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Grasping isolated DC/DC converter voltage regulation

Posted: 05 Dec 2014 ?? ?Print Version ?Bookmark and Share

Keywords:Isolated? DC/DC converters? optocoupler? flyback converter? load regulation?

Isolated DC/DC converters are needed in a broad range of applications including power metering, industrial programmable logic controllers (PLCs), insulated-gate bipolar transistor (IGBT) driver power supplies, industrial fieldbus, and industrial automation. These converters often are used to provide galvanic isolation, improve safety, and enhance noise immunity. Moreover, they can be used to generate multiple output voltage rails including dual-polarity rails.

In terms of output voltage regulation accuracy, isolated DC/DC converters usually fall into three categories: regulated, unregulated, and semi-regulated. This article discusses various regulating schemes and the corresponding topologies. Factors affecting regulation accuracy are examined in detail. This leads to some design tips to improve regulation accuracy in practical designs. Additionally, the pros and cons of each scheme are presented to provide a guideline for choosing an appropriate solution for a specific application need.

Feedback and control of isolated DC/DC converters
Isolated DC/DC converters typically use a transformer to electrically isolate the output from the input of the power stage (figure 1).

 Isolated DC/DC converter power stage

Figure 1: Block diagram of an isolated DC/DC converter power stage.

In a closed-loop isolated DC/DC converter (figure 2), the feedback circuitry senses the output voltage and generates an error by comparing the sensed voltage with its target (feedback voltage reference). The error is then used to adjust the control variable (duty cycle in this example) to compensate the output deviation. Galvanic isolation between control circuitry on primary side and secondary side is also essential. Such isolation can be achieved by utilising either a transformer or an optocoupler. Assuming the reference voltage VREF is precise and stable over temperature, regulation accuracy mainly depends on output voltage sensing accuracy (in other words, how well VSENSE resembles VOUT).

 Isolated DC/DC converter

Figure 2: Feedback and control of a closed-loop isolated DC/DC converter.

Unregulated isolated DC/DC converters
Unregulated isolated DC/DC converters, also known as open-loop isolated DC/DC converters, are widely used in applications that don't require precise output voltage.

A typical example is the push-pull converter with fixed 50 per cent duty cycle (figure 3). The control circuitry consists of only an oscillator along with two gate drivers, which generates two complimentary fixed 50 per cent duty cycle gate signals to drive Q1 and Q2. The transformer turns ratio is selected to deliver the desired output voltage. Neither a feedback circuitry nor a signal isolator is required, which reduces cost and solution size.

 Unregulated push-pull converter

Figure 3: Unregulated push-pull converter with fixed 50 per cent duty cycle.

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