The TDHBG2500P100 half-bridge evaluation board from Transphorm provides the elements of a simple buck or boost converter for basic study of switching characteristics and efficiency achievable with Transphorm’s 650V GaN FETs. In either buck or boost mode the circuit can be configured for synchronous rectification.
Jumpers allow use of a single logic input or separate hi/lo inputs. The high-voltage input and output can operate at up to 400Vdc, with a power output of up to 2.5kW. The inductor provided is intended for efficient operation at 100kHz, although other inductors and other frequencies may be easily used.
The circuit comprises a simple half-bridge featuring two TPH3212PS GaN FETs. Two high-voltage ports are provided which can serve as either input or output, depending on the configuration—boost or buck.
In either case one FET acts as the active power switch while the other carries the freewheeling current. The latter device may be enhanced, as a synchronous rectifier, or not.
With GaN FETs the reverse recovery charge is low and there is no need for additional freewheeling diodes. Two input connectors are provided which can be connected to sources of logic-level command signals for the hi/lo gate driver.
Both inputs may be driven by off-board signal sources; or alternatively, a single signal source may be connected to an on-board pulse-generator circuit which generates the two non-overlapping pulses. Jumpers determine how the input signals are used.
An inductor is provided as a starting point for investigation. This is a 440μH toroid intended to demonstrate a reasonable compromise between size and efficiency for power up to 2.5kW at a switching frequency of 100kHz.
The board can be used for evaluation of basic switching functionality in a variety of circuit configurations. It is not a complete circuit, but rather a building block. It can be used in steady-state dc-dc converter mode with output power up to 2.5kW. When operating the board at high power (>1000W), an external fan should be used to cool the heatsink
For buck mode operation, with input voltage of 400V and output voltage of 48V; 50A max output current is achievable at 2500W with duty cycle of 12%. A typical 400Vin – 200Vout buck operation with 50% duty cycle, 6.5A max output current is seen at 2500W. On the other hand, for 200Vin – 400Vout boost mode operation at 2.5kW, 12.5A max output current can be reached with a duty cycle of 50%. Thermal cooling must be enforced for high current switching at all times.
The required form of the gate-drive signals is shown in the following figure.
The times marked A are the dead times when neither transistor is driven on. The dead time must be greater than zero to avoid shoot-through currents. The Si8230BB gate drive chip ensures a minimum dead time based on the value of a resistor connected to the DT input.
The dead time in ns is equal to the resistance in kΩ x 10, so the default value of 12k corresponds to 120ns. This will add to any dead time already present in the input signals. The on-board pulse generator circuit; for example, creates dead times of about 60ns. The resulting dead time at the gate pins of Q1 and Q2 is about 240ns. Either shorting or removing the resistor will reduce the dead time to 60ns.