Power supplies generally sense output voltage and regulate it. However, it is almost always necessary to measure some sort of current. There are two main reasons why you would measure current:
There are several ways to measure the current, but the methods can be split into two categories: loss current sense that adds an element and creates loss, or a loss-less current sense that uses existing components. Today’s blog will focus on loss current-sense methods, Part 2 will focus on loss-less current sense techniques.
Without further ado, two "loss" current-sense methods for measuring current…
Simply use a resistor to measure current. Everyone knows ohms law: V=IR. By measuring the voltage across a known resistor, the current can be determined. Figure 1 shows a very simple diagram of how to measure the current in the output of a power supply.
Figure 1: Measuring Output Current of a Power Supply Using a Resistor
Measuring current in this method might be useful for limiting or regulating the DC level of the output current. For control purposes, it is usually necessary to make very fast AC current measurements. Figure 2 shows how to measure the current in a FET of a synchronous buck converter.
Figure 2: Measuring FET Current Using a Resistor
This current measurement is necessary for current mode control. Using a resistor in the source of the low-side FET makes the measurement very easy because it is referenced to GND.
Although using a resistor is simple, they come with a fatal flaw - Resistor burn power!
While very simple and possibly very accurate (resistors are available in 0.1% tolerances), resistors do cause losses in the system. For very large currents, this is usually unacceptable. This can quickly add up to a significant amount of loss in the system. Resistors can also cause problems with regulation and voltage tolerances do to the voltage drops. There has to be a better way! There is..
Alternatively, use current-sense transformers for very-low loss and the ability to measure AC currents. A current sense transformer can be used in place of a sense resistor to improve efficiency in cases where a resistor is not practical. Figure 3 shows how to use a current sense transformer to sense the primary current in an isolated converter.
Figure 3: Measuring Primary Current Using a Current Sense Transformer
A current sense transformer allows the circuit designer to include a gain factor as well by using the step up ratio of the turns. In order to use a current sense transformer, some additional circuitry and care are needed. The transformer can only measure AC current, so there needs to be a finite amount of time in which the primary switch is off. During this time, the transformer resets and the voltage across the winding can go negative. Because of this, you need to include a diode in series with the winding. The fact that the transformer needs to be reset which prevents a current-sense transformer from being used for DC measurements. If there is no off time for the transformer to reset, it will saturate and the information is not useful. The placement of the transformer in the circuit and on the PCB is also very important. Care must be taken to make sure that circulating current loops are minimized.
Both methods discussed here require the addition of extra loss components. In my next blog post, I will talk about a couple of methods that can be used to measure current with elements that are already included in the circuit. If you have any questions/comments on this method, please leave them in the comment section below.
Related resources:
- Read: All PowerLab Notes
- Easily find and use one of nearly 1500 reference designs: PowerLab Reference design library
- Find: Power Management products
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