Earth Leakage Current in a Bi-Directional Totem Pole Converter Using Wide Band Gap Devices

A bi-directional totem pole converter with hybrid PWM using GaN (WBG) offers higher power density and higher efficiency as compared to other topologies. However, as a drawback, this converter draws leakage current, of the line frequency. This article explains the generation mechanism of this leakage current along with its path through appropriate circuit diagrams.

 

Introduction to Bi-directional Totem Pole Topology

The bi-directional totem pole using only GaN is shown in Figure 1.1, includes two half-bridge legs. One half-bridge switch at high switching frequency and is denoted as the high-frequency leg. The other half-bridge switches at the line frequency and is denoted as the low-frequency leg [1]. In the PFC mode, the power is drawn from the grid and in the grid-tie mode the power is fed to the grid with near to unity power factor.

 

Figure 1.1: Bi-directional totem pole

 

The DC side Y-caps represents EMI filter or they can be a representation of parasitic capacitors of PV panel or load. The high-frequency switching devices are hard switched in this topology in both the modes, PFC as well as grid tie mode. This topology yields superior efficiency and power density. Newly emerging wide bandgap semiconductor devices can be a perfect match for this topology since they have almost zero reverse recovery charge. Relatively smaller switching loss in wide bandgap devices makes them particularly suitable for CCM operation in bi-directional totem-pole. Conduction loss in totem-pole converter is also reduced compared to conventional PFC, due to absence of line rectifier. A bi-directional totem-pole using all GaN devices is thus preferable for high efficiency and high-power density applications.

 

Application of bi-directional totem pole

• High efficiency servers and telecom power supply
• Bi-directional On-Board Battery Chargers (Bi-OBC)
• Solar inverters

 

Generation of Earth Leakage Current in Totem Pole

From the Figure 2.1 it is clear that the midpoint of the DC bus Y-caps “Cy1” and “Cy2” is connected to earth. This means “Cy2” is connected between the DC negative terminal and earth as shown in Figure 2.2. Any change in the voltage between DC negative and earth will charge or discharge “Cy2” [2]. The voltage between DC negative and earth will change because of switching of “G2” and “G4”.

 

Figure 2.1: Bi-directional totem pole with Y-caps

 

Due to the low impedance path offered to current through “G4” the charge and discharge of “Cy2” is done through “G4” only. In hybrid PWM “G4” makes transition from ON to OFF or vice versa at the zero crossing only. For the entire positive or negative half cycle the voltage across “G4” is either VDC or zero. This means during the positive or negative half cycle, there is no abrupt change in drain to source voltage of “G4” and according to capacitor current equation in (1), “Cy2” will not charge or discharge resulting into no leakage current flow. At the zero crossing, due to the abrupt change in drain to source voltage of “G4”, “Cy2” will see a sudden voltage change resulting into current spike to charge or discharge itself as per equation (1).

$$i_{CY2} = CY2 \times \frac{dv_{CY2}}{dt} \quad (1)$$

 

This phenomenon give rise to leakage current only at the zero crossing. Leakage current increases the conducted and radiated electromagnetic emission. The leakage current will also be imposed on the line and neutral current and can increase or decrease depending on the direction of flow resulting into distortion near to zero crossing. The leakage current is directly proportional to the rate of change of voltage across the parasitic or Y-cap. The rate of change of voltage depends on the switching speed of “G4” and if “G4” is a GaN then the switching speed is faster than any other semiconductor devices. The selected GaN takes 9ns to turn ON and 15.5ns to turn OFF [3]. The voltage fluctuations will charge and discharge the parasitic or DC side Y-caps.

In case of PV panels – There exist a parasitic capacitance between the PV cell and the metal frame which is grounded. The ground voltage (potential between DC negative and earth) using hybrid PWM is same as the drain to source voltage of ‘G4” and it goes through sharp transition at the zero crossing which will charge and discharge the parasitic capacitance. If someone accidently touches the surface of the PV panel, a leakage current will flow through the body of the person (due to the absence of any other low impedance path). If the leakage current is excessively high, then it can give shock or personal injury.

In case of DC Bus capacitor – The Y-caps across DC Bus capacitor, generally placed as EMI filter in designs, will play the same role as the parasitic capacitance play in case of PV panel. The earth current of the DC supply will increase and if this current goes above the protection limit, the DC supply can randomly trip and disturb the entire operation of the bi-directional totem pole.

 

Figure 2.2: Bi-directional totem pole with Y-cap across G4

 

Bi-directional Totem Pole in PFC Mode

During positive half cycle (refer Figure 3.1) – G2 is the main switch and G1 conducts in the reverse direction with synchronous rectification and G4 being continuously ON for the entire half cycle. When G2 is ON, the differential current path is “P-Ls-A-G2-G4-B-N”. Energy is stored in inductor (Ls). Capacitor CDC supplies the load.

 

Figure 3.1: Bi-directional totem pole in PFC mode (during positive half)

 

When G1 is ON, the differential current path is “P-Ls-A-G1-CDC-G4- B-N”. Capacitor CDC is charged from the grid. Energy stored in the inductor and from grid is supplied to DC bus (CDC). The Y-cap “Cy2” which was charged during positive to negative transition (at the zero crossing) will get quickly discharged through G4 and the return path for the current is through earth and line side Y caps. The leakage current path is “Cy2-G4-B-Cz2-through earth”.

