Download A Comparative Study of Switching Loss in IGBT and

A Comparative Study of Switching Loss in IGBT and
CoolMos Power Switch Devices
Nor Zaihar Yahaya and Ahmet Jumanazarov
University Teknologi PETRONAS, Malaysia. Email: [email protected]
ABSTRACT
In high voltage and current applications, the power switch,
IGBT has become more attractive due to its high voltage and
current ratings than MOSFET. However, due to its slow turnon and high turn-off losses it is unsuitable for high switching
application. A newly developed device, CoolMos, using
improved MOSFET feature, with lower RDS(ON) is introduced.
Whilst turn-off switching loss is determined to be constant,
the turn-on switching losses in these two power switches is
found to be 78.5% better in CoolMos. A conventional buck
converter using IGBT (IRG4BC20W 6.5 A/600 V) and
CoolMos (SPP07N60C3 7.3 A/650 V) were investigated to
show that the total switching loss in the circuit could be
significantly improved using CoolMos as the switch. A 40
kHz, 1.5 Watts prototype converter has been constructed to
verify the simulated analysis. It can be seen that using
CoolMos as the switching device, the total switching loss
indicates an improvement of 68%.
Keywords: PSpice simulation software, switching loss,
reverse recovery, CoolMos, IGBT.
1.
INTRODUCTION
D
c/dc converter have been widely used in computer related
accessories such as computer power supplies and
servomotor drives. Normally the basic circuit consists of a
transistor switch, a freewheeling diode and other passive and
active components. For high power and high frequency
applications, Insulated Gate Bipolar Transistor (IGBT) is
considered to be one of the most popular switches. However,
latest advancement of semiconductor technology has
introduced CoolMos, said to have better performance than
IGBT.
Power converter study demands very efficient switches and
diodes for high switching speed application. An efficient
dc/dc converter requires minimal switching power loss.
Normally, Bipolar Junction Transistors (BJT)s are used for
high voltage applications whilst MOSFETs are used for low
voltage. MOSFETs are fast switching devices. However, for
high voltage applications, they exhibit high on-state resistance
as the breakdown voltage increases. BJTs are rarely used for
fast switching applications even though they have low on-state
resistance. Due to this, IGBTs are the preferred device since
they have the advantages of both BJT and MOSFET.
IGBT has a MOS gate input structure, with a simple gate
control circuit design and is capable of fast switching speed.
Additionally, IGBT has current conduction capability that
makes it widely used in applications exceeding 300V [1].
However, this device does not have an integral reverse diode.
Because of this, it needs to be connected to an appropriate fast
recovery diode when necessary [2]. Even though it can drive
low power input and easily be controlled, the tailing tail
during the turn-off and possible latch-up due to the internal
device’s structure could lower the speed. [3].
CoolMos, on the other hand, is a new power switch
developed by Infineon Technology for high power
application. It is a revolutionary technology for MOSFET.
The main feature is primarily an improvement of the on-state
resistance and literally, this breaks the rules for the RDS(on)
limitations. The improvement of the on-state resistance is
achieved due to reduction of the resistance of the voltagesustaining drift zone in the device’s internal structure [4].
Having lower on-state resistance, the switching energy losses
will low.
This paper is to compare switching stress level (energy
losses) of CoolMos and IGBT in conventional buck converter
circuit. The simulation work involving buck converter circuit
is used to gather data and the experimental waveform results
are compared.
2.
METHODOLOGY
2.1 The Buck Converter Circuit.
A Pspice simulation circuit for a buck converter is shown in
Fig. 1 below. The circuit uses two switches, mainly IGBT and
CoolMos, the switch-under-test (SUT) which are simulated
separately. The operating parameters are: Vin=50 V, f=40 kHz,
L=40 mH, C=470 nF, R=20 Ω and the load resistance, RL is
varied from 200 Ω to 2000 Ω. The variation of RL is applied to
determine the best possible minimal difference of value in
reverse recovery turn-off current losses in Diode employing
the SUT. Then the determined RL will be used as the
optimized value.
