When you need to design a power supply but you can’t source enough current, what can you do to increase the current output? A simple solution is to use a pair of power regulator circuits wired in parallel. The use of parallel power regulator circuits provides a simple way to source higher currents than would be available from a single regulator.
Paralleled regulators can be formed in multiple ways. For example, discrete components can be paralleled when building a power regulator circuit, or entire integrated circuits can be paralleled. If paralleling integrated circuits, there are certain precautions to take to ensure the regulators are switching in-phase rather than in a multi-phasing configuration.
In this article, we’ll look at the finer points needed to understand regulator design for parallel operation. Contrary to the series configuration, the paralleled regulators must be very similar and should be specifically designed for wiring in a parallel configuration. The goal is to provide guidance on how to select regulators that allow paralleling, as well as how to design regulators with discrete paralleled switching elements.
Selecting Parallel Voltage Regulators
The basic topology involving parallel power regulators is shown below. In this example, the regulators placed in the parallel arrangement provide additive total current; the combined current is nearly equal to the sum of the currents from the two supplies. The total power output is also the sum of the two power outputs.
In the above parallel circuit, it’s assumed that the supplies are ICs, and they could have integrated switches and/or inductances. They could also be power modules, where all the important control, switching, and inductive components are included on-die or in-package. Not all modules are appropriate for parallel arrangement, so it’s important to select regulators with the right capabilities that enable parallel operation.
Series vs. Parallel Specifications
In a series arrangement, the main consideration is the current handling of the regulators. Essentially, the maximum current limit of one regulator can limit the total current supplied by all other regulators. This is not the case with parallel regulators.
In order to construct working parallel regulators, we have the following requirements on the regulator specifications:
- Switch-mode controller ICs are typically used with integrated switching
- The controllers must provide the same output voltages
- The controllers must have PWM frequencies that are integer multiples of each other
- External reference oscillators or sync pins may be used to synchronize the regulators
- The regulators must have the same ground reference; their outputs should not be isolated
- The regulators typically require their own feedback loops; sharing a single loop is not recommended
- Each regulator could have different supply voltages
In order to ensure the best-case operation of the parallel supply arrangement, the best approach is to just duplicate a single regulator circuit across N stages. This will ensure the output voltage from each stage very closely match to each other within very small tolerance. Typically any output voltage mismatch reaching 300 mV has the potential to drive one of the regulators into excessive reverse polarity and could damage the power converter IC.
It should be well understood that many power regulators use discrete MOSFETs that are external to the power regulator IC. The external MOSFETs make up the switching stage, and the function of the regulator IC is to switch the MOSFETs on/off by controlling the driving PWM signal’s duty cycle. The gate driving signal in these FETs still needs to be synchronized so that the converter waveforms are aligned in time, which requires built-in synchronization or a reference oscillator.
Why would you use discrete MOSFETs with paralleled switching regulator controllers? The answer is: you shouldn’t! If you switching regulator enables the required synchronization needed for parallel operation, it most likely will not require external discrete MOSFETs.
Parallel MOSFETs Offer an Alternative
Another option for designing a regulator circuit with higher current output capability is to use parallel MOSFETs. Some power regulator ICs are not actually ICs, they are just gate drivers with an integrated feedback and control mechanism. These regulator circuits use the following set of components:
- A switched-mode controller/gate driver with feedback
- A resistive feedback loop for measuring voltage or current
- An inductor or transformer
- Discrete capacitors
- One or more MOSFETs for switching and rectification
An example regulator circuit that uses external FETs is shown below. By using the FETs in parallel, the total current that can be provided during switching during a single swing of the driver’s duty cycle is much higher. If you have N FETs in parallel, the total current is a factor N larger than that of a single FET.
This can be challenging if your regulator IC operates within an open drain configuration and includes current monitoring. This is generally implemented to ensure that the current can be measured in the IC, and the IC can be shut off if the current is excessive. This protects the IC and the other components from excessive heating due to switching losses and R_ON losses. A better option may be some kind of thermal monitoring when paralleled MOSFETs are used to achieve higher currents.
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