As electronics devices move toward portable and mobile, battery becomes the major power source. However, due to characteristics of batteries, an output voltage of a battery pack could vary in a wide range between a fully charged state and a fully depleted state. Depending on the state of a battery, a charging voltage could be higher or lower than the battery voltage.
In addition, as USB Type C begins infiltrate the major market, voltage from a USB port is no longer fixed at 5V, and could vary in a wide range between 3.5V and 20V. In the meanwhile, downstream devices connected to such type of USB port may still need a voltage substantially around 5V, or close to the middle of 3.5V to 20V.
In all these situations, input voltage and output voltage of a power converter of an electronics device may crossover during a normal operation. Traditional BUCK (step down) converter or BOOST (step up) converter can only work with Input voltage either higher or lower, respectively, than output voltage. Thus, four-switch BUCKBOOST converter becomes the choice due to its flexibility with input and output voltage ranges.
A traditional control method for a BUCKBOOST converter is shown in FIG. 1. In the BUCKBOOST converter, all four switches are turned on and off once in every switching cycle. Also, energy of input power source is never transferred directly to an output. Instead, energy of input power source need to be stored in an inductor, and then passed on to the output. Thus, efficiency of the traditional BUCKBOOST converter is low. Further, the traditional BUCKBOOST has a high cost due to the need for high current rating devices. Other control methods based on peak current mode (PCM) or voltage mode (VM) control have also been used in controlling four-switch BUCKBOOST. However, all these control methods are based on fixed frequency control with clock signal to determine the timing of four switches.
Therefore, a new method is desired to control a four-switch BUCKBOOST converter.