Photovoltaic Power Supply Design Fundamentals
Power systems are normally designed to plug into the electrical grid or a battery, but some newer systems are being designed as photovoltaics.
A photovoltaic power supply is essentially a miniature version of a PV array with multiple panels, an inverter, and power conditioning features.
The power conditioning and power output tracking portions of the design are the most critical to ensuring highly efficient power conversion and output.
A photovoltaic power supply intends to miniaturize a PV array, inverter, and power point tracking equipment into a small unit with regulated power output
Today, much of the world has largely agreed that the transition to green energy is inevitable, and many companies are starting to develop a range of power systems to support photovoltaic power. PV arrays on homes, commercial buildings, and solar farms have been around for many years, but a newer development is in smaller, more compact power supplies that can support photovoltaic power conversion. These smaller units can be challenging to design and operate due to the nature of photovoltaic cells and power conversion.
A photovoltaic power supply incorporates many elements that are not seen in other power systems or in power supplies that accept power from the AC electrical grid. These designs convert insolation directly into electricity in a very small form factor, yet they intend to provide some of the same features found in a typical PV array. If you want to start designing photovoltaic power supply units, we’ve compiled some important high-level design guidelines that can help you get started.
Photovoltaic Power Supply Architecture
A photovoltaic power supply operates on a simple concept: take DC input power from a solar module, regulate it to remove noise and variance, and output stable DC power to a charge controller, inverter, battery, or other component that requires DC power. Aside from the interface with other systems like inverters, a photovoltaic power supply performs some of the same basic functions as a standard regulated DC power supply.
To see the exact functions a photovoltaic power supply needs to perform, it helps to compare it with a conventional power supply, as shown in the table below.
To satisfy all of these requirements, a photovoltaic power supply needs to be designed to accommodate some important facts about the behavior of solar modules. This goes beyond the need to accommodate intermittent insolation and power generation.
Power Point Tracking
One important aspect of DC power conversion in photovoltaic power systems is tracking the system’s power point to ensure it always outputs maximum power. In a typical residential or commercial solar array, a maximum power point tracking (MPPT) circuit is a type of DC-DC converter that minimizes the impedance mismatch between the solar array and a load component or system, thereby delivering maximum power to the load. In a residential setting, the load is normally a battery bank or an inverter, the latter of which will provide power to the home or directly to the utility grid.
To see why power point tracking is important, take a look at the graph below. Solar cells are nonlinear devices running in the photovoltaic mode, similar to the case of a biased photodiode in photovoltaic mode. The current vs. voltage curve below (red curve) reveals the nonlinear nature of photovoltaic modules. Here, the output current depends on whether the load is a short circuit (0 Ohm load) or an open circuit (infinite Ohm load), as indicated by the intersections at the axes of this graph.
DC power output from a typical solar cell
In between these two extreme load values, there is a transition region where the voltage and current together deliver maximum power to a load component. The power output corresponding to this IV curve is shown in the blue curve, and there is a specific pair of voltage and current values where the system will output maximum power. This pair of values is called the maximum power point.
If the insolation changes throughout the day, the power output from the cell will also change. From the graph below, we can see that the maximum power point does not stay constant as insolation changes, nor is the maximum power output point a strictly linear function of insolation.
DC power output from a typical solar cell
The DC-DC regulator section shown above can have a standard switching regulator topology. The PWM driver in the above circuit is meant to adjust the switching elements (normally power MOSFETS) so that they have the correct impedance to maximize power delivery. The sense and PWM driver stage can involve complex algorithms to ensure accurate power point tracking and maximized power output.
Some of the important components required in the control loop to provide precision sensing and control include:
- Sensing resistors (usually mOhm values)
- Current sense amplifier for precision power output measurements
- A PWM driver, possibly integrated with an MCU
- Power MOSFETs in the regulator section
Once the physical layout is finished, it should be evaluated using DC power integrity tools. This will help you verify that the regulator section does not experience excessive temperature rise when running at maximum power output. More advanced 3D electromagnetic field solver tools can help you spot excessive EMI and determine how to solve them so your design will pass EMC tests. Both points are important for consumer-grade and commercial-grade photovoltaic power supplies.
If you’re developing innovative photovoltaic power supply systems, you can use Cadence’s PCB design and analysis software to build the highest quality products and prepare them for manufacturing. Cadence provides the industry’s best CAD tools for PCB design and power integrity analysis tools that help automate many important tasks in systems analysis. Cadence’s suite of pre-layout and post-layout simulation features gives you everything you need to evaluate your system.
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