1. Implementation of Unipolar PWM Modulation for H-Bridge Inverter
EE462L, Spring 2014
Implementation of Unipolar PWM Modulation for H-Bridge Inverter
(pre-fall but discrete components provide a better sense of how this circuit operates)

2. H-Bridge Inverter Basics – Creating AC from DC!
H-Bridge Inverter Basics – Creating AC from DC
Switching rules
Vdc
Either A+ or A –
is closed,
but never at th
e same time
Either B+ or B –
is closed,
but never at the same time A+ B+
Can use identical isolated firing signals for A+, A–, with inverting and non-inverting drivers to turn on, turn off simultaneously Va
Load Vb
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:
Vcont > Vtri , close switch A+, open
switch A –
, so voltage Va = Vdc
Vcont < Vtri , open switch A+, close
switch A –
, so voltage Va = 0 –
Vcont > Vtri , close switch B+, open
switch B
, so voltage Vb = Vdc –
Vcont < Vtri , open switch B+, close
switch B
, so voltage Vb = 0
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!
Implementation of Unipolar PWM Modulation for H-Bridge Inverter
Progressivelywider pulses at the center (peak of sinusoid)
Progressively narrower pulses at the edges
Vdc
−Vdc
Vload
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 A −
reference for B −
firing circuit
firing circuit S S

7. This year’s circuit

8. Comparator Gives V(A+,A–) wrt. Common (0V)
Output of the Comparator Chip
+12V
–12V
Comparator Gives V(A+,A–) wrt. Common (0V)
Vcont > Vtri
Vcont < Vtri
V(A+,A–)
+12V
from DC-DC chip
1.5kΩ
1.5kΩ
270kΩ
Since the comparator compares signals that can be either positive or negative, the comparator must be powered by ±V supply
Vtri
Vcont
1kΩ
Comp
+24V 0V
Vcont > Vtri
Vcont < Vtri
Use V(A+,A–) wrt. –12V
270kΩ
–Vcont
–12V
from DC-DC chip
Common (0V) from DC-DC chip

9. Comparator Gives V(B+,B–) wrt. Common (0V)
Output of the Comparator Chip
+12V
–12V
Comparator Gives V(B+,B–) wrt. Common (0V)
–Vcont > Vtri
– Vcont < Vtri
V(B+,B–)
+12V
from DC-DC chip
1.5kΩ
1.5kΩ
Since the comparator compares signals that can be either positive or negative, the comparator must be powered by ±V supply
270kΩ
Vtri
Vcont
1kΩ
Comp
+24V 0V
– Vcont > Vtri
– Vcont < Vtri
Use V(B+,B–) wrt. –12V
270kΩ
–Vcont
–12V
from DC-DC chip
Common (0V) from DC-DC chip

10. This year’s circuit

15. +4V 0V
−4V

16. +24V 0V
+24V 0V

17. +24V 0V
+24V 0V
+24V 0V
+24V 0V

18. +24V 0V
−24V

19. +24V 0V
−24V
Flat toping indicates the onset of overmodulation
+24V 0V
−24V

20. Approaching a square wave