 

Figure 3.2: Bi-directional totem pole in PFC mode
(during negative half)

 

During negative half cycle (refer Figure 3.2) – G1 is the main switch and G2 conducts in the reverse direction with synchronous rectification and G3 being continuously ON for entire half cycle. When G1 is ON, the differential current path is “P-B-G3-G1-A-Ls-N. Capacitor CDC supplies the load. Energy is stored in inductor (Ls). When G2 is ON, the differential current path is “P-B-G3-CDC-G2-A-Ls-N”. Capacitor CDC is charged. Energy stored in the inductor and from grid is supplied to DC bus. The Y-cap “Cy2” will quickly get charged during positive to negative transition (at the zero crossing) and the return path for the current is through earth. The leakage current path is “Cz2-B-G3-CDC-Cy2-through earth”.

 

Bi-directional Totem Pole in Grid Tie Mode

During positive half cycle (refer Figure 4.1) – G1 is the main switch and G2 conducts in the reverse direction using synchronous rectification and G4 being continuously ON for entire half cycle. When G1 is ON, the differential current path is “DC(+)-G1-A-Ls-P-N-B-G4-DC(-)”.

Capacitor CDC supplies to the grid. When G2 is ON, inductor energy freewheels through “Ls-P-N-B-G4-G2-A”. The leakage current path is similar to the PFC mode in the positive half cycle.

During negative half cycle (refer Figure 4.2) – G2 is the main switch and G1 conducts in the reverse direction using synchronous rectification and G3 being continuously ON for entire half cycle. When G2 is ON, the differential current path is “DC(+)-G3-B-P-N-Ls-A-G2-DC(-)”. Capacitor CDC supplies the grid. When G1 is ON, the inductor will free wheel through “Ls-A-G1-G3-B-P”. The leakage current path is similar to the PFC mode in the negative half cycle.

In the absence of the AC side Y-caps, the earth leakage current will continue to flow and find another path through the AC side neutral which is grounded to the earth and shown in Figures 4.3 and 4.4.

 

Simulation Results of Bi-directional Totem Pole in Grid Tie Mode

The circuit of Figure 1.1 is simulated in the grid tie mode to confirm the path of the earth leakage current. With reference to the grid voltage, it is seen that the earth leakage current is observed only at zero crossings.

From the simulation, it is clear that the current which flows through the AC side Y-cap “Cz2” is the same current that flows through the DC side Y-cap “Cy2”. The simulation result also shows the voltage across “Cy2” which changes state only when “G4” changes state from ON to OFF or vice versa and this voltage is same as the drain to source voltage of “G4”.

 

Figure 4.1: Bi-directional totem pole in grid tie mode
(during positive half)

Figure 4.2: Bi-directional totem pole in grid tie mode
(during negative half)

Figure 4.3: Earth leakage current path in the absence of AC side Y-caps
(negative to positive transition)

Figure 4.4: Earth leakage current path in the absence of AC side Y-caps
(positive to negative transition)

 

 

Hardware Result of Bi-directional Totem Pole in Grid Tie Mode

The circuit in Figure 1.1 is built and run as a grid tie inverter to prove the simulation results. The sharp current pulse at the zero crossing is seen in Figure 6.1 similar to Figure 5.1. The sharp pulse is the result of abrupt change in the ground voltage which charge or discharge “Cy2” and matches with the simulation result.

 

Figure 5.1: Simulation results using hybrid PWM in grid tie mode

Figure 6.1: Experimental earth current of DC Power Supply

 

Conclusion

This article explained the root cause for the generation of leakage current having sharp edges at line frequency along with its path. Since the leakage current finds the return path through the earth, so it interferes with leakage current protection of the programmable DC source or load and results into malfunctioning of this equipment in addition to endangering the human safety.

Acknowledgement: The authors would like to thank VisIC Technologies, Israel for supporting this work.

 

About the Authors

Milind Dighrasker holds a Master's Degree in Power & Control at IIT Kanpur and a Bachelor's Degree in Electrical Engineering at Government Engineering College Bilaspur. He co-founded Enstin lab Pvt Ltd, a Design services company. He currently works there as the CTO responsible for leading and enabling a highly customer-focused technology team to develop innovative and energy-efficient products.

Raj (Thiagarajan) Venkatachalam holds a Master's Degree in Power and Applied Electronics at the Indian Institute of Science and a Bachelor's Degree in Industrial Electronics at PSG College of Technology. He specializes in leading technically qualified teams through all phases of development from technical requirements elicitation to product roll-out, including cost analysis and planning. He is currently the CEO and Co-Founder of Enstin Labs Pvt. Ltd. since July 2017.

Kapil Bhise is an experienced engineer with a demonstrated history of implementing embedded control system focusing on latest technology in electric power processing/conversion products for Green Technology Segments like Solar and Electric Mobility Applications. He holds a Master's Degree in Power Electronics and a Bachelor's Degree in Electrical Engineering. He currently works as the Technical Lead at Enstin Labs Pvt. Ltd.

 

References

1. Ray-Shyang Lai, K. D. T. Ngo, “A PWM Method for Reducing the Switching Loss in a Full-Bridge Inverter, IEEE Trans. Power Electronics, vol. 10, issue 3, no. 4, pp. 326-332, May. 1995.
2. Kerekes, T. (2009). Thesis on “Analysis and Modeling of Transformer less Photovoltaic Inverter Systems”. Aalborg University.
3. VisIC Technologies, V22TC65S1A - GaN Power Switch Datasheet, Feb 2020.