Categories Lighting

Comparing Critical Conduction Mode and Discontinuous Mode PFC Designs for LED Lighting Applications

Engineers building LED lighting applications have to be constantly aware of evolving regulatory requirements for power factor (PF), Total Harmonic Distortion (THD) and efficiency. Today regulatory agencies generally require PFC to be >.9 for designs > 5W at the nominal line voltage for that design. But the trend is for regulatory agencies to continually push the requirements for PFC down to lower and lower power levels.

A similar movement is underway in THD requirements. Currently regulatory agencies require <25 percent THD for applications > 25W. But we are already beginning to see agencies worldwide push that requirement down to < 10 percent in many applications. While there are no specific THD requirements for light bulb applications currently, it seems clear designers can expect them in the near future.  Given these trends it appears clear that designers building LED lighting applications today need to achieve as high a PF as they can and a THD as low as they can over as wide an operating voltage as possible.

In terms of efficiency, regulatory agencies have set some standards for power levels <20W.  A non-dimmable power supply for that voltage needs to be at least 85 percent efficient across nominal line voltages. Regulatory agencies reduce that requirement for lighting applications using phase-modulated dimming because the dimming circuitry requires external components that run at a fixed loss. But, again, regulatory requirements are clearly moving toward higher efficiency requirements.

Most AC-input and isolated power supply designs for lighting applications use a flyback circuit to meet PFC requirements. This single stage flyback PFC solution typically incorporates a Critical Conduction Mode (CrCM) control IC, which operates with a fixed on time, variable off time. This approach offers excellent constant current or constant voltage regulation at the output. But it was originally intended for boost PFC applications, not single stage flyback applications. With circuit modifications for load control and the addition of a fast start circuit, designers can achieve passable efficiencies and reasonable PF and THD. But the CrCM approach has some inherent characteristics that impose upper limits on how well it can perform in those applications. Furthermore, the CrCM architecture adds a number of additional components that impact reliability and cost.

Another popular option is the Discontinuous Mode (DCM) PFC approach. In this topology the power supply operates with a fixed on-time and fixed frequency for any given line load configuration. By doing this, it eliminates some of the limiting factors that occur in a CrCM approach and allows designers to achieve higher PF and lower THD.

Recently Fairchild took two evaluation boards and built two comparable 50W LED lighting solutions using both the CrCM and DCM PFC architectures. Both solutions worked well and each offered distinct advantages. But the CrCM approach offered PF, THD and efficiency performance that barely met current requirements for a class C lighting application. In contrast, the DCM board, based on Fairchild’s new FL7733A single stage, primary-side-regulated LED driver, significantly outperformed the first design in all three categories with a significantly simpler design. Given the trend in regulatory requirements, the DCM PFC approach may offer a better fit for LED lighting designers looking for more performance headroom in future applications.