Violating limited conversion ratio of boost controllers causes all sorts of proplems. Consider interleaving boost power stages instead to increase effective switching frequency.
Running the boost stages 180 degrees out of phase provides a number of advantages. With 280W of output power and a 90+ percent efficient power supply, there is 18W of power dissipations. Two phases provide the opportunity for precise control of currents and hence dissipations in the semiconductors and inductors. It also spreads the heat to facilitate cooling.
Since the phases are running 180 degrees apart, there is input and output ripple current cancellation, which reduces both the peak and the RMS ripple current in the capacitors. Since the inductor currents are out of phase, the effective ripple current frequency is twice the frequency of each phase. Basically, you double the effective switching frequency of the power supply with no impact on its efficiency.
The chart below shows the efficiency of this power supply versus load current. There are three domains to the efficiency curve. The lower domain efficiency is limited by the overhead losses of control and gate drive. As the current is increased, these losses become less dominant, and switching losses become more significant, At higher current levels, the efficiency falls due to increased conduction losses in the FETs and inductors. The efficiency peak could be moved to the right with lower resistance parts.
High efficiency is demonstrated by a 24-140V, two-amp boost.
Boost controllers have a limited conversion ratio set by minimum off time of the controller and the operating frequency. It is important to be mindful of this limit. Violating it will cause pulse skipping, increased output ripple with lower frequency content, and erratic power supply operation. Interleaving boost power stages provides a method of increasing the effective switching frequency. Very high ratios are possible, and this article has provided a design example with a ratio of 6:1.
Please join us for the next Power Tip, where we will look at using boost converters for higher conversion ratios. For more information about this and other power solutions, click here.