Symptoms
Use gate driver "1EDS5663H" to drive 4 GaN switches "IGT60R070D1" in a PSFB DC-DC converter.
When design this converter for ZVS (soft switching), it occurs that the FET sometimes turns on unexpectedly in the spice simulation if the deadtime is long. Below is the driver circuit of GaN.
Diagnosis
Notice that GaN driver is different with general unipolar and bipolar drivers. The waveform of V_GS with high frequency is shown below.
The turn off negative voltage is not equal -V_DDO, which is inverse of driver supply, but the "off" voltage V_N.
Furthermore, this "off" voltage remains for a short time t1 which can be calculated with t1=Rt1*10.8pF. Now we can know the t1 with the default resistance on demo board is 190ns. And this "negative voltage time" t1 should be longer than dead time.
Obviously, the cause of issue is: The GaN hasn't been turned off completely when the dead time longer than default t1.
Solution
The "negative voltage time" t1 can be configured by adjusting resistance on TNEG PIN with a RC circuit referring to the mentioned formular t1=Rt1*10.8pF. 10.8pF is the capacitance of an internal capacitor in driver IC.
After setting up a simulation circuit and test, we can find a proper value for external resistance on TNEG PIN.
For an adaptive delay which has a dead time range from 70ns to 250ns, it is recommend a 33k resistor on TNEG PIN, which fills at least 340ns dead time.
At the end there is an important reminder: the "negative voltage time" t1 will not always increase with resistance on TNEG PIN. It will reach a maximal value and then decrease in simulation.