CMOS Isolators for Home Appliance Motor Control

CMOS Isolators for Home Appliance Motor Control
Introduction
The home appliance market uses three-phase, pulse-width-modulated (PWM) motors in a
number of end applications including air conditioners, washers, dryers and garage door openers.
Many of these applications require variable speed and/or torque, and the controllers that provide
this capability must be low-cost, reliable and efficient. Such controllers frequently require isolation
for several circuits, such as the gate drive, current measurement, and control feedback paths.
While optocouplers have traditionally provided this isolation, most designers have begun moving
to complementary metallic oxide semiconductor (CMOS) isolators, which offer substantial
improvements over optocouplers in the areas of performance, power, size, reliability and costper-channel.
Optocoupler Technology vs. CMOS Isolator Technology
An optocoupler is a hybridized device containing a light-emitting diode (LED), optically
transparent insulating film (dielectric) and an output die containing a photo detector and output
stage. Optocoupler operation is simple: the output-side photodetector converts light to current,
which drives the output stage in proportion to LED brightness. In spite of this simple operating
principle, optocouplers are notorious for relatively poor performance and reliability due to their
underlying process and packaging technologies. Light emissions from the Gallium Arsenidebased (GaAs) LED change with temperature and device age, complicating design and often
forcing design compromises. LEDs also have an intrinsic wear-out mechanism (“LOP”) that
permanently reduces LED emissions by 20% or more and is worsened by elevated temperature
and LED current. This reduction in LED output further worsens optocoupler timing and output
drive performance. The single-ended architecture of optocouplers (and high internal capacitive
coupling) results in poor common-mode transient immunity (CMTI), which can increase
optocoupler error rates in electrically noisy environments. These and other issues (e.g. high
power consumption, external BOM and large footprint per channel) require added design efforts
to compensate for the fundamental weaknesses of optocouplers.
Silicon Labs CMOS isolators use conventional CMOS process technology and ON/OFF keying
modulation to transmit digital data through the isolation barrier and offer superior performance
and reliability compared to optocouplers. These key technology differences are:
•
The Use of Mainstream, Low-Power CMOS Process Technology Instead of GaAs
CMOS is arguably the most robust, best performing and most widely sourced process
technology in the world. CMOS offers very high device integration and speed, low-power
operation and exceptionally high reliability. The combination of advanced circuit design
techniques and CMOS processing enable Silicon Labs’ fast 150 Mbps data rate (tPD = 10
ns), 5.6 mW/channel power consumption and resistance to temperature and age effects.
The isolation barrier time-dependant device breakdown (TDDB) is in excess of 60 years
at the full data transmission speed of 150 Mbps, worst case operating temperature and
maximum VDD.
Silicon Laboratories, Inc.
Rev 1.0
1
•
The Use of a High-Frequency Carrier Instead of Light
The use of a high-frequency carrier further enables low operating power and high-speed
operation and adds the benefits of precise frequency discrimination for higher noise
rejection and simplified packaging compared to optocouplers.
•
The Use of a Fully Differential Isolation Path Instead of Single-Ended
The differential signal path and high receiver selectivity provides high rejection of
common-mode transients (CMTI > 25 kV/µs), external RF field immunity to 300 V/m and
magnetic field immunity beyond 1000 A/m for error-free operation.
•
The Use of Proprietary Design Techniques to Suppress EMI
Devices in this family meet the emission standards of FCC Part B and are tested using
automotive J1750 (CISPR) test methods. For more information on CMOS isolator
emissions, susceptibility and reliability vs. optocouplers, see Silicon Labs white paper
“CMOS Isolators Supersede Optocouplers in Industrial Applications” available at
www.silabs.com/isolation.
CMOS isolator operation is straightforward; an isolator channel (Figure 1) consists of a two-die
structure in which the transmitter and receiver are separated by a differential capacitive isolation
barrier. Logic high at the isolator input turns the transmitter on, sending a carrier across the
isolation barrier to the receiver, which asserts logic high at the output when sufficient in-band
carrier energy is detected. Conversely, logic low at the input inhibits transmitter operation,
causing the receiver to drive the output low.
