Microtransformer Isolation
Benefits Digital Control
Integrated microtransformer isolators eliminate
digitally controlled design constraints caused
by the use of optocouplers and discrete pulse
transformers.
D
esigning a closed-loop, controlled power supply or motor-control system presents two main
challenges. First, the design requires sending
feedback voltage or current information across
an isolation barrier. Second, it requires providing isolated gate-drive signals for high-side switches.
In the past, the possible isolation solutions included
optocouplers or discrete pulse transformers. However, they
posed significant constraints on system performance, cost
and reliability. Linear optocouplers have been used to send
analog error signals from the secondary to the primary in
isolated power supplies, but their gain varies significantly
from part to part and by temperature. Because of this,
feedback-loop design becomes difficult, causing phase
margins to vary under worst-case conditions.
Today, digitally controlled systems can use microtransformer isolators that provide a better solution, because they
are easily integrated with many circuit functions. Such an
isolator eliminates the dependence of loop performance
on the optocoupler and error amplifier, simplifying the
feedback-loop design. Moreover, a shorter propagation
delay for microtransformer isolators allows the closed-loop
By Dr. Baoxing Chen, Senior Staff Engineer, Analog Devices,
Wilmington, Mass.
converter to achieve optimum dynamic performance.
Typically, a digital controller is located on the secondary side, so the microtransformer isolator also can provide
initial bias for controller startup, which eliminates the need
for an auxiliary power supply. Either pulse transformers or
optocouplers can be used to provide isolated gate-drive signals for the high-side switches. Unfortunately, optocoupler
approaches need a floating supply, and pulse transformers
do not provide dc correctness.
Moreover, pulse transformers need additional discrete
components for ac-coupled drives, posing significant constraints on duty-cycle variations. However, gate drivers
based on microtransformer isolators, with an integrated
high-side supply, impose no constraints on duty-cycle
variation.
Microtransformer Isolators
An example of a microtransformer isolator is Analog
Devices’ coreless transformer iCoupler, which provides
fully integrated signal and power isolation. Fig. 1 shows a
quad-channel isolator with a fully integrated isolated dcdc converter in a 16-lead SOIC package. Stacked windings
are built on top of these
Input
Output
CMOS substrates. The
power
power
chip on the left has highRECT
OSC
voltage CMOS switches,
while the chip on the right
has the rectifier diodes
Output
Input
data
data
and a converter controller.
Two cross-coupled switches and the transformers
form the oscillator, with
Fig. 1. Four-channel iCoupler isolator with integrated isolated dc-dc converter in a 16-lead SOIC pack- Schottky diodes used for
age. The OSC chip on the left has high-voltage CMOS switches; the RECT chip on the right has the
fast, efficient rectification.
rectifier diodes and a converter controller. In the top-middle are two larger transformers for power,
The transformers sit in
while the lower small transformer transmits the pulse-width-modulation feedback signal.
the middle. This imple20
Power Electronics Technology October 2008
RECT
OSC
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isolator solution
VOB
VIA
VI
VIB
VOA
VOB
DC-DC converter
(ADuM5000)
ISO 5 V
PWM out 1
PWM out 2
PWM out 3
PWM out 4
(a)
Voltage sense
AC
input
VIA
Half-bridge gate driver
with dc-dc
(ADuM5230)
VO
Half-bridge gate driver
with dc-dc
(ADuM5230)
PFC
Secondary
controller
PFC
4
3
1
AC
input
2
PWM out 1
PWM out 2
PWM out 3
PWM out 4
VDD
Secondary
controller
Voltage sense
VSS
(b)
Fig. 2. The iCoupler implementation for power supplies. An ac-dc power supply with a
secondary digital controller uses two ADuM5230 isolated half-bridge drivers (a). The reddashed lines indicate the isolation barrier. An ac-dc power supply with a secondary digital
controller and a single discrete pulse transformer gate-drive circuit (b). The gate-drive circuits
for the other pulse transformers are similar.
mentation has the transformers on separate chips, but the
transformer process is compatible with standard CMOS
processes, so in principle, they can be put on the same chips
as the switches or Schottky diodes. On the top transformer
chip, the two larger transformers are for power, while the
small transformer transmits the feedback PWM signal.
The bottom transformer chip holds four additional microtransformers for the four-channel isolator. The left and right
chips also hold the encoding and decoding circuit for the
four-channel isolators.
The transfer of logic signals across the isolation barrier
occurs by the appropriate encoding on the primary side
and decoding at the secondary side that recovers the input
logic signals. Specifically, short pulses (about 1 ns) are
transmitted across the transformers with two consecutive
short pulses indicating a leading edge and a single short
22
Power Electronics Technology October 2008
pulse indicating a falling edge. A
nonretriggerable mono-stable at
the secondary generates detection
pulses. If it detects two pulses, the
output goes high, whereas a single
pulse causes the output to go low.
