June/July 2021

22 POWER SEMICONDUCTORS www.infineon.com/coolsic-mosfet-discretes Issue 3 2021 Power Electronics Europe www.power-mag.com As the supply paradigm shifts towards renewables, traditional generation from carbon-based fuels reduces, but also interacts to its advantage by using distributed storage to feed AC back into the grid through inverters for ‘peak- shaving’, to make generation more cost effective and reliable. To achieve this, batteries need to be able to charge from a cheap or convenient energy source and then discharge to a local load or back into the utility grid as ‘feed in’. AC/DC chargers and DC/AC inverters are established products, but if they can be efficiently combined, then there are costs to be saved. As a result, there is intense interest in ‘bi-directional converters’, with the volume market set to be in households with a local renewable energy source and storage, which may be an EV battery. Bi-directional converter requirements A major concern is to maximize the energy from solar or wind sources, therefore any losses in electronic power conversion stages must be kept to a minimum, not least to shorten payback time for the capital costs involved. This has always been true for power processing in any application, so over the years, conversion topologies have evolved towards better efficiency, with 99 % or more now realistic for single stages. For bi-directional converters however, high efficiency has to be maintained with forward and reverse energy flow, which is an added complication. Fortunately, one of the enablers for better efficiency also facilitates bi-directional flow - the use of MOSFETs as synchronous rectifiers in ‘third quadrant’ operation. A typical bi-directional converter outline that might be used as a battery charger and feed-in inverter is shown in outline in Figure 1. The symmetry of the circuit is evident with bridges of MOSFETs able to act as rectifiers, an inverter or DC/DC converter dependent on drive arrangements. AC/DC stages must also feature power factor correction (PFC) and this is best achieved at medium power levels by the bi-directional ‘totem-pole PFC’ topology where MOSFETs double as line AC rectifiers and boost switches in AC/DC mode and inverter switches in DC/AC mode. This characteristic of a MOSFET to change function hinges on its ability to not only conduct through its channel in the ‘normal’ direction from drain to source but also in reverse from source to drain with low loss, all under the control of the gate drive. MOSFETs also however feature a parasitic body diode from drain to source which can be an advantage; some circuits that require reverse conduction naturally ‘commutate’ to forward bias this diode to pass energy to the output at the appropriate stage of the switching cycle. The diode is not ideal however and, when conducting, stores significant charge in its junction which is released when reverse biased during each cycle. This results in ‘recovery current’ which causes losses, reducing efficiency, and increased EMI. The diode also has a high forward voltage drop compared with a Silicon rectifier which causes extra dissipation. Turning on the MOSFET channel bypasses the diode so if this is done with little delay, after the complementary MOSFET in the leg of a Benefits of CoolSiC MOSFETs in Bi-Directional Inverter Applications With the move to renewable energy, there is an increased focus not only on generation but also storage, to make the most of the intermittent supply from wind and solar. Batteries are the common solution and costs are dropping, driven by the technology improvements stemming from the EV market. This opens up opportunities for energy storage at any scale, from domestic to utility. David Meneses Herrera, Senior Staff Application Engineer; and Nico Fontana, Senior Staff Product Definition Engineer, Infineon Technologies Figure 1: MOSFETs in bridge arrangements suit bi-directional power converters