4-Switch Buck-Boost DC-DC Converter: A Case Study

The need for DC-DC non-inverting Buck-Boost converters is ever growing to supply tiny systems, i.e., a regulated input voltage at 3.3 V while the battery voltage can vary widely from 4.2 V down to about 2 V. Efficient power management solutions are required to extend the battery life before recharging, to successfully address line and load transients and to be cost attractive. The use of a ripple-based control loop is advocated to meet stringent dynamic performances. However, this kind of control loop is more complex to analyze and the converter still subject to bifurcations. Stable operations whatever the operating conditions, are mandatory but not straightforward to assess. Literature covers abundantly to the authors' knowledge the stability analyses of Buck converters or Boost converters but not the Buck-Boost mode. The Buck-Boost mode's stability issues are pointed out with their consequences in terms of generated Electro-Magnetic Interferences through the study of a monolithic 4-Switch Buck-Boost converter (only the L-C output filter and some of the controller's capacitors are out-of-chip). An improvement to the Sampled-Data Modeling approach for stability analysis is proposed to enable the designer to adjust adequately controller's parameters and comprehensively verify the circuit stability other than by performing repeated transient simulations. The results of this augmented method are confronted to experimental measurements on the one hand and simulation at transistor-level of the full converter on the other hand.