Simplified Analysis Of A DCM Boost Converter Driving An LED String Part I: Theoretical Analysis

Focus:

The design of a DCM boost converter involves certain challenges such as properly stabilizing the design. Although small-signal models exist for performing this type of analysis, some difficulty arises in trying to apply these models to LED driver applications such as those encountered in automotive LED backlight designs. That’s because the ac analysis of a boost converter driving LEDs differs from that using a standard resistive load. As the series diodes impose both dc and ac loading conditions, deriving the final transfer function is not simple. In this article, the authors present an easier method for analysis. Rather than using the classical small-signal model of the DCM boost converter, they derive a simplified model based on the output current expression for the converter and then use this model for analysis. This small-signal model represents the first-order response of the boost converter in DCM operation driving an LED string. This model is sufficient for stabilizing the control loop. After deriving the model, its associated transfer function, and operating points, the authors run a Spice simulation to test the model on an example circuit.

What you’ll learn:

• How to simply model a boost converter driving an LED string under DCM operation

Notes:

In part II of this article, (Practical Considerations), the authors delve into an implemented solution and verify empirical results versus the theoretical derivation.

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Author & Publication:

Christophe Basso, ON Semiconductor, Toulouse, France and Alain Laprade, ON Semiconductor, East Greenwich, R.I., How2Power Today, Feb 15 2013

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