ISOdriver Channel
Die #1
VDD
Die #2
ISOLATION
IN
INPUT
CONDITIONING
+
+
-
-
XMITTER
RECVR
DRIVER
DIFFERENTIAL
ISOLATION BARRIER
OUT
GND
Figure 1. CMOS Isolator Top-Level Block Diagram
Silicon Labs CMOS isolators can service most applications currently served by digital
optocouplers. Like optocouplers, CMOS isolators can be used for safely isolation, voltage level
shifting and ground noise mitigation.
Consumer appliances using variable-speed, three-phase motors represent a significant
opportunity for CMOS isolators. Specialized devices, such as high-speed isolated drivers
(“ISOdrivers”), isolated inductive current sensors and multi-channel digital isolators (up to six
channels per package) provide high performance, low external BOM, high reliability and
competitive cost for many applications.
Silicon Laboratories, Inc.
Rev 1.0
2
Isolation in Consumer Motor Control Applications
There are many areas of the home that require the use of small three-phase motors. In addition
to appliances, such as washers and dryers, there are other areas, such as heating and air
conditioning systems, air movers (fans, exhaust blowers) and pool pumps that require controls of
one form or another. Table 1 lists the more common end applications.
Table 1. Home Appliances
TYPICAL APPLIANCE MOTOR APPLICATIONS
- Adjustable Bed
- Air-Conditioner Blower/Compressor
- Attic Ventilator
- Ceiling Fan
- De-Humidifier
- Dishwasher
- Dryer
- Electric Tools (Saw, Grinder, Sander, Compressor)
- Exhaust Fan
- Freezer
- Garage Door Opener
- Garbage Disposal
- Heater Blower
- Humidifier
- Jet Pump
- Pool Pump
- Portable Electric Heater
- Refrigerator Fan/Compressor
- Sump Pump
- Trash Compactor
- Treadmill
- Washer
Motors used in home appliance applications are typically fractional or low-horsepower types with
power ratings from 0.25 hp (186 W) to 3 hp (2,238W). While safety certification agencies
mandate the use of isolators to protect consumers, isolators are also used in these systems for
signal level shifting and electrical noise (ground loop) reduction.
SYSTEM CONTROLLER
CONTROL
INTERFACE
ISOLATION
PROTECTION &
CONTROL
RECTIFIER & INVERTER
ISOLATED BIAS
SUPPLIES
BIAS SUPPLIES
ISOLATED
GATE
DRIVERS
ISOLATED
CURRENT
SENSOR
MOTOR
ISOLATED
CURRENT
SENSOR
POWER SUPPLIES
ISOLATED
GATE
DRIVERS
ISOLATED
CURRENT
SENSOR
ISOLATED
CURRENT
SENSOR
120/240VAC
LINE
Figure 2. Appliance Motor Control Top-Level Block Diagram
Silicon Laboratories, Inc.
Rev 1.0
3
Figure 2 is a typical three-phase home appliance motor control system block diagram showing
where galvanic isolation is used. Some systems include isolated and/or non-isolated power
supplies to provide bias or to provide high-voltage dc to the switching circuits. These supplies
typically require isolated gate drivers and/or high-frequency current sensors for current limit
protection and/or feedback control. The system may use a small microcontroller for system
management, which is powered from an isolated supply. This controller requires isolation to
protect against voltage surges into low-voltage areas exposed to the user. The rectifier and
inverter together convert the line-derived dc input into ac, which, ultimately, drives the threephase electric motor. These circuits require both safety isolation and level shifting to drive
switches riding on high common-mode voltages. Power stage circuits like these typically require
isolated gate drivers and current sensors.
Isolated Gate Drivers
A three-phase motor usually has three high-side/low-side IGBT transistor pairs plus one motor
brake IGBT for a total of seven isolated driver channels. At a minimum, IGBT isolated gate drive
motor applications require competitive installed cost, high peak output drive, high reliability over
elevated temperature conditions and high CMTI. Fast propagation delay times may also be
required for fractional-horsepower applications having a high modulation frequency. The three
most popular isolated gate drive options are: 1) a single-package optocoupler plus driver (i.e.
