HIGH VOLTAGE TESTING
BASIC OPERATION
Burr Brown Corporation has adopted a partial discharge test
criterion that is similar to the German VDE0884 optocoupler
standard. This method requires that less than 5pC partial
discharge across the isolation barrier with 1200 Vrms 60Hz
applied. This criterion confirms transient overvoltage (1.6 x
750 Vrms) protection without damage to the PWS727 or
PWS728. Lifetest results verify the absence of high-voltage
breakdown under continuous rated voltage and maximum
temperature.
PC BOARD LAYOUT CONSIDERATIONS
These converters have low output ripple so that standard
layout methods are all that is necessary for good circuit
performance. Bypass capacitors should be placed close to
each device powered by the converters. To minimize the
noise pickup by sensitive (12 bit resolution) circuits, a 10Ω
resistor can be placed in series with the isolated power
traces. The resistor in conjunction with the bypass capacitor
acts as a low pass filter, to minimize isolation amplifier
PSRR related ripple voltage feedthrough. A π filter can also
be used at the input (+VIN) to minimize input supply ripple
current.
The minimum AC barrier voltage that initiates partial dis-
charge above 5pC is defined as the “inception voltage”.
Decreasing the barrier voltage to a lower level is required
before partial discharge ceases and is known as “extinction
voltage”. We have developed a package insulation system to
yield an inception voltage greater than 1200 Vrms so that
transient voltages below this level will not damage the
isolation barrier. The extinction voltage is above 750 Vrms
so that even overvoltage-induced partial discharge will cease
once the barrier voltage is reduced to the rated value.
Previous high voltage test methods relied on applying a
large enough overvoltage (above rating) to break down
marginal devices, but not so high as to damage good ones.
Our partial discharge testing gives us more confidence in
barrier reliability than breakdown/no breakdown criteria.
5V OPERATION
With 5V operation (PWS728) the transformer winding
ratio is 3:1, therefore generating much greater currents in
the primary. The input ripple will be larger, so an input π
filter will be necessary to isolate the supply noise from
the rest of the circuit. For example, when the output load
is ± 15mA the input current will be approximately
120mA.
OUTPUT CURRENT RATING
The PWS727 and PWS728 contain soft start circuitry to
protect the FETs from high inrush currents during turn on.
The internal input current limit is 250mA peak to prevent
thermal overload of the MOSFETs. The maximum output
current rating is ±30mA. Total current, which can be drawn
from each isolation channel, is the total of the of the power
being drawn from both the +V and –V outputs. For example,
if one output is not used, then maximum current can be
drawn from the other output. In all cases the maximum
current that can be drawn from any individual channel is:
(+IOUT) + (–IOUT) < 60mA
It should be noted that many analog circuit functions do not
simultaneously draw equal current from both the positive
and negative supplies.
The information provided herein is believed to be reliable; however, BURR-BROWN assumes no responsibility for inaccuracies or omissions. BURR-BROWN assumes
no responsibility for the use of this information, and all use of such information shall be entirely at the user’s own risk. Prices and specifications are subject to change
without notice. No patent rights or licenses to any of the circuits described herein are implied or granted to any third party. BURR-BROWN does not authorize or warrant
any BURR-BROWN product for use in life support devices and/or systems.
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PWS727/728
