isolated current voltage transducers

Miscellaneous
8 Miscellaneous
Here are additional concerns when using the types of
measurement devices described in this document.
8.1 Power supply polarity inversion
A LEM transducer may be damaged by an inversion of the
power supply voltage or connection of supply voltages to the
output or common pins. If this is a concern, LEM advises the
user to insert a diode on each power supply line, both
positive and negative, or to look for a specific LEM transducer
that incorporates these protection diodes.
8.2 Capacitive dv/dt noise
Any electrical component with galvanic isolation has
capacitive coupling between the isolated potentials.
Applications with fast switching speeds, and consequently
fast voltage changes (dv/dt), across this capacitance
experience some coupling of the primary transient to the
secondary side creating undesirable interferences.
For example, a voltage change of 10 kV/µs in combination
with a 10 pF coupling capacitance generates a parasitic
current of i = C • dv/dt = 100mA. This represents an error of
two times the nominal output current for a transducer with a
50 mA nominal output.
This issue typically occurs with power converters where a
power component, such as a MOSFET (Metal Oxide
Semiconductor Field Effect Transistor) or IGBT (Insulated
Gate Bipolar Transistor), switch the voltage at frequencies in
the 10 kHz to 1 MHz range, generating dv/dt values in the
5 to 50 kV/µs range.
Figure 53 is an example of a primary dv/dt and the resulting
output noise:
- the time scale is 200 ns/div;
- CH1 is the primary voltage of 250 V/div, a 1 kV excursion
creating a 6 kV/µs transient (800 V in 133 ns);
- CH2 is the output of an LAS 50-TP representing a primary
current of 16 A/div;
- CH3 is the output of an LAH 50-P representing a primary
current of 4 A/div.
For these two transducers, the settling time is close to 800 ns
and the peak disturbance is 50% and 7% of the nominal
current, for the LAS 50-TP and LAH 50-P respectively.
dv/dt disturbances can be minimized at two different levels:
• at the transducer level the design is optimized to limit the
primary-secondary capacitive coupling and to minimize
the settling time after a dv/dt disturbance
• at the user level care must be taken in the application of
the transducer into the system
For the latter, good EMI control practices must be observed.
Figure 53: dv/dt disturbance and transducer signal
The following points can be highlighted:
• when long cables are used for the secondary connections
of the transducer, it is advisable to use a shielded cable
connecting the shield to a driven common at the
measurement end of the cable
• when possible, it is recommended to desynchronize the
measurement from the dv/dt occurrence, never performing
a measurement during the dv/dt disturbance and settling
time
• it is possible to attenuate the dv/dt disturbances using a
low pass filter; while this is detrimental to the transducer
bandwidth, in many cases it is a reasonable and desirable
solution
• during printed wiring board (PWB) layout it is important to
keep secondary traces and primary traces, wires, or bus
bars separated and avoid long parallel runs to minimize
capacitive coupling; the best scenario has the secondary
traces running perpendicular to the primary providing
immediate separation; it is also advantageous to include a
screening trace or plane, connected to a star ground point,
between the primary and secondary to shunt the
disturbance to the screen rather than the secondary
8.3 Magnetic disturbances
Because the majority of transducers use magnetic coupling,
it is important to be concerned with external magnetic fields
likely to disturb a measurement. Potential sources include
transformers, inductors, wires, busbars, as well as other
transducers. Typical power electronics equipment has
multiple conductors, oftentimes in close proximity to each
other. A transducer may be disturbed by the magnetic fields
from adjacent conductors, with the disturbance being the
largest with short distances and high currents. A key
parameter in this case is the relative position between the
field sensing element (e.g. Hall or Fluxgate cell) and the
conductor creating the disturbance.
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