# COTS Battery Charge Equalizer for Small Satellite Applications
## 3 Implementation of the Selected Balancing Methods
### Single Flyback Converters Based Equalization  
This section outlines the design of the SFCBE topology. This architecture consists of '\(n\)' flyback converters, where '\(n\)' is the number of battery cells used to transfer energy from the most-charged cells to the less-charged ones. Figure 1B shows an electric schematic diagram of the SFCBE topology. This topology is not faithfully based on any equalizer in the literature. The main idea is based on the MFCBE topology presented in [32], and a general overview is presented in [15]. In this proposed topology, a single flyback converter was implemented per cell. This choice allowed the use of COTS transformers instead of custom ones. A transformer with one primary winding and one secondary winding with a specific transformer ratio was easier to find than one with multiple secondary windings. Choosing this type of transformer offered additional advantages. For instance, compactness is a key factor in microsatellite applications, and COTS transformers are generally smaller. Another benefit is the precision of the inductance values; the primary and secondary windings of COTS transformers tend to exhibit more consistent inductance, which enables better matching across converters. When selecting a COTS transformer, it is crucial to consider the nominal input and output voltages of the flyback converters. In this design, the primary winding is connected to the battery voltage, and the secondary winding is connected to the terminals of a battery cell. Consequently, the required turn ratio is defined by the number of cells; in this case, a 4:1 ratio was used. With this information, a study of the existing commercial transformers was conducted. In Table 3, a summary of some of the most suitable COTS transformers is presented. The selection of the correct transformer was driven by the primary inductance. This was because discontinuous conduction mode (DCM) was required for this design. This mode ensures the transformer fully discharges its stored energy before the next switching cycle begins, avoiding core saturation. Under this mode, the peak current through the secondary winding in each switching cycle is given by Equation (4). \[Is_{max}=\frac{V_{BAT}\cdot D}{fsw\cdot n\cdot L_{p}} \tag{4}\]  
Assuming a preliminary duty cycle of 40% to ensure DCM, it can be deduced that higher primary inductance values require lower switching frequencies. Higher values of frequencies are associated with higher switching loses, so it is critical to find a compromise between frequency and switching losses. Moreover, each transformer has a range of frequencies in which it can operate that must be respected. \begin{table}
\begin{tabular}{c c c} \hline
**Component** & **Manufacturer** & **Manufacturer Number** \\ \hline Driver & Texas Instruments (Dallas, TX, USA) & UCC2742 SQDRQ1 \\ Transformer & Coilcraft (Cary, IL, USA) & DA2320-AL \\ Transistors & Nexperia (Nijmegen, The Netherlands) & PMV19 XNEAR \\ Balancing capacitors & KYOCERA AVX (Fountain Inn, SC, USA) & 12103C106K4Z4A \\ \hline \end{tabular}
\end{table} 
Table 2: Main components of the design of the DSSCE topology implemented. According to Table 3 and Figure 3, the only transformer capable of providing the required balancing current at an appropriate switching frequency (250 kHz) was the Wirth Elektronik 750311558 (Niedernhall-Waldzimmern, Germany), which had a primary inductance of 300 \(\upmu\)H with a leakage inductance of 3 \(\upmu\)H.  
Concerning the control circuit design, we implemented an analog circuit that enabled the use of dc-dc converters associated with the most discharged cells, more specifically, the cells that had a voltage value that was below the average voltage of all cells. In Table 4, the main characteristics of the flyback converters implemented for this equalizer are presented, and Table 5 provides the references for the main components in the implementation.