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Battery management deep dive on-demand technical training
Battery chargers: Fundamentals Current and voltage dynamic power management of multi-cell charges
Battery management deep dive on-demand technical training
Current and voltage dynamic power management of multi-cell charges
[CHIME] Hello. And welcome to the multi-cell charge controllers training on dynamic power management. Here we see a typical battery charging system. There's an input source, usually a power adapter, a battery, and a system power rail. The charge controller's responsible for partitioning the power from the adapter between the system and the battery.
What happens then if the power required by the system and the power required to simultaneously charge the battery is larger than what the adapter is able to provide? If the charger were to supply both the system and the battery charging at full rate, then it would overload the adapter, which might crash. What we want is for the charger to scale back the battery charging rate when this occurs so that the adapter isn't overloaded. This is called dynamic power management, and it's the topic of this training.
There are two techniques that our multi-cell charging devices used for dynamic power management, or DPM for short. They are input current DPM and input voltage DPM. Which one to use depends on whether or not the current capability of the attached adapter can be determined.
There are many different types of power adapter. For some types such as USB and USB PD, the maximum current capability of the adapter is known, because it's either defined by the standard, or it's communicated via a standardized mechanism. In cases such as this, input current DPM may be used to limit the input current drawn by the battery charger to ensure that it remains at or below what the adapter is capable of supplying.
In many cases, however, the current capability of the adapter is not known. With barrel jack adapters, the size of the barrel jack indicates the voltage. But unless a proprietary mechanism is used, the current that the adapter is able to provide is not known. How then can we ensure that the adapter isn't overloaded when the adapter's capability is unknown? In this case, as we're about to see, input voltage DPM is the best method.
The USB power delivery protocol, or PD for short, is an optional extension to the USB standard that allows a host and a device to negotiate a known voltage and current limit. Voltages between 5 and 20 volt may be offered and accepted in increments of 50 millivolts. And currents between 0 and 5 amps may be negotiated in increments of 10 milliamps.
This voltage and current limit may even be dynamically updated without breaking the USB connection. USB PD is a perfect application for input current DPM, since the current supplied by the adapter is always known. Furthermore, it's important that the current syncing device doesn't exceed the negotiated limit, or else this would be violating the PD spec.
With this example using the BQ25710, we can see how input current dynamic power management works. The BQ25710 is an excellent device for USB PD applications, as it is able to provide the 5 through 20 volt PD range and its On the Go mode. And its buck boost architecture allows it to charge the battery from input voltages that are either above or below the battery's voltage level.
Once a current limit is negotiated using USB PD, this current limit is programmed into the input current register of the BQ25710 using SM bus. If we look at T 0 in the chart at the right, we can see that at this time, the system current requirement is low, so that the BQ25710 can charge the battery at full rate without hitting its input current DPM level. In this situation, the input current DPM loop is inactive. And the device is regulating the charge current using the RSN sense resistor that's close to the battery's positive terminal.
But as the system load increases, the total adapter current also increases until T 1, where the adapter current just reaches the input current limit. At this point, the input current DPM loop becomes active. The input current DPM loop, or In DPM for short, measures the adapter current using the differential voltage that's measured across the RAC adapter sense resistor.
Since the value of this current sense resistor is known, the voltage drop across the resistor indicates the current being drawn from the adapter. This current is fed into an error amplifier that compares the measured current to a reference value and modifies the duty cycle of the buck boost regulator accordingly. The current reference that is used in the error amplifier is generated from the input current registers setting so that the feedback loop regulates the adapter current to this level.
The effect of limiting the current through the regulator is that the voltage of the system node will drop slightly, which will reduce the charging current into the battery. As the system current continues to increase, this slight voltage drop at sys continues to increase as well, resulting in less and less charging current into the battery. But once the system load begins to reduce, the charging current is correspondingly increased until, at T 4, the maximum charging current is once again reached. At this point, the In DPM loop is exited.
We've seen that the input current dynamic power management feature allows us to keep from overloading an adapter when its current limit is known. But what about the many cases in which an adapter is used, and the current limit isn't known? How can the BQ25710 be configured to ensure that it doesn't overload the adapter so far that its voltage crashes?
The answer is input voltage dynamic power management or V in DPM for short. V in DPM takes advantage of the fact that, as an adapter becomes overloaded and reaches its maximum power output, a further increase in current will cause a drop in the output voltage of the adapter. This is shown in the VI graph on the right. By setting a voltage point that is a little below the nominal level of the adapter, maybe a 10% margin, the BQ25710 can detect when the adapter is becoming overloaded. At this point, the V in DPM loop becomes active.
The V in DPM loop works by monitoring the voltage at V in and reducing the current flow through the regulator whenever the adapter voltage drops below this level. V in DPM does overload the adapter slightly, since it requires this voltage drop at the adapter in order to enter the V in DPM loop, but it limits the overloading so that the adapter won't crash, while still maximizing the utilization of the available power that's in the adapter.
Generally speaking, if the adapter's current limit is known, input current DPM is the preferred approach, since it doesn't require this limited overloading of the adapter. It should be noted that even if In DPM is use the V in DPM may also be programmed as a backup protection. And when the input current limit is not known, In DPM can't be used, so V in DPM is the primary mechanism.
Thank you for watching this training. Don't forget that this and many other technical training videos are available at www.TI.com.
2019년 9월 29일
In this training you will learn the details of how input current and input voltage dynamic power management work in Texas Instruments multi-cell charger ICs. The BQ25710 device is used as an example, and the benefits of each method of dynamic power management are explained.
This course is also a part of the following series