Power Regulators in Series and Parallel

When designing power supplies for demanding applications, sometimes a regulator circuit does not meet the voltage or current requirements. Although there are ways to scale a single regulator circuit to higher power levels, sometimes multiple regulators need to be used together to reach higher power levels.

Multiple regulators operating together can be wired up in two ways: using series configurations to reach higher output voltage, or parallel configurations to reach higher output current. We will show how regulators can work together in these configurations and some example parts from real designs.

Series Regulators Give Summed Output Voltages

The series regulator configuration enables higher output voltages, similar to stacking batteries. This works because voltages sum along a loop (Kirchhoff's voltage law) while the current through each element remains identical (Kirchhoff's current law).

To implement this topology, the output ground terminal of one regulator connects to the input power terminal of the next regulator. In a typical implementation, both regulators share a single input power source, with the common input distributed to each stage. While sharing the same input source is not strictly required to achieve summed output voltage, it represents one practical method for implementing the arrangement and simplifies the overall system architecture.

The configuration can theoretically extend beyond two regulators when applications demand even higher voltages. This cascade of series-connected voltage regulators exhibit several important characteristics:

• Voltage summation: The total output voltage equals the sum of individual regulator target voltages
• Shared current path: All regulators in the series chain carry identical current
• Independent voltage targeting: Each regulator can regulate to a different output voltage
• Isolation and protection: Protection is needed to prevent overdriving successive regulators in the cascade

Series regulators require some isolation between stages. Protection diodes serve a critical role in this topology, providing the necessary isolation between regulators, as shown above. The diodes need to have sufficiently high reverse breakdown voltage such that each regulator does not drive its output protection diode into reverse bias. In general, sufficient margin is needed in the event one of the regulators experiences a fast transient, otherwise there will be a risk of reverse bias failure during normal operation and transient conditions.

Regulation and Feedback in Series

Implementing proper regulation feedback in series configurations follows standard control approaches, though each regulator requires its own feedback loop. These would be done in the standard arrangements:

• Voltage-mode control: place a resistor divider network for each regulator, connected back to that regulator's feedback pin. The resistor divider measures the voltage across that specific regulator's output to maintain regulation.
• Current-mode control: the feedback voltage is taken as the voltage drop across a current-sense resistor (CSR). This is much more difficult as it requires placing CSRs between each regulator and on each output pin.

In the voltage-mode control feedback lines, each divider needs to connect to the regulator’s GND pin and not the shared GND net. Otherwise, the feedback measurement will reference the summed voltage, and not the individual regulator’s voltage. This ensures each regulator can adjust its output independently.

Beyond the basic series connection, higher voltages can be achieved by cascading boost regulators, where one stage's output directly feeds the next stage's input. In cascaded boost stages, output voltages do not sum. Instead, each stage multiplies the voltage based on its duty cycle, and total system efficiency becomes the product of individual stage efficiencies.

Cascaded boost configurations are not common, primarily due to the lower efficiencies of cascaded stages. The total efficiency of the cascaded boost stages is the product of the individual efficiencies, resulting in a large amount of loss when many stages are cascaded. As stages are cascaded to N > 2, the required input voltage must increase to reach the target output voltage.

Parallel Regulators Give Summed Current

When a single regulator cannot deliver enough current, paralleling two or more regulators provides a straightforward method to increase the total current output. This the total current provided by sources of current in parallel is the sum of the individual currents (Kirchoff’s current law).

Parallel regulator arrangements can be implemented in two ways:

1. Paralleling the discrete components within a regulator design
2. Paralleling complete regulator ICs and their passives

In approach #1, we would essentially be paralleling components that sit in the output current path, i.e., paralleling the FETs in the switching stage.

When paralleling ICs, the designer must ensure the regulators switch in-phase rather than operating as an unintentional multiphase system. Some regulators ensure this with a synchronization (SYNC) pin, which may be optional. Another option is to tie the enable pins on the two regulators to the same input.

Parallel regulators must be closely matched in specifications and have specific configurations that enable parallel operation. The regulators should have several common specifications:

• Both controllers must regulate to identical output voltages
• Each regulator should have its own feedback loop to ensure they regulate to the same voltage
• PWM switching frequencies must relate as integer multiples
• Synchronization requires either an external reference oscillator or dedicated sync pins
• Both regulators will share the same ground connection
• The regulators may operate from different input supply voltages

In the basic parallel topology, output currents add across the paralleled regulators. Similarly, total output power increases as the sum of individual regulator power contributions. This behavior contrasts with series arrangements, where current-handling capability of each regulator restricts the total delivered current.

The best approach is to duplicate a proven regulator circuit across multiple stages. This will ensure mismatches between the output voltages are seen so that none of the parallel regulators are driven in reverse.

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