“optodriver”), 2) a two-chip solution consisting of an optocoupler and an external high-voltage
driver IC, or 3) a gate drive transformer circuit. Optocoupler-based drivers exhibit performance
and reliability deficiencies regardless of how they are implemented. For example, low CMTI
remains an issue that can be addressed with additional external circuitry, but these circuits tend
to overdrive the optocoupler, reducing service life. Optodrivers, such as the Avago HCPL-3120,
are essentially optocouplers with a higher drive output buffer forming a single-package isolated
gate driver. Heat dissipated by the internal driver is easily transferred to the optocoupler,
degrading performance and contributing to shorter service life. The two-chip solution (optocoupler
plus external HVIC driver) externalizes the driver and improves optocoupler reliability but at an
increased cost. Many designers choose lower cost gate drive transformer-based isolated driver
solutions because they provide more uniform timing than optocouplers and at lower cost.
However, a transformer-based drive topology cannot transmit dc or low frequency and, therefore,
imposes maximum duty cycle and ON-time limitations. In addition, they require additional external
reset circuitry or a dc blocking capacitor to prevent transformer core saturation. These timing
restrictions and added reset BOM overhead make gate drive transformers most useful in systems
operating with maximum duty cycles of 50% or less and/or relatively short ON-times.
The Si823x ISOdriver is an integrated CMOS multi-channel isolator with on-chip output gate
driver circuit that offers higher reliability, substantial timing improvements and higher CMTI
compared to optocouplers. It also has no timing restrictions like gate drive transformer designs.
These devices are offered in three base configurations: a high-side/low-side isolated driver with
separate control inputs for each output (Figure 3a), a single PWM input (Figure 3b) or a dual
isolated driver (Figure 3c).
Silicon Laboratories, Inc.
Rev 1.0
4
VDDI
VDDI
ISOLATION
LPWM
VOA
UVLO
VOA
UVLO
GNDA
VDD1
ISOLATION
DISABLE
VDD1
VDDI
VDDB
UVLO
UVLO
VOB
DISABLE
GNDB
VDD1
VDDB
VDDI
UVLO
GNDA
VDDI
VDDI
ISOLATION
VDDI
UVLO
STEERING
LOGIC &
DT CONTROL
DT
VDDI
VOA
UVLO
GNDA
DT CONTROL
&
OVERLAP
PROTECTION
DT
VDDA
VIA
VOB
DISABLE
UVLO
VOB
UVLO
GNDB
GNDB
LPWM
VDDB
ISOLATION
ISOLATION
VDDI
VDDA
PWM
ISOLATION
VDDA
VIA
VIB
GND
VIB
GND
GND
Dual ISOdriver
HS/LS PWM Input ISOdriver
HS/LS Two Wire Input ISOdriver
A) Two-Wire Input High-Side/Low-Side
B) One Wire (PWM) Input High-Side/Low-Side
C) Dual ISOdriver
Figure 3. ISOdriver Family
All devices are offered with 0.5 A and 4.0 A peak output current options and isolation ratings of
1 kV, 2.5 kV and 5 kV. The high-side/low-side versions have built-in overlap protection and an
integrated adjustable dead time generator. The dual ISOdriver version has no overlap protection
or dead time generator.
M
+HV
Isolated VDDA
Isolated VDDA
VIB
GNDI
VIA
Isolated
VDDB
DISABLE
VDDB
VIB
GNDI
VIA
Isolated
VDDB
DISABLE
VDDB
VOA
GNDA
Isolated
VDDB
DISABLE
VIB
VDDB
DT
VOB
VOB
RDT
VOA
GNDA
DT
DT
OUT
5V
Control
Control
VOA
GNDA
Control
Control
VIA
GND
VDDI
5V
GNDI
VDD2
VDDA
VDDI
VDDI
5V
IN
Si823x
VDDA
VDDA
Si822x
Isolated VDDA
Si823x
Si823x
VOB
RDT
RDT
GNDB
GNDB
GNDB
GND2
-HV
Dynamic
Brake
Inverter Stages
Figure 4. Three-Phase Motor Control Power Stage
Figure 4 shows an example three-phase ac motor drive for home appliances in which each IGBT
pair is driven by a high-side/low-side ISOdriver. The DT input of each driver determines the
amount of dead time added between switching phases and can be adjusted over a 4 to 950 ns
range with an external resistor to ground. (If dead time is not used, DT should be connected to
VDD). The dynamic brake is driven by a single-channel Si822x ISOdriver that is available with
either a conventional digital input, or an “optocoupler” input that mimics LED behavior. The
Si822x is pin-compatible with many standard optocoupler-based optodrivers, including the HCPL3120. Table 2 below compares the attributes of CMOS isolated drivers with those using
optocoupler and gate drive technologies.
