to a point that current will conduct through the triac. Then skip several other line cycles. The result is that the LED will flicker on and off. I have seen a transistor circuit that will turn on a resistive load when the instantaneous line voltage drops below a certain voltage. This load current needs to be greater than the triac's hold current. This circuit will waste a good part of a watt (not good for your efficiency).
Another issue is that when the triac conducts, the inrush current along with the line inductance and ballast's input capacitance will create a damped oscillation on the line current. This will prematurely terminate the triac's conduction which will cause the LED to flicker. Another circuit uses a series resistor (about 100 Ohms) with the line to reduce the inrush. A transistor with a RC delay circuit will turn on a few microseconds after a voltage is present and shunt the current by the resistor. This helps reduce the losses of the circuit. You will need to check if someone has patented these circuits for LED applications.
As far as the actual LED ballast design, there are several options. For non-isolated designs, look at our website for RDR-131. For an isolated design look at DI-185 on our website.
Z. Cochrane