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Limitations of conventional Isolated converters


Conventional power converters employed for 500VAC (~710VDC) iso- lation have a limited ability to support industrial and commercial applica- tions. 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 assis- tance, 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 con- verters offer common fixed output voltages such as 3.3V and 5V, they do not provide any flexibility to accom- modate the 0.1V or greater dropout voltage of an external 3.3V or 5V refer- ence 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 limita- tions 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).


Figure 4: Simplified block diagram of the LTM8048


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 volt- age 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 feed- back pin and the secondary side ground. The output voltage features a ripple of less than 1mV providing a stable power rail for ADCs and ana- logue sensors for more accurate and


Figure 5:


Output ripple less than 1mVp-p from the linear regulator


provides a clean power supply (CBYP COUT


= 0.01µF, = 10µF)


Figure 3: LTM8048 typical application


repeatable measurements. The internal 725VDC galvanic isolation barrier suit- able for 500VAC (~710VDC) require- ments 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 effec- tive 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 offer- ing advancements in input voltage range, output voltage range, and oper- ating temperature in a compact surface mount solution.


Moreover all µModule power prod- ucts 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 www.linear.com


Enter 201


Willie Chan is Senior Product Marketing Engineer at Linear Technology Corporation


A Glimpse inside the LTM8048


ithin the LTM8048 is an isolated flyback controller, power switch, 725VDC isolated transformer, a modest amount of input and output capacitance, compensation and a low output ripple linear regulator supporting up to 1.5W of output power (Figure 4 above). The controller architecture and voltage feedback loop allows this device to create an output voltage on the secondary side that is above, below or equal to the input voltage. Some isolated controller ICs use opto-isolators or extra transformer windings to feedback voltage information. Opto-isolator circuits waste output power and the extra components increase the cost and physical size of the power supply. Moreoever, opto-isolators can also exhibit trouble due to limited dynamic response, nonlinearity, unit-to-unit variation and aging over life. Circuits employing extra trans- former windings experience an increase to the transformer’s physical size and cost,


W Electronics OCTOBER 2013


and dynamic response is often mediocre. In contrast, the LTM8048 control loop examines the switch voltage reflected to the primary side of the transformer to ascertain the secondary side voltage when the secondary side current is near zero. This novel approach to regulation enables this device to support a wide range of secondary side output voltage which is selected simply by adding a resistor. A switching cycle begins with the internal switch turning on. The inductor current increases until an internally set current limit is reached. The voltage across the power switch rises to the output voltage divided by the secondary-to-primary transformer turns ratio plus the input voltage. When the secondary current through the diode falls to zero, the voltage across the power switch pin voltage falls below VIN


. A discontinuous conduction mode (DCM) comparator detects this event and turns the switch back on. 13


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