Power Through the Isolation Barrier Datasheet

Power Through the Isolation Barrier:
The Landscape of Isolated DC/DC
Bias Power Supplies
Ryan Manack
Business Lead,
Texas Instruments
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Power Through the Isolation Barrier 2 January 2020
The electronic designer’s toolbox is ever-growing. Finding
the right tool for the job requires understanding the task at
hand, knowing which tools exist and, finally, how to best use
those tools.
Moving signals and power across an isolation barrier is a
common challenge for designers. Isolation might be required
for safety, noise immunity or large potential differences
between system domains. For example, a cellphone charger
is internally isolated to prevent humans from becoming
electrically tied to the mains if the connector short-circuits.
In other applications like factory robots, sensitive control
circuitry sits on a separate ground and is isolated from the
motors that draw large DC currents and create noise and
ground bounces.
Communication and sensing are commonly implemented
across an isolation barrier. Automotive applications with
controller area network (CAN) or CAN flexible data rate (FD)
protocol communications can isolate these signals from
the high-voltage side of the automobile using an isolated
CAN transceiver that integrates isolation and transceiver
components. Industrial applications may also utilize the CAN
protocol, but can also use the RS-485 protocol for long-
distance serial communications. Similar to isolating CAN and
CAN FD signals, designers can leverage isolated transceivers
designed for the RS-485 protocol. Protection relays use
isolated current and voltage sensors to sense power moving
through the grid. Traction inverters and motor drives take a
pulse-width modulated signal from the motor controller and
pass it through an isolator to tell the gate driver to turn an
insulated gate bipolar transistor on or off.
Isolated bias converters enable isolated communication
and sensing by providing bias power from one side of the
isolation barrier to the other. Current and voltage sensors,
digital isolators and gate drivers typically require less than
15 W and as little as tens of milliwatts of power. Figure 1
shows an example of each of these applications.
Isolated DC/DC bias supply requirements
There are many solutions that can provide isolated bias
power, from controllers, which have external power
switches, to converters, which integrate a controller
with power switches, to finally power modules, which
integrate controllers, power switches and transformers in
one package. Because of this wide variety of bias power
solutions and diverse applications that they go into, it is
important to fully understand the application requirements in
order to meet the specifications at the lowest cost.
VISO
VISO
VIN
RXD
CANH
CANL
VISO
VIN
TXD
VIN
DC/DC
Converter
DC/DC
Converter
VISOVIN
RSHUNT
To Load
REF
REF
ADC
VIN VISO
VSW
VSW
VISO
HV
PWM
VIN
DC/DC
Converter
Figure 1. Isolated bias applications.
Power Through the Isolation Barrier 3 January 2020
At a minimum, the designer should understand the
bias supply input voltage range, the output voltage and
the output power requirements. Some applications will
require more than one bias voltage so it is important to
define the acceptable regulation for each output. System
requirements such as insulation rating, ambient operating
temperature range, electromagnetic interference (EMI) and
electromagnetic compatibility (EMC) will further drive design
decisions. Table 1 shows just four examples of an extremely
broad landscape of isolated bias converter specifications.
Let’s review some example isolated bias supply topologies.
Flyback
The flyback converter is a well-known topology that has
been widely used for decades. This power converter can
support a wide variety of applications due to its flexibility and
low cost. Advancements such as field-effect transistor (FET)
integration and primary-side control make this topology even
more attractive.
Compared to buck-derived topologies like forward, push-
pull and half-bridge, the flyback topology requires only
one primary switch, one rectifier and one transformer-like
coupled inductor. Figure 2 shows a simplified schematic
of the converter. When the primary switch is on, the input
voltage is applied across the primary winding, storing energy
in the transformer’s air gap. The output load is supported
only by the output capacitor in this state. When the primary
switch turns off, the energy stored in the transformer is
delivered to the secondary through the rectifier to supply
the load and recharge the output capacitors.
Figure 2. A flyback converter.
A flyback converter performs well as a bias converter for a
number of reasons. It provides regulation and isolation in
one conversion stage. Its flexibility is also useful for multiple
outputs. You can choose the number of output windings and
wind the transformer to support your chosen configuration.
