Term 1 Test

Question 1

Whats the definition of power electronics?

Answer 1

Study of circuits which are able to efficiently process and transfer electric power using semiconductor switching devices

Question 2

Power devices: diode

• Is it controllable
• How to control it
• Power handling

Answer 2

Uncontrollable
Turned on and off by voltage applied across it
Depends of type and how its packaged

Question 3

Power devices: thyristor

• Is it controllable
• How to control it
• Power handling

Answer 3

Partially controllable through a gate signal
To turn it on, brief pulse of current apllied at gate
Device turns off when current through it goes negative
Very high power handling

Question 4

Power devices: transistors

• Is it controllable
• How to control it
• Power handling

Answer 4

Fully controllable
Independently turned on and off by applying correct signal across third terminal
Medium to high power handling

Question 5

What is a switch controller and a linear controller in a circuit?
Why is switching better?

Answer 5

Using a switch with a controlled on/off ratio
A variable resistor
No power loss when switching as power is lost in resistor

Question 6

Attributes of an ideal switch:
Zero…
Infinite…

Answer 6

Zero: period between on and off switching states
ON state voltage drop across switch
OFF state current through the device

Infinite: power-control ratio
OFF state voltage withstand capability
ON state current handling capability
Power handling capability

Question 7

Attributes of a practical switch-3 points

What problem do all of these create?

Answer 7

Cannot instantaneously switch from on to off state or vice versa
When it is turned on, finite voltage drop across it
When turned off, small leakage current flows through it
All of this leads to power loss

Question 8

Equations for switchs:
c = crossover of voltage and current
tc(on), tc(off) (think about switch graphs)
Wc(on): energy dissipated during tc(on)
Wc(off): energy dissipated during tc(off)

Answer 8

tc(on): tri + tfv (rising current time + falling voltage time)
tc(off): trv + tfi (rising voltage time + falling current time)

Wc(on) = 0.5 * Vdc * Io * tc(on)
Wc(off) = 0.5 * Vdc * Io * tc(off)
Vdc = max voltage level
Io = max current level

Question 9

Equations for switchs:
Ts,Fs,t(on),t(off)
Average switching power loss Ps
Average conduction power loss Pon (uses Won)
Average power dissipation in switch Pdis

Answer 9

Ts = one switching period = t(on) + t(off)
Ts = 1/Fs = 1/switching frequency

Ps = Fs * (Wc(on) + Wc(off))
Fs = switching frequency

Won = Von * Io * ton = energy dissipated during ON state time
Pon = Fs*Won

Pdis = Ps + Pon

Question 10

Why do you need a diode when using a switch? Think about voltage and current

Answer 10

When diode is there, when switch is turned off current continues to flow through it.

When diode is removed, when switch is turned off voltage across inductor becomes infnite and will eventually destroy it.

Question 11

Two types of PWM signals: think about shapes

Answer 11

1. Edge-aligned modulation. Sawtooth shaped wave. If Vc (threshold voltage) above V(saw), signal = high. Else signal = low
2. Centre-aligned modulation
   Triangle shaped wave. If Vc above V(tri) signal = high. Else signal = low

Question 12

Duty ratio/cycle, K.

Two equations for it, involves Ts

Answer 12

K = t(on)/Ts
1 - K = t(off)/Ts
As Ts = t(on) + t(off)

Question 13

Graphs for buck and boost converter

Answer 13

Revise

Question 14

Buck converter equations:
Voltage across inductor V(L)
Duty ratio using: 
Voltage in and out
Current in and out

Answer 14

V(L) = L * (Δi/Δt)
Δi = change in current 

K = Vo/Vi
K = Ii/Io
Other way round

Question 15

Buck converter equations:
Inductor L (two equations)
Capacitor equation Co (two equations, similar logic to inductor equations)

Answer 15

L = ((Vi - Vo)*K) / (Fs*Δi) OR
L = ((1 - K)*Vo) / (Fs*Δi)

Co = ((1 - K)Vo) / (8(Fs)^2ΔvoL)
OR
Co = ((Vi - Vo)K) / (8(Fs)^2ΔvoL)
Δvo = output voltage ripple (peak to peak)

Question 16

Boost converter equations:
Duty ratio equation (similar to buck) using voltage and current
Voltage through inductor for switch on and switch off

Answer 16

Vo/Vi = 1/(1 - K)
Ii/Io = 1/(1 - K)

V(L) = Vi = L * Δi(on)/t(on)
V(L) = Vi - Vo = L * Δi(off)/t(off)

Question 17

Boost converter equations:
Inductor L (two equations, second is kinda pointless)

Capacitor Co (one equation but how can you rewrite it to get time constant?)

