10-08-2012, 09:44 AM
AN39 Current measurement applications handbook
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Introduction
Current measurement or monitoring within electronic circuitry is a common requirement spanning many types of applications. These may include anything from portable, handheld equipment through to automotive applications. This application handbook explores factors that are relevant for AC and DC current measurement and the implications on cost and performance for different approaches, and how to best use ZETEX current monitors in your applications. Basic application topologies are explored including typical example calculations.
Application types
There are two basic application types,
•Closed loop
•Open loop
Closed loop systems
In closed loop systems, current is measured compared with a reference value and then modified as necessary by some control element. Response time can be critical here especially where immediate actions need to be taken based on instantaneous current value.
Examples of closed loop current monitoring include:
•Switching power supplies with current limiting functions or switch mode battery charging circuits.
•PWM control of solenoids (automotive valve applications).
•RF transmit control loops in portable cellular equipment, where transmitted power is adapted with distance.
•Control of bias currents in (RF) power amplifiers.
•Electronic fuses for internal fault limiting in equipment on distributed power systems.
•Auto shutdown functions for DC motor control (replacing slipping mechanical clutches).
Open loop systems
Open loop current monitoring systems are characterised by the fact that the measured value is not acted upon immediately. It may, for example, be made available for some other system, usually less time critical. Examples include,
•Current measurement in instrumentation (e.g. bench power supplies, ammeters,
current probes).
•Power consumption indication, especially portable battery powered consumer items.
Detailed discussion of methods
This Application Note is primarily concerned with resistive method because this is by far the most frequently used and also since it is supported by Zetex' wide range of current monitors. However, for completeness, the other two methods will be very briefly discussed before we look extensively at the resistive method compared to Zetex' range of current monitors.
Optically isolated resistive method
In the strictest sense, this can not really be considered as a current sensing method in its own right. This is because the opto-isolating device (usually an optically isolated transistor) does not directly measure the current but merely transfers the already sensed current information across a galvanically isolated barrier. It is discussed here to illustrate an often used method in isolated current monitoring.
Various configurations may be used but the simplest method is illustrated in Figure 1. Resistor R is the current sensing resistor. It is chosen such that, at the current that we want to limit the output to, it develops a voltage equal in magnitude to VF, the forward voltage drop of the opto-isolator, IC1. VF is typically approximately 2V.
This circuit is however so simple and basic that it is of rather limited use. The main reason being the 2V or so VF that is needed to drive the opto-coupler. For example, if this were to be a power supply required to supply a very modest 5A, R would have to dissipate at least 10W of power. This is just not acceptable for many practical reasons. If the output voltage were 5V, it would mean 40% of the available power is lost in the current sensing resistor!
Magnetic method
Like the optical method, the magnetic method also offers isolation but, unlike it, also directly senses its own current without the need for a current sensing resistor.
The magnetic method commonly uses a current transformer which produces an output voltage that is proportional to the current. A change in topology is immediately obvious when we compare Figure 2 with Figure 1. This type of magnetic method can only be used with AC measurements unlike resistive and optical methods which can be either. Even so, the use of the magnetic method is only practical at high frequencies rather than low frequencies. This is because the current transformer that would be required at low frequency would be so bulky and expensive as to make it a non-practical solution. To put things into perspective, you could imagine a scenario where the current monitoring transformer could be nearly as big or bigger than the circuit to be monitored.
For high frequencies however (e.g. switch mode power supplies) magnetic current monitoring becomes feasible and is often used although, even here, it is being replaced by other more cost effective methods such as the use of intelligent FET's to implement cycle-by-cycle current limiting and indeed the now ubiquitous current monitoring solutions from Zetex.
Resistive method
This is the simplest, cheapest and the most basic method of current sensing. It is also by definition the most accurate and linear method of all. Inserting a resistance into the current path has the advantage of converting that current into voltage in a linear way that inherently follows Ohm's law of V = I x R.
It is however not without its own faults although these can be minimised for many given applications. The first and obvious one of these drawbacks is that it introduces additional resistance into an electrical circuit. This can result in unacceptable power loss manifested as heat and loss of efficiency.
Since power dissipation is a square function of resistance (P=I2R), this power loss increases as an exponential function of current which is why the resistive method is rarely used beyond the low/medium current application. Figure 3 illustrates just how very quickly power dissipation builds up in a circuit using resistive current monitoring.
Another drawback is that the method inherently increases the source output resistance. The effect of this may range from the mildly undesirable (such as slightly reduced terminal voltage) to catastrophic, especially where the introduction of the resistor would interrupt the circuit from the ground plane (e.g. a very noisy design which fails to meet statutory EMC requirements).Resistive method
Low-side resistive measurement
Low-side (negative or ground potential) measuring circuits generally offer the simplest solutions because the resulting signal is already ground referenced. One such method is to insert a small resistance into the ground plane between the supply's ground and the load to be measured as illustrated in Figure 4. The resulting proportional voltage developed across that resistance can be used directly or amplified.
Care must be taken to add further circuitry on the correct side of the sense resistor. Circuit C will contribute additional current to that of Circuit B. This may or may not be desirable. To avoid this happening, other circuits should be placed in the position of circuit A so that their currents do not pass through the sense resistor.