The input pulse, VGG triggered to the SUT is adjusted to
turn-on the switch. High-powered Silicon Carbide Schottky
diode (SDP06S60, 6 A/600 V) is used as a freewheeling
device. This Diode exhibits very low-reverse recovery effect,
which makes it superior among other high-powered diodes
[5]. The comparison data being investigated will be influenced
by this reverse recovery current, as it directly contributes to
the switching loss of the SUT, hence total switching loss.
Fig. 1: Buck Converter Circuit
VGG provides an appropriate pulse signal with duty ratio
40%. This pulse signal will turn-on the SUT. The initial start
of the pulse signal will trigger a spike current through R goes
to SUT. The gate-drive signal (GDS) for IGBT is the current
flowing in R whilst gate-source voltage (VGS) for the
CoolMos. The SUT will slowly turn-on and when the pulse
ends, the GDS will drop to zero. This is because as the pulse
signal decreases rapidly to zero, the GDS through the SUT
also decreases rapidly. When the GDS of SUT is below the
threshold value, the junction will not be forward biased
anymore. Hence, the SUT is turned off and no GDS will flow
[6].
When the SUT is turned on, current from the DC source
will flow through the RL and L completing the loop. Hence, no
current will flow to the diode. The diode is turned off when
SUT is on. As current flows through L, the inductor will start
to develop current diL/dt. During the turn-off of the SUT, the
loop is disrupted, as junction of the SUT is opened. The SUT
is now acting as open circuit. The current stored in L will then
start to discharge and will go through the diode. The diode is
now forward biased. Diode is turned on when SUT is turned
off. When the diode is again turned off and the SUT is turned
on, the initial current flowing through the diode is forced to
flow in the reverse direction and this is reverse recovery
current of the diode. This is repeated for every cycle during
the subsequent turn-on and off of the SUT device.
RL
(Ω)
2000
800
400
270
100
Table 1: Comparison of Turn-off Loss of Diode
Diode Turn-off Diode Turn-off % Difference
(IGBT) (nJ)
(CoolMos) (nJ)
305.62
255.00
16.56
311.02
263.15
15.39
317.5
272.25
14.25
321.23
319.46
0.55
336.30
468.65
-39.33
From Table 1, when RL decreases, the turn-off loss
increases. This is obviously true since the increase of energy
loss is due to higher current conduction in the circuit.
However, the turn-off losses of the diodes are almost equal at
RL =270 Ω, having the difference of just 0.55%. This is the
optimized RL value that will be used throughout the next
simulations, as the main study is to compare the losses in the
SUT. For this reason, the losses in the diodes must be close to
each other with respect to the reverse recovery overshoot
current (Irr) which at –345 mA for IGBT and –340 mA for
MOSFET (1.4% difference). At lower RL, (100 Ω), diode loss
(CoolMos) increases rapidly more than Diode (IGBT). This
indication of high percentage difference between them makes
the 100 Ω or lower load resistance value not suitable for
optimization.
The output voltage ripple and current ripple are maintained
at around 5% from its nominal value of 20 V. In general,
diodes switching losses are generated during the turn-off
transition, while the SUTs, during the turn-on. When these
two sets of data are added, the total energy loss in the circuit is
obtained. The switching energy losses in the SUT during the
turn-on and total energy losses are illustrated in Table 2 and
Table 3 respectively.
Table 2: Simulation of Turn-on Loss (SUT )in Circuit
RL(OPT)
Turn-on Loss (SUT) (nJ)
(Ω)
IGBT
CoolMos
% Diff.
270
1923
384.94
80
Table 3: Simulation of Total Switching Loss in Circuit
RL(OPT)
Total Switching Loss (nJ)
(Ω)
IGBT
CoolMos
% Diff.
