Which supply design for what application?
For analog applications including amplifiers, analog-to-digital and digital-to-analog converters, a stable, regulated converter output voltage is a must. Applying a low dropout linear regulator (LDO) at the unregulated converter output minimizes output ripple, but also reduces overall converter efficiency. Also component count increases due to the input capacitor at the LDO input that enables fast load regulation and the buffer capacitor at the LDO output for LDO stability and output voltage buffering. In addition, the transformer’s turns-ratio might have to be increased to assure LDO line- and load-regulation across a specified load current range. Otherwise, if the voltage at the regulator input is too low, the LDO drops out of regulation. In the regulator output follows the input voltage without regulation, and the LDO behaves like a simple piece of wire.
Figure 4. 3V-to-5V push-pull converters with LDO ensure stable output voltages for analog circuits.
Digital interfaces, on the other hand, present somewhat less challenging designs. Logic gates, microcontrollers, and bus transceivers often allow for a wide range in supply voltage variation. Take for example an isolated RS-485 transceiver that must provide a minimum differential output of VOD = 1.5V across a 54 Ohm load for a supply voltage ranging from 4.5V to 5.5V.
In this application an LDO might be superfluous, unless of course current-limiting and/or thermal shutdown of the supply deems necessary. Without LDO, it needs to be assured that the output voltage curve of the unregulated converter output is sufficiently flat. To maintain reliable device operation, the converter output must remain within the transceivers supply range across the specified range of load current.
How much load current is required depends on the application. For our RS-485 example, however, we can assume that every bus node is isolated and common-mode loading does not exist. With that in mind we can picture the input resistance each receiver presents to the A and B lines lying parallel to the bus termination resistors. Allowing for the maximum DC-load of 375 Ohm on each line (specified in the EIA-485 standard), Figure 5 shows the series circuit of two 375-Ohm resistors in parallel to the effective bus termination of 60 Ohm (2 x 120 Ohm in parallel).
Assuming now the specified minimum differential bus voltage of VOD
= 1.5 V across the parallel combination of 60 Ohm || 750 Ohm yields a total driver output current of IOUT
= 27 mA. For more powerful drivers with VOD
= 2V, this current increases to 36 mA. Then adding 15 mA of quiescent current yields a total load current in the range of 42 mA to 51 mA. That’s all the converter output must be able to drive.
Figure 5. The maximum load current in an isolated RS-485 bus is the sum of the quiescent current, the current through the termination resistors, and the current through the receiver inputs connected to the bus.
With the recent introduction of low-saturation isolation transformers, such as the one in Figure 6, converter efficiencies of up to 85 percent have been accomplished at low-load currents. Figure 6 shows a low-cost, highly efficient, 3V-to-5V push-pull converter with minimum component count.
Figure 6. 3V-to-5V push-pull converter with unregulated output supplies digital circuits at high efficiency
Ease-of-use, low-noise, small size, and inexpensive development make push-pull converters the preferred solution for modern, isolated power supply designs. The freedom to choose between transformers with as low as 1 kV isolation and as high as 6 kV isolation is not limited by the low isolation voltage of a noisy feedback circuit. Furthermore, matched the transformer design with the drive capability of the transformer driver ensures high-efficiency converters.
About the Author
Thomas Kugelstadt is a senior systems engineer with Texas Instruments. He is responsible for defining new, high-performance analog products and developing complete system solutions for industrial interfaces with robust transient protection. He is a Graduate Engineer from the Frankfurt University of Applied Science. Thomas can be reached at firstname.lastname@example.org.