VL IX - Power electronics

Difference between electrical and information transport

• minimum loaa of energy for energy transport

What does PWM mean

Pulse Width Modulation

V×s theorem

• Voltage can be controled by duty cycle

definition of duty cycle term:

Duty cycle = ton/T

T = ton + toff

whats pulse width modulation

• modification constant signal into alternating sinusoidal signals

PWM for AC

• through suitable modelling of duty cycle AC can be generated

• Demodulation of signal by motor inductances

• Smoothness can be set via switching frequency

» Required for power electronics

different type of switches

• only activatable

• Actively controllable

• Uncontrolled

why are semiconductor switches necessary

Regular switches would be to slow

where is a rectifier used

• AC to DC

• used in onboard charger

how does a rectifier work no smoothing

• Using 4 diodes the current is kept positiv

• disadvantage high voltage fluctuation

How to smooth rectifier

• using inductive or capacitive smoothing

• If smoothing is large enough it is considered a constant voltage source

pros and cons of smoothed rectifier

Pros:

• constant output voltage

Cons:

• harmonic waves have feedback on grid

• Only usable for low power applications

what does obc mean

Obnboard charger

Step by step how OBC works

• rectifier converts AC to DC

• DC Converter adapts DC to HV-DC

  • Contains high frequency transformer for safety reasons

• Bidirectional AC/DC + bidirectional DC/DC + control + grid/communications regulations

  • V2G applications possible

components of DC/DC (Buck) converter

Operation cycle 1 DC/DC converter

1. Switch S closed

2. Coil L is charged

3. Current flow through L and R driven by voltage

4. Diode D is blocking

operation cycle 2 DC/DC Converter

1. Switch S is opened

2. Coil L is discharged

3. Diode D is forward bias

Types of DC/DC converter

• Buck

• Boost

Buck Converter Mode of Actions

• Switch closed

  • Current increases exponentially

• Switch open

  • Current drops exponentially

• Ratio between working cycles determines effective output current

Boost Converter Components

Mode of Operations Boost converter

• Coil L provides voltage, which ads to input voltage

• Coil L charged in the first and discharged in the second cycle

• Smoothing of output cycle through capacitor C

Cycle 1 Boost converter

1. Switch S is closed

2. Coil L is charged

3. Current flows through L and S

4. D is blocking

5. Load R is buffered via backup Capacitor

cycle 2 boost converter

1. Switch S is open

2. Coil L discharges through D, C and R

3. D is forward biased

problems caused by DC/DV Ripple

• Overvoltage on sensitive components

• Strong noise for sensitive analog components

• Noise generation

• Flashes of light

how trade of reduction of DC/DC ripple

• increasing switching frequency

• more switching losses

inverter DC/AC mkde of operation

• Three phase AC required

• Power electronic switches used for conversion

• Reversible operating principle

what are the different Inverter numbers affected by

• Voltage by duty cycle and DC-link voltage

• Frequency by motor speed

• Switching frequency affects current ripple

Power factor inverter causes and consuquence of low power factor

• If current and voltage are not in phase reactive power generated

Low factor » higher RMS current required for same power

On what to adjust AC voltage for inverter

• specification of maximimum DC-Voltage

Or

• Specification of the maximum RMS of AC-voltage

on what to adjust max. Power/current inverter

• Inverter power > motor power

• Max current of inverter and motor must match

  • Power ASM 1,5×P_engine

  • Power PSM 1,2×P_engine

• Electric motor & inverter must be designed together

what does RMS mean

Root Mean square

Semiconductor Materials ins BEVs

Elemental:

• Silicon (Si)

Compound:

• Silicon Carbide (SiC)

• GaN

whats n-dopimg semiconductor

Adding Atom with Excess valenz electron

whats p-doping in Semiconductors

Adding Atom with missing valenz electron