The corresponding voltage on the output windings is a
function of the duty cycle and the primary-to-secondary
windings’ turns ratio. It is also possible to reference each
output to a different ground in order to meet system isolation
requirements. Other flyback benefits include its relatively low
cost and wide input-to-output operating range.
It is important to properly design a flyback transformer
for best performance. The transformer should be very
well-coupled, with low leakage inductance for the highest
efficiency and best regulation, especially in multiple
outputs. However, it is also necessary to limit the parasitic
capacitance from the primary to secondary in order to
prevent excessive electromagnetic interference (EMI).
Traction inverter SiC
gate driver bias
Isolated current or
voltage sensing
Isolated CAN
communication
Industrial motor IGBT
gate driver bias
Input Voltage 12V + 10% 5V 5V 24V + 10%
Output Voltages +20V / -5V 5V 5V +15V / -5V
Output Power 1.5W 100mW 350mW 1W
Regulation + 5 % + 10% + 5% + 10%
Insulation Rating Basic Reinforced Reinforced Reinforced
Ambient Temperature Up to 105°C -55°C to 125°C -40°C to 125°C -40°C to 85°C
EMI needs CISPR25 Class 5 CISPR32 Class B CISPR25 Class 5 CISPR32 Class B
Table 1. Example isolated bias converter specifications.
Control IC
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Power Through the Isolation Barrier 4 January 2020
Fly-Buck™ Converter
A Fly-Buck converter is a Texas Instruments-specific
topology used to create an isolated bias supply. It is
capable of operating from input voltages as high as 100 V.
Like a flyback converter, metal-oxide semiconductor
field-effect transistors (MOSFETs) are typically integrated
inside the integrated circuit (IC), and it is very simple to
realize primary-side control. Figure 3 shows a Fly-Buck
converter. The topology uses a synchronous buck converter
with a coupled inductor to create one or multiple isolated
outputs. When the high-side switch is on, the primary side
works like a buck converter and the secondary winding
current is zero. In the off-state when the low-side switch is
on the secondary side is driven from energy stored in
the primary.
Figure 3. A Fly-Buck™ converter.
Synchronous buck converters are widely available,
making the Fly-Buck converter an attractive topology. The
converter does not require an additional auxiliary winding
or optocoupler for control as the feedback loop can be
closed on the primary output voltage. The coupled-inductor
construction is flexible. The turns ratio, insulation rating,
number of secondary windings and PWM duty cycle are
controllable for use in a wide variety of applications.
Like the flyback converter, the coupled inductor must be
properly designed. It is important to manage the leakage
inductance while limiting the parasitic capacitance from
primary to secondary. For applications requiring greater than
100-V inputs, you can use a Fly-Buck converter with an
external MOSFET.
Push-pull transformer driver
A push-pull transformer driver is a commonly used solution
for low-noise, small-form-factor isolated power supplies. It
is supplied from a tightly regulated input rail and operates
in an open loop at a fixed 50% duty cycle. MOSFETs are
integrated into the IC, enabling a compact solution.
Figure 4 shows the push-pull topology. The push-pull
topology is a double-ended variant of the forward topology
with both MOSFETs ground-referenced, eliminating the need
for external bootstrap circuitry. Similar to the single-ended
forward converter, the voltage stress at the FETs is two
times the input voltage. The MOSFETs switch at a 50% duty
cycle during alternate half cycles, driving the center-tapped
winding of the transformer.
Figure 4. A push-pull transformer driver.
The push-pull transformer driver is a prevalent isolated bias
power-supply solution for many reasons. It offers flexibility
and the ability to support multiple outputs. The open-loop
configuration provides design simplicity by eliminating the
feedback loop. The push-pull transformer offers lower
primary-secondary capacitance, which enables a reduction
in common-mode noise compared to flyback and Fly-
Buck converters. Additionally, the push-pull topology more
efficiently uses the transformer core magnetizing current,
resulting in a smaller magnetic solution compared to flyback
and Fly-Buck converters.
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Power Through the Isolation Barrier 5 January 2020
Although the transformer driver carries a number of
advantages, it is also important to take into account the
trade-offs. Unlike the flyback and Fly-Buck converters, the
transformer driver cannot support a wide input voltage
range, and instead requires a tightly regulated input voltage.
