Application Note - Dynex Semiconductor Ltd.

2014
Application Note
Authors
Noman Rao & Dinesh Chamund
AN6156-1 September 2014
LN31943
Application Note
AN6156-1 September 2014 LN31943
Contents
Abstract: .................................................................................................................................................. 3
Introduction:............................................................................................................................................ 3
Types of Power Loss............................................................................................................................... 3
Losses Hierarchy:.................................................................................................................................... 3
IGBT .................................................................................................................................................. 4
Conduction Losses .................................................................................................................................. 5
IGBT output characteristics .................................................................................................................... 6
Calculating IGBT Losses ........................................................................................................................ 7
Switching Losses .................................................................................................................................... 8
Total Losses ............................................................................................................................................ 8
Freewheeling diode ........................................................................................................................... 8
Reverse Recovery Time .......................................................................................................................... 8
Diode forward characteristics: ................................................................................................................ 9
Thermal Model...................................................................................................................................... 11
Application Example ...................................................................................................................... 12
Two Level Converter ............................................................................................................................ 12
Boost Converter .................................................................................................................................... 13
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Power Losses in an IGBT Module
Abstract:
The Insulated Gate Bipolar Transistor (IGBT) is an active power semiconductor switch
which is well suited for high power applications such as controlling a motor, traction drives,
converters, wind turbine etc. This application note will show you how to calculate the losses
in the converter/inverter by using Dynex datasheets. If we know the working condition and
with the help of some parameters in the datasheet, we can easily calculate the total power loss
of the module and hence calculate the junction temperatures.
Introduction:
Power loss in an IGBT mainly consists of steady – state conduction loss and switching
loss. The switching loss in the IGBT is given by
whereas, in the Diode it
is given by the reverse recovery loss. All these switching energies can be added together
multiplied by the switching frequency to give the total module switching losses. This note
describes the theory behind the calculation and show how to calculate the power losses for
the IGBT and Diode and the junction temperatures respectively.
Types of Power Loss
In an IGBT module there are many IGBT die and diode die depending on the module and
requirements of the application. All chips dissipate power when they are conducting or
switching from one state to another.
Losses Hierarchy:
Figure 1: Module Losses Hierarchy
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The conduction losses for the IGBT and freewheeling diode are the product of the current
flowing through the collector or anode and saturation voltage (on state voltage) over the
conducting period. In contrast, the switching losses happen as a result of energy loss during
the transition and switching frequency.
IGBT
An IGBT is a voltage-controlled device which combines the advantages of a MOSFET and a
BJT. It is a three terminal device; collector, emitter and gate terminal. It is a four layer
semiconductor that uses the drive characteristics of a MOSFET and voltage characteristics of
BJT. For high power IGBT modules it is necessary to provide a suitable heatsink, otherwise,
it may go into the thermal runaway. IGBTs works in two states and produce losses in those
states; conduction losses and switching losses.
Conduction losses mainly depend on the duty cycle, load current and junction temperature,
whereas, switching losses depends on the load current, dc link voltage, junction temperature
and switching frequency. If the switching frequency is higher, then the losses will be higher.
Figure 2: Switching Waveform
Power losses for different stages give significant amount of power loss in an IGBT module if
driven carefully.
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Figure 3: Turn on losses
Figure 4: Turn off losses
The total average power of the IGBT is the sum of the conduction loss, turn on and turn off
losses as shown in Eqn.1.
When the IGBT turns on, collector current increases rapidly and the voltage across the
collector to emitter decreases. During this switching it takes time for the current to go from
zero to its rated level, also current overshoot can be seen. This overshoot is the mirror image
of diode added current and the voltage drops to the saturation level. This transition of voltage
and current produce losses called
turn on power loss. For turn off condition the device
behaves in vice versa.
Conduction Losses:
Conduction losses are the losses that occur while the IGBT or freewheeling diode is on and
conducting current, the total power dissipation during conduction is computed by multiplying
the on-state voltage
and the on-state current
. In PWM applications the
conduction loss must be multiplied by the duty factor to obtain average power dissipation. A
first order approximation of conduction losses can be obtained by multiplying the IGBT’s
rated
by the expected average device current.
Conduction loss is the on-state loss or steady state loss. The average power dissipated by the
IGBT is given by Eqn.2.
