DESIGNSPARK PCB CONTEST 2014
Optical Isolation for Three-Phase Inverter
(Team No. 85)
Madishetti Sandeep [1] and Vashist Bist [2]
Research Scholars, Electrical Engineering Department,
Indian Institute of Technology Delhi, Hauz Khas, New Delhi -110016.
[1]
sandeep.madishetti@gmail.com, [2] vashist.bist@gmail.com
I.
Introduction
This works deals with the design and development of the high frequency optical isolation circuitry
for three phase inverter. The main purpose of this circuit is to isolate the control circuit (low voltage)
and power circuit (high voltage). This circuit provides a galvanic isolation between the digital signal
processor (DSP) or Micro-Controller Unit (MCU) and the gate drivers of the solid state switches of
inverter. This also amplifies the DSP voltage which is of the order of 3.3V (VDSP) i.e. CMOS
(Complementary Metal Oxide Semiconductor) to 15 V for gate drivers of solid state switches. The
schematic diagram of the optical isolation circuit for interfacing the PWM signals from DSP to the
inverter solid state switches is shown in Fig. 1.
VCC =+15V
220Ω
NC
1
8
2
7
1.5kΩ
NC
4
VDSP
6
3
PN2222A
Optical
Isolation
1.2kΩ
NC
4.7kΩ
4.7kΩ
VOUT
5
PN2222A
6N136
Opto-Isolator
Fig. 1. Schematic diagram of an Optical-isolation circuitry.
Output to Inverter
Input from DSP/MCU
VEE=+5V
Here, one input transistor is used to not to load the DSP I/O pins. 6N136 opto-coupler IC is used for
isolation. The opto-coupler will gives compliment output and the output transistor is used to
compliment the output of opto-coupler to get the output which is equal to the input.
II. Application of Optical Isolation Circuit
This circuit is primarily used in nearly every kind of gate drivers and isolators circuit. A typical
example of controlling the three phase voltage source inverter via a PWM based current control
technique for motor drive is discussed in this section. Fig. 2 shows the circuit configuration of
brushless DC (BLDC) motor drive. The DSP generates the gating pulses to the voltage source inverter
(VSI) feeding BLDC motor has to be isolated from the high power circuitry of the BLDC motor drive
[2].
DBR +
DC Link Capacitor
PWM-VSI
S1
Single
Phase AC
Supply
S5
BLDC
Motor
Vdc
Cd
S2
Speed Controller
ω*
(Reference
Speed)
S3
ωe
ω
S4
Hall Effect
Position Signals
S6
Optical-isolation circuit
S1 S2 S3 S4 S5 S6
Speed PI
Controller
i*
Ref. Current
Generator
Speed
Calculation
PWM Generator
ω
ix* ma mb mc
Current
Controller
ia
ib
APPLICATION OF OPTICAL-ISOLATION CIRCUIT
Fig. 2. An application of optical-isolation in BLDC motor drive.
Other applications of this optical isolation circuit can be summarized as follows:
1) Used in input of gate driver cards for solid state switches of any AC-DC, DC-DC, DC-AC and ACAC power converters.
2) Used in isolation of various digital signals (such as encoder and Hall-effect position signals) from
the power circuitry.
III. Schematic Design of Optical Isolation Circuit Using DesignSpark PCB
Fig. 3. Schematic Design of Optical Isolation Circuit Using DesignSpark PCB.
IV. PCB Layout of Optical Isolation Circuit Using DesignSpark PCB
Fig. 4. Schematic Design of Optical Isolation Circuit Using DesignSpark PCB.
V.
3D View of Optical Isolation Circuit Using DesignSpark PCB
(a)
(b)
Fig. 5. 3D View of Optical Isolation Circuit Using DesignSpark PCB
VI.
Bill of Material (BOM) of Optical Isolation Circuit
The bill of material (BOM) of the designed optical isolation circuit (one unit) is shown in Table I.
TABLE-I
BOM of Optical Isolation Circuit
S. No.
Component Name
RS Stock
No.
Unit Price
(INR)
Qty
Price
(INR)
-
100
1
100
1.
PCB Manufacturing Cost
2.
Opto-coupler (6N136)
805-1255
88.43
6
531
3.
Transistor (PN-2222A)
295-028
79.38
12
953
4.
Resistors (4.7 kΩ, 0.25W)
-
0.78
12
10
5.
Resistors (1.5 kΩ 0.25W)
-
0.78
6
5
6.
Resistors (1.2 kΩ 0.25W)
-
0.78
6
5
7.
Resistors (220 Ω 0.25W)
-
0.78
6
5
Total (INR)
1609
VII. Conclusions
A design of an optical isolation circuit for proving the galvanic isolation between the power and
signal circuitry has been presented. The designed optical isolation circuit has been validated at wide
range of frequencies and has shown a good performance. The cost of this product is expected to go
down drastically during the mass-production.
VIII. References
[1]
Datasheet: Fairchild Semiconductors, High Speed Transistor Optocouplers-6N136/6N137,
available online- http://www.fairchildsemi.com, Aug. 2008.
[2]
Vashist Bist and Bhim Singh, “An Adjustable Speed PFC Bridgeless Buck-Boost Converter Fed
BLDC Motor Drive”, IEEE Transactions on Industrial Electronics., vol.61, no.6, pp.2665-2677,
June 2014.