Meeting the output-voltage regulation requirements for
feedback can be challenging due to the absence of a closed
loop and may require a low-dropout post-regulator (LDO).
Power modules
Power modules have existed for decades. These solutions
are widely available and offer significant integration
compared to discrete implementations. Power modules exist
in many varieties, with input voltage, output voltage, output
power, number of outputs, isolation rating and regulation
options.
Figure 5 shows the block diagram of the inner workings
of one power module. The topology includes a transformer
driver similar to the discrete version. Some devices may
integrate an output LDO for regulation.
Figure 5. A power module.
With many options available, you can use a power module
in most isolated bias converter applications. They greatly
simplify the design process because you do not need to
specify, design or choose a transformer; you only need to
include an input and output decoupling capacitor to start the
design. Other options like synchronization, output voltage
selection, enable and error signaling are available as well.
You will lose some flexibility with modules, specifically to
configure the number of outputs and transformer turns
ratios. The selection of modules rated for a 125°C ambient
temperature range is less than for the 55°C and 85°C
options. Similarly, the number of modules available with
fully reinforced insulation ratings is less than those modules
available with functional or basic isolation.
A next-generation bias solution
Innovations in transformer design and higher frequency
topologies have enabled IC designers to integrate a
transformer and silicon into one IC. For the end user you
get a small, lightweight isolated DC/DC bias power supply
without having to design a transformer or compromise on
system performance.
Figure 6 shows the block diagram of the Texas Instruments
UCC12050. Though it looks similar to a power module
with integrated power stage and rectifier, a closer look
at the UCC12050 operation shows that the switching
frequency is much higher compared to power modules.
This allows significant height and weight reduction versus
lower switching frequency alternatives. The internal topology
control scheme runs closed-loop without an LDO or external
feedback components.
Figure 6. UCC12050 isolated DC/DC bias power supply.
The UCC12050 brings many benefits to the wide variety of
isolated DC/DC bias supply applications. It is designed
with an EMI-optimized transformer with only 3.5 pF of
primary-to-secondary capacitance and a quiet control
scheme. On its own the solution can pass CISPR32 Class
B on a two-layer PCB without ferrite beads or LDOs. The
device is robust, rated for reinforced isolation of 5 kVrms
and 1.2 kVrms working voltage and will operate at 125°C
ambient temperature. The family of devices also includes
UCC12040, which is rated for basic isolation of 3 kVrms
and 800 Vrms working voltage.
UCC12050 is targeted for 5-V input, 3.3-V to 5.4-V output
applications requiring 500 mW. Applications requiring higher
input or output voltages will need to provide pre-or-post
conversion. Also, for designs requiring power above the
UCC12050’s derating curve, alternative topologies should
be explored.
Power
Stage
VOUTVIN
Power Module
VOUTVIN
UCC12050
Rectifier
Control
Power
Stage
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Table 2 compares each of the above topologies. It is clear
that topologies with external transformers offer the most
flexibility while power modules and UCC12050 provide the
most ease of use.
Table 2. Isolated bias power supply topology comparison.
Conclusion
You have many options to provide power across an isolation
boundary. Understanding system-level specifications like the
number of outputs, regulation requirements, output power,
insulation rating, operating temperature and input voltage
range are critical. From there, you can derive the lowest-cost
solution that meets all of your system requirements.
# of Outputs Regulation Output Power Insulation Rating Operating
Temperature
Flyback Converter Flexible – XFMR
Dependent
PSR or Optocoupler
Only One Winding
Regulated
Up to 15W Flexible – XFMR
Dependent
Flexible – XFMR
Dependent
Fly-Buck
TM
Converter Flexible – XFMR
Dependent PSR or Optocoupler 5 to 10W Flexible – XFMR
Dependent
Flexible – XFMR
Dependent
Transformer Driver Flexible – XFMR
Dependent Unregulated 1 to 5W Flexible – XFMR
Dependent
Flexible – XFMR
Dependent
Power Modules 1 to 2 Outputs Regulated or
Unregulated 1 to 3W Mostly Basic or
Functional Typically 85°C
UCC12050 1 Output Regulated 0.5W Reinforced 125°C