We almost always need to measure some types of current. In the last article, I introduced two main reasons for measuring current and several methods for measuring using loss current sensing techniques. This article will focus on lossless current sensing technology.
Use existing circuit components! We will introduce two methods for current sensing using existing circuit components. The two methods are inductor DCR sensing and FET sensing.
Inductor DCR sensing is not particularly accurate, but it is sufficient. The DCR of the inductor is usually approximately +/- 10%. The temperature coefficient changes due to copper and you may get some very inaccurate measurements. The good thing is that after the DCR network, you end up with a very clean signal without switching noise. Figure 1 is the network needed to extract current information from the inductor's DCR.
figure 1
The components of this network are selected by the following formula:
There are several factors to consider when setting up a DCR network:
The maximum amount of signal that the controller or sensing circuit can handle - it may need to be split.
Temperature Compensation - Components with a negative temperature coefficient can be used to help keep the DCR constant during temperature changes.
DCR sensing is commonly used in multi-phase configurations for circuit mode control. It is easy to implement current sharing between multiple phases using this technique.
Use FET sensing, but pay close attention to switching noise! When the FET switches, it generates a lot of noise. This noise needs filtering. Figure 2 shows the FET sensing scheme and how noise interferes with the measurement. There are several ways to reduce noise, but they are all flawed.
figure 2
Use RC network filters. It can be used, but it will round the current signal and make the edges unclear. Rounded-off current sense signals can cause jitter and other noise problems.
Leading edge blanking. This is a technique that ignores the first part of the current sense signal. The main problem with this approach is that it leads to a minimum on-time, and if there is a problem, the duty cycle can only be reduced so much.
Figure 3 shows the defects of the above two methods.
image 3
Lossless current sensing technology is less accurate than precision resistors, but it avoids efficiency and power loss issues. Lossless methods are generally suitable for applications where efficiency gain is more important than accuracy. Current sensing methods are by no means limited to the methods described in the previous two blogs. Please comment and let us know how you did it!
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