The body spreading
resistance in the IGBT is reduced by making body region heavily doped.
How the latching problems in the IGBT can be avoided?
The latching problem in the IGBT is avoided by modifying doping level and geometry of the body layer.
When the inversion layer formed in the IGBT?
When the gate – emitter
voltage exceeds threshold voltage, the inversion layer is formed in the p type
body region ( for n channel IGBT)
Why the doping level of the drift region is kept low in the n –
channel IGBT?
Drift region
The doping level and
thickness of the drift layer determines the current amplification factor of the
PNP transistor. The doping level and thickness of the drift layer is kept low
resulting most of the current flows through the MOSFET. Therefore, the voltage drops across body
spreading resistance reduces and possibility of static latch up of the IGBT is
eliminated.
When the
IGBT is operated under cut off region?
OR
Explain the term : Cut off region
Cut off region
When the gate – emitter
voltage is less than the threshold voltage, the collector – emitter voltage
almost equal to supply voltage and only leakage current flows through the
device. This is called as cut off region of the IGBT.
Why the maximum reverse voltage in the non – punch through IGBT
is very small?
Due to heavily doped n+
drift layer
Explain the term : Active region
Active region
When the gate – emitter
voltage is greater than the threshold voltage, the IGBT operated in the active
region. The collector current in the region depends upon transfer
characteristic of the IGBT.
In which operating region the voltage across IGBT remains constant?
Saturation region
Explain the term : Forward breakdown voltage
Forward breakdown voltage
It is collector to
emitter voltage at which avalanche breakdown occurs in the IGBT.
Why the IGBT is operated below forward breakdown voltage?
Importance of
the forward breakdown voltage
The voltage across the
IGBT and current passes through IGBT is high at forward breakdown voltage. This
will result in high power loss in the device and it may cause damage it. Therefore,
the IGBT is always operated below forward breakdown voltage.
How the dynamic latch up of the parasitic thyristor in the IGBT
avoided?
The
gate emitter voltage of the IGBT is kept negative during its turn off period in
order to avoid dynamic latch up of the thyristor.
How the punch through IGBT and non-punch through IGBT affect the
tail current?
Tail current
The punch through IGBT
shortens the duration of the tail current time. The n+ buffer layer
acts as sink for holes and it removes the holes from the drain drift layer
therefore the tail time is reduced. The tail current in the non-punch through
IGBT is reduced by minimizes the magnitude of the current during failing
interval. This is done by designing the MOSFET of the IGBT such that it carries
as much as of the total current.
Why IGBTs can be easily connected in parallel without any
compensating device?
The IGBTs can be easily
connected in parallel without any compensating devices due to flat resistance
temperature coefficient.
Describe the advantages of flat temperature coefficient of
resistance in the IGBT.
Due to flat temperature coefficient
of resistance in the IGBT
Resistance remains
constant under all temperature conditions
Secondary breakdown does
not take place
The parallel operation of
the IGBTs can be possible without any compensating devices.
State the
advantages of the IGBT over the BJT.
Advantages of the IGBT
over the BJT
The IGBT is voltage-controlled
device so gate drive circuit is very simple.
The IGBT can operate at
higher frequency.
Secondary breakdown does
not take place.
Snubber circuit does not
require.
Describe the different modes of latching in the IGBT.
Latching modes in
the IGBT
Static
latch up
Dynamic
latch up
Give reason : The switching losses in the IGBT is determined
only by turn off losses for pure inductive load.
Switching losses
The turn on losses in the IGBT is very small because the transistor is turned on at zero current therefore the total switching losses consists of only turn off losses particularly at pure inductive load.
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