Power loss in semiconductors Flashcards
What do almost all semiconductor devices require
cooling
What are the 2 types of losses in semiconductor devices
Conduction and Switching
What sort of voltage drop do Diodes, SCRs and TRIACs have
Constant
What is the conducting power of a Diode etc
Pcond = Vf x Iave
What do MOSFETS behave like
Resistors
What is the conduction power for a MOSFET
Pcond= R X I^2r.m.s
In a real switch what happens when the switch is open
Some current is still leaked
In reality what happens to the voltage drop of a diode
It is not 0
For a real switch what is different about switching time than for an ideal switch
Switching takes a finite time
What causes the Conduction losses (Pcond)
Non 0 voltage drop when switching
What causes the switching losses (Psw)
Finite time to switch on and off
When are conduction losses significant in diodes
When in forward biased
When are switching losses significant
When switching off (not on)
When calculating the average current what period is the integration done over
The period that the diode is conducting
What is the equation for Pcond
Pcond = Vf x Iavg
For a half wave rectifier what do the 2 half cycles make different contributions to
Pcond and Iavg
Describe the first positive half of half wave rectification
Diode is forwars biased, Vd(t)=Vf and large id flows
Does the 2nd half of the half wave rectification make a contribution to the Pcond
No
What is the energy loss during turnoff equal to
Esw=QrrVr
What is the Psw equal to
Psw = Qrr x Vr x f(switching)
What is the equation for total power loss in a diode
Ptot = Pcond +Psw
= Vf x Iavg + Qrr x Vr x f(switching)
What is Vf the symbol for
The voltage across the forward biased diode (0.7-1.0V)
What does the symbol Qrr mean
Reverse recovery charge
the value for this will be on the data sheet
What is Vr the symbol for
reverse blocking voltage across diode when it switches off
What happens when the TRIAC or SCR is off
No current flows and all the voltage appears across the semiconductor
When the SCR or TRIAC is On
The voltage across semiconductor Vf falls to 2V so most of the voltage is across the Load
How can the conduction power loss for a BJT be calculated
In the same way as the it would for a diode
How can the switching power loss be calculated for BJT
Same as it would for a MOSFET
Briefly explain the operation of a MOSFET
The channel between source and source and drain is a resistor controlled by the voltagve on the gate
What is the resistance of the MOSFET when the MOSFET is on
R(DSon)
What cause conduction losses in a MOSFET
R(DSon)
What causes the switching loses in a MOSFET
the MOSFET switching on and off
What is the equation for Pcond for a MOSFET
Pcond = R(DSon) x 1^2(Dr.m.s)
Reminder, what are the equations for average current for PWM with a resistive load
Iave = D x Ion Ir.m.s= root(D) x Ion
Reminder, what is the equation for average power for PWM with a resistive load
Pavge = D x Pon
What is the average current equation for PWM with an inductive load
Iave = D x (Imin + Imax)/2
What happens to the drain current and voltage when the MOSFET switches on
Drain current raises from 0 to I(Don)
Drain voltage drops from V(Ds) to nearly 0
From the MOSFET power loss equations what values are found on the data sheet
R(DSon)
t(on)
t(off)
What does V(DSoff) mean
The voltage across MOSFET when it is off
