TECHNICAL ARTICLE
Aengus Murray and
Robert Zwicker
Analog Devices, Inc.
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ISOLATION ARCHITECTURE,
CIRCUIT, AND COMPONENT
SELECTION IN POWER
INVERTER APPLICATIONS
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Motor and power control inverter designers share the common problem
of isolating control and user interface circuits from dangerous power line
voltages. The key elements of this are the basic requirements to prevent
power line voltages from damaging control circuits, and more importantly,
to protect the user from dangerous voltages. Systems must comply
with safety requirements defined by international standards such as
IEC 61800 and IEC 62109 that cover applications like motor drives and
solar inverters. Standards primarily focus on compliance testing. How
does the standards’ compliance testing give freedom to the engineer?
It guides the engineer regarding safety but how does it give freedom
so the engineer has the option to choose the appropriate architecture,
circuit, and components required to meet the target system specification,
including standards compliance? These decisions are influenced by the
capability of the circuits to deliver the required system performance in
terms of efficiency, bandwidth, and precision while still meeting safety
isolation requirements. The challenge in designing innovative systems is
that the design rules developed for existing architectures, circuits, and
Power (HV)
AC
components may no longer apply. Therefore, the engineer needs to invest
time in carefully evaluating the capability of a new circuit or component
to meet standards for EMC and safety. In some regions there is a more
onerous burden because the engineer may be personally liable in the case
of an injury resulting from the failure of a safety function on a system he
designed. This article explores the impact of system architecture choice
on the power and control circuit design and the implications on system
performance. It will show how performance improvements in recently
available isolation components are enabling alternate architectures to
provide system performance improvements without compromising safety.
Isolation Architectures
The problem is that you need to safely control the flow of power from
the ac supply to the load based on commands provided by the user.
This problem is described in the high level motor drive system diagrams
shown in Figure 1 of three power domains: command, control, and
power. The safety constraint is that the user command circuits must
be galvanically isolated from the dangerous voltages on the power
circuit. The architectural decision is based on whether to place the
isolation barrier between the command and the control circuits or place
Power (HV)
U
Power (HV)
AC
V
U
Gate Drive
V
AC
Motor
W
W
iv,iw
Power Ground
Power (G)
Control
PWM
Bus Voltage
and Current
PWM
Mod
Motor
Control
iv,iw
Power Ground
Power (G)
Winding
Currents
Control
ADC
PWM
Bus Voltage
and Current
PWM
Mod
Motor
Control
Setpoint
Command
System and
Communications
AC
Motor
Winding
Currents
ADC
Setpoint
Safety Earth
Command
System and
Communications
(b) Isolated control circuit.
(a) Nonisolated control circuit.
Figure 1. Isolation architectures in motor control systems.
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Safety Earth
Isolation Architecture, Circuit, and Component Selection in Power Inverter Applications
it between the control and the power circuit. The introduction of an
isolation barrier between any circuits risks signal integrity and adds
cost. The isolation of analog feedback signals is particularly challenging
because the classical transformer approach rejects the dc signal
component and introduces nonlinearities. Digital signal isolation is
relatively easy at low speeds but much more challenging and power
hungry at high speeds or when low latency is demanded. Power supply
isolation is surprisingly challenging in systems with 3-phase inverters
because of the multiple power domains connected to the power circuit.
The power circuit has four different domains that need to be functionally
isolated from each other; so high-side gate drive and winding current
signals need to be functionally isolated from the control circuit even
though both may be referenced to power ground.
The nonisolated control architecture has a common ground connection
between the control and power circuits. This gives the motor control ADC
access to all signals in the power circuit. The ADC samples motor winding
currents at the midpoint of the center-based PWM signals when they flow
in low-side inverter legs. The drivers for the low-side IGBT gates may be
a simple nonisolating type, but the PWM signal needs to be isolated from
the three high-side IGBT gates with functional isolation or level shifting.
The complexity introduced by isolation between the command and control
circuits depends on the end application, but it typically involves using
a separate system and communications processor. This is acceptable
in home appliance or lower end industrial applications where a simple
processor manages a front panel interface and sends speed commands
over a slow serial interface. The nonisolated architecture is less common
in high performance drives used in robotics and automation because of
the high bandwidth requirements for the command interface.