Silicon Laboratories, Inc.
Rev 1.0
5
Table 2. Isolated Driver Technology Comparison
Si823x ISOdriver
Opto + Driver
Gate Drive XFMR
ADuM Isolated
Gate Driver
Digital Isolator +
Driver
Prop Delay (nS)
50
300+
10
160
~100
Stability Over Time
& Temperature

POOR: Up to 70%
prop delay variation
in over temp.



External BOM
Components
5
17
12
5
8
Reliability

POOR: LED wearout
with temp/ageing



Peak IOUT(max)
4
Various
Various
0.1
Various
Dead Time
Generator
Built-In
-
-
-
-
Overlap Protection
Built-in
-
-
-
-
Summary
Best Solution
Poor performance Bulky, no integrated
over temp, poor
protection, EMI
reliability, high BOM
source
Low drive strength,
liimited choice of
isolation ratings
Multi-package
design, large
footprint
For more detailed ISOdriver information, please see the Si823x ISOdriver data sheet and the
Silicon Labs white paper, “Improving Isolated SMPS, UPS and other Power Systems with
CMOS Isolation Products”.
Current Sensing Background
Switch mode power control requires current measurement at the system modulation frequency for
either feedback control (typically in current mode control) or system protection (typically in voltage
mode control). Key considerations in choosing a current sensor are installed cost, reliability,
performance, and size. Installed cost includes the sensor and surrounding external components.
Excessive current can damage a power system or injure users; so, reliability is a paramount
concern. Performance includes attributes, such as power loss, measurement accuracy, and
input/output latency. Size is important mostly in small power regulator modules for embedded
system applications. The most favored current sense solutions tend to be: 1) current-sense
transformers (CT’s), 2) Hall-Effect devices and 3) Shunt + differential amplifiers. While there are
other, less expensive, “non-intrusive” methods (e.g. DCR current measurement across the output
choke and low-side FET VDS sensing), these techniques tend to have very low accuracy (>40%
error) and find use only in highly cost-sensitive applications, such as POL modules.
As the name implies, a current transformer (CT) induces current flow in the secondary winding
circuit when ac current is present in its primary winding. A burden resistor connected across the
secondary scales converts the current to a voltage suitable for connection to a control circuit. CTs
tend to have relatively high primary series resistances that reduce power efficiency in large
systems and high series inductances that worsen ringing. Small CTs tend to have package lead
splay problems and other quality issues.
Silicon Laboratories, Inc.
Rev 1.0
6
Like gate drive transformers, CTs require a core reset to prevent saturation, and these circuits
range from simple to complex, depending on the host system (Figure 5). The self-reset circuit is
used in relatively simple, low-frequency applications; the CT is reset by reverse current flow
through the CT primary or by the blocking action of D1. Forced Reset resets the CT by initiating
reverse current flow from VCC through RC during the CT’s negative-going half-cycle. The
controlled reset circuit is used in high-frequency and/or high duty cycle applications where the CT
is reset by generating a reverse field when no current is flowing through its primary.
C1
R1
CT
RS
SELF- RESET
C1
OUTPUT
R1
RS
OUTPUT
D1
VCC
VCC
RC
D1
C1
RS
OUTPUT
Q1
R1
VREF
CT
Q2
CURRENT SOURCE
CONTROLLER
CT
FORCED- RESET
CONTROLLED RESET
Figure 5. Common CT Reset Circuits
Hall Effect devices generate a voltage proportional to an applied perpendicular magnetic field
and, without physical contact with the current-carrying conductor being sensed, provide intrinsic
isolation. This isolation and the ability to measure both ac and dc currents are the chief
advantages of Hall Effect current sensors. However, Hall Effect current sensors are easily
affected by external magnetic field interference; they have large temperature drift issues and they
have offset issues around zero. The output signals of Hall devices tend to be low-amplitude with a
poor signal-to-noise ratio, and their device footprint is often prohibitively large. Closed-loop Hall
Effect sensors tend to remedy some of these issues, but at a substantially higher price.
