A single PMIC can control multiple power rails and manage the way they come online at power-up. This can protect devices that are sensitive to power being applied to their connections before they are fully running. PMICs can also integrate DC/DC converters to buck or boost a single supply, to generate other voltages.
There is no “typical” PMIC. The features will vary, which makes them specific to a type of application. In energy harvesting, PMICs are used to control the energy source and maximize efficiency. This type of PMIC needs to have low operational power consumption, but more importantly it needs to work in conjunction with the energy source.
Sources of harvested energy are often unpredictable and the power they provide can vary. Photovoltaic (PV) cells are a good example. Running an application from PV cells makes a lot of sense, as it can potentially run continuously if the PMIC and PV can generate more energy than the application uses, and it stores excess energy in a battery.
Even if the PMIC/PV subsystem only provides part of the total energy used, it means the application will run that much longer on a single charge or primary cell.
Using a PMIC to charge a battery
Charging a battery from a PV cell requires some form of power management. One way to characterize the PV module behavior is by measuring it’s “I-V curve” as well as the “P-V curve,” as shown.
Figure 1: I-V and P-V curve shows a photovoltaic cell.
The shape of the power curve is dependent on the number of photons hitting the PV cell. As the amount of incident light changes, the curve will change. And as the load changes, the total power supplied will also vary.
To get the most power out of a PV cell, the load must be varied in relation to the amount of incident light. This is called maximum power point tracking, or MPPT. This is the technique used in large PV systems, such as those found on the roofs of buildings or in fields. It isn’t normally used in a PMIC designed for energy harvesting.
Instead, most energy-harvesting PMICs use a simple and course formula to fix the MPPT maximum power point estimating a fixed Maximum Point Voltage - Vmpp. The simple formula assumes the maximum power output occurs at a fixed ratio between Vmpp and the open-circuit voltage – Voc. This fixed ratio can be set to be 75% or 80% for instance, and this ratio can be set by the engineer using different methods. One method is to use two external resistors or using pre-set values in the PMIC.
However, this approach has two problems. The first is that it requires disconnecting the load to measure the Voc, causing instantaneous supply interruptions. The second is that the engineering team must characterize the PV to define the Vmpp/Voc ratio, which changes depending on the temperature, aging and light incidence.
The NEH2000BY from Nexperia has been designed specifically for this application. Rather than estimate and fix the MPPT, the NEH2000BY adjusts the load to locate the MPPT on the power curve. It takes just 10 ms to recalculate the MPPT and it performs this calculation every 0.7 seconds.
The datasheet refers to this as the hill-climbing algorithm. The calculation takes place automatically, the design engineer using the PMIC doesn’t need to configure the algorithm or use external resistors to set the MPPT.
Here, we dive deeper into what’s going on behind the datasheet, to explain in more detail how the hill-climbing algorithm in the NEH2000BY works.
What’s behind the hill-climbing algorithm?
When a PV cell is open-circuit (no load) it will deliver maximum voltage and zero current. This gives the upper point on the power curve. By connecting the load, the output power will increase as current is flowing, but the voltage will drop.
By adjusting the load, the amount of output power will change. It can be plotted on the curve and will either go up or go down, based on the shape of the curve at that moment. Incident light will change the shape of the curve.
To find the maximum power point on the curve, the NEH2000BY selects an initial power point and measures the output voltage. It then adjusts the load and re-measures the output power. If it goes up, it knows it is going in the right direction. If it goes down, it backs off.
The MPPT circuit repeats this process until it settles on the optimum load, by measuring and re-measuring the output power. The process is repeated every 0.7 seconds and takes just 10 ms to complete.
The NEH2000BY can adjust the load because it uses a capacitor switching topology. The frequency used to switch the capacitor is adjusted based on the measured output power. As the switching frequency changes, the load presented by the output capacitor also changes.
Nexperia has implemented the hill-climbing algorithm in custom logic, and it is integrated alongside the oscillator and the converter functional blocks. Because it is optimized for low power energy-harvesting applications, the entire solution is only 3mm by 3mm.
Today, the NEH2000BY can deliver 80% conversion efficiency thanks to its hill-climbing algorithm, dynamic load management and switched capacitor topology. But there are further advantages to using the NEH2000BY.
Harvesting PV energy without inductors
Unlike most PMICs designed for energy harvesting, the NEH2000BY uses a switched capacitor topology. Consequently, it doesn’t integrate a buck/boost converter. This means it doesn’t need any external inductors.
This is a big deal in small, portable applications where this kind of PMIC would be used. The PCB area needed for the PMIC is much smaller than the area needed for a PMIC with external inductors. Also, an inductor could easily be the second-most expensive component on the board, so removing the inductors saves board space and BOM cost.
Energy harvesting using PMICs and PVs is becoming more popular in wearables and other small devices. Combining the consistent efficiency, small PCB area and low BOM cost of the NEH2000BY makes energy harvesting viable for many new applications.
Why is MPPT important?
How efficient are solar cells?
Energy harvesting options
"Unlike most PMICs designed for energy harvesting, the NEH2000BY uses a switched capacitor topology. Consequently, it doesn’t integrate a buck/boost converter. This means it doesn’t need any external inductors."
Energy harvesting using PMICs and PVs is becoming more popular in wearables and other small devices.
What’s next for low power design?
If your design engineers put low power high on the agenda, they may find the International Conference on Ultra-Low Power Embedded Systems a useful event. It is an opportunity for engineers, researchers and academic scientists to share their latest innovations.
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