Answer 17

L = (Vi*K) / (Fs*Δi) OR
L = (1 - K)*(Vo - Vi) / (Fs*Δi)

Co = (Vo/Δvo) * K/(RFs)
Δvo = output voltage ripple
Rearrange so you get CoR and you have time constant

Question 18

Phasor diagrams: current and voltage lines on a graph for

• resistor
• inductor
• capacitor

Answer 18

Resistor: 0° between them
Inductor: Voltage leads current by 90°
Capacitor: Current leads voltage by 90°

Question 19

Complex impedances ofr resistors, inductors and capacitors

Answer 19

Z = R
Z = jωL
Z = -j/ωC

Question 20

RLC circuit power equations:
Instantaneous power
Average power
RMS current (similar to AP)

Answer 20

IP: p(t) = v(t)i(t)
AP: Pavg = 1/T ∫ v(t)i(t) dt between T and 0

RMS: I = √( 1/T ∫ (i(t))^2 dt ) between T and 0

Question 21

Those super weird angle forms for power:
Instantaneous power (two equations, one just cos, one cos and sine)

Answer 21

p(t) = V*I*cos(σ) - V*I*cos(2ωt - σ) OR
p(t) = (V*I*cos(σ))*(1 - cos(2ωt)) - (V*I*sin(σ))*sin(2ωt)

Question 22

Angle equations for:
Active power
Reactive power (whats the σ sign?)
Apparent power
Power factor

Answer 22

AP: P = V*I*cos(σ)
RP: Q = V*I*sin(-σ)
σ negative if current lags voltage
Positive if current leads voltage
AP: |S| = V*I
PF: cos(σ) = P/S

Question 23

What equation gives displacement power factor (DPF) (simple)

Whats another equation for power factor involving DPF?

Answer 23

cos(σ1)

PF = I(1)/I * DPF
I(1)/I: usually given but ask Henry

Question 24

Whats the point of a rectifier (normal diode one)?

What heppens when you add an inductor?

Answer 24

Takes an AC voltage and current wave and makes all values positive (reverses humps below 0)
Makes supply current a square wave and output current constant

Question 25

Thyristor Bridge Rectifier, fully controlled: Equation for average voltage out Whats α? Which thyristors fire first?

Answer 25

Vo(avg) = 0.9 Vs/√2 cos(α) α is the angle difference between 0 + nπ (0, π, 2π...) and the point at which the pulse is applied to the gate T1 and T4 first at 0 phase T2 and T3 at π phase

Question 26

Power equations for thyristor rectifier, fully controlled (similar to RLC ones): Active/average Power Displacement Power Factor Power Factor

Answer 26

``` AP = 0.9*Vs*Io*cos(α) DPF = cos(α) PF = Is1/Is DPF = 0.9*cos(α) ```

Question 27

Whats a half controlled thyristor rectifier? What does it do to output voltage in comparison to normal thyristor rectifier? What do you have to remember to revise?

Answer 27

T3 and T4 (bottom two thyristors) are replaced with normal diodes No negative output voltage-flat line of 0 volts where previously it was negative Revise shape of graphs

Question 28

``` Half controlled thyristor rectifier equations: Average voltage out (half is a clue) Power in/out Power factor RMS value for supply current ```

Answer 28

``` Vo(avg) = 0.45*vs*(1 + cos(α)) Pin = Pout = Vo(avg)*Io PF = Pout/(Vs*Is) Is = Io √(1 - α/π) ```

Question 29

What does an inverter do? What order do the switchs turn on and off and what does this do to the wave? Whats the RMS value for output voltage? (simple equation)

Answer 29

Converts a DC voltage signal to an AC square wave where the two different levels are Vi and -Vi S1 and S4 turn on which causes Vout to be Vi S2 and S3 turn on which causes VOut to be -Vi Vout = 0.9*Vi

Question 30

DC-AC Full Bridge Converter (inverter) output current: Shape of graph against time Can be divided into 4 time periods. Whats going on in each?

Answer 30

Looks like a shark fin starting at -Iout. Not quite triangular but close T1: S1 and S4 turned on, Io goes from -Io(max) to 0 T2: S1 and S4 still on, Io goes from 0 to Io(max) T3: S2 and S3 turned on, Io goes from Io(max) to 0 T4: S2 and S3 still on, Io goes from 0 to -Io(max)

Question 31

Bipolar PWM signals for inverters: What are the names of the two signals? Whats the switching signal look like? How to interpret it for control over S1 to S4

Answer 31

Vm-modulating signal, a normal AC wave VC-carrier signal, a triangle wave that has a much higher frequency than Vm Both signals are overlayed on top of each When Vm > Vc , S1 and S4 are on When Vc > Vm, S2 and S3 are on

Question 32

In relation to inverters and Vm/Vc whats the bipolar PWM equations for: Frequency modulation ratio Amplitude modulation ratio Peak output voltage

Answer 32

FMR: Mf = Fc/Fm Fc = frequency for Vc Fm = frequency for Vm AMR: Ma = v̂m/v̂c v̂m = max voltage level/magnitude for Vm v̂c = max voltage level/magnitude for Vc √2 * Vout = Ma*Vin

Question 33

Hows does unipolar PWM switching differ from bipolar? What do the waves look like? How does this control the switchs? Its quite different from bipolar

Answer 33

Instead of one Vm wave there are two. The second is 180° behind the first. There are three waves on the graph, Vc, Vm1 and Vm2 When Vm1 > Vc, S1 = 1, S3 = 0 When Vm2 > Vc, S2 = 1, S4 = 0 And vice versa