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Big input with big output


Willie Chan at Linear Technology Corporation explores the design benefits of an isolated µModule power converter and how this improves signal measurement accuracy performance in hostile electrical & thermal environments


Properly imple- mented, galvanic isola- tion is an effective defence against distur- bances in the ground plane often referred to as ground loops, which occur as a result of vary- ing electrical potentials. Physical limitations require that electrical components on a PCB connect to the ground plane at different physi- cal locations.


As a result, pockets of varying electrical poten- tials are created when


each components’ ground plane connection acts in combination with circuit board parasitics.


Another significant contributor to the creation of ground loops is con- ducted EMI created by high current motors, pumps, switching regulators and digital processors with their char- acteristically fast changes in power demand often in the tens of amperes of current (Figure 1). These ground plane disturbances can result in significant measurement inaccuracies.


The ground potential where the meas- urement sensor is located may not be the same as the ground potential where the ADC converted the analogue signal to a digital signal. Thus the resulting digital signal is now skewed by the voltage delta between the two ground poten- tials. While compensation for the delta could theoretically be added at the signal processor, the magnitude of the


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ground potential delta changes over time as neighbouring loads constantly vary their current consumption. This situation makes compensation a challenging proposition at best. Moreover, isolation offers protection


Figure 1. Isolating power & ground rails for sensitive data converters, amplifiers & references from other high current power paths improves signal resolution & reliability


for down-stream devices from poten- tially damaging supply rail transients or short circuit events.


Dividing a large control system circuit design into smaller galvanically isolated compartments is a smart strategy to protect components from the risks of damage from electrical overstress. The isolation barrier prevents any transfer of electrically charged particles therefore communi- cation between compartments would be performed using other means such as optical, wireless, capacitive or magnetic methods.


Any supply rail and/or ground dis- turbances can easily damage low power 5V or less sensor units com- prised of ADCs, amplifiers, voltage ref- erences and transducers, which often have a combined power consumption below 1W depending on performance. As a precautionary measure, 500VAC (~710VDC) of galvanic isolation is inserted to protect these devices should a short cause the input supply voltage to exceed the components’ absolute maximum voltage rating. In the event of such a failure, the resulting damage is limited to a small compartment or section of the overall control system. Additionally ground disturbances are also minimised which will be discussed in the next section. The damaged sub-sections may then be stocked or purchased as standard ‘off-the-shelf’ replacement units enabling a complete system recovery in a shorter amount of time with less effort.


Figure 2. Power Consumption range for each component in a sensor unit consisting of amplifier, ADC, transducer & Reference. Total power consumption is less than 1W.


The right balance of power Isolated power converters preserve signal accuracy by creating an electri- cal barrier between noisy high current and low current sections of the system where a cleaner more stable ground is available. In particular isolated DC/DC converters delivering under 2W pro- vide sufficient power for one or more sensor units consisting of an amplifier, ADC converter, transducer and voltage reference (Figure 2).


While isolated compartments consuming more than 2W may start to experience the same ground loop issues which called for galvanic isola- tion in the first place. Furthermore, as the isolated compartment increases in complexity, the additional wires and PCB traces inside become more sus- ceptible to electrical noise generating sources such as radiated EMI from neighbouring electronics.


Given a stable ground plane pro- tected by an isolation barrier, more accurate readings can be made by the sensor unit, improving system control. Accuracy may even be improved to the point of permitting system performance upgrades with higher resolution ADCs.


OCTOBER 2013 Electronics


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