Term 1 Test Flashcards
Whats the definition of power electronics?
Study of circuits which are able to efficiently process and transfer electric power using semiconductor switching devices
Power devices: diode
- Is it controllable
- How to control it
- Power handling
Uncontrollable
Turned on and off by voltage applied across it
Depends of type and how its packaged
Power devices: thyristor
- Is it controllable
- How to control it
- Power handling
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
Power devices: transistors
- Is it controllable
- How to control it
- Power handling
Fully controllable
Independently turned on and off by applying correct signal across third terminal
Medium to high power handling
What is a switch controller and a linear controller in a circuit?
Why is switching better?
Using a switch with a controlled on/off ratio
A variable resistor
No power loss when switching as power is lost in resistor
Attributes of an ideal switch:
Zero…
Infinite…
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
Attributes of a practical switch-3 points
What problem do all of these create?
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
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)
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
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
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
Why do you need a diode when using a switch? Think about voltage and current
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.
Two types of PWM signals: think about shapes
- Edge-aligned modulation. Sawtooth shaped wave. If Vc (threshold voltage) above V(saw), signal = high. Else signal = low
- Centre-aligned modulation
Triangle shaped wave. If Vc above V(tri) signal = high. Else signal = low
Duty ratio/cycle, K.
Two equations for it, involves Ts
K = t(on)/Ts
1 - K = t(off)/Ts
As Ts = t(on) + t(off)
Graphs for buck and boost converter
Revise
Buck converter equations: Voltage across inductor V(L) Duty ratio using: Voltage in and out Current in and out
V(L) = L * (Δi/Δt) Δi = change in current
K = Vo/Vi
K = Ii/Io
Other way round
Buck converter equations:
Inductor L (two equations)
Capacitor equation Co (two equations, similar logic to inductor equations)
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)
Boost converter equations:
Duty ratio equation (similar to buck) using voltage and current
Voltage through inductor for switch on and switch off
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)
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?)
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
Phasor diagrams: current and voltage lines on a graph for
- resistor
- inductor
- capacitor
Resistor: 0° between them
Inductor: Voltage leads current by 90°
Capacitor: Current leads voltage by 90°
Complex impedances ofr resistors, inductors and capacitors
Z = R Z = jωL Z = -j/ωC
RLC circuit power equations:
Instantaneous power
Average power
RMS current (similar to AP)
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
Those super weird angle forms for power: Instantaneous power (two equations, one just cos, one cos and sine)
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)
Angle equations for: Active power Reactive power (whats the σ sign?) Apparent power Power factor
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
What equation gives displacement power factor (DPF) (simple)
Whats another equation for power factor involving DPF?
cos(σ1)
PF = I(1)/I * DPF
I(1)/I: usually given but ask Henry
Whats the point of a rectifier (normal diode one)?
What heppens when you add an inductor?
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
Thyristor Bridge Rectifier, fully controlled:
Equation for average voltage out
Whats α?
Which thyristors fire first?
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
Power equations for thyristor rectifier, fully controlled (similar to RLC ones):
Active/average Power
Displacement Power Factor
Power Factor
AP = 0.9*Vs*Io*cos(α) DPF = cos(α) PF = Is1/Is DPF = 0.9*cos(α)
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?
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
Half controlled thyristor rectifier equations: Average voltage out (half is a clue) Power in/out Power factor RMS value for supply current
Vo(avg) = 0.45*vs*(1 + cos(α)) Pin = Pout = Vo(avg)*Io PF = Pout/(Vs*Is) Is = Io √(1 - α/π)
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)
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
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?
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)
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
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
In relation to inverters and Vm/Vc whats the bipolar PWM equations for:
Frequency modulation ratio
Amplitude modulation ratio
Peak output voltage
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
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
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