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IGBT output characteristics:
Figure 5: IGBT output characteristic
The value for
the datasheet.
resistance can easily be calculated from the IGBT characteristic curve from
The
value received from the above formula should match with the datasheet value to
justify the correct calculation from the graph. The average power losses in PWM sinewave
switching in given by Eqn.4.
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Calculating IGBT Losses:
The average power loss in an IGBT is given by the Eqn.2 below
Time period ‘T’ is inversely proportional to frequency ‘f’
The total average power loss incurred in the IGBT can be obtained by integrating all the
values of power losses over a period of time.
The total average power loss for the IGBT can be split into three phases; 1) turning on the
device, 2) conducting period and 3) turning off the device.
The conduction losses are independent of the switching frequency but dependant on the duty
cycle, whereas the switching losses are dependant of the switching frequency and therefore
they are directly proportional to each other.
The values for the energy loss
are given in the Dynex datasheet, therefore there
is no need to calculate these values. The switching energies are then simply multiplied by the
switching frequency to give the power loss for on and off time as shown in Eqn.7.
The total average conduction loss can also be calculated by another method if we can
calculate the values from
and
for the system.
and values can be measured from the datasheet by just drawing a tangent line passing
through the rated value as shown in Fig 5. These values vary with the temperature therefore
at 125°C it is higher than 25°C.
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Switching Losses:
In power electronics switching losses typically contribute a significant amount to
the total system losses. Therefore, omitting switching losses in the calculation or weighting
the conduction losses with an estimated factor to take into account switching losses, might
result in large errors concerning the total losses switching losses occurred because the
transitions from on-state to off-state and vice versa do not occur instantaneously. During the
transition interval both the current through and the voltage across the device are substantially
larger than zero which leads to large instantaneous power loss. The curves show the
simplified current and voltage waveforms and the dissipated power during one switching
cycle of an IGBT in an inverter leg. If one plans to calculate the junction temperature time
behaviour to improve reliability of the design, it is necessary to calculate accurately the
switching losses.
The switching power loss needs to be normalized with the conditions provided for any
application with the nominal values of datasheet.
Total Losses:
The more points we add in the calculations, the more accurate will be the losses.
Freewheeling Diode
A diode is a two-terminal pn–junction device; anode and cathode. It allows current to pass in
one direction (conduction state), while blocking current in the opposite direction
(the reverse direction). The average total power losses in diode is given by Eqn.10.
Reverse Recovery Time:
When switching from the conduction to the blocking state, a diode or rectifier has stored
charge that must be discharged first before the diode blocks reverse voltage. This discharge
takes a finite amount of time known as the Reverse Recovery Time, or trr. During this time,
diode current may flow in the reverse direction.
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Figure 6: Reverse recovery time
When the device turns off it generates losses called recovery loss and the time required to
recover is called the reverse recovery time.
Diode forward characteristics:
Figure 7: Forward characteristic of diode datasheet
Change of forward voltage and forward current can give the
resistance value.
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Diode threshold voltage can be calculated from the above graph see Fig. 7 which is used for
calculating the forward voltage see Eqn.12. This forward voltage can be checked against the
datasheet value to give the confirmation of calculation. The average power losses in diode are
when operated under PWM sinewave switching is given by Eqn.13.
The switching power loss needs to be normalized with the conditions provided for any
application with the nominal values of datasheet.
The total average power loss for the diode is the sum of conduction loss in diode and reverse
recovery.
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Thermal Model:
The junction temperature Tj is given by the Eqn.15 and is illustrated in Fig.8.
Figure 8: Transient thermal model
Where;
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Application Example
Two Level Converter:
Here are some graphs and results for the Dynex IGBT module DIM1200ASM45-TS000
using the 2- level converter topology and PWM sinewave switching.
The software is calculating the power losses for IGBT and Diode and the junction
temperatures. The key parameters from the datasheet are also displayed in the datasheet
table.
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Boost Converter:
This is an example using PLECS software showing how the Dynex devices can be used for a
boost converter. The electrical and thermal circuit are shown above. This will show you how
to calculate the power losses for IGBT and Diode.
The switching time for an IGBT can be control by the duty cycle;
The duty cycle is
Therefore
With the diode continuously conducting and the IGBT behaving as an open switch
Then duty cycle will be
Therefore
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Since the average voltage across the inductor is zero
Figure 9: Simulation waveform
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The conduction losses for the Boost converter can be calculated by,
Figure 10: Boost converter IGBT module power losses
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