GS66508T-EVBHB 650V
GaN E-HEMT Half Bridge Evaluation Board
User’s Guide
_____________________________________________________________________________________________________________________
GS66508T-EVBHB UG rev. 150917
© 2015 GaN Systems Inc.
www.gansystems.com 7
Please refer to the Evaluation Board/Kit Important Notice on page 21
Gate capacitor and clamping Clamping
diode
An additional gate capacitor per drive circuit (C11/C18) is used to help shunt the Miller current and
reduce the Miller effect. The recommended value is between 100-220pF.
A clamping diode is placed
close to thebetween the
gate
and source
of each E-HEMT (DZ
31
/DZ
42, on the
location of C11 and C18
) for clamping negative gate voltage spikes induced by negative dv/dt on the
drain. It should be a fast Schottky diode or a zener diode (6.8V, 200mW, P/N: MMSZ5235BS-7-F).
Added
Zener or schottky diode may contribute to gate oscillation combined with the parasitic inductance so it is
recommended to insert a ferrite bead in between clamping diode and the gate if to suppress any
unwanted gate oscillation is observed.
Ferrite bead
Ferrite bead on the gate helps to damp the gate ringing and reduce the risk of gate oscillation. On the
other hand adding ferrite bead increases the gate inductance and risk of miller turn-on. A small surface
mount device (SMD) EMI suppression ferrite bead with impedance of 10-20
Ω
@100MHz is sufficient for
suppressing gate oscillation while having minimal impact on the gate miller voltage. On this board a
ferrite bead is used on the
gate for each devicehigh side only
, FB1
/FB2
, (15
Ω
@100MHz, 0.5A, Wurth
Electronics P/N: 74279268) to stabilize the gate driver and suppress the gate ringing.
Testing your own gate driver
Figure 8 - Layout of testing pads for gate driver
Six square pads are provided around the gate driver as testing points as well as providing connection
pads for a user designed add-on gate driver board. The pads are on a 0.1” grid to allow users to
experiment with their own gate driver circuit. In this case the existing gate drive circuit(s) should be de-
populated from the EVB.
GATE
VDD
IN+
VSS
IN-
ISOLATION
VSS
0.3 [7.62]
0.2 [5.08]
0
.3
[7
.62
]
Inch [mm]
GATE
INPUT
GATE
OUTPUT