270
2244
704
68.6
2.2 The simulation of a Buck Converter Circuit.
The circuit in Fig. 1. was simulated in Pspice. The aim is to
first obtain the expected reverse recovery overshoot currents
during the SUT turn-on. The load resistance, RL is varied from
2000Ω to 200Ω. In each of the simulation, the reverse
recovery current energy loss of the diode is observed and
calculated. Table 1 shows the simulated results in comparing
the turn-off loss of diodes at different load resistance.
From Table 2, the turn-on loss in IGBT switch is high,
almost 80% higher than the CoolMos. Hence, the total
switching loss is higher in IGBT switch by 68.8% as shown in
Table 3. It can be explained by the characteristics of both
IGBT and CoolMos. The switching losses are due to their
switching time and current overshoot. The main factor that
contributes to this amount of switching loss is the switching
characteristics. CoolMos has better switching characteristics
compared to IGBT (having turn-on spikes), which results in
lower power loss than of CoolMos. The simulation results
confirm that CoolMos has better performance compared to
IGBT at RL=270 Ω with input power kept constant at 50 V.
This suggests that CoolMos could be a substitute for IGBT. In
the simulation, the output forward current is maintained at
around 75mA (±5%) with average output power of 1.5 W
(±5%) for Vin=50 V.
Table 4: Experimental Results of Turn-off Loss of Diode
Using (SUT)
Diode turn-off
loss (IGBT)
(nJ)
Diode turn-off
loss (CoolMos )
(nJ)
Experiment
325
310
With Irr
-330 mA
-316 mA
2.3 The Experimental Results.
The simulated results are used to compare and verify
experimentally using the optimized load of RL=270 Ω. The
other parameters are set as constant as described in the
simulation methodology. In this experiment, reverse recovery
current waveforms and Diode turn-off losses were obtained.
Equipment limitations were encountered when collector and
drain currents of IGBT and CoolMos were tried to measure.
The currents were measured using the voltage probe, having
put across in parallel with the 1 Ω-resistor. The waveforms
are shown in Fig. 2 and Fig. 3.
Irr = -330 mA
The current is then multiplied with the voltage during its
turn-off time. Then the switching energy loss of the Diode can
be calculated. While the diode is turn-off, the switch (SUT) is
turn-on. This duration of turn-on of the switch will bring
difference between them. As the diode loss is kept constant,
the SUT switching loss can be easily obtained. For the same
switching turn-on time, the energy losses in both SUT are
measured.
Fig. 4: IGBT Turn-On Loss at RL(OPT)
[Experiment (1V/div, 100ns/div)]
Fig. 2: Reverse Recovery Overshoot Current using IGBT
During Turn-Off at RL(OPT) [Experiment (100mV/div, 100ns/div)]
Irr = -316 mA
Fig. 3: Reverse Recovery Overshoot Current using CoolMos
During Turn-Off at RL(OPT) [Experiment (100mV/div, 100ns/div)]
Fig. 2 and Fig. 3 show the experimental reverse recovery
overshoot current during Diode turn-off. Clearly, the peak
reverse overshoot using IGBT is -330 mA, which is about
only 4.2% difference to CoolMos, -316 mA. Diode turn-off
losses are tabulated in Table 4. The corresponding diode turnoff loss employing IGBT is 325 nJ in which 15 nJ higher
compared to CoolMos. For this, the search for finding the
correct value of RL has been proven; hence the results for RL
optimization are verified.
Fig. 5: CoolMos Turn-On Loss at RL(OPT)
[Experiment (1V/div, 100ns/div)]
As shown in Fig. 4 and Fig. 5, the turn-on loss of the SUT
were measured. From calculation, it shows that the turn-on
energy loss in the IGBT is higher by 78.5% than the CoolMos.
This is true since IGBT has spikes during its turn-on as well as
higher turn-on resistance, RDS(on). The difference in
simulation between SUT turn-on loss is 80% in which close to
the experimental work (Table 5).
