## 5 Control of the Proposed Converter  
The basic idea of the control strategy for HESS is that the SC supports the high frequency/high amplitude components in the load power, which is usually estimated by a load voltage controller, and the battery supports the low-frequency power/high energy component. Therefore, a low pass filter (LPF) is usually employed to separate the slow varying average power component, which is used to derive the reference current to the battery current control loop whilst the highly dynamic power component is used to produce the reference value for the SC current control loop [36; 37]. In order to further reduce the stress of the battery, an asymmetric charge/discharge current limiter was considered, as in [38]. The resulting mismatch in the battery power is added to the error component of the battery current to produce the SC reference current, which facilitates a faster and more accurate regulation of the output load voltage. The control strategy of the proposed converter prototype follows the above philosophy to accommodate the limited current capability of the high specific energy batteries used in HESS, and the resulting block diagram is shown in Figure 11, where the symbols for the state variables are indicated in the circuit diagram shown in Figure 6. First, the converter output voltage (Vo) is compared with the reference voltage (Vo_R), and the error is inputted to a PI controller. This controller estimates the current that is consumed by the load and supplied by the converter and detects any imbalance that causes deviation of load voltage from the reference. This is then used to calculate the total power (Pt) that has to be supplied by both the battery and SC whilst taking into account the available power extracted from the PV. This total power required from the energy storage is then separated into low and high-frequency components by an LPF. The reference battery power (P\({}_{\text{b-R}}\)) is obtained by applying an asymmetric limiter to the low-frequency power component to control the charge/discharge power of the battery based on specified limits to preserve its lifetime. Given that the battery power is defined by the battery stage inductor current (I\({}_{\text{b}}\)) multiplied by the average voltage seen at point A (shown in Figure 6), which is estimated using (7), the reference value for the battery stage inductor current I\({}_{\text{b-R}}\) that is required to deliver this battery power is determined and passed to the battery current control loop that generates the PWM for the switches S3 and S4 accordingly. On the other hand, the SC's reference power (P\({}_{\text{SC-R}}\)) is obtained by subtracting the reference battery power P\({}_{\text{b-R}}\) (subject to specific battery limitations) from the total energy storage power requirement (P\({}_{\text{t}}\)). Based on the required power from the SC and the average voltage seen at point A, the reference value for the SC stage inductor current (I\({}_{\text{SC-R}}\)) is determined and given to the SC's current control loop, which generates the PWM for the switches S1 and S2 accordingly. The switch S5 has dual functionality: (i) to control the operating point of the PV to follow the MPPT and (ii) to ensure the controllability of the SC and battery stages' inductors (L1, L2) is maintained, which depends on the voltage level of the SC and the battery, as described in Section 3. As the latter is a system function, therefore it has higher priority. Accordingly, the duty cycle of S5 is selected as the maximum value (that corresponds to a minimum average voltage V\({}_{\text{A}}\)