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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 implemented, galvanic isolation is an effective defence against disturbances in the ground plane often referred to as ground loops, which occur as a result of varying electrical potentials. Physical limitations require that electrical components on a PCB connect to the ground plane at different physical locations.
As a result, pockets of varying electrical potentials 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 conducted EMI created by high current motors, pumps, switching regulators and digital processors with their characteristically 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 measurement 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 potentials. While compensation for the delta could theoretically be added at the signal processor, the magnitude of the 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 for down-stream devices from potentially 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 communication between compartments would be performed using other means such as optical, wireless, capacitive or magnetic methods.
Any supply rail and/or ground disturbances can easily damage low power 5V or less sensor units comprised of ADCs, amplifiers, voltage references 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.
The right balance of power
Isolated power converters preserve signal accuracy by creating an electrical 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 provide 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 isolation in the first place. Furthermore, as the isolated compartment increases in complexity, the additional wires and PCB traces inside become more susceptible to electrical noise generating sources such as radiated EMI from neighbouring electronics.
Given a stable ground plane protected 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.
Limitations of conventional isolated converters
Conventional power converters employed for 500VAC (~710VDC) isolation have a limited ability to support industrial and commercial applications. Many have a maximum internal operating temperature of 85°C. Including the effects of internal power losses and package thermal resistance, the output power of a conventional converter may start de-rating at an ambient temperature between 50°C to 65°C leaving little margin.
Cooling systems can provide some assistance, however it raises other concerns in terms of cost, size and reliability should the fans fail. Other isolated solutions require a ±10 percent accurate 12V or 24V input which is incompatible with power from an unregulated power supply or an industrial Li-Ion battery whose usable voltage range varies by ±12 percent to ±14 percent.
While conventional isolated converters offer common fixed output voltages such as 3.3V and 5V, they do not provide any flexibility to accommodate the 0.1V or greater dropout voltage of an external 3.3V or 5V reference nor a similar output voltage LDO post regulator. The latter may be implemented to reduce input power ripple for an A/D converter.
As control systems become more complex, additional isolated sensor compartments are required to support a greater number of signal channels providing further information on system performance. At the same time board space is limited requiring a smaller solution size to fit more features into less space. A new advancement in isolated DC/DC power converters addresses these concerns.
New 500VAC (~710VDC) isolated converters
A solution which addresses these limitations is the LTM8048, a 725VDC isolated µModule power converter. This device includes the power switch, controller, transformer, and compensation in a 9 by 11.25 by 4.92mm BGA package requiring few external components (Figure 3).
The wide input supply voltage from 3.1V to 32V allows this device to be powered directly from less expensive unregulated switching power supplies or a wide range of battery stacks. Moreover, the primary side input voltage on the converter may be above, equal to, or below the desired output voltage on the secondary side.
An internal LDO offers any output voltage from 1.2V to 12V adjustable simply by applying the appropriate resistor between the LTM8048’s feedback pin and the secondary side ground. The output voltage features a ripple of less than 1mV providing a stable power rail for ADCs and analogue sensors for more accurate and repeatable measurements. The internal 725VDC galvanic isolation barrier suitable for 500VAC (~710VDC) requirements is 100 percent production tested for guaranteed circuit protection.
Isolated power is a proven method to protect and preserve the accuracy of low power sensor units comprised of ADC converters, references, amplifiers and transducers whose performance may otherwise be adversely affected. Correct and reliable data gathering by the sensor units is critical to control system operation. In most cases, the entire sensor unit consumes less than 1W using the latest components.
Although conventional low power galvanically isolated DC/DC converters have provided a trustworthy and effective barrier they have shortcomings in the areas of input voltage range, output voltage range, maximum operating temperature and size. A new 1.5W µModule isolated converter broadens the application possibilities by offering advancements in input voltage range, output voltage range, and operating temperature in a compact surface mount solution.
Moreover all µModule power products are backed by extensive reliability testing with the results available online. A more flexible and compact option is now available for design engineers seeking a 725VDC isolated DC/DC power solution.
Linear Technology Corporation