For transmitting power across
the isolation barrier, the microtransformers are switched resonantly at
high frequency to achieve efficient
energy transfer. Energy regulation is
obtained by a low-frequency pulsewide-modulation (PWM) feedback
signal, which controls the duty cycle
for this high-frequency resonant action. The transformer switches and
Schottky diodes used for rectification are all implemented on chip.
Isolation up to 5 kV rms is provided
by 20-μm-thick polyimide layers
sandwiched between the primary
and secondary coils.
Similarly, fully integrated halfbridge gate drivers, isolated analogto-digital converters (ADCs) and
isolated transceivers can be integrated. Integrating the isolated dcdc converter with a signal isolator
solves the common problem with
optocouplers, which is the need to
design a discrete isolated power supply. The discrete dc-dc converter is
relatively large, expensive, difficult to
design and, in many cases, provides
insufficient isolation. Combined
signal and power isolation provides
possibilities for functional integration that reduce the complexity, size
and total cost of isolated systems.
Isolation for AC-DC Supplies
Digital power supplies provide programmability, ease
of use and higher performance. And they can leverage
powerful modern processors to implement complex control
algorithms that lead to higher efficiency and faster transient
response. One of the major barriers in adopting digital
control for isolated power supplies is the need to send
digital rather than analog information across the isolation
barrier. Traditional optocouplers, which are typically used
in analog interfaces, are slow and difficult to integrate. On
the other hand, microtransformer-based isolators are fast
and easily integrated with many circuit functions, including
digital interfaces.
Fig. 2a shows the isolation needs for a typical ac-dc power supply implemented with a secondary digital controller.
The PWM signals generated from the secondary controller
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isolator solution
are sent directly across the isolation barrier to drive the full
bridge through two half-bridge gate drivers. This example
uses two ADuM5230 isolated half-bridge drivers, whose
two isolated outputs are also isolated from each other. The
red-dashed lines in the figure indicate the isolation barrier.
The half-bridge gate drivers provide isolation between the
primary and secondary and between the high side and
low side. The integrated 200-mW, 15-V high-side supply
eliminates the need for a floating power supply to bias the
high-side gate driver. This implementation eliminates the
need for four discrete isolated pulse transformers, eight
discrete catch diodes and eight discrete gate-clamping
zener diodes in a traditional gate-drive approach based on
pulse transformers.
Fig. 2b shows the complete gate-drive circuit for pulse
transformer 2. The gate-drive circuits for the other three
pulse transformers are the same. In some cases, only two
transformers with two secondary windings may be needed
if the same drive signal can be used for two diagonal
switches. Two windings can be used for two complementary gate-drive signals also, but only if the duty cycle is
fixed at 50%, as in some phase-shift topologies. Four pulse
transformers would allow four independent gate-drive
signals that are required for many phase-shift and resonant
topologies.
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The gate-drive amplitude is dependent on the duty cycle
of the gate-drive signals with capacitive transformer drivers (Fig. 2b). This amplitude change makes the gate drive
inefficient, as the peak-to-peak swing can be twice that of
the gate-drive amplitude at 50% duty cycle, so it wastes essentially half of the power. Adding the driver and winding
loss, the pulse transformer approach can be inefficient. The
microtransformer half-bridge gate drivers encode and decode the gate-drive signals with dc correctness, eliminating
the limitation on the PWM duty cycle seen in the discrete
pulse transformer implementation.
Optocoupler-based gate drivers do not pose limitations on the PWM duty cycle, but the signal distortion
they introduce may be too large to be useful for sending
accurate PWM pulses. Also, additional discrete isolated
power supplies are needed for powering the optocouplers.
The use of a bootstrap circuit again poses limitations on the
PWM duty cycle, because it needs bootstrap capacitors to
hold enough charge during the high side on cycles. Highspeed microtransformer half-bridge gate drivers are very
important for transmitting the highly accurate digital PWM
with very little distortion. The low-propagation delay will
ensure maximum achievable loop response and matching
between different gate-drive channels for maximum converter efficiency.
A secondary controller is typically used because of its
close proximity to the load. Also, the drive signals for the
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Power Electronics Technology October 2008
isolator solution
synchronous rectification switches need not go through the
isolation barrier. Implementing secondary control is difficult, in part because of the need to provide initial bias for
the controller before the converter starts to deliver regulated
power. The ADuM5000 500-mW isolated dc-dc converter
uses microtransformers that provide startup power for the
controller, eliminating the need for an auxiliary supply at
startup.
For power supplies with secondary controllers, there is
Half-bridge gate
driver with dc-dc
(ADuM6132)
15 V
also a need to communicate primary voltage information
to the secondary. For power supplies using the SMBus or
PMBus, fully integrated I2C iCoupler devices can be used
such as the ADuM1250/1251 with 2.5-kV isolation or the
ADuM2250/2251 with 5-kV isolation.
