Working with a Shunt Voltage Regulator in Your PCB Layout

Key Takeaways

• Linear regulator circuits can have low efficiency, but they are simple to build and are useful at low voltages/currents.

• One simple linear regulator is a shunt voltage regulator, which regulates voltage by diverting some current to ground.

• These components are low-noise and will provide a simple way to quickly regulate low voltages without placing an LDO.

Caps, transistors, and heatsinks in a voltage regulator circuit.

One thing they never tell you in an electronics class is that DC power can be noisy. You often work with regulated, isolated power supplies that provide clean output, so it’s easy to assume that all DC sources operate in this way. The real world is not so simple, and designers need a range of methods to regulate DC power for components in their circuit boards. Among the various methods, we have switching regulators and linear regulators.

Among the class of linear regulators, a shunt voltage regulator gives a designer a simple way to regulate power in a DC circuit by diverting some current to ground. By modulating the regulating element, the DC voltage can be maintained at a relatively stable value. Similarly, some shunt regulators can provide overvoltage protection back to the input section of the regulator, a feature that might only be found in an integrated circuit. Here’s how you can design your own shunt voltage regulator and how to place it in your PCB layout.

A Shunt Voltage Regulator Circuit Example

The circuit diagram below shows a simple but powerful shunt voltage regulator. In this circuit, the Zener diode runs in reverse bias and feeds its saturation current into the NPN transistor, which turns on the conductive channel in the transistor and allows current to flow through the emitter to ground. This provides the step-down regulation action required to maintain a stable output voltage by maintaining the current in the circuit I at a nearly constant level.

The output voltage is set by choosing the appropriate resistor value R, which drops some of the input voltage while the remainder is delivered to the output according to Kirchoff’s voltage law. If you want to filter any noise on the input and output, you can place large shunt capacitors on the input and output. Note that the circuit below shows an unregulated DC input is used, but this circuit could provide the same functions as an LDO by stepping down a regulated DC voltage to a lower value.

Basic shunt voltage regulator circuit with a Zener diode and an NPN transistor.

The graph below shows the efficiency vs. series resistance in the example shunt voltage regulator shown above. Note that there are many different kinds of shunt voltage regulators, and a similar efficiency graph can be generated from simulation data or measurements. In this particular example, the efficiency of the circuit is heavily dependent on the input voltage headroom with respect to the output voltage. This matches the behavior of a typical LDO regulator.

Efficiency vs. series resistance in a shunt voltage regulator.

Diode and Transistor Operation

The key to making a shunt voltage regulator work properly is to select the appropriate diode and transistor. The saturation current in the Zener diode shown above will turn on the NPN transistor, which then allows some current to flow into the collector. As the input voltage and current fluctuate, the diode current will also slightly fluctuate , which will then modulate how much current is drawn into the transistor. This is what allows an upward current fluctuation to be dropped back to ground, rather than having it delivered to the load component. Adding a shunt capacitor on the output is useful as this helps maintain a constant voltage despite should the input current exhibit a slight decreasing fluctuation. 

In order to accommodate large current fluctuations in this circuit, the transistor needs to provide high gain. However, if the gain is very high, more power is dropped across the transistor, meaning it is wasted as heat. As an alternative, a transistor with lower gain can be used with a Zener diode that has larger saturation current. Careful balancing of these two parameters is needed to ensure you can damp any expected voltage fluctuations without wasting too much power as heat.

Advantages and Disadvantages

As a linear regulator, a shunt voltage regulator circuit will have lower efficiency than a switching regulator because some of the voltage is dropped across the series resistor and across the transistor. In other words, some of the power is lost as heat across both components. The efficiency is just the ratio of the output to input voltages, as defined in the equation below. Despite the lower efficiency, it has some other advantages that make it desirable in certain situations:

• No switching element: These regulators do not require a switching element for regulation, in contrast to various switching topologies. This eliminates the need to select a PWM driver for a large transistor.

• Low noise: Because there is no switching element and no transient current draw, a shunt voltage regulator produces very low noise. The source of noise is conducted EMI from the input to the output, which can be suppressed using filtration.

• Low component count: The low component count in a shunt voltage regulator allows its cost to be brought down compared to a more complicated and physically larger switching regulator.

• Low output ripple: The collective action of the Zener diode and transistor allow wideband regulation of the current up to the relevant bandwidth in the transistor. This helps keep output ripple low as long as the input current is not too large.

Layout for a Shunt Voltage Regulator

Once you’ve designed and simulated your shunt voltage regulator, you need to place it in your PCB layout. Here are some basic tips that should be followed when laying out these circuits:

• Parasitics: Shunt voltage regulator circuits for low voltages are small enough that they can fit in small board space, and you should keep components closely packed to reduce parasitics to low levels. Place a ground plane below your circuit to ensure the circuit will have isolation from noise. 

• Component size: If you’re expecting high input voltage/current fluctuations in your system, you’ll need to use physically larger components to provide regulation and withstand heat generation. Keep this in mind when selecting components and planning your layout. 

• Heat dissipation: Because these components damp current fluctuations through a resistor and transistor, these circuits generate heat. You may need to use a heatsink to dissipate enough heat to keep components cool. 

Once you’ve designed your shunt voltage regulator circuit, you can use the best PCB layout and design software to capture your designs as an initial PCB layout and begin arranging components. Allegro PCB Editor includes the features you need to layout boards for any application, as well as advanced routing and design verification tools. You can then use Cadence’s analysis tools to simulate and analyze the behavior of your power electronics.

If you’re looking to learn more about how Cadence has the solution for you, talk to us and our team of experts.