How to sense two types of current losslessly to measure current? Look here!

We almost always need to measure some type of current. This article will focus on lossless current sensing techniques. Use circuit components you already have! We will look at two methods of current sensing using circuit components you already have. These methods are inductor DCR sensing and FET sensing. Inductor DCR sensing is not super accurate, but it is adequate. The DCR of an inductor is usually on the order of +/- 10%. The temperature coefficient will vary due to the copper, and you may get some very inaccurate measurements. The good news is that after the DCR network, you end up with a very clean signal that is free of switching noise. Figure 1 shows the network needed to extract current information from the DCR of an inductor.  The components of this network are selected using the following formula: 
 
There are several factors to consider when setting up the DCR network: 1. The maximum amount of signal that the controller or sensing circuit can handle - splitting may be required. 2. Temperature compensation - components with negative temperature coefficients can be used to help keep the DCR constant as temperature changes. DCR sensing is often used in multi-phase configurations to implement circuit mode control. Current sharing between multiple phases is easy to implement using this technique. Use FET sensing, but keep an eye on switching noise! When FETs switch, a lot of noise is generated. This noise needs to be filtered. Figure 2 shows a FET sensing scheme and how noise can interfere with the measurement. There are several ways to reduce noise, but they all have drawbacks.   1. Filters using RC networks. This can be used, but it rounds the current signal, making the edges less sharp. The rounded current sense signal can cause jitter and other noise issues. 2. Leading edge blanking. This is a technique that ignores the first portion of the current sense signal. The main problem with this method is that it causes a minimum on time, and if there is a problem, the duty cycle can only be reduced so much. Figure 3 shows the drawbacks of the above two methods.   
Lossless current sensing techniques are not as accurate as using precision resistors, but they avoid efficiency and power loss issues. Lossless methods are usually more suitable for applications where efficiency gains are more important than accuracy. Current sensing methods are by no means limited to the methods described in the last two blogs. Please leave a comment and let us know how you do it!