It can be summarised from the literature that there are two origins of GaN-HEMT dynamic
[
26]: the
bias effect, which is associated with device operation when it is in OFF-state, and the hot electron effect, which is associated with device switching transients (overlapping of
and
). As commercial GaN-HEMT is driven by
V in power electronics converters, the impact of threshold voltage
shift for dynamic
can be almost neglected [
27]. In this section, GaN-HEMT dynamic
characterisation and modelling when the device is operated under zero voltage soft switching (ZVS) will be presented. Soft turn-ON is used in preference to hard switched turn-ON for the following reasons: (1) as presented in
Table 4, GaN-HEMT
is higher than
. Therefore, device total switching losses can be greatly reduced under soft switching, so as to operate device in higher switching frequency, which is close to device application for high power density power converters. (2) Due to hot electron effect, dynamic
can be greatly reduced and device junction temperature can be easily controlled in soft switching. Therefore, the
bias effect can be studied with little interference.
3.1. Model Principle and Parameters Extraction
The
bias effect on the GaN-HEMT dynamic
value can be illustrated in
Figure 8. At each switching cycle
T, during the OFF-state (
), the charge will be trapped by
bias voltage in device structure, which will increase its
value when it is in ON-state (
). This is known as the trapping effect. Then, during the ON-state, those trapped charges will be gradually released, therefore the device
value will decrease towards its static value. This is known as the detrapping effect. In order to model the device dynamic
value for power electronics converters, it is necessary to obtain dynamic
trapping and detrapping time constants.
Dynamic
of the presented GaN-HEMT is then measured by a proposed electrical circuit shown in
Figure 9a. There are mainly two parts in this circuit: device switching circuit (DSC) and voltage clamping circuit (VCC). DSC and DUT form a standard H-bridge circuit. Under the control signal given in
Figure 9b (control signals of T2 and T3 are complementary to those of T1 and DUT), DUT hard or soft switching conditions as well as its OFF-state (
) and ON-state time (
) can be precisely controlled.
can be precisely controlled at interval
–
. As DUT is under ZVS soft switching at
by negative drain current,
includes its reverse (
–
) and forward conduction (
–
). For VCC, its function is to reduce measurement voltage
when DUT is in OFF-state, so as to increase measurement resolution for its ON-state voltage
by using a 8–12 bit oscilloscope. When DUT is in the OFF-state, depletion MOSFET
voltage
; therefore, a small voltage of a few volts which is equal to Zener diode
Zener voltage is measured. When DUT is in the ON-state under both reverse and forward conduction,
, which guarantees accurate measurement of
(
). Device dynamic
is then obtained by
. More details on the measurement accuracy of the proposed measurement circuit can be found in our previous publication [
28]. In this paper, we focus on the use of this measurement circuit to extract parameters for GaN-HEMT dynamic
model.
When
V, device dynamic
values under different
are presented in
Figure 10a, where dynamic
values are quickly obtained within 50 ns after device
reaches ON-state gate voltage (6 V). This result illustrates the trapping effect on device dynamic
value. It is observed that dynamic
quickly increased more than 25% after 100
s bias time. Then, it increases slowly with
until 1 s. From 1 s to 10 s, dynamic
increase rapidly until 70% more than static
value. After 30 s, dynamic
is stabilized to reach its maximal value
.
Device dynamic
values under different
and
is presented in
Figure 10b. This result illustrates the detrapping effect on dynamic
value. It is observed that dynamic
decreases about 20% until 10
s, then it is almost stable until 10 ms. After that, it starts to decrease again to static
value until
s.
It can be concluded from the above measurements that multiple time constants of trapping and detrapping effect are observed, which can be further expressed by an analytical Equation (
3).
represents device static
value, and
represents the increase of resistance value.
and
represent the time constant of the trapping and detrapping effect, respectively. The numbers of the unit
n correspond to the observed multiple time constants. Therefore, dynamic
value increases with
towards
and then decreases with
towards static
. Based on measurement results,
,
and
, and
n can be determined by fitting method and they are given in
Table 5.
The comparison between the model and measurement for GaN device dynamic
is illustrated in
Figure 10. Five terms are finally used in the model, in which the obtained
and
correspond to the above analysis. It is also shown that the model represents well the evolution of device dynamic
under different bias time
and ON-state time
. Note that even though there is no measurement data for
below 100
s, the data obtained by the model represent a reasonable dynamic
evolution. The analytical model will be implemented into the GaN-HEMT compact model, which will be presented in the next subsection.
3.2. Model Implementation
GaN-HEMT dynamic
compact model (DCM) is shown in
Figure 11. It is constituted by a standard GaN-HEMT compact model (SCM) (e.g.,
Figure 1) and a behavioural voltage source
. When GaN-HEMT is in conduction, its
is obtained by Equation (
4), where the term
corresponds to the exponential function and the term
corresponds to
of Equation (
3).
is then expressed by Equation (
5), where
corresponds the voltage increase of each RC unit. Time constant
and
of each unit is represented by a RC circuit. In order to transform the resistance increase (
in Equation (
3)) to that of
, it is introduced one parameter
k (unit:
A and
).
As proposed, GaN-HEMT DCM is a compact model that can be used in different simulation platforms (e.g., PEVP, LTspice, Pspice, SIMetrix, ADS, etc.), and proposed
can be also used alone with device manufacturer model. The comparison between DCM and SCM for dynamic
simulation after 10 switching periods (100 kHz, D = 50%) is presented in
Figure 12. Note that the device dynamic
value increases to approximately 20% more than its static
value in the proposed DCM, and it decreases slightly with ON-state time, which illustrates the detrapping effect. By contrast, only static
value is obtained in SCM. In the next subsection, proposed DCM will be validated by experimental measurements for dynamic
value at transient and steady state when device switches at different operation conditions.