Posted on 24 May 2012

Concerns When Using Transducer Measurement Devices

 

 

 

 

 

 

 

Below are some concerns that one should be aware of when using transducer based measurement devices.

Power supply polarity inversion

A 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, the user should insert a diode on each power supply line, both positive and negative, or select a transducer that incorporates these protection diodes.

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 twice 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), switches the voltage at frequencies in the 10 kHz to 1 MHz range, generating dv/dt values in the 5 to 50 kV/μs range.

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 integrating the transducer into the system

For the latter, good EMI (electromagnetic interference) control practices must be observed.

The following points should 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 busbars 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.

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, often 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.

To minimize the disturbance, one should employ the following:

• Increase the distance between the transducer and the external conductor as much as possible

• Modify the transducer and/or conductor layout to optimize the conductor position with regard to the field sensing element of the transducer

• Twist, layer, or parallel, if possible, the source and return conductors to minimize the external field

• Divide the external conductors into equal parts and place them symmetrically on each side of the transducer, canceling or minimizing the external magnetic influence

• Magnetically shield the transducer

 

For more information, please read:

Transducers - Basic Principles of Selection

Selecting Current Transducers

Selecting Voltage Transducers

Basic Hall Effect Technologies

 

 

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