With energy storage systems prices becoming more affordable and electricity prices going up, the demand for renewable energy sources is increasing. Many residences now use a combined solar energy generation and battery energy storage system to make energy available when solar power is not sufficient to support demand. Figure 1 illustrates a residential use case and Figure 2 shows how a typical solar inverter system can be integrated with an energy storage system.
Figure 1: A residential solar energy generation and energy storage system installation
Figure 2: A typical solar inverter system with an energy storage system
In the bestcase scenario, this type of system has highly efficient power management components for AC/DC and DC/DC conversion and high power density (with the smallest possible solution size) that are highly reliable (with the lowest losses) and enable fast time to market. Those requirements are not always achievable at the same time, however, and you will need to make tradeoffs on the best powerconversion topologies for these subblocks.
What existing power topologies for AC/DC and DC/DC buck and boost power converters have in common are half bridges or converter branches that run interleaved, either to increase power levels in a DC/DC converter or to achieve threephase operation in an AC/DC inverter or power factor correction stage by placing three branches running in 120degree phase shifts. Figure 3 shows simplified schematics of five power topologies.
Figure 3: Power topologies for halfbridge and branch equivalent
 Topology No. 1: In the twolevel converter topology, pulsewidth modulation (PWM) signals are applied complementary (with a deadtime delay to avoid shootthrough because of overlapping switching signals) to power devices Q1 and Q2. For the positive sine wave at the output, the duty cycle applied is >50% at Q1. For the negative sine wave at the output, Q2 has a >50% duty cycle. It is a simple concept to control the output power, but output signals before the line filter have a full bus voltage swing, which requires a larger filter to reduce electromagnetic interference. The ripple frequency into the filter is the PWM frequency, which affects the size of the filter.
Threelevel topologies allow the use of smaller passive components and have lower EMI compared to twolevel converters. There are four threelevel topologies:
 Topology No. 2: The Ttype topology is named for the way that the transistors are arranged around the neutral point (V_{N}). Q1 and Q2 connect between the DC link, and Q3 and Q4 are in series with V_{N}. The ripple frequency seen by the filter is equal to the PWM frequency applied to switches Q1 through Q4. This defines the size of the filter components to achieve the required low total harmonic distortion at the AC line frequency. Q1 and Q2 see the full bus voltage and need to be rated at 1,200 V for an 800V DClink voltage in the system. Since Q3 and Q4 connect to V_{N}, they see only half the bus voltage and can be rated at 600 V in an 800V DClink voltage system, which saves costs on this converter type. See the 10kW, Bidirectional ThreePhase ThreeLevel (TType) Inverter and PFC Reference Design.
 Topology No. 3: In the active neutral point clamped (ANPC) converter topology, V_{N} connects with active switches Q5 and Q6 and sets V_{N} in the middle between the DClink voltage. Like the Ttype converter, the ripple frequency seen by the filter is equal to the PWM frequency defining the size of the AC line filter. What’s nice about this architecture is that all switches can be rated at half the maximum DClink voltage; in an 800V system, you can use 600V rated switches, which positively impacts cost. When turning off this converter, it’s important to limit all voltages across each switch to half the DClink voltage. In other words, the control microcontroller (MCU) needs to handle the shutdown sequencing. TI’s TMS320F280049C and other devices in the C2000 product family have configurable logic that allows the realization of shutdown logic in hardware to offload software tasks for the MCU. See the 11kW, Bidirectional, ThreePhase ANPC Based on GaN Reference Design.
 Topology No. 4: The neutral point clamped (NPC) converter topology is derived from the ANPC topology. Here, V_{N} connects through diodes D5 and D6 and sets V_{N} in the middle between the DClink voltage. The output ripple frequency seen by the filter is equal to the PWM frequency defining the size of the AC line filter. Like the ANPC topology, all switches can be rated at half the maximum DClink voltage, but instead of two more switches, there are two fast diodes. The NPC topology’s slightly lower cost compared to the ANPC topology comes at the expense of slightly lower efficiency. The requirements for shutdown sequencing are also identical to the ANPC topology. It is easy to derive an NPC topology from the ANPC reference design mentioned above.
 Topology No. 5: The flying capacitor topology already tells you what’s happening in this converter; a capacitor connects to the switch nodes of the stacked half bridges realized by Q1 and Q2 and Q3 and Q4. The voltage across the capacitor is limited to half the DClink voltage and shifts periodically between V+/V–; power transfers when shifted. This topology uses all switches during the positive and negative sine wave. In this topology, the output ripple frequency seen by the filter is twice the PWM frequency given each cycle shift of the flying capacitor, resulting in a smallersized AC line filter. Again, all switches can be rated at half the maximum DClink voltage, which positively impacts cost.
Table 1 lists the benefits and challenges of the different topologies.
2L TIDA01606 in 2L 
TType 3L TIDA01606 
ANPC TIDA010210 
NPC 3L derived from ANPC 
FC3L Flying capacitor 3L 

Benefits 





Challenges 





Table 1: The benefits and challenges of different converter topologies
All four threelevel topologies have clear advantages on power density (with the smallest possible solution size), highly reliable operation, and fast time to market over traditional twolevel converters. Using wide bandgap devices and highperformance MCUs increase these advantages even further, at a comparable cost.
Additional resources
 Learn more about how you can accelerate your development of solar energy systems.
 Check out the Bidirectional, Dual Active Bridge Reference Design for Level 3 Electric Vehicle Charging Stations.
 Discover our battery management and power conversion technology for energy storage systems.