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A New Inverse Charge Constant On-Time (IQCOT) Control with Fast and Optimized Transient Response
Year: 2018 | Author: Syed Bari | Paper: D2.10c
Fig. 1. (a) Proposed IQCOT control structure. Fig. 1. (b) Waveforms at load step-up transient. Fig. 1. (c) Waveforms at load step-down transient.
The proposed structure with conventional COTCM control, the IQCOT, is presented in Fig. 1. As shown, the difference between Vc and IL × Ri is converted into current by using a gm amplifier, and this current is used to charge a capacitor. Then, this capacitor voltage (Vramp) is compared with a fixed threshold voltage (VTH) to create pulse frequency fsw. When Vramp touches VTH, the off-time ends and a fixed on-time (Ton) is started. In case of a large load step-up transient, when Vc-IL × Ri becomes very large, fsw pulses can occur even before the end of the previous on-time. If these very close pulses are allowed to merge together to create a longer on-time (Fig. 2), significant under-shoot reduction can occur at the output. Another important feature is that, since the fsw pulse increment is proportional to Vc-IL × Ri (Fig. 2), the Ton extension is eventually linearly proportional to the Vout under-shoot. This will eliminate any chance of overcorrection or ring-back of Vout, which is a major problem in Ton extension method by nonlinear controls. Fig. 4 shows that the IQCOT not only reduces under-shoot by naturally increasing the Ton, but also its response is naturally optimized at different transient conditions. Fig. 3 shows that when an over-shoot is created in Vout at the load step-down, Vc decreases very quickly and crosses the IL × Ri. This can be used to create a logic (like Vos in Fig. 3), and can be used to immediately truncate the constant Ton, thus reducing the Vout over-shoot (as shown in Fig. 5).
Fig. 2. (a) IQCOT transient response at different slew rates (a) 20 A/200 µs, (b) 20 A/1 µs, and (c) 20 A/2 µs. Fig. 2. (b) Overshoot reduction at load step-up using proposed control.
