Reducing the size, component cost, and power loss of the solution is becoming increasingly important for industrial applications. Analog I/O modules for programmable logic controllers (PLCs) are good examples of meeting such requirements.
Industry 4.0 points to the trend of in-depth automation combined with intelligent communications. Therefore, in process engineering, industrial automation and equipment management, PLCs need to be equipped with more I/O ports. If space is limited and the controller cannot place more substrate faces, then we must increase the module density to support more I/O requirements. Optimizing the power supply design clearly contributes to achieving these goals.
Let us look at the power requirements of the analog I/O module power supply design.
Figure 1: PLC I/O module array
Analog I/O modules typically use a 4-20 mA current loop or +/- 10V signal transmission. We need a DAC/ADC for conversion, usually 5V. In order to protect the device and also to ensure that the required performance can be achieved, the supply rail is isolated to offset ground displacement or noise.
Therefore, the analog I/O module requires a 5V rail and +/-12 to +/- 15V split rail for voltage signal transmission or current loop sink/pull, isolated from the 24-V rail. Figure 2 shows a general embodiment.
Figure 2: General power supply implementation
Is there room for optimization in power supply and design?
The analysis is best where to start, the transformer is the most suitable. Since it has two secondary windings, the production cost and floor space are large. More importantly, its side height is about 5mm. Space savings enable smaller switchgear cabinets or accommodate more I/O modules, while also significantly reducing costs. Ideally, the active module can be as thin as a passive terminal. Ultimately, the side height of the development board also needs to be reduced.
Let us start with this idea. If we use a 1:1 single secondary winding transformer, we can use a cost-optimized component with a side height of only 1.8mm in the flyback converter configuration. By setting the LM5160 flyback converter output to 5V, you don't have to use a buck converter. To produce a +/- 12V to +/- 15V split rail output, the fully integrated TPS65130 split rail converter enables further optimization. The device integrates a boost converter for positive voltage output and an inverting buck boost stage for negative voltage output. Since the output voltage is tightly controlled, there is another positive side effect: we can remove both LDOs.
From the point of view of power conversion efficiency, it is a disadvantage that the split rail passes through two conversion stages. However, the 5V rail is now optimized - in the first design, it needs to go through two stages. Moreover, this design also removes two LDOs, each of which can reduce the voltage by about 2V. Therefore, the LDO power loss that causes the system to heat up is also gone.
Figure 3 shows the new solution.
Figure 3: New power supply implementation
The solution requires only two active devices, with a maximum component height of only 1.8 mm, so that the flat design can easily pass through the door seam. The required area for the development board is reduced by half. Ultra-thin analog I/O modules can be built at the best cost, the output voltage is precisely controlled, and voltage ripples as low as 5mV are available.
This concept is mentioned in the small-sized, isolated analog DC/DC converter reference design (TIDA-00689), including system descriptions, schematics, layout recommendations, and bill of materials. Test reports include test data and thermal measurements.
More resources:
LM5160 Wide Input 65V, 1.5A Synchronous Step-Down DC-DC Converter
TPS65130 positive and negative output split rail converter
SHAOXING COLORBEE PLASTIC CO.,LTD , https://www.colorbeephoto.com