11 EE462L Isolated Firing Circuit for H Bridge Inverter
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Transcript of 11 EE462L Isolated Firing Circuit for H Bridge Inverter
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EE462L, Spring 2014
Isolated Firing Circuit for
H-Bridge Inverter
(partially pre-Fall 2009 approach)
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Isolation is needed because we have threeseparate
MOSFET source nodes, and these threenodes are
ground references for the respective firing circuits
One logic
signal
toggles
A+,A
One logic
signal
toggles
B+,B
Vdc
(source of power delivered to load)
Load
A+
B+
A B
Local ground
reference for A+
firing circuit
Local ground
reference for B+
firing circuit
Local groundreference for B
firing circuit
Local groundreference for A
firing circuitS
S
S
S
!
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8 5Comp
1 4
270k
Vtri
Vcont
Vcont
270k
1k
1.5k
1.5k
V(A+,A)
12V
from DC-DC chip
+12V
from DC-DC chip
Common (0V) from DC-DC chip
+12V
12V
Comparator Gives V(A+,A)
wrt. Common (0V)
Vcont> Vtri
Vcont< Vtri
+24V
0V
Vcont> Vtri
Vcont< Vtri
Use V(A+,A) wrt.12V
Output of the Comparator Chip
Since the comparator
compares signals that can beeither positive or negative, the
comparator must be powered
by V supply
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8 5Comp
1 4
270k
Vtri
Vcont
Vcont
270k
1k
1.5k
1.5k
V(B+,B)
12V
from DC-DC chip
+12V
from DC-DC chip
Common (0V) from DC-DC chip
+12V
12V
Comparator Gives V(B+,B)
wrt. Common (0V)
Vcont> Vtri
Vcont< Vtri
+24V
0V
Vcont> Vtri
Vcont< Vtri
Use V(B+,B) wrt.12V
Output of the Comparator Chip
Since the comparator
compares signals that
can be either positive ornegative, the
comparator must be
powered by V supply
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AC wall wart
(marked with yellow paint)
DC
wallwart
V(A+,A)
control
signal
V(B+,B)
control
signal
Reference (is 12V
from DC-DC chip)
The control signals at the open-circuited output
of the PWM control circuit are +24V, or 0V
When V(A+,A) is 24V,MOSFET A+ is on,
MOSFET A is off
When V(A+,A) is 0V,
MOSFET A+ is off,
MOSFET A is on
MOSFETs B+ and B work
the same way with V(B+,B)
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Figure 14. Output control voltage V(A+,A) on top, and V(B+,B) on bottom, with respect
to protoboard12V reference, with ma> 0 (the situation shown is where Vcontis negative)
Figure 13. Output control voltage V(A+,A) on top, and V(B+,B) on bottom, with respect
to protoboard12V reference, with ma> 0 (the situation shown is where Vcontis positive)
Save screen
snapshot #3
0V
+24V
0V
+24V
0V
+24V
0V
+24V
Look for symmetry of
pulse centers
Look for s
ymmetry of
pulse centers
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One firing circuit for each MOSFET, with each firingcircuit mounted on a separate protoboard. Protoboards
Aand Bcan share a power supply and ground. A+
andB+must each use separate power supplies andgrounds. Do not connect any of these grounds to the
ground of the control circuit.
O+ O
(see Figure 2 for connections)
Powered by +12V that is isolatedfrom thePWM control circuit
10k
0.1F101.2k
MOSFET
G D S
100k
5 4
Opto8 1
5 4
Driver
8 1
Outline ofprotoboard
A+and B+ use inverting drivers
(1426s). A and B use non -inverting drivers (1427s) . Theoptocouplers provide an additional
inversion.
green
green
green
blue for A+
,B+
,violet for A,B
blue
blue
red
blue
Grounds (isolatedfrom control circuit)
Wait until
next week
Switching diode
14mA
Optocoupler is current-
controlled. Gate current
turns on the transistor,
which pulls down the
collector voltage.
Isolating
barrier
Once the MOSFET is
connected, this
asymmetrical circuit will
add blankingby making
the turn-on slower than
the turn-off. (blanking isthe opposite of overlap)
Overlap is the time that
A+and Aare
simultaneously on,
which should be avoided.
Hence, some blanking
(time between one
turning off and the other
turning on) is desirable.
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14mA from
control circuit10k
+12V
+
Vdriver = 0V
Isolatingbarrier
14mAto Opto Input Yields 0Vto Input of Driver Chip,
so Inverting Driver Chip Turns MOSFET ON
To
driver
1.2mA (will pull down Vdriverto zero)
Spec. sheet current transfer ratio
0.2 to 0.3 (times 14mA)
!
