One of the most critical design considerations to make for a 48V DC-DC converter using GaN transistors is to minimize the dead time between one transistor turning off, and the other turning on. This is because in a GaN E-HEMT transistor there is no intrinsic, parasitic diode, nor is there a need for one. When the GaN transistor is forced to conduct current in the reverse direction, the reverse voltage can be as high as -2V or more. Therefore, the conduction losses during this time can be high. One might consider using a diode in parallel with the GaN transistor, but this is not required and might decrease efficiency and increase noise due to Qrr effects. Because it has no diode, the GaN E-HEMT has a higher reverse voltage. But because GaN has no Qrr (reverse recovery charge) it saves power, and possibly more importantly in communications systems, reduces noise and EMI significantly. Figure 3 shows the dead time, Td, of approximately 20 nsec.
Figure 3. Minimizing dead time (Td) can improve efficiency greatly
In order to study the effects in efficiency of both the gate voltage, as well as the dead time, the circuit of Figure 2 was simulated so that parameters could be varied. The output power was set to 240W (12V, 20A) and the gate drive and dead time varied. Table 2 shows that the ideal (highest efficiency) operation of the GS66108P