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Controlling Output With a Voltage Divider Power Supply

Published Date
Author Cadence PCB Solutions

Key Takeaways

  • What is voltage division and how does it apply to circuit design?

  • Why can’t a voltage divider itself supply power?

  • Incorporating voltage division to unlock additional functionality from power supplies.

Network view of a voltage divider

Voltage divider power supplies allow users to set the output voltage to most values below the maximum.

Oftentimes in engineering disciplines, designers are faced with tradeoffs. The old adage rings true: there is no such thing as a free lunch. Improvements in one facet of design must be offset by losses in another, and it is up to the development team to figure out how best to distribute their resources to best achieve the design intent of the device. Power is a common subject for efficiency evaluation due to its importance in overall performance, not to mention basic operability.

However, efficiency is not the only metric by which to measure power supply systems, and in fact, there is circuitry intentionally designed to flout efficiency in favor of other critical parameters. Voltage divider power supplies do just this in order to allow users to adjust voltage outputs during testing or in-circuit.

Voltage Division and Its Functions

A voltage divider is a simple mechanism to adjust voltage using the most basic tenets of circuit analysis: Ohm’s Law and Kirchoff’s Laws. Starting with the latter, in any branch of a linear circuit with idealized elements, the current remains constant while the voltage drops throughout the circuit elements; more specifically, this is Kirchoff’s Junction Rule and Loop Law, respectively. With these in mind, it’s possible to step the voltage down to fractional increments of the input. Starting from the source voltage:

Derivation of voltage divider output.

A quick derivation of the impedance function at the heart of the voltage divider transfer function.

As seen above, there exists a simple relationship between the input-side and output-side impedances: the voltage at the output always decreases towards zero, with a larger input impedance causing a larger voltage drop at the output, and a smaller input impedance leading to a smaller voltage drop. For the case where the impedances match, the output voltage is exactly half of the input; this is a common technique to establish max power transfer as well as remove signal reflection in transmission lines.

Is a Voltage Divider Power Supply an Option?

Max power transfer is a topic heavily associated with both voltage division and power supplies, but there are many misconceptions about the practice, namely the difference between maximum efficiency and maximum transfer. As stated, an impedance match (that is, equal resistance and offsetting reactance, also known as the complex conjugate) will maximize the rate of power transfer but will result in a system efficiency of exactly 50%. Adjusting the load impedance up increases efficiency (more input power is dissipated at the load instead of elsewhere in the circuit), but the increased impedance drops the overall power of the circuit. Conversely, decreasing the impedance at the load drops efficiency towards zero, as the majority of the power is dissipated in the source rather than the load. Therefore, although maximum power transfer results in losses, the availability of maximum power at any given moment to the load will trump this concern.

Since voltage dividers are so intrinsically linked with the idea of power transfer, they may seem an excellent choice to act as a power supply. By themselves, voltage dividers have little control over their output, as they lack the ability to regulate current draw at the load. Variations in the voltage will have a direct effect on the current available at the load, which can starve electronics during moments of high current usage. Even under ideal operating circumstances, the power used to operate a voltage divider will be magnitudes higher than the voltage delivered to the load.

Combining Voltage Division and Regulation for Power Supply Output

Although the voltage divider circuit alone is tremendously unsuitable as a power supply, the value of maximum power transfer is a crucial concept to power supply operation. The ability to adjust an output voltage with only a pair of resistors is an extremely inexpensive and easy-to-implement control for a variety of power supply circuits. Often, this is achieved with a feedback pin on the power supply IC that acts as a voltage reference point for the circuit. Engineers can quickly determine the ratio to diminish the output voltage by using the impedance relationship between input and output. For example, converting a 5V supply into a 3.3V network would require a 34% voltage drop from input to output impedance, or alternatively, a 66% remainder of the voltage at the output. Expressed as decimals, the input impedance needs to be .34 of the total impedance. Therefore, the output impedance must be .66 of the total. Calculating roughly, the output impedance should be about twice that of the input to achieve the desired voltage output.

While the relationship between the resistors is known, the actual values remain an open choice. The first thought might be to maximize the resistance value (while maintaining the pre-determined relationship) to avoid power losses. However, high input impedance, as seen by the feedback pin, can result in intermittent performance due to the reduced current dictated by Ohm’s Law. Further, the decreased signal-to-noise ratio can result in run-time issues and remove feedback from the voltage divider. On the other hand, lower resistance values are likely to be more responsive but can be highly inefficient and lead to premature aging of nearby components and board materials due to excessive heat dissipation. The best solution is a compromise between control and efficiency, similar to the balance of efficiency and power availability in the max power transfer theorem.

Cadence’s Design Tools Power PCB Innovation

An enabled voltage divider power supply offers a high level of modularity for little cost and board space, allowing engineers to more easily produce voltage levels best suited for the signal and power requirements of the board. Applying the concept of voltage division is key to many aspects of maximizing power system performance, but without real-world parameters, the ideal use case can be nonrepresentative of actual operating conditions. 

Cadence’s PCB Design and Analysis Software suite affords design teams a collaborative space to accurately model component functions and board materials before entering into manufacturing, saving time and money on product development cycles. Designers can easily roll these tested and verified circuits into OrCAD PCB Designer, which offers a smooth and powerful layout environment to expedite time to market.

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