A sliding-mode switching power supply (24) having N phases (28) and a method of operating the power supply (24) are provided. N switches (30) are coupled to a bipolar power source (22), with each switch (30) effecting one phase (28). An inductance (32) is coupled to each switch (30), and a capacitance (36) is coupled to the inductances (32). A load (26) is coupled across the capacitance (36). A monitor circuit (38) is coupled to the inductances (32) and the capacitance (36) and configured to monitor currents (I.sub.L) through the inductances (32) and/or a voltage (V.sub.C) across the capacitance (36). A sliding-surface generator (78) is coupled to the monitor circuit (38) and generates a single sliding surface (.sigma.) for all phases (28). A constant-frequency control (104) forms a variable window (.DELTA..sigma.) for the sliding surface (.sigma.). A switching circuit (138) switches the switches (30) at a switching frequency (f.sub.S) determined by the variable window (.DELTA..sigma.). The constant-frequency control (104) adjusts the variable window (.DELTA..sigma.) to maintain the switching frequency (f.sub.S) substantially constant.