Magnetics Business & Technology - May/June 2023 - 32

TECH TIPS
Low Profile Embedded Transformer Provides 7.5kV Isolation
By Jim Quilici, Shennan Circuits Corporation (SCC)
Figure 2 Cut-away view of an embedded transformer
Battery stacks used in automotive and energy storage systems
can have voltages ranging from 400 to over 1000 volts. These
systems require continual monitoring and management to
maintain the integrity of each battery. Voltage and temperature
sensors on the battery stack have a floating reference potential,
which can vary depending on where the sensors are located on
the stack. Data channels from the stack continually feed data
to a controller which manages the battery status and integrity.
Often, these data channels cross need to cross a high voltage
isolation barrier before connecting to the control circuitry that is
referenced to either chassis or earth ground. The isolation barrier
is required to protect the controller circuitry, which often runs at
+5V volt levels or lower. Digital isolators are commonly used to
move data and control signals between the stack and controller.
Digital isolators have been around for over a decade and utilize
capacitive coupling to isolate input voltages from the output. Silicon
Oxide is quite a good insulator and it is practical for implementing
small capacitors within an integrated circuit that have
breakdown voltages exceeding 2kV. This is fine for passing logic
level signals across the isolation barrier. However, power still
needs to cross the isolation barrier to bias the output circuitry on
the high side. SiO2 capacitors are too small for this function and
an isolated switch mode power converter is required. The power
converter is built around a transformer, which translates voltage
levels and establishes the isolation barrier. Figure 1 depicts gate
drive circuity that uses digital isolators for the control channels
and an auxiliary power converter to deliver supply voltages to the
high side. Historically, the high isolation DC/DC converter has
been implemented as a discrete component independent of the
digital isolator function. However, in recent years the trend has
been to integrate the DC/DC converter next to the isolators as
32 Magnetics Business & Technology * May/June 2023
a System in a Package (SIP). The benefit of this integration is
convenience and a small application footprint.
Figure 1 Digital Isolators and Isolated Power Supply
Traditionally, transformers have been constructed by winding
wire around a magnetic core or bobbin. For high voltage isolation,
this often requires using double insulated " reinforced "
wire, which is difficult to work with. SIP packaging also requires
transformers with a low profile and small footprint. For such small
transformers, the added insulation makes the wire especially
difficult to wind. In addition, the wire gauges can be in the range
of 28 AWG (6 mil diameter) to 32 gauges (4 mil diameter). It is
difficult to strip the insulation off such small wires so that they
may be soldered or bonded onto the system circuit. An Embedded
Magnetics construction provides a robust alternative to winding
wire around a core. Rather than building transformers one
at a time, small power transformer can be fabricated on a PCB
production line using batch processes.
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Magnetics Business & Technology - May/June 2023

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