Issue
149 December 2002
Quad
Bench Power Supply
Start
•
The Analog Core • The
Zetex ZXCT1009
• An Ideal Isolator •
MCU and User Interface • Firmware
•
Sources and PDF
AN
IDEAL ISOLATOR
After
spending years servicing and designing electronics devices,
I have to say that I’m as impressed with some of the
amazing things that were done with vacuum tube circuits
back in the old days, as I am with some of the modern,
miniature ICs that are available today.
For
this project, though, I pampered myself with state-of-the-art
devices rather than depending on clever, but more involved,
circuits using conventional devices. I’ve already described
the Zetex current monitor, which is one example of this.
I continued with this trend in choosing the isolation
technique for the floating power supplies.
The
digital control and monitoring signals for the two floating
supplies have to be electrically isolated from the ground-referenced
MCU circuit. Thanks to the clever design of Microchip’s
SPI digital potentiometer and SPI ADC, each power supply
needed only four control signals: three outputs from
the MCU and one input.
My
first inclination was to use optoisolator chips. I had
just finished another project using optoisolators to
interface the same Microchip SPI ADCs. In that project,
meeting the ADC’s SPI timing considerations given the
rather slow response of the optoisolators was a bit
tricky, although possible.
Luckily,
Jeff Bachiochi had just written a column about isolation
in which he outlined a novel line of isolators made
by Nonvolatile Electronics ("You’re Not Alone—Dealing
with Isolation," Circuit Cellar 142). Rather
than using an optical method to achieve galvanic isolation,
these isolators use magnetism. Although pulse transformers
have been around for ages and can perform isolation
using magnetism, they are comparatively bulky, expensive,
and don’t pass DC levels.
The
IsoLoop isolators, on the other hand, use GMR or giant
magnetoresistive devices to sense the magnetic field
change produced by an excitation coil, which is nearby
but electrically isolated. The change in resistance
of the magnetic thin film layer is used, along with
other on-chip circuitry, to implement the isolation
function of the device. The IsoLoop devices actually
differentiate the input signal, and send only short
magnetic pulses through their excitation coils during
input signal transitions. The resulting resistance changes
in the magnetic thin film layer—configured in a Wheatstone
bridge—are measured, and the resulting output signal
is actually the output of an on-chip latch device.
Don’t be fooled by the use of the term "giant"
in GMR; these devices are tiny. Typically, four isolators
will fit into a 16-pin wide SOIC package. The wide package
is needed, presumably, to allow the devices to withstand
the 2500 VRMS at which they are rated.
With
regard to the packaging, I was impressed with NVE’s
decision to produce several different device configurations.
They sell the normal quad devices with all four channels
configured in the same direction (IL715); however, they
also sell quad devices containing two channels in each
direction (IL716). My favorite, the IL717, has three
channels in one direction and the remainder going in
the other direction. This configuration is perfect for
SPI device isolation, which needs a Chip Select, Clock,
and Data Out lines coming out from the MCU and a Data
In line going back into the MCU.
Given
the modest voltage isolation I needed for this supply,
I could have used a quad optical isolator and wired
up one section "backwards," so to speak, but
the PCB layout would have been much less neat. In cases
where input and output signals have to be isolated and
substantial voltage isolation is required, the only
way to achieve this—apart from using separate devices—is
to use an appropriately configured device like those
in this IsoLoop family.
I’ve
actually saved the best part for last: these IsoLoop
devices are fast! The IL700 family exhibits a 100-Mbps
data rate. In addition, it has only 2-ns pulse width
distortion and 10-ns pulse delay.
Unlike optoisolators, which require LED drive voltage/current
and often don’t provide logic-level output signals,
the IsoLoop devices work directly with 3.3- or 5-V logic
devices including MCUs. Although an optical isolator
requires a steady drive current whenever its LED is
turned on, the IsoLoop devices use only a short pulse
of magnetism whenever the input signal changes state
(even though a small but steady current is required
for the detection and latching circuitry in the chip).
The IL717 that I used requires only a 2.5-mA power supply
current on its input side, and 6 mA on its output side.
This difference arises from the fact that the device
has three channels in one direction and only one in
the other.
In
my design, I did not have to give any more thought to
the SPI timing on the floating channels than I did to
the channel that wasn’t isolated. Basically, what goes
into the IL717 is what comes out the other side!
There
are only two cautionary notes that I would add regarding
these devices. First, IsoLoop devices transmit their
signal across the isolation barrier only on signal transitions.
The recovered signal on the other side of the barrier
is then electrically latched. Practically, this means
that the output of the devices is indeterminate until
input transitions occur. For some applications, this
means that an initialization routine must be performed
to ensure that the device’s outputs are in a known state
after power-up.
The
second cautionary note is just as important. Because
the devices rely on sending a short magnetic pulse at
each input transition, it is important to place at least
a 47-nF ceramic decoupling capacitor between VDD and
ground on both input and output ports of the device.
The capacitors should be placed close to the actual
device pins.
I
tried to share one capacitor between two IsoLoop devices
on the common MCU port side of the two devices. This
didn’t work. There were random output errors on the
device farthest away from the sole capacitor that disappeared
completely when I followed directions!