The shunt plus differential amplifier is the most intuitive current sense solution, in which a
differential amplifier measures the voltage drop across the shunt and generates a current
waveform proportional to the measured current. The problem with this method is usually
excessive power loss in the shunt and/or an overly narrow measurement frequency range. In
addition, these devices tend to have low common-mode voltages relegating them to lower voltage
applications.
Silicon Laboratories, Inc.
Rev 1.0
7
Si850x/1x Current Sensor
The Si850x/1x (Figure 6) is a CMOS unidirectional ac current sensor having an accuracy of ±5%,
a low 1.3 mΩ series resistance for low loss and direct measurement of ac current from 50 kHz to
1 MHz to a maximum of 20 A.
Gate Control
Timing
Current
Si850x/1x
CHIP (DIE)
METAL SLUG
RESET
LOGIC
INTEGRATOR
TEMP
SENSOR
ADC
SIGNAL
CONDITIONING
Output
AUTO
CALIBRATION
LOGIC
Figure 6. Si850x/1x Block Diagram
This device consists of a metal slug and CMOS die in a 4 x 4 x 1 mm QFN package. (The
isolation rating of the QFN package device is 1 kV; devices in 16SOW package are certified to 5
kV.) The slug and on-die pick-up coil form a coupled inductor in which the ac current flowing
through the slug induces a voltage proportional to Lmdi/dt into the pick-up coil. An analog
integrator performs an integration of this signal over the switching cycle period and generates a
real-time voltage representation of the current through the slug. This signal is further conditioned
by an on-chip temperature compensator and gain stage, resulting in a 2 V full-scale waveform at
the output pin.
The on-chip integrator must be reset prior to the start of each current measurement cycle using
only local gate control signals for reset and no external components. The integrator reset criterion
is simple: Reset must begin after current measurement and end prior to the start of the next
current measurement. For rated accuracy, this reset event should last a least 150 ns. (On-chip
one-shot allows the user to trade reduced measurement accuracy for shorter reset time. See the
Si850x/1X data sheet for details.) On-chip integrator reset logic provides flexibility to allow the
sensor to be used with virtually any power system topology (for more information, see application
note AN398: “Using Si85xx Current Sensors in Switch-Mode Power Supplies”). The
Si850x/1x is offered in two output versions: ping-pong output and single output. The ping-pong
output version (Si851x) is targeted for use in topologies where there are two current paths
through the power train, such as in a full-bridge. The single output version (Si850x) is applicable
to virtually all other power topologies, such as buck, push-pull, etc.
Silicon Laboratories, Inc.
Rev 1.0
8
Example Si850x/1x Applications
Figure 7 shows a phase-shift-modulated full-bridge application using the Si851x operating in
“Ping-Pong” output mode, which enables a single Si851x to replace two CTs (typically used to
monitor transformer flux balance). Ping-Pong output mode routes current signals from each leg of
the bridge to separate output pins.
VIN
VDD
IIN
VDD
MODE
GND
OUT1
OUT1
OUT2
OUT2
Si851x
PH1
VDD
PH2
TRST
R1
R2
R3 R4
IOUT
PH3
PH4
Q1
Switches Turned ON
PH1
Q2
PH2
Si85xx State
1-4
MEASURE
1-2
RESET
2-3
3-4
MEASURE
RESET
T1
OUT1
Q3
PH3
Q4
PH4
OUT2
Figure 7. Si851x (Ping-Pong Mode) in Phase-Shifted Full Bridge Application
Measured current flowing when Q1 and Q4 are on appears on OUT2, and current flowing when
Q2 and Q3 are on appears on OUT1. Integrator reset occurs during the current circulation phase
(i.e. when Q1 and Q2 are on or when Q3 and Q4 are on). The relatively low-frequency operation
of the full bridge allows ample reset time; so, TRST is tied to VDD, causing reset time to be a
function of the states of R1-R4.
