1 Introduction

Power conversion systems in micro inverters are typically designed using a two-stage approach, as shown in Figure 1-1.

Figure 1-1 Micro Inverter Two-Stage Topologies

In this approach, there is a first DC/DC stage (flyback or push-pull boost stage) followed by another AC/DC-stage (self-commuted AC/DC or totem pole PFC) to convert DC-power available from a PV-panel to an interim DC-Bus very often around 400V_DC. This DC-Bus is then converted to an AC-voltage (110V_AC..230V_AC), depending on the grid available in a country or region. Power levels used to be often between 300-400W, however, there are recent implementations with up to 600W per input as well as multi input systems. Micro inverters were traditionally built as unidirectional converters since power flows from thePV panel to AC-grid. Mainly two implementations can be found: isolated current source inverter (CSI) and Isolated Voltage Source Inverter (VSI). The VSI is slightly more complex, but provides better efficiency at comparable power levels. An isolation barrier is needed to isolate the PV-panel from high-voltage AC-connections to avoid electrical hazard when someone touches the panel. Additionally, an isolation barrier is needed to isolate the capacitive behavior of a PV panel from the AC-connections to avoid residual current.

The changes needed to make the isolated DC/DC-stage bi-directional for usage in an energy storage system are to replace the push-pull or flyback stages with bi-directional converters like CLLLC or dual active bridge (DAB) as shown in Figure 1-2. The AC/DC stage remains the same. Either a totem pole PFC/Inverter or a full bridge running in unipolar or bi-polar operation. The differences of AC/DC stages can be studied in the design guide for TIDA-010938 (configurable AC/DC stage).

Figure 1-2 Bi-Directional Power Stage Topologies for Portable Power Stations

The operating principle of a dual active bridge is described in reference design TIDA-010054 and for the LLC or CLLLC converter in reference design TIDA-010933. The CLLLC is a resonant converter where the control MCU uses frequency modulation to control the output power. A resonant controller shows good efficiency when operated in a narrow range near the resonant frequency. A dual active bridge is running typically at a fixed frequency and power flow is controlled by the phase shift between the input and output bridges on each side. Both have advantages and disadvantages. Which one is finally selected depends on the requirements for the system, like input and output voltage ranges.

Two-stage converters are often limited to power efficiencies in the range of up to 96% from DC to AC, especially when operated uni-directional with diodes on rectification stage. From a pure power switch count perspective, two-stage converters can easily end up in 10-12 high-voltage switching elements.

This article presents a new single-stage converter reference design TIDA-010954 that makes implementation of the above end equipment more efficient and smaller in size while reducing cost. The power conversion control algorithm is based on extended-phase shift which lowers the requirements on MCU speed and software complexity.