Microtransformer half-bridge gate drivers are also useful
for power supplies with primary controllers, because they
provide the isolation for the high side even though the low
side and the primary controller can share the same ground.
Additional two-channel isolators
would be needed to send the synchronous rectification switching signals
from the primary to the secondary.
HVDC
Isolation for Motor-Drive
Applications
Buffer
In motor-drive applications, two
main parts of the circuit require
isolation: the gate drive for IGBTs of
bridge inverters and motor-phasecurrent sensing. Phase-current sensing
provides IGBT protection and linear
current feedback information for the
controller to maintain closed-loop
Buffer
current control. Series shunt resistors,
together with high-precision ADCs at
the inverter output, are typically used
to sense the phase current. Isolated
Fig. 3. In this low-power motor-drive implementation, the red-dashed lines show the iso- power supplies are needed to provide
lation barrier. Inverter outputs must be isolated from each other, which is accomplished the bias for the current-sensing ADC
with multiple half-bridge gate drivers.
and gate-drive circuit, and separate
supplies are needed for
each phase. The iCoupler
devices can simplify this
complicated signal and the
power isolation needs of ac
motor drives.
Fig. 3 is an example of
5V
HVDC
an implementation for a
low-power motor drive
using microtransformers.
The half-bridge gate driver
with an integrated 300M
mW high-side, 15-V supply provides isolated 15-V
gate-drive output for the
high-side IGBT and nonisolated 15-V gate-drive
output for the low-side
IGBT. The 15-V high-side
supply, generated through
an integrated dc-dc conFig. 4. In a medium- to high-power motor drive, low-side isolation protects the motor controller
verter, provides power for
from damage caused by the IGBT’s inductive switching transients. Also, the six gate-drive signals
the buffer circuit to drive
are usually isolated through logic isolators, which provides further level shifting or isolation for the
the IGBT, and also can be
high-side IGBTs.
used with a zener diode to
ADC
Gate-driver module
Quad isolator
with dc-dc
(ADuM5400)
Bidirectional
quad isolator
(ADuM2401)
Controller
Isolated
ADC
(AD7401)
Isolated
ADC
(AD7401)
Controller
M
24
Power Electronics Technology October 2008
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isolator solution
generate a 3-V to 5-V supply to power the current-sensing
ADC. An isolated second-order sigma-delta modulator
converts the analog input to a high-speed single-bit data
stream that can interface directly with the controller. It
receives a clock signal from the controller and sends the
clocked data stream back to the controller.
Without an integrated ADC, multiple optocouplers
would be needed; slow optocouplers are usually not suitable
for transmitting this high-speed data stream. High-voltage
level shifters can be faster than optocouplers, but they do
not provide galvanic isolation, which is important to prevent latchup in the presence of negative transients. Both
the high-side gate drivers and the current-sensing ADC
have their grounds referred to inverter outputs that can be
switching very fast. iCoupler isolation with high commonmode transient immunity is important to maintain data
integrity for high-side switching and current sensing.
The red-dashed lines in Fig. 3 show the extent of the
isolation barrier. The circuit components shown in the
blue box can be replicated in bridge inverters for additional
phases. The inverter outputs need to be isolated from each
other and multiple half-bridge gate drivers will achieve that.
Each of the half-bridge gate drivers will generate its own
gate-drive signal and high-side supply.
For medium- to high-power motor-drive applications,
isolation is also typically required for low-side gate drive,
as shown in Fig. 4. Low-side isolation protects the controller from being damaged by IGBT inductive switching
transients. The six gate-drive signals are usually isolated
through logic isolators. They provide inputs to a gate-drive
module, which provides further level shifting or isolation
for the high-side IGBTs.
Logic isolation facilitates communication between the
controller and dc-link ground, such as passing the dc-link
voltage or current-sensing information to the controller.
The four-channel isolator provides isolation for four of
the six gate-drive signals from the controller. The other
four-channel isolator provides reinforced isolation for
the other two gate-drive signals. Two unused isolation
channels can be used for serial communication between
the controller, and a nonisolated ADC can be used for
high-voltage direct-current voltage sensing. The 500-mW
isolated power from the ADuM5400 can be used to power
any logic circuits referenced to the low-side ground, such
as the output side of the ADuM2401 and the ADC used
for voltage sensing.
Hall-effect sensors are typically used for high-power
motor drives because of the power dissipation limitation of
shunt resistors. If a series shunt resistor is used for phasecurrent sensing in medium-power motor drives, an isolated
second-order sigma-delta modulator can be used to send
the phase-current information to the controller. PETech
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Power Electronics Technology October 2008