The isolated control architecture has a common ground connection
between the control and command circuits. This allows the very tight
coupling between the control and command interface and allows the
use of a single processor. The isolation problem shifts to the power
inverter signals with a different set of challenges. The gate drive signals
require relatively high speed digital isolation to satisfy inverter timing
requirements. Magnetic or optically coupled drivers work well in inverter
applications where isolation requirements are quite severe because of
the very high voltages present. The requirements for the dc bus voltage
isolation circuit are modest because of the low dynamic range and
bandwidth demanded. Motor current feedback is the biggest challenge
in high performance drives because high bandwidth, linear isolation
is required. Current transformers (CT) are an obvious choice because
they provide an isolated signal that can be easily scaled. CTs have
nonlinearities at low currents and do not transmit the dc level but are
used extensively in low end inverters. CTs are also used in high power
inverters with nonisolated control architectures because current shunt
losses become excessive. Open-loop and closed-loop Hall effect current
sensors can measure dc, so they are more suitable for high end drives
but suffer from offset. Resistive shunts provide a high bandwidth, linear
signal with low offset but need to be matched with a high bandwidth,
low offset isolation amplifier. Typically the motor control ADC directly
samples the isolated current signal, but the alternative measurement
architecture described in the next section shifts the isolation problem to
the digital domain and provides significant performance improvements.
Inverter Feedback Using Isolated Converters
A common approach to improving the linearity of the isolation system
is to move the ADC to the other side of the isolation barrier and isolate
the digital signal. In many cases this uses a serial ADC combined
with a digital signal isolator. The specific requirements of high frequency
for motor current feedback combined with a fast response for drive
protection steers the ADC selection to a Σ-Δ type. The Σ-Δ ADC has
a linear modulator that converts the analog signal to a single bit stream,
followed by a digital filter that reconstructs the signal as a high resolution
digital word. The beauty of this approach is that two different digital
filters can be used; a slower one for high fidelity feeback and a second
low fidelity fast filter for inverter protection. In Figure 2, winding shunts
measure the motor winding currents and an isolating ADC transmits
a 10 MHz data stream across the isolation barrier. The sinc filters
present high resolution current data to the motor control algorithm,
which calculates the inverter duty cycles needed to apply the required
inverter voltages. A second lower resolution filter detects current
overloads and sends a trip signal to the PWM modulator in the case of
faults. The sinc filter frequency response curve illustrates how appropriate
parameter selection enables the filter to reject PWM switching ripple from
the current samples.
Power
AC
Motor
iv,iw
CTL
Σ-Δ
Isolated
Gate Drives
Isolated
ADCs
Trip
Control
PWM
Mod
Σ-Δ Sinc
Filters
Motor
Control
Safety Earth
Figure 2. Isolated current feedback.
Frequency Response for 10 MHz Modulation
0
D= 125
–50
Gain (dB)
2
–100
–150
–200
–250
–0
50
100
50
100
150
200
250
300
150
200
Frequency (kHz)
250
300
0
Phase (radians)
–10
–20
–30
0
Figure 3. Sinc filter frequency response.
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The common problem in both control architectures is the requirement
to support multiple isolated power domains. This is more difficult if
multiple bias rails are required in each domain. The circuit of Figure 4
efficiently produces +15 V and –7.5 V for gate drive and +5 V for
powering an ADC, all in one domain using only one transformer winding
and two pins per domain. This design makes it possible to use a single
transformer core and bobbin to create dual-supply or triple-supply rails for
four different power domains.
Primary
Winding
C5
Flyback
Transformer
Control
Winding
D2
C4
D3
+7 V
C2
In
Out
LDO
C1
D1
+15 V
+5 V
C6
C
E
C3
Coupled Inductor
Aengus Murray is the Motor and Power Control Applications Manager
for the automation, energy, and sensors unit at Analog Devices. He
is responsible for the complete Analog Devices signal chain offering
for industrial motor and power control. Dr. Murray holds a bachelor’s
and doctorate’s degree in electrical engineering from University
College Dublin in Ireland. He has over 30 years of experience in the
power electronics industry and has also worked with International
Rectifier, Kollmorgen Industrial Drives, and Dublin City University. He
can be reached via email at aengus.murray@analog.com.
G
Gate
Driver
LPD6235–473
About the Author
–7 V
Figure 4. Isolated power supply circuit for the gate driver and current
feedback converter.
Σ-Δ
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