Understanding isolated DC/DC converter voltage regulation

-November 15, 2014

Isolated DC/DC converters are required 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).



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 utilizing 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).



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% duty cycle (Figure 3). The control circuitry consists of only an oscillator along with two gate drivers, which generates two complimentary fixed 50% 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.



Figure 3. Unregulated push-pull converter with fixed 50% duty cycle


A push-pull converter is essentially a forward-derived topology. When it operates with a fixed 50% duty cycle, the output voltage regulation can be elaborated using the equivalent circuit in Figure 4. R is the equivalent resistance of secondary transformer winding and trace. The output voltage can be expressed by (1):


where VR is the voltage drop across resistor R and VF is diode forward voltage drop, which are both load- current-dependent. Moreover, VR and VF also vary with the ambient temperature, and so does VOUT. Equation 1 indicates VIN, in addition to load current and ambient temperature, is also a factor of VOUT. These factors are not compensated at all which could result in significant output voltage variation. That is why such converters are called unregulated.



Figure 4. The equivalent circuit of unregulated push-pull converter


Similar to a push-pull converter, other topologies commonly used for unregulated isolated DC/DC converters are half-bridge and full-bridge (H-Bridge) converters. Due to the low cost and circuitry simplicity, these unregulated isolated DC/DC converters are commonly used as DC transformers to provide galvanic isolation. A low-dropout (LDO) regulator is often used as the post regulator to provide low noise and low ripple power supply.


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