TSTE25 Power Electronics · 2016-12-07 · TSTE25/Tomas Jonsson 2016-12-06 4 m f odd multiple of 3 Reduce harmonics. Harmonics of 3-ph vs 1-ph PWM • 1-ph bipolar PWM ... 2016 -12

TSTE25 Power Electronics

• Lecture 12

• Tomas Jonsson

• ISY/EKS

Outline

2016-12-06 2TSTE25/Tomas Jonsson

DC-AC switching inverters

3-phase inverter

Snubber circuits

Exercises

Project work

Three-phase inverters


Three legs

Controls phase shifted 120°

Midpoint not used

Three-phase inverter


mf odd multiple of 3

Reduce harmonics

Harmonics of 3-ph vs 1-ph PWM

• 1-ph bipolarPWM

• 3-ph PWM


Harmonics of 3-ph vs 1-ph PWM

• 1-ph unipolarPWM

• 3-ph PWM


Three-phase inverter harmonics


1-ph PWM 3-ph PWM

Output voltage magnitude

• Single phase ac-voltage:

𝑉𝐴𝑁1 = 𝑚𝑎

𝑉𝑑2

• Phase-phase (Line-Line):

𝑉𝐿𝐿1 = 𝑚𝑎

𝑉𝑑2

3


3rd harmonic PWM

• ma = 1

• Subtract a 3rd harmonic withmagnitude 1/6

• Peak of total output voltagereduced to

87%=3

2


0 0.005 0.01 0.015 0.02 0.025 0.03 0.035 0.04-1

-0.8

-0.6

-0.4

-0.2

0

0.2

0.4

0.6

0.8

1

time [s]

3rd

harmonic PWM

cos(wt)

-1/6*cos(3wt)

cos(wt)-1/6*cos(3wt)

3rd harmonic PWM, cont

• ma = 1.15

• Single phase ac-voltage:

𝑉𝐴𝑁1 = 𝑚𝑎

𝑉𝑑

3

• Phase-phase (Line-Line):

𝑉𝐿𝐿1 = 𝑚𝑎𝑉𝑑

• Peak of total = 1


0 0.005 0.01 0.015 0.02 0.025 0.03 0.035 0.04

-1

-0.8

-0.6

-0.4

-0.2

0

0.2

0.4

0.6

0.8

1X: 0.0201

Y: 1.154

3rd

harmonic PWM

time [s]

1.15cos(wt)

-1.15/6*cos(3wt)

1.15(cos(wt)-1/6*cos(3wt))

Lecture 12

Snubber circuits

Problem with switching converters

TSTE25/Tomas Jonsson

Switches turns on/off while conducting large currents Recovery time create large power dissipation

di/dt generates EMI (Electromagnetic interference)

Small size requires higher switching frequencies

2016-12-07 12

Example, full bridge leg


Finite di/dt and dv/dt

Parasitics: L, C, R

IO can be both positive and negative

2016-12-07 13

Hard switching waveform Stray inductance gives voltage overshoot

Stray capacitance gives current overshoot

Parasitics limits di/dt and dv/dt

PT- = vT- iT-

2016-12-07TSTE25/Tomas Jonsson 14

Switch voltage and current

Short moments of high power dissipationDevice must cope with the power dissipation

Overshoot increases required SOA

2016-12-07TSTE25/Tomas Jonsson 15

Snubber circuits for controlled switches


Step-down example Lσ = L1 + L2 + L3 stray inductances

Voltage and current overshoot due to inductances

Three snubber types Turn-off, turn-on, over-voltage

2016-12-06 16

Turn-off snubber circuit for controlled switches


Diode to only include Rs

at switch turn-on

Simplified circuit for switch turn-off

Switch current at switch turn-off not affected by snubber circuit

𝑖𝑠𝑤 𝑡 = 𝐼0 1 −𝑡

𝑡𝑓𝑖

2016-12-07 17

Overvoltage snubber function


At transistor switch-off completed t = 0 VCOV = VD

I0 goes through Df

Fast switch give iLσ(t=0) = I0

2016-12-07 18

Lecture 12

Exercises

12-100

a) Calculate the short circuitcurrent in the Lab 3 circuit setup. Consider T1 and T2 to conduct at the same time. The dc-voltageis 15V.

b) Is this peak currentacceptable for the MOSFET without failure?


D1

D2

560 µF+

Ud

-

Iv

T2

T1

+Uv

-

D3

D4T4

T3

+Uac

-Rload (10 ohm)

0.04 ohm

0.04 ohm

50nH

0.56 ohm

12-102

a) In an DC/AC converter calculate the MOSFET case and junctiontemperature at 25 C ambient for 0.95A rms ac-side 50Hz load current. Use the data sheet for thermal and electrical data of the MOSFET.

– For switching loss calculation:tri = 38 ns, tfv = 690 ns, trv = 24 ns, tfi = 32 ns, mf=19

– Consider a MOSFET IRF540 without heatsink.

b) Adding a heatsink. Calculate the required thermal resistance (Rth) ofthe heatsink to keep the case temperature below 80 C when the peakrms current of the MOSFET is 20A. What is the junction temperaturein this case?


22-13

• A MOSFET step-down converter such as shown in Fig. 22-10 operates at a switching frequency of 30 kHz with a 50% duty cycle at an ambient temperature of 50°C.

• The power supply Vd = 100 V and the load current I0 = 100 A.

• The free-wheeling diode is ideal but a stray inductance of 100 nHis in series with the diode.

• The MOSFET characteristics are listed below:BVDSS = 150 V; Tj,max = 150°C; Rth,j-a = 1 K/W; rDS(on) = 0.01 ohm, tri = tfi = 50 ns; trv = tfv = 200 ns; ID,max = 125 A

• Is the MOSFET overstressed in this application and if so, how? Be specific and quantitative in your answer.

2016-12-06 22TSTE25/Tomas Jonsson100 nH

Project work flow2016-12-07 23TSTE25/Tomas Jonsson

• Converter circuit layout and selection of components.

– Define how to connect input and output to the source and the load.

– Which MOSFETs and diodes in the base circuit shall be used

– Calculate the operating range: Input/output voltage & current, duty

cycle or ma

– Determine a suitable switching frequency to satisfy requirements. The

boundaries typically set by distortion and losses.

– Select inductor size and required filter capacitors to meet the

requirements.

• Basic verification, simulation in MULTISIM

• Modification of the control design in Labview to meet the dedicated

application.


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TSTE25 Power Electronics · 2016-12-07 · TSTE25/Tomas Jonsson 2016-12-06 4 m f odd multiple of 3 Reduce harmonics. Harmonics of 3-ph vs 1-ph PWM • 1-ph bipolar PWM ... 2016 -12