How to reduce switching losses in Photovoltaic (PV) inverters

With rising energy costs and requirements for portability, efficiency is key to every power system. This is particularly true for photovoltaic (PV) inverters where designers and operators want to convert as much of the Sun's energy to electrical power as possible. While the power sector is relatively mature there remains room for incremental innovation. Innovation such as this new technique to control switching losses in power converters based on half-bridge circuits.

Synchronous Reverse Blocking (SRB) adds a second switching transistor in series with the main switch to block the reverse current in the associated freewheeling diode. This second switch must be synchronized with the main switch so that reverse current is passed through a parallel silicon carbide (SiC) Schottky diode. This diode has a high breakdown voltage and an extremely low reverse recovery charge so that the effect of Q_rr on switching losses is significantly reduced.

With Advanced SRB (A-SRB¹) the losses caused by recharging the output capacitance of the main switch are significantly reduced by pre-charging it to a lower voltage. The output capacitance C_OSS strongly depends on the drain-source voltage V_DS. When this is increased from 0V to 40V the capacitance is reduced by a factor of about 100. During turn-on, this voltage dependency causes the main portion of the charging current to flow for low V_DS. However, in a half-bridge configuration, a low V_DS across the switch being in off-state means a high voltage across the switch turning on,  leading to high turn-on losses due to the charging current peak.

If C_OSS (Q1) of the switch remaining in off-state, is pre-charged before turn-on of the other switch of the half-bridge (the low-side switch in our example) most of the charging current does not flow through the transistor turning on and thus cannot contribute to turn-on losses. The pre-charging is performed by an additional voltage source, which is realised by a charge pump in the gate driver IC.

Based on the A-SRB technology, Toshiba has developed a system solution for PV inverters with an output power of up to 5kW. It consists of four main components:
 

• The inverter bridge with A-SRB technology
• One MCU for controlling the entire system
• Two analog front-end ICs (AFE) for controlling the DC/DC converter input stages as well as the output inverter.

In addition to the efficiency gains achieved by A-SRB, the two highly integrated AFE ICs contribute to a compact, cost-optimised inverter system.

To learn how Toshiba's integrated A-SRB solution for PV applications can help you convert energy more efficiently, please click here: