The voltage-ripple dependence relationship on x-ray energy-spectral values (energy fluence per unit interval of photon energy) and exposures at 70-kV peak were obtained theoretically by using the semiempirical formula of emission spectra given by Birch and Marshall [Phys. Med. Biol. 24, 505 (1979)]. The calculations were performed with and without various thicknesses of aluminum. As the ripple increases, the energy-spectral values decrease as expected. When the ripple is large, however, energy-spectral values (per mAs) take the minimum values; therefore, the exposure (per mAs) also reaches the minimum value for the unsaturating current modes, contrary to expectation. The reasons for this phenomenon were clarified. Exposures clearly take the minimum value in 2-pulse units. This phenomenon was experimentally verified.