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EE462L, Spring 2013

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EE462L, Spring 2013 Implementation of Unipolar PWM Modulation for H-Bridge Inverter (pre-fall 2009 - but discrete components provide a better sense of how this ... – PowerPoint PPT presentation

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Title: EE462L, Spring 2013


1
EE462L, Spring 2013 Implementation of Unipolar
PWM Modulation for H-Bridge Inverter (pre-fall
2009 - but discrete components provide a better
sense of how this circuit operates)
2
H-Bridge Inverter Basics Creating AC from DC

Switching rules



Vdc
A

B

Can use identical isolated firing signals for A,
A, with inverting and non-inverting drivers to
turn on, turn off simultaneously
Va
Vb
Load



Same idea for B, B
A


B


The A, A firing signal is a scaled version of Va
The B, B firing signal is a scaled version of Vb
The difference in the two firing signals is a
scaled version of Vab
3


Implementation of Unipolar PWM
Vcont is the input signal we want to amplify at
the output of the inverter.
Vcont is usually a sinewave, but it can also be a
music signal.
Vcont
Vtri
-Vcont
The implementation rules are




Vtri is a triangle wave whose frequency is at
least 30 times greater than Vcont.
Ratio ma peak of control signal divided by peak
of triangle wave
Ratio mf frequency of triangle wave divided by
frequency of control signal
4
Implementation of Unipolar PWM Modulation for
H-Bridge Inverter
Progressivelywider pulses at the center (peak of
sinusoid)
Progressively narrower pulses at the edges
Unipolar Pulse-Width Modulation (PWM)
5
The four firing circuits do not have the same
ground reference. Thus, the firing circuits
require isolation.

Vdc

(source of power delivered to load)



A

B

Local ground
Local ground


reference for A

reference for
B

firing circuit

firing circuit

S

S

Load



A

B

Local ground
Local ground
-
-
reference for
B

reference for A



firing circuit

firing circuit

S

S

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7
This years circuit
8

Output of the Comparator Chip
12V from DC-DC chip
1.5k?


1.5k?



270k?

Since the comparator compares signals that can be
either positive or negative, the comparator must
be powered by V supply
Vtri

Vcont

1k?
8 5

Comp


1 4
270k?

Vcont
12V from DC-DC chip
Common (0V) from DC-DC chip
9

Output of the Comparator Chip
12V from DC-DC chip
1.5k?


1.5k?



Since the comparator compares signals that can be
either positive or negative, the comparator must
be powered by V supply
270k?

Vtri

Vcont

1k?
8 5

Comp


1 4
270k?

Vcont
12V from DC-DC chip
Common (0V) from DC-DC chip
10
This years circuit
11
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19
Flat toping indicates the onset of overmodulation
20
Approaching a square wave
21
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