© 2015 World Scientific Publishing Company. This paper deals with controllers of fast-scale instabilities in DC-DC switching power converters from a frequency domain standpoint with the aim of understanding their working principle and hence simplifying their design. Some approaches for controlling fast-scale instabilities and their limitations are revisited. Considering the frequency domain transfer function of already existing controllers, a simple and extended notch filter centered at half of the switching frequency is proposed to avoid these instabilities. However, a switching converter under this controller may still exhibit the undesired slow-scale instability. Accordingly, the paper explores an alternative approach based on amplifying the harmonic at the switching frequency. Numerical simulations show that the new proposed controller can concurrently improve both fast-scale and slow-scale stability margins. The results from the different controllers are contrasted in terms of stability boundaries, indicating that the last one presents a wider stability range.