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10k
+12V
+
Vdriver = 12V
0mA
Isolatingbarrier
0mAto Opto Input Yields 12Vto Input of Driver Chip, so
Inverting Driver Chip Turns MOSFET OFF
To
driver
0mA from
control circuit
!
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A+
Firing
A
Firing
B+
Firing
B
Firing
Control Circuit from Previous Lab
Jack for
DC wall
wart
Figure 2. Physical layout of firing circuits
(A+opto and driver are powered by a +12V isolated DC converter chip. Likewise, B+ is powered
by another +12V isolated DC converter chip. Aand Bare powered by the DC wall wart.)
Individual protoboard for
each firing circuit
Optically-isolated firing
circuits. Mount drivers
near the MOSFETs
O+ O O+ O O+ O O+ O
V(A+,A) V(B+,B)
12Vdc
regulated
blue
blue blue
violet violet
Jack for
AC wall
wart
8
We use the control signals to send 14ma through
optocouplers on each of the four firing circuit boards
A+ and Aare
daisy chained
B+ and Bare
daisy chained
(for complementary
outputs)
So, each 14mA control signal passes through two optocouplers in series
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24V control signals from the comparators, less 3.2V drop across twoseries optocoupler LEDs, and with 14mA, requires about 1.5kof
resistance in series with the daisy-chained optocouplers
With 14mA, the LED of each optocoupler has about 1.6Vdrop
If applied half the time, 24V across a 1.5k resistor would produce
about 0.2W. So, it is a good idea to size up to W resistors.
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Thus, you use W series resistors between the
comparator chip and the output terminals
8 5
Op Amp1 4
1
en.14
0.01F 100k
Approx
22kHz
triangle
wave
Filtered and
buffered
triangle wave
igh-pass filter to
block DC
8 5
Comp1 4
1.5k, W
270k
1.5k, W
1k
+12Vdc regulated from 2W,
DC converter chip
V(A+,A)
red
green
blue
bl
red
These W resistors can get hot - keep them offthe surface of the protoboard
Vcont
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Layout of inverter control circuit and isolated firing circuits
AC wall wart(marked with yellow paint)
DC
wallwart
A+
A
B+
B
No MOSFETs connected yet (i.e., the drivers are open-circuited)
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Keep the 0.1F capacitorsacross the drivers to prevent
driver failure
Use the same pattern for B+
and B
One DC converter chip feeds
A+
Another DC converter chip
feeds B+
Wall wart feeds Aand B
DC converter chip
feeds A+circuit
Inverting driver
for A+(1426s)
Non-inverting driver
for A(1427s)
Wall wart feeds Acircuit
12Vr
ailfedb
DCco
nverterchi
12Vr
ailfedb
wallwart
groundrailfedby0Vo
utpu
tpinofDC
converterchip
groundrailfed
bywallwart
Zoom-in view of A+ and
Aisolated firing circuits
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wallwartinput
chipoutput
+ +
Side view of A+ and Aisolated firing circuit and single 12V
isolated DC-DC converter chip that powers A+
Socket each single DC-DC converter chip,
using one half of an 8-pin SIP socket.
Carefully break an 8-pin SIP socket in
half. Do this by clamping on one-half
with your long-nose pliers, and then
bending the other half down with your
fingers. It should be a clean break.
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V(A+,A)
Opto A+
output
Save screen
snapshot #1
0.5V
3.2V
Input and Output Voltages of Optocoupler
Vcont= 0 (i.e., ma= 0) in this Snapshot
12V
0V
Opto Input (the 1.5kresistor drops the voltage
from 24V to 3.2V)
As expected, the opto
output is inverted
This phototransistor turn off delay will limit
your PWM operating frequency
Phototransistor
turning on
Phototransistor
turning off
Look for Symmetry Among all Four Circuits
!
Different time-constants to avoid shoot-through
(i.e. to provide a dead-time)
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Look for Nearly Perfect Alignment Between V(A+,A) Signal to
Optocoupler, and Output of A+Inverting Driver Chip
V(A+,A)
A+driveroutput
In
phase
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Look for Nearly Perfect Out of PhaseAlignment Between V(A+,A)
Signal to Optocoupler, and Output of ANon-Inverting Driver Chip
V(A+,A)
Adriveroutput
Out of
phase
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Now the present circuit based on PCBs:
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IRS21844
High output
Low output
Actual pinout
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IRS21844
Blanking time and isolation already integrated in a single IC