Power Regulators in Series Give Higher Voltages
Just like connecting passives, discrete semiconductors, and FETs, it is possible to connect power regulator circuits in series and in parallel. This can be done in regulator circuits built from discrete components, or by connecting integrated circuits together. In this way, the total power output from the group of power regulators can be increased above the rating for a single regulator.
To reach a higher voltage than might be available from a single power regulator, it is possible to connect two power regulators together in series. In other words, the output (GND) from one regulator is connected to the input (PWR) of the next regulator. While there are EMI and challenges that can arise from this arrangement, and a certain type of isolation should be enforced, this strategy allows a higher voltage to be reached when needed in a power electronics system.
How to Connect Voltage Regulators in Series
Voltage regulators are placed in series with the idea of increasing the output voltages. This is similar to connecting two batteries in series: the two battery voltages will sum according to Kirchoff’s voltage law, but the current in each battery would be the same according to Kirchoff’s current law. These voltage regulator arrangements have the following characteristics:
- Regulators connected with series outputs will have summed voltages
- Regulators connected in series will share the same current
- Each regulator can target different output voltages
- Each regulator must have a maximum current rating that exceeds the load current
The sections below outline how to connect voltage regulators in series in order to provide protection for each regulator and achieve the intended summing of voltages.
Circuit Design
A simple example of the circuit design required to connect two regulators in series is shown below. In this example, the negative terminal of one supply is connected to the positive terminal of the other supply. This set of cascading connections can theoretically be extended to any number of regulators in series, although the total power that can be achieved would be limited by the power conversion efficiency of each regulator.
The above circuit diagram assumes the two supplies are non-isolated and that they share the same input power source. In general, they are not required to share the input power source in order to provide a summed output voltage, but the circuit above does show a single source V(in) being distributed across the two regulators.
The above circuit has the following properties:
- The output voltage is the sum of the two target voltages from each regulator
- The output current is shared across the two regulators
- The diodes provide isolation between the two regulators
- Each supply can target a different output voltage
The diodes used in the circuit must not be driven into reverse bias when placed across the terminals in the design. Therefore, choose diodes with reverse breakdown voltage greater than the target output voltage of each regulator.
Feedback Targeting
Feedback into the regulators can be implemented with a standard voltage divider technique (assuming voltage-mode control) or with a small sense resistor (assuming current-mode control). The implementation of voltage-mode control requires a resistor divider on each regulator individually with a connection back to the feedback pin on each regulator. An example of this is shown below.
One method to implement regulation through a resistor divider feedback loop.
The above regulation method involves a measurement across the full voltage Supply #1. However, it may be advantages to instead use the measurement to the negative terminal of Supply #1 rather than the shared ground connection.
Alternative Series Method: Cascaded Boost Regulators
There is one other method to achieve additive voltages that involves cascaded regulators. This involves the implementation of cascaded boost regulators. In this type of topology, the output in one stage of the regulator is connected to the input of the next regulator. This is commonly done in a buck-boost -> VRM - >LDO type of topology for embedded systems to bring wide inputs down to logic levels. However, to get to higher voltages, boost regulators would need to be cascaded to hit much higher voltages.
The output voltages do not sum in this case. Instead, the input voltage is multiplied by the duty cycle factor for each regulator. The total efficiency of the regulator stages is multiplicative, as shown in the above figure. As more regulators are cascaded together, the required input voltage continues to rise in order to hit the target output voltage. If you plan to cascade more than two boost regulators, make sure your efficiency is considered when calculating regulation requirements.
Whenever you need to select and connect power regulators in a series configuration, use the complete set of simulation tools in PSpice from Cadence. PSpice users can access a powerful SPICE simulator as well as specialty design capabilities like model creation, graphing and analysis tools, and much more.
Subscribe to our newsletter for the latest updates. If you’re looking to learn more about how Cadence has the solution for you, talk to our team of experts.