Implementing Hybrid ANPC Inverters With Synchronous Rectification

Manufacturers in the field of PV inverters and energy storage systems are increasingly utilizing silicon carbide power modules to enhance the efficiency of their products. This article explores the implementation of hybrid active neutral point clamped (ANPC) inverter topology with synchronous rectification to achieve a balance between efficiency and cost in common applications.

This article was published by EE Power as part of an exclusive digital content partnership with Bodo's Power Systems.

PV installations with battery storage are becoming more popular as there is a growing need for a resilient energy infrastructure that can cater to demand fluctuations. Unlike traditional solar inverters that only require a power factor range of 0.8 to 1, bi-directional inverters supporting battery charging from the power grid need to operate at a power factor of -1. The active neutral point clamped (ANPC) inverter topology has proven to be advantageous for such bi-directional operations, and leading PV inverter manufacturers have already incorporated it into their solutions.

With the increasing cost per kWh, equipment manufacturers are seeking ways to improve the efficiency of their solutions while minimizing modifications to their designs. To achieve this, they are turning to silicon carbide components, which offer higher power density and reduced ohmic losses compared to standard silicon components.

Synchronous rectification is a technique that reduces conduction losses in the freewheeling path by replacing the diode PN-junction voltage drop with the resistive channel of a MOSFET. Figure 1 illustrates the conduction loss of a diode and a MOSFET channel at different temperatures. Synchronous rectification provides significant conduction loss savings until the cross-over point of the two characteristics.

The active neutral point clamped (ANPC) inverter topology converts a DC link voltage to an alternating voltage with variable frequency. It differs from a half-bridge or six-pack topology as it offers an additional voltage level at the output, which can jump to DC+, DC-, or zero. This topology is commonly used in high-efficiency three-phase PV inverters and applications with bi-directional operation requirements like battery storage. The ANPC inverter offers a blocking voltage of 950 V or 1200 V, making it suitable for 1500 V applications.

Figure 1. Until the cross-over point of the I-V characteristic of the diode and the MOSFET channel, synchronous rectification offers significant conduction loss savings. Image used courtesy

Figure 2. Comparing the outer switch modulation and inner switch modulation in terms of topology (left and right) and cost (middle). Image used courtesy

There are two main modulation techniques used in the three-level ANPC topology: outer switch modulation and inner switch modulation. Outer switch modulation requires SiC MOSFETs in four positions, making it more expensive compared to inner switch modulation, which only requires two. Outer switch modulation also requires a more complex pulse width modulation (PWM) scheme when implementing synchronous rectification. Inner switch modulation is the more cost-effective solution, especially for applications utilizing SiC MOSFETs and synchronous rectification.

The implementation of the ANPC topology with inner switch modulation depends on the specific design constraints of the target application. There are two variations of the topology: one utilizes the MOSFET's conducting channel as the freewheeling diode, while the other incorporates an additional freewheeling SiC Schottky barrier diode (SBD) parallel to the MOSFET body diode. Cost-sensitive applications typically prefer the first variant, while applications sensitive to re-turn on are better served with the second variant.

Figure 3. Comparing inner switch modulation with synchronous rectification (left) with the use of additional freewheeling SiC SBDs in parallel to the MOSFET body diode (right). Image used courtesy

Synchronous rectification in ANPC with inner switch modulation involves separating the freewheeling time into three phases. These phases ensure proper commutation and avoid cross-conduction during the switching cycles. The MOSFET is turned on in one phase, current flows through its conducting channel in another phase, and it is turned off in the remaining phase.

The PWM Pattern for ANPC With Synchronous Rectification

When implementing the ANPC with synchronous rectification, the basic PWM pattern and duty cycle definition correspond to that of a conventional ANPC. The difference is the separation of the freewheeling time into three phases. The following section shows the freewheeling phases using the example of a positive line voltage and current.

Figure 4. The PWM pattern for ANPC with synchronous rectification. Image used courtesy

t1-t2: The current flows through the MOSFET body diode (T14).

This phase ensures proper commutation and avoids cross-conduction during the turn-off of T13 and the turn-on of T14. With perfect drive signal matching, this time could theoretically be optimized to zero.

t2-t3: The MOSFET is turned on, and current flows through the conducting channel. This phase should be made as long as possible to maximize efficiency gains.

t3-t4: The MOSFET is turned off, and current flows through the body diode. This last phase ensures proper commutation and avoids cross-conduction during the turn-off of T14 and the turn-on of T13. With perfect drive signal matching, this time could theoretically be optimized to zero.

 
Comparing the efficiency of ANPC with synchronous rectification to implementations using the MOSFET's body diode and freewheeling SiC SBDs, it was found that ANPC with synchronous rectification offers the highest efficiency over the entire power range. The use of freewheeling SiC SBDs almost matches the efficiency of synchronous rectification but comes with an 18% increase in module cost. Using the MOSFET's body diode for freewheeling at high currents is not recommended due to its high forward voltage.

Figure 5. The typical I-V characteristic of a SiC MOSFET at a junction temperature, Tj, of 150 °C. Image used courtesy

In conclusion, this article highlights the optimal ANPC inverter topology for emerging applications such as PV inverters and battery storage solutions. ANPC with inner switch modulation is a cost-effective solution compared to outer switch modulation. By eliminating the need for a freewheeling diode and utilizing the SiC MOSFET's reverse conducting channel, efficiency is further enhanced without requiring changes to the system architecture. ANPC with synchronous rectification, such as the Vincotech flowANPC S3 power module, offers the best price-performance ratio for a wide range of power electronics applications.

Figure 6. Comparing the efficiency in terms of power and cost of ANPC with (i) SR, (ii) the MOSFET’s body diode and (iii) an additional SiC SBD. Simulation conditions: Vdc=1350V, Vac=460V, cosphi=0.8, fsw=16kHz, Rg=4R, Ths=80°C. Image used courtesy

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