# An LLC Converter with Capacitive Insulation
## 1 Introduction
### Prior Arts of Capacitive Insulation in Power Converters  
As shown in Figure 2, a high-inductance common-mode choke in [17] works as a line-frequency low-pass filter to suppress touch current under the requirement limitations. However, the high-inductance common-mode choke on the DC output path usually induces lower conversion efficiency and higher cost. The experimental result of touch current is 0.734 mA. The hipot test is not discussed in [17]. In [18], there are two Y3 capacitors used  
\begin{table}
\begin{tabular}{l c} \hline \hline
**Type of Equipment** & **Maximum Touch Current** \\ \hline Information equipment without earth ground connection & 0.25 mA \\ \hline Hand-held information equipment with earth ground connection & 0.75 mA \\ \hline Stationary, pluggable information equipment with earth & 3.5 mA \\ ground connection & \\ \hline \hline \end{tabular}
\end{table}
Table 2: Limitation of touch current.  
\begin{table}
\begin{tabular}{l c c} \hline \hline
**Insulation Level** & **Creepage Distance** & **Clearance Distance** \\ \hline Basic or supplementary insulation & 3.2 mm & 2.0 mm \\ \hline Double or reinforced insulation & 6.4 mm & 4.0 mm \\ \hline \hline \end{tabular}
\end{table}
Table 3: Creepage distance and clearance distance.  
as secondary-side switch zero voltage switching (ZVS) auxiliary components. Because the Y3 capacitors [19] cannot satisfy the requirement for reinforced or double insulation shown in Table 1, the main safety insulation and the energy transmission path are still via the transformer.  
The traditional LLC converter is widely used because of its simple circuit structure and ZVS function. The basic structure is shown in Figure 2(a). The resonance between the capacitor \(C_{r}\), the resonant inductor \(L_{r}\), and the transformer magnetizing inductance \(L_{m}\) can realize the ZVS of the switches \(Q_{1}\) and \(Q_{2}\), achieving high conversion efficiency. In many low-power applications, the leakage inductance \(L_{lk}\) of the transformer is also used as the resonant inductor \(L_{r}\). On the other hand, the resonant capacitor \(C_{r}\) generally adopts the plastic film capacitor, which is characterized by its high capacitance and low cost.  
Figure 3: _Cont._  
Figure 2: Conventional capacitor isolation converter.  
In the LLC converter presented in [20], the resonant capacitor \(C_{r}\) is replaced by the Y1 capacitor [21] to meet the clearance and creepage isolation requirements defined by the safety regulations. However, considering the design of EMI common mode noise, insulation current, and LLC voltage gain curve, the transformer is retained, but the transformer does not need to adhere to the electrical isolation requirements. The transformer as a whole can be regarded as a secondary component and only requires a basic insulation grade. Therefore, this transformer does not use triple insulated wire, and can use high-efficiency wires such as Litz wire and flat wires that are not reinforced insulation grades to minimize the insulation material in the transformer. Hence, the transformer structure and volume can be reduced so that a compact design of the magnetic component can be achieved.  
In addition, since the rated withstand voltage of a typical triple insulated wire is only 3 kVac, applications requiring higher-voltage insulation such as power supplies for industrial equipment requiring 4.2 kVac [22] or auxiliary power supplies for solid state transformer systems with an insulation voltage of 25 kV [23] require expensive transformer insulation materials, resulting in more volume and cost. The proposed method only requires increasing the number of Y-capacitors in series to achieve higher insulation voltage without redesigning the transformer design.