High-Isolation DC/DC Converters Meet Green Requirements

Solar energy is the cleanest, most abundant global renewable energy source, and plays a key role in the development of sustainable energy systems. Global photovoltaic (PV) generation capacity has undergone exponential growth over the past decade. Approximately 183GW was added in 2021, bringing estimated global PV installations to over 1000GW (one terawatt). The US Dept of Energy plans to install 5TWh of grid storage capacity by 2050 to meet US energy needs alone.

Solar power conversion systems transform solar energy into electrical energy that is either fed to utility grids or used by off-grid electrical networks. The harvested electrical energy is processed by power converters.

The PV solar panel produces a high-voltage DC output that is converted to AC by a high-voltage three-phase inverter. Figure 1 shows the block diagram of a typical transformer-less solar power conversion system with IGBTs in the inverter stage.

The converter uses both high-voltage and low-voltage circuits. The DC rail voltage can be 1000V or higher, and the inverter module is often floated by a few hundred volts above ground, so isolation is required for both functional and safety purposes.

Optocouplers provide isolation for control signals, but the gate driver power supply must also be isolated. This isolated supply consists of a DC/DC converter that supplies typically +15/-9V for IGBT devices, +20/-5V for SiC MOSFETs (+15/-3 V for 2nd Gen. devices), or +6 V and +9 V for GaN HEMTs. The simplest (functional) isolation of a DC/DC converter can withstand 1 kV DC for one second. However, this is often not sufficient for high-side gate drivers.

A typical DC/DC isolation voltage would be at least twice the working voltage, but the high ambient temperature and fast switching edges generated by these high-power transistors additionally stress the insulation barrier. As a result, highly isolated DC/DC converters are needed to power the gate drivers.

A RECOM isolated DC/DC converter provides a simple power solution for an isolated gate driver design, no matter whether IGBT, SiC MOSFET, or GaN power devices are used.

RECOM offers a range of isolated DC/DC converter modules that provide an efficient power solution for IGBT, SiC MOSFET, or GaN HEMT power devices. Features of these converters include asymmetric output voltages, high insulation voltage, and low isolation capacitance.

What are the optimum RECOM DC/DC converter families for the most popular power switch technologies?

IGBT power devices: RECOM’s DC/DC converters RH, RV, RP, RGZ, RKZ, RxxPxx, and RxxP2xx are specially designed for IGBT driver circuits. Their asymmetric outputs of +15V and -9V make them ideal to power IGBT drivers with only a single DC/DC converter. RECOM offers both SIP and DIP packages in a variety of pinouts to accommodate different IGBT applications.

SiC MOSFETs: The RxxP22005D and RKZ-xx2005D series feature asymmetric outputs of +20V and -5V to switch the SiC MOSFET efficiently and effectively. The RxxP21503D series provides asymmetric output voltages of +15V and -3V, which are needed to efficiently switch second-generation SiC MOSFETs.

GaN HEMTs: High slew-rate GaN transistor drivers reach optimal performance, switching at +6V from RECOM’s DC/DC converters in the RP-xx06S and RxxP06S series featuring high isolation voltage and low isolation capacitance. In GaN applications where higher noise and interference must be considered, RECOM also offers converters with +9V output, which can be split up via a Zener diode to +6V and -3V to provide a negative gate voltage on turn-off, ensuring that the gate voltage stays below the turn-on threshold.

The RA3 Series is optimized for gate drivers: RECOM’s new RA3 family of unregulated 3-watt DC/DC converters is specially designed to power transistor gate drivers. The modules are available with input voltages of 5, 12, or 24VDC with single or dual asymmetric outputs to cover the latest Si, SiC, and GaN transistors on the market today. The compact SMD design ensures that required board space is minimal—especially on multilayer PCBs. The modules offer a robust 5.2kVDC/1min. isolation and an isolation capacitance of less than 10 pF. With typical efficiencies from 78% to 82%, the operating temperature range of -40°C to +85°C at full load meets strict environmental requirements for solar inverters as well as induction heating, telecom, EV battery chargers, and motor drives.

R-REF01_HB evaluation board: The R-REF01-HB Half-Bridge Gate-Drive Power Supply Reference Design can be used to compare the real-life performance of various high power-switching technologies. The design consists of a half-bridge suitable for voltages up to 1kVDC and a fully isolated driver stage with isolated power supplies for the low-side and the high-side switching transistors. The R-REF01-HB includes two R12P22005D, R12P21503D, R12P21509D, and R12P06S DC/DC converters and can generate gate drive voltages suitable for IGBTs and SiC and GaN devices.

RECOM gate driver ecosystem:
RECOM offers a robust set of resources in support of the gate driver portfolio. For more information visit RECOM’s gate drivers. There is also a related whitepaper, “Designing Robust Transistor Circuits with IGBTs and SIC MOSFETs”, available for download, as well collections of application notes.