MPPT vs. PWM for Solar Charge Controllers

Key Takeaways

• Learn what MPPT and PWM are.

• Explore the key differences between MPPT and PWM.

• Find out what’s important to know when designing MPPT and PWM circuits.

We all know that when there are different options for one job, they each usually have their own strengths and weaknesses. However, once you think more about your specific needs and resources for the job, one of the options typically starts to stand out as the better choice.  

When looking at solar charge controllers, the same thing is true. A common comparison for this job is MPPT vs. PWM. In this piece, we will discuss the differences between these two technologies, help you decide which is better for you, and even provide some design tips for working with these technologies. 

What Are MPPT and PWM?

MPPT and PWM are different technologies for solar panel chargers.

Both MPPT (maximum power point tracking) and PWM (pulse width modulation) are terms used when describing solar charge controllers. They refer to the technology that is powering the devices. PWM achieves this function by altering the duty cycle of the connection to charge a battery from a solar panel. 

A PWM charge acts like a switch, where a MOSFET is used to bridge the connection between the solar panel and the battery. A microcontroller is often used to control the charging stages by altering the duty cycle of the charging current. Typically, a PWM charge goes through three stages: bulk charge, absorb charge, and floating charge. 

The bulk charge stage is where the charger lets a reasonably large amount of current through to charge the battery. The next stage, absorb charge, involves letting the battery voltage settle down before the charge releases a constant amount of current. Finally, the floating charge stage ensures that losses to the battery self-discharge are compensated by trickle charging.

An MPPT charger goes through the stages as it charges a battery. However, it does so dynamically by ensuring that it is delivering a combination of voltage and current that results in maximum power efficiency. 

MPPT vs. PWM Solar Charger

Both MPPT and PWM solar chargers exhibit differences in various areas.

Operation

An MPPT charger produces a charging voltage and current that correlates to the maximum power on the I-V curve of a solar panel. The charging process also takes the panel temperature, angle, and exposure into account. 

Meanwhile, a PWM charger behaves like a switch between the solar panel and the battery. It is not designed to keep track of the maximum power on the I-V curve, and thus, operates rigidly regardless of circumstances.

The MPPT charger is more efficient but at a much higher cost. 

Efficiency 

An MPPT charger is way more efficient than its PWM counterpart. You can expect up to 99% efficiency from an MPPT charger, but a PWM charge only gives you 50%-75% efficiency. Let’s explore why that is the case. 

A PWM charger is built to behave like a switch, and thus, the solar panel will always charge at the battery’s voltage. A solar panel with an open circuit voltage of 24V, when connected to a PWM charger, is pulled down to almost the battery’s voltage of 12V. This reduces its efficiency by 50%. 

On the other hand, MPPT adjusts the charging voltage and current to the maximum deliverable power. It behaves like a DC-DC transformer instead of a switch, which allows it to minimize charging losses.

Cost

With a simpler build, the PWM charger is cheaper than an MPPT charger. A PWM charger typically costs less than $100 while an MPPT charger costs hundreds of dollars. 

Key Considerations When Designing MPPT or PWM Chargers

Ensure sufficient trace width for high current. 

Whether you’re designing around an MPPT or PWM charging circuit, there are a couple of things to keep in mind. Both circuits utilize high-frequency switching and so it’s important to design around lowering EMI emission. Minimize the loop inductance by keeping traces short and keep components close to each other.

Both types of chargers can be designed to handle a high charging current that can be anything from 1A to 60A. It is vital to ensure the trace width is sufficient to handle that maximum current. Otherwise, the trace may overheat or break when it’s stressed.

It’ll be helpful if you’ve run some checks with a good PCB design and analysis software. You’ll find OrCAD Celcius Thermal Solver helpful in identifying thermal hotspots on the traces, which can help inform your design decisions. You can also use InspectAR to interactively assess and improve your PCB design process using augmented reality. Inspecting, debugging, reworking, and assembling PCBs has never been simpler or more accurate.

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