3.
the project.
COMPARISON & DISCUSSION
The numerical comparison data in turn-on switching energy
loss between IGBT and CoolMos is tabulated in Table 5.
Table 6 shows the total switching loss in the circuit.
Table 5: Comparison of Turn-on Loss of SUT
IGBT TurnON Loss
(nJ)
CoolMos TurnOn Loss
(nJ)
%
Improvement
Experiment
1868
402.50
78.5
Simulation
1923
384.94
80
% Diff
2.9
4.4
-
Table 6: Comparison of Total Switching Loss in Circuit
Using IGBT
(nJ)
Using CoolMos
(nJ)
%
Improvement
Experiment
2193
712.50
67.5
Simulation
2244
704
68.6
% Diff
2.3
1.2
-
From Table 5, having the experimental turn-on data of the
SUT, the total switching energy loss experienced by the
circuit experimentally is 2193 nJ (IGBT) and 712.50 nJ
(CoolMos). Both results, based on Table 6 are close to the
simulation data for 2.3% difference in IGBT (2244 nJ) and
only 1.2% in CoolMos (704 nJ) respectively. The experiment
has proven that CoolMos is better than IGBT switch around
68% in generating less energy loss, hence less switching stress
to the circuit.
4.
CONCLUSIONS
The work has proven that the total switching energy loss in
a buck converter circuit is reduced when the CoolMos is used
as a switch as compared to the IGBT switch. The total loss is
primarily due to the loss during turn-off of the diode and
during the turn-on of the switch. The diode switching loss
during its turn-off is maintained to be almost equal at
optimized load resistance of 270 Ω for easy comparison of
turn-on switching loss in the switch. Experimental result
shows that CoolMos has improved the switching stress by
68% hence shown to be better switching device in dc/dc
converter applications.
5.
ACKNOWLEDGEMENT
The authors wish to thank the Universiti Teknologi
PETRONAS (UTP) for providing financial support for
presenting the paper. Also to Mr. Musa Md. Yusuf and Miss
Siti Hawa Tahir for the technical assistance in accomplishing
6.
REFERENCES
[1] K. S. Oh, “IGBT Basic I”, Fairchild Semiconductor
Application Note 9016, Maine, NE, Feb. 2001
[2] K. S. Oh, “IGBT Basic II”, Fairchild Semiconductor
Application Note 9020, Maine, NE, Apr. 2002
[3] H. Iwarnoto, H. Kondo, H. Hongtao, and A. Kawakami,
“An Analysis of Turn-Off Behavior of Planar and Trench
Gate IGBTs Under Soft and Hard Switching Conditions,”
Power Electronics and Motion Control Conference, vol.
1, pp. 84-88, 15-18 Aug. 2000.
[4] C. Xing-Bi and J. K. O. Sin, “Optimization of the
Specific On-State Resistance of the CoolMos,” IEEE
Transaction on Electron Devices, vol. 48, iss. 2, pp. 344348, Feb. 2001.
[5] J. Richmond, “Hard Switched Silicon IGBT’s?”, Cree
Power Application Note AN03, 2003.
[6] N. Z. Yahaya and K. K. Chew, “Comparative Study of
The Switching Energy Lossess Between Si PiN and SiC
Schottky Diode”, National Power & Energy Conference,
Kuala Lumpur, pp. 216-229, 29-30 Nov. 2004.
7.
BIOGRAPHIES
NOR ZAIHAR YAHAYA was born in
Lumut, Malaysia. He went to the University of
Missouri-Kansas City, USA to study
electronics. He graduated with BSc in
Electrical Engineering in 1996. After that he
served 5 years in industry in his country. In
year 2002, he was awarded the MSc in Microelectronics from
University of Newcastle Upon Tyne, UK. Currently he is a
lecturer at Universiti Teknologi PETRONAS, Malaysia. His
main teaching/research areas are the study of Power
Electronics Converter & Devices.