# An LLC Converter with Capacitive Insulation
## 1 Introduction
### Test Methods and Requirements of Electrical Safety  
For a commercial converter product, it is necessary to satisfy the requirements of the electrical safety test. There are two important certification items as shown in Figure 1: the dielectric strength test and touch current, which is also called the hipot test earth leakage current.  
\begin{table}
\begin{tabular}{c c c c} \hline \hline \multirow{2}{*}{**Subclass**} & **Type of Insulation** & **Range of Rated** & **Peak Impulse Voltage** \\  & **Bridged** & **Voltages** & **before Endurance Test** \\ \hline \multirow{2}{*}{Y1} & Double insulation or & \multirow{2}{*}{\(\leq\)500 V} & \multirow{2}{*}{8.0 kV} \\  & reinforced insulation & & \\ \hline \multirow{2}{*}{Y2} & Basic insulation or & \multirow{2}{*}{\(\geq\)150 V} & \multirow{2}{*}{5.0 kV} \\  & supplementary insulation & & \\ \hline \multirow{2}{*}{Y3} & Basic insulation or & \multirow{2}{*}{\(\geq\)150 V} & \multirow{2}{*}{None} \\  & supplementary insulation & & \\ \hline \multirow{2}{*}{Y4} & Basic insulation or & \multirow{2}{*}{\(\leq\)150 V} & \multirow{2}{*}{2.5 kV} \\  & supplementary insulation & & \\ \hline \hline \end{tabular}
\end{table}
Table 1: Classification of class Y capacitors.  
Figure 1: Electrical safety test: (**a**) hipot, (**b**) touch current, (**c**) human body model.  
In the design process of a commercial converter product, it is important to satisfy the requirements of electrical safety. For example, for information and communication equipment products, the Underwriters Laboratories Inc. (UL), (Northbrook, IL, USA) and the International Electrotechnical Commission (IEC) define the standard documents of UL60950-1 [11] and IEC62368-1 [12], respectively. One of the requirements in these two standards is the dielectric strength test, commonly called a \"dielectric withstand\" or \"hipot\" test. The hipot test is a stress test of the insulation of a device under test (DUT).  
As shown in Figure 1a, the hipot test applies a voltage that is much higher than the normal operating voltage to the DUT. For reinforced insulation products with a universal input voltage ranging from 85 Vac to 264 Vac, the required test voltages [11] may be 3000 Vac or equivalent to 4242 Vdc. When the equipment operates, the DUT is applied with a high voltage between the primary side and the secondary side or the earth ground to test its insulation breakdown status.  
Table 2 and Figure 1b simply illustrate the touch current defined in the standards [11; 12; 13]. The touch current test simulates the effect of a person touching exposed metal parts of a product and detects whether or not the safe level of leakage current flows through the body to the earth. A high enough leakage current can cause an uncontrolled muscular spasm or shock. The equivalent circuit of the human body in [11; 12; 13] is defined in Figure 1c.  
Converters used in data centers or communication centers are defined as grounded products, so the maximum touch current is 3.5 mA. The maximum touch current for an ungrounded (2 pin plug) product is 0.25 mA.  
The creepage distance and clearance distance are also specified in the standard documents [11; 12; 14], as shown in Table 3. In offline converters used in homes or businesses, components such as PCBs, photo couplers, Y capacitors, transformers, etc., are required to comply with the electrical insulation system standards specified in safety regulations [11; 12; 14], and must have sufficient clearance distance and creepage distance, as shown in Table 3. In general, the primary-side winding to the secondary-side winding adopts double insulation or reinforced insulation, and the primary-side winding to the core adopts basic insulation. In order to reach a high insulation voltage, the insulation materials [15; 16] and internal structure of a transformer are critical. The transformer contains many insulating materials, such as insulation tape [15], bobbins, margin spacers, and triple insulation wire [16], and so the transformer is bulky.