Electricians know all about current clamps, but for the circuit designers and PCB designers, something more compact and practical is needed. Whether you need to measure very low DC currