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A New Inverse Charge Constant On-Time (IQCOT) Control for Noise Performance Improvement in Multiphase Operation

Image of conventional COTCM control structure, ripple-cancellation effect, and small I<sub>sum</sum> ripple at <i>D</i> ≈ 0.4.
Fig. 1. (a) Conventional COTCM control structure. Fig. 1. (b) Ripple-cancellation effect. Fig. 1. (c) Small Isum ripple at D ≈ 0.4.
Recently, ripple-based constant on-time current-mode (COTCM) control has been widely used for its excellent small-signal property. Fig.1 shows an example of COTCM multiphase operation using a two-phase structure with a pulse-distribution method, in which the summation of all phase inductor currents (Isum) interacts with the voltage-loop compensator output (Vc) to generate the duty cycle. The issue with this control strategy is that when the duty cycle approaches the ripple cancellation point (where summation of the inductor current ripple becomes zero, i.e., D = 0.5 for two-phase operation), the ripples become smaller and smaller. This ripple cancellation effect at different multiphase operations is shown in Fig. 2. Fig 3 illustrates that when the duty is close to the ripple cancellation point (D ≈ 0.4 for two phases), the inductor current ripple becomes very small. In that case, any noise in Vc or Isum can create jittering at the output, and control becomes very noise-sensitive.

This paper proposes a new COTCM control method based on the inverse charge control concept (IQCOT control). In Fig. 4, a two-phase COTCM control with the proposed structure is shown. In this control, unlike conventional COTCM, Vc-Isum is used to charge a capacitor and in every cycle, the cap voltage (Vramp) is compared with a VTH to generate the duty cycle for control. The advantage of the proposed IQCOT control is that, as it is not a ripple-based control, when the duty cycle is approaching the ripple cancellation point, and the inductor current ripple becomes very small, there is no noise impact, since modulation depends on the Vramp signal, which is still very large. Furthermore, at the ripple cancellation point, when the ripple is zero, the converter can still operate normally, as Vramp can still be generated by the voltage difference between Vc and IL × Ri. This is shown in Fig. 5, where for four phases, the converter is operating at the ripple cancellation point (at D = 0.25), and the ripple current summation ILsum is zero. But fsw is determined by the Vramp signal, which is 2V in this case. Fig 6 shows the test results for the proposed control. As illustrated, in two-phase operation with D = 0.5, Isum is almost negligible.
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