Power loss in semiconductors

Question 1

What do almost all semiconductor devices require

Answer 1

cooling

Question 2

What are the 2 types of losses in semiconductor devices

Answer 2

Conduction and Switching

Question 3

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

Answer 3

Constant

Question 4

What is the conducting power of a Diode etc

Answer 4

Pcond = Vf x Iave

Question 5

What do MOSFETS behave like

Answer 5

Resistors

Question 6

What is the conduction power for a MOSFET

Answer 6

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

Question 7

In a real switch what happens when the switch is open

Answer 7

Some current is still leaked

Question 8

In reality what happens to the voltage drop of a diode

Answer 8

It is not 0

Question 9

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

Answer 9

Switching takes a finite time

Question 10

What causes the Conduction losses (Pcond)

Answer 10

Non 0 voltage drop when switching

Question 11

What causes the switching losses (Psw)

Answer 11

Finite time to switch on and off

Question 12

When are conduction losses significant in diodes

Answer 12

When in forward biased

Question 13

When are switching losses significant

Answer 13

When switching off (not on)

Question 14

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

Answer 14

The period that the diode is conducting

Question 15

What is the equation for Pcond

Answer 15

Pcond = Vf x Iavg

Question 16

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

Answer 16

Pcond and Iavg

Question 17

Describe the first positive half of half wave rectification

Answer 17

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

Question 18

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

Answer 18

No

Question 19

What is the energy loss during turnoff equal to

Answer 19

Esw=QrrVr

Question 20

What is the Psw equal to

Answer 20

Psw = Qrr x Vr x f(switching)

Question 21

What is the equation for total power loss in a diode

Answer 21

Ptot = Pcond +Psw

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

Question 22

What is Vf the symbol for

Answer 22

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

Question 23

What does the symbol Qrr mean

Answer 23

Reverse recovery charge

the value for this will be on the data sheet

Question 24

What is Vr the symbol for

Answer 24

reverse blocking voltage across diode when it switches off

Question 25

What happens when the TRIAC or SCR is off

Answer 25

No current flows and all the voltage appears across the semiconductor

Question 26

When the SCR or TRIAC is On

Answer 26

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

Question 27

How can the conduction power loss for a BJT be calculated

Answer 27

In the same way as the it would for a diode

Question 28

How can the switching power loss be calculated for BJT

Answer 28

Same as it would for a MOSFET

Question 29

Briefly explain the operation of a MOSFET

Answer 29

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

Question 30

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

Answer 30

R(DSon)

Question 31

What cause conduction losses in a MOSFET

Answer 31

R(DSon)

Question 32

What causes the switching loses in a MOSFET

Answer 32

the MOSFET switching on and off

Question 33

What is the equation for Pcond for a MOSFET

Answer 33

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

Question 34

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

Answer 34

``` Iave = D x Ion Ir.m.s= root(D) x Ion ```

Question 35

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

Answer 35

Pavge = D x Pon

Question 36

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

Answer 36

Iave = D x (Imin + Imax)/2

Question 37

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

Answer 37

Drain current raises from 0 to I(Don) | Drain voltage drops from V(Ds) to nearly 0

Question 38

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

Answer 38

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

Question 39

What does V(DSoff) mean

Answer 39

The voltage across MOSFET when it is off