Chapter 2 Minimum Switch Number Structure with the Load-Dependant
2.2 LDPCC Method for Improving Light-Load Efficiency, Stability, and
The inductor waveform (IL) represents status of energy storage and the order of system.
Four inductor waveforms are depicted in Fig. 27~30. At first, the inductor current waveform (IL) of DCM is shown in Fig. 27. The characteristic of inductor current (IL) is that current level decreases to zero before the end of each switching cycle. The order of the system is equal to one. That is, only one low-frequency pole exists in the close loop. When load current changes from light to heavy load condition, the peak value of inductor current (IL) is increased for storage more energy. There is a maximum peak current level existed, because of the disappearance of zero-current condition and thus a maximum power limitation exists in the operation of DCM. As a result, the characteristic of inductor current waveform (IL) changes from DCM to CCM operation as shown in Fig. 28 when the sudden power is larger than maximum power limitation of DCM operation. The order of system becomes two and the
Fig. 27. The inductor current waveform in DCM operation when the load current changes from light to heavy load conduction.
Fig. 28. The inductor current waveform in CCM operation when the load current changes from light to heavy load conduction.
compensation of system needs a complicated compensation like proportional integral differential (PID) compensator to make sure large low-frequency gain and suitable phase margin. Another serious problem is disappearance of isolation period which is zero-current stage works as freewheeling stage. Since disappearance of isolation period may cause worse cross regulation. The cross regulation and instability of system comes from the charge accumulation of inductor. A summary of the two operation modes is that disadvantage of the DCM operation is maximum power limitation while the disadvantage of CCM operation is charge accumulation of inductor L.
The PCCM operation was proposed to improve the disadvantages in DCM and CCM operation [10]. The PCCM technique sets a constant inductor current DC level to store enough energy in inductor as depicted in Fig. 29. Thus, the order of system is similar to that of DCM operation while the maximum power delivered by PCCM operation is larger than that of DCM operation. That is, the order of system is one, and thereby simplifying compensation skill. After the usage of P-I compensator, bandwidth can be extended to have better performance for transient response. Nevertheless, the advantages of the PCCM operation only exist when freewheeling stage exists at end of each switching cycle. Once disappearance of freewheeling stage happens when load current exceeds maximum power limitation or when sudden load current changes from light to heavy load level, the stability and minimized cross regulation isn’t guaranteed since the order of system becomes two. The solution of scenario is that pre-defined and fixed inductor current level (IDC) is required large enough to provide maximum power to all output terminals. That is, the value of the pre-defined and fixed inductor current level (IDC) causes the power conversion lower than that by CCM or DCM operation at light load condition since the freewheeling stage with high inductor current occupies the most period of switching cycle. Thus, the light-load efficiency is decreased. As well, the power conversion efficiency at light-load determines usage time of battery for portable devices.
Fig. 29. The inductor current waveform in PCCM operation of work [2] when the load current changes from light to heavy load conduction.
Fig. 30. The inductor current waveform in proposed LDPCC technique operation when the load current changes from light to heavy load conduction.
To enhance the power conversion efficiency at light-load becomes most important issue in today’s portable devices. The LDPCC technique is needed to effectively improve the power conversion efficiency at light-load condition. As illustrated in Fig. 30, the LDPCC technique is proposed to adaptively store suitable energy in inductor L. When load current changes from light to heavy load condition, the LDPCC current level (Ipeak) is raised to a higher current level to store enough charge in inductor L. On other hand, when the load current becomes small, the LDPCC current level (Ipeak) will be decreased to a small current level for ensuring high power conversion efficiency at light load condition. Besides, a minimum inductor current level (IDC(min)) is defined to prevent output terminal from large transient drop voltage.
Thus, the LDPCC technique can have advantages of simple compensation, large driving
Fig. 31. The inductor current variation when the load condition changes from light to heavy in output terminal VOA.
Fig. 32. The inductor current variation when the load condition changes from light to heavy in output terminal VOB.
capability, and high power conversion efficiency at light-load condition. The LDPCC technique can minimizes the cross-regulation, which looks alike the QC method in [2], as is demonstrated in Fig. 31. Suppose that the current required by output terminal VOA suddenly increases, the duration of time interval (φa) will then increase so that more current can be delivered to output terminal VOA. The inductor current (IL) then reaches the LDPCC level (Ipeak) sooner. As the load for output terminal VOB remains unchanged, the output of the corresponding error amplifier remains the same. Although the whole duration of time interval
(φb) is shifted to the right, the same amount of charge is still delivered to output terminal VOB. The LDPCC level (Ipeak) is then higher at the start of the next period. Hence, time interval (φa) is extended left, so as the subsequent time interval (φb). In the third period, the inductor current (IL) assumes the new profile as in the second period and a new steady state is reached.
All along, the current delivered to output terminal VOB is not affected, and cross-regulation is minimized. Similarly, Fig. 32 demonstrates the minimized cross-regulation in that output load current required by output terminal VOB suddenly increases.