Silicon Laboratories, Inc.
Rev 1.0
9
VDD
VIN
VDD1 VDD2
RTRST
IIN
TRST
R2
R1
OUT1
Si850x
IOUT
OUT1
GND1
GND2
GND3
PWM
PH2
Si85xx State
MEASURE
120nS
Q1
PH1
OUT1
L
VOUT
C
Q2
PH2
Figure 8. Buck Regulator
Figure 8 shows a simple buck regulator application where reset is provided by the low-side switch
gate control signal. In this case, a high-frequency, high maximum duty cycle system is assumed
with a maximum reset time allowance of 120 ns. In this case, one-shot timing resistor RTRST is
installed between the TRST input and ground with a value chosen to provide the 120 ns one-shot
period. Note the timing diagram in Figure 8; the rising edge of the PH2 reset provides the oneshot trigger. Table 3 summarizes the Si850x/1x selection by switch mode topology and shows the
reset logic equations and setup of the reset and mode pins.
Table 3. Recommended Si850x/1x Configurations by Power Topology
Power
Topology
Recommended
Part Number
Reset Input(s)
Required
Input States
Reset Logic
Expression
Output
Configuration
Buck or Boost
Si850x
R1
R2 = 0,
MODE = 1
XOR(R1, R2)
Single Output
Half Bridge
Si850x
R1
R2 = 0,
MODE = 1
XOR(R1, R2)
Single Output
Full Bridge
Si851x
R1, R2, R3, R4
MODE = 0
[R1 & R2] | [R3 & R4]
4-Wire Ping-Pong
Mode
2-Switch
Forward
Si850x
R1
R2 = 0,
MODE = 1
XOR(R1, R2)
Single Output
Push-Pull
Si851x
R1, R2
R3 = 0, R4 = 1
MODE = 1
XNOR [R1, (R2 | R3)]
2-Wire Ping Pong
Mode
Silicon Laboratories, Inc.
Rev 1.0
10
Brushless dc motors (BLDC), also called permanent magnet dc synchronous motors, have rapidly
gained popularity because of their desirable characteristics. From a performance perspective, the
BLDC behaves like a dc motor with linear relationships between current and torque and voltage
and rotational speed. BLDC motors offer advantages over brushed dc motors and induction
motors including better speed versus torque characteristics and dynamic response, high
efficiency and reliability, long operating life, noiseless operation, higher speed ranges and
reduced electromagnetic interference (EMI) emissions. In addition, the ratio of delivered torque to
the size of the motor is higher, making it useful in applications where space and weight are critical
factors. The BLDC speed controller shown in Figure 9 regulates BLDC speed by varying the
average voltage across the motor phases using pulse-width modulation. This single-sided PWM,
120 degree conduction mode, two-quadrant controller approach is simple and capable of driving
the motor in both directions.
G1
G1
G2
G3
G5
M
VDC
G3
G4
G4
BLDC
Speed
Sensor
G2
G6
G5
G6
Q2
Q4
Q6
74HC4002
Controller
Position
Sensing
Q1 Q2 Q3 Q4 Q5 Q6
ISOdriver
Isolated Gate Drivers
Rotor Sensor Inputs
Q1
Q2
Q3
Q4
Q5
Q6
PWM
Q
SET
5V
VDD
MODE
R2
Si850x
AC Current
Sensor
GND
PWM
OSC
2 Quadrant
Commentator
(Firmware)
IIN
R1
IOUT
OUT
S
Vcp
Q
CLR
Speed
Command
R
DIR
Direction
Control
Q
VDD
D
CLK
Overcurrent
Detector
VREF
Q
Direction
VREF
Figure 9. BLDC Feedback Speed Control Using Si850x AC Current Sensor and Si8xxx ISOdrivers
Silicon Laboratories, Inc.
