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Power Elecronics 2 > Power loss in semiconductors > Flashcards

Power loss in semiconductors Flashcards

1
Q

What do almost all semiconductor devices require

A

cooling

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2
Q

What are the 2 types of losses in semiconductor devices

A

Conduction and Switching

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3
Q

What sort of voltage drop do Diodes, SCRs and TRIACs have

A

Constant

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4
Q

What is the conducting power of a Diode etc

A

Pcond = Vf x Iave

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5
Q

What do MOSFETS behave like

A

Resistors

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6
Q

What is the conduction power for a MOSFET

A

Pcond= R X I^2r.m.s

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7
Q

In a real switch what happens when the switch is open

A

Some current is still leaked

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8
Q

In reality what happens to the voltage drop of a diode

A

It is not 0

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9
Q

For a real switch what is different about switching time than for an ideal switch

A

Switching takes a finite time

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10
Q

What causes the Conduction losses (Pcond)

A

Non 0 voltage drop when switching

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11
Q

What causes the switching losses (Psw)

A

Finite time to switch on and off

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12
Q

When are conduction losses significant in diodes

A

When in forward biased

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13
Q

When are switching losses significant

A

When switching off (not on)

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14
Q

When calculating the average current what period is the integration done over

A

The period that the diode is conducting

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15
Q

What is the equation for Pcond

A

Pcond = Vf x Iavg

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16
Q

For a half wave rectifier what do the 2 half cycles make different contributions to

A

Pcond and Iavg

17
Q

Describe the first positive half of half wave rectification

A

Diode is forwars biased, Vd(t)=Vf and large id flows

18
Q

Does the 2nd half of the half wave rectification make a contribution to the Pcond

A

No

19
Q

What is the energy loss during turnoff equal to

A

Esw=QrrVr

20
Q

What is the Psw equal to

A

Psw = Qrr x Vr x f(switching)

21
Q

What is the equation for total power loss in a diode

A

Ptot = Pcond +Psw

= Vf x Iavg + Qrr x Vr x f(switching)

22
Q

What is Vf the symbol for

A

The voltage across the forward biased diode (0.7-1.0V)

23
Q

What does the symbol Qrr mean

A

Reverse recovery charge

the value for this will be on the data sheet

24
Q

What is Vr the symbol for

A

reverse blocking voltage across diode when it switches off

25
Q

What happens when the TRIAC or SCR is off

A

No current flows and all the voltage appears across the semiconductor

26
Q

When the SCR or TRIAC is On

A

The voltage across semiconductor Vf falls to 2V so most of the voltage is across the Load

27
Q

How can the conduction power loss for a BJT be calculated

A

In the same way as the it would for a diode

28
Q

How can the switching power loss be calculated for BJT

A

Same as it would for a MOSFET

29
Q

Briefly explain the operation of a MOSFET

A

The channel between source and source and drain is a resistor controlled by the voltagve on the gate

30
Q

What is the resistance of the MOSFET when the MOSFET is on

A

R(DSon)

31
Q

What cause conduction losses in a MOSFET

A

R(DSon)

32
Q

What causes the switching loses in a MOSFET

A

the MOSFET switching on and off

33
Q

What is the equation for Pcond for a MOSFET

A

Pcond = R(DSon) x 1^2(Dr.m.s)

34
Q

Reminder, what are the equations for average current for PWM with a resistive load

A 
Iave = D x Ion 
Ir.m.s= root(D) x Ion
35
Q

Reminder, what is the equation for average power for PWM with a resistive load

A

Pavge = D x Pon

36
Q

What is the average current equation for PWM with an inductive load

A

Iave = D x (Imin + Imax)/2

37
Q

What happens to the drain current and voltage when the MOSFET switches on

A

Drain current raises from 0 to I(Don)

Drain voltage drops from V(Ds) to nearly 0

38
Q

From the MOSFET power loss equations what values are found on the data sheet

A

R(DSon)
t(on)
t(off)

39
Q

What does V(DSoff) mean

A

The voltage across MOSFET when it is off

Power Elecronics 2 (11 decks)

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