Buck-boost and boost converters in wireless security cameras and video doorbells, Part 3
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Welcome to the third part of our series about wireless security cameras and video doorbells. In this session you will become familiar with lithium ion battery discharge curves, and learn in which use cases a buck-boost converter will help significantly increase the battery operating time.
This chart shows discharge curves for a typical lithium ion battery. The x-axis shows the used battery capacity from 0 amp hours up to 2.5 amp hours. And the y-axis shows the battery voltage. The plot shows several discharge curves that vary in the C rate. The C rate is a measure for the speed of battery discharge. And 1C is the rate of current that fully discharges the battery within one hour. A 0 to 2 C rate discharges the battery within five hours.
This plot also shows that a lower C rate leads to a higher battery output voltage and increases the usable battery capacity. Let's assume for the moment a 3 to 6 volt rate has to be generated for the system, and check how different DC to DC converter types are able to generate that rate.
A buck converter can provide output voltages that are lower than the input voltage. This means the buck converter will start generating 3 to 6 volts when the battery is fully charged. However, as soon as the battery voltage falls below 3 to 6 volts, the buck converter's no longer able to generate 3 to 6 volts, and the system has to turn off.
A boost converter can provide output voltages that are higher than the input voltage, and often supports a bypass function to connect the battery directly to the system if the input voltage is higher than the required output voltage. This means any voltage higher than 3 to 6 volts is either directly passed to the system or an additional LDO is required to regulate down to 3 to 6 volts. Any battery voltage lower than 3 to 6 volts can be efficiently boosted to 3 to 6 volts.
A buck-boost converter can work as a buck converter or a boost converter. If the input voltage is almost equivalent to the output voltage, the device works in the buck-boost mode. The buck mode and boost mode are alternating. This means the buck-boost converter can generate 3 to 6 volts across the whole battery operating range with high efficiency.
In general, a buck-boost converter's an excellent choice if the required system voyage is within the battery operating range. Buck-boost converters can also be used if two or more power sources are available in the system, with some of them being higher and others being lower than the required system voltage. A buck-boost converter will generate the required output voltage independently of the input voltage being provided and will operate well during the transition phase from one power source to another one.
Let's now take one specific use case as an example. The left side of the slide shows the LAB bench setup. A 2,500 milliamp hours lithium ion battery has been connected to a DC DC converter. The DC DC converter provides 3 to 6 volts at 1 amp current to a constant current load, which is typical for supplying Wi-Fi and other system blocks that run from the same or a similar supply voltage.
The test was repeated with three DC DC converters. The TPS63802 buck-boost converter, the TPS61280A boost converter with integrated bypass or path remote, and the TPS62826 buck converter. The right side of the slide shows the trend of the battery voltage while the battery capacity is being used up.
The output voltage of the boost converter is out of regulation and higher than 3 to 6 volts for the fully charged battery, and starts getting regulated to 3 to 6 volts as soon as the battery voltage starts going below 3 to 6 volts. An additional LDO is needed to keep the voltage for the system at 3 to 6 volts across the whole battery operating range, which impacts efficiency.
The output voltage of the buck converter is out of regulation for any battery voltage less than 3 to 6 volts, which usually causes the system to either shut down or to operate at reduced performance. Any battery voltage higher than 3 to 6 volts is regulated efficiently down to the target voltage.
The output voltage of the buck-boost remains at 3 to 6 volts independently of the battery output voltage [INAUDIBLE]. The result is maximum system performance and a very efficient output voltage generation that is independent of the battery state.
Let's have a look now how this translates into battery life or battery operating time. The x-axis shows the output voltage of the DC to DC converter. The y-axis shows the battery life in minutes.
For the 3 to 6 volts range there seems to be no significant difference between a boost and a buck-boost converter in terms of battery life. However, the blue line assumes any voltage higher than 3 to 6 volts is directly passed to the system. In many cases, this is not acceptable.
An additional LDO with a drop off voltage of 200 millivolts was added after the boost converter to supply 3 to 6 volts to the system across the whole battery operating range. In this case, the buck-boost increases the battery operating time by about 7 minutes, compared to the combination of boost converter and LDO. The buck converter's only able to support a very short operating time at 3 to 6 volts.
The buck-boost converter is clearly the best choice at 3 dot 3 volts. While close to 3 volts the buck converter starts to perform very well. As mentioned previously, a buck-boost converter performs best if the required output voltage is within the battery voltage range. At higher or lower voltages, pure buck and boost converters are the best choice.
Another important benefit of a buck-boost converter is its ability to run off power sources with voltages higher or lower than the output voltage, which can be important if multiple power sources are combined in a system like 5 volt USB for line power, and double A or lithium-ion batteries for backup, or remote location operation.
Here is a similar chart that shows the battery operating time if two high power IR LEDs in series are driven from a buck, boost, or buck-boost converter. In this case, the voltage cannot be higher than 3 to 6 volts, which is why only the boost converter plus LDO is shown in blue. Even if the load current changed from 1 amp to 200 milliamps, the relative position of the lines is almost identical to the previous example.
Thank you for watching part 3 of this video series.
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Buck-boost and boost converters in wireless security cameras and video doorbells
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