Rev 1.0
11
Figure 10. 120 Degree Motor Commutation Timing Diagram
The voltage switching scheme is simple as well; only two of the six switches are on at any time,
alternately switching the voltage to motor phases. The voltage waveforms for all six gates of the
Figure 9 controller are shown in Figure 10 (the gate voltage timing sequence is: G1 and G2, G2
and G3, G3 and G4, G4 and G5, G5 and G6, G6 and G1). As shown in Figure 9, the Si850x ac
current sensor can be used to sense current in each BLDC motor phase. The required
modulation frequency for the controller in Figure 9 is less than 70 kHz, and the maximum PWM
duty cycle can be clamped to a maximum of 80%, allowing a more than ample amount of time to
perform the cycle-by-cycle Si850x reset. Figure 12 shows a simplified feedback torque controller
that is only a slight variation of the speed controller shown in Figure 9.
PWM
Gate Drive
Si850x
Output
Waveform
Si850x
Reset Period
Figure 11. Reset Timing for Si851x AC Current Sensor
Silicon Laboratories, Inc.
Rev 1.0
12
G1
G1
G2
G3
G5
M
VDC
G3
G4
G4
BLDC
G2
G6
G5
G6
Q2
Q4
Q6
74HC4002
Controller
Position
Sensing
Q1 Q2 Q3 Q4 Q5 Q6
ISOdriver
Isolated Gate Drivers
Rotor Sensor Inputs
Q2
Q4
Q5
VDD
MODE
Si850x
AC Current
Sensor
Q1
Q3
IIN
R1
2 Quadrant
Commentator
(Firmware)
Q6
Q
SET
IOUT OUT
S
Gain
Amplifier
CLK
Q
CLR
Speed
Sensor
GND
PWM
OSC
PWM
5V
R
DIR
Direction
Control
Q
VDD
D
Torque Command
CLK
Q
Direction
VREF
Minimum Speed
Figure 12. Torque Control Using the Si850x AC Current Sensor and Si823x ISOdrivers
Extending Si850x/1x Full Scale Range
AC current measurements beyond 20 A can be realized using the circuit board layout
modification shown in Figure 10. The image on the left is an “X-ray view” of the Si850x/1x
mounted on a circuit board, where all of the current flows through the slug. The image on the right
adds a small current bypass trace in parallel with the slug, forming a current divider where the
width and thickness of the bypass trace determine the current divider ratio. For example, a 1 mm
wide trace shunts enough current around the slug to increase Si85xx full-scale 1.8 times to 36 A.
(For more information on extending Si850x/1x range, please see application note AN329:
“Extending the Full-Scale Range of the Si85xx” .) Note also that the Si850x/1x can be over
ranged up to 50% with no damage and without the bypass trace shown in Figure 10. A 50% over
range causes the output voltage to rise to 3 V instead of 2 V. See the Si850x/1x data sheet for
more details.
Silicon Laboratories, Inc.
Rev 1.0
13
CONDUCTOR
Si85xx
CURRENT
BYPASS
Si85xx
SLUG
SLUG
Figure 10. Extending Full Scale Range Using Current Bypass Trace
Summary
There are many areas of the home that require the use of small three-phase motors, including
washers, dryers, heating and air conditioning systems and more. These end applications
frequently require galvanic isolation for safety, ground noise mitigation and/or voltage level
shifting. Legacy technologies, such as optocouplers and Hall effect current sensors, have
traditionally been used for such applications. Silicon Labs CMOS isolation technology has given
rise to isolated gate drivers, multi-channel digital isolators and ac current sensors that offer higher
capability and significant gains in performance and reliability compared to legacy devices.
Silicon Laboratories, Inc.
Rev 1.0
14
Related Documents
1.
Silicon Labs application note AN441: “Using the Si8232/5/6 Dual ISOdrivers in Power
Delivery Systems”
2. Silicon Labs application note AN486: “High-Side Bootstrap Design Using Si823x
ISOdrivers in Power Delivery Systems”
3. Silicon Labs application note AN490: “Using ISOdrivers in Isolated SMPS, UPS, AC
Inverter and Other Power Systems.”
4. Silicon Labs application note AN497: “Adding Overcurrent Protection to ISOdrivers”
5. Silicon Labs application note AN583: “Safety Considerations and Layout
Recommendations for Digital Isolators”
6. Silicon Labs white paper: CMOS Digital Isolators_WP.pdf; Title: “CMOS Digital Isolators
Supersede Optocouplers in Industrial Applications”
Silicon Laboratories, Inc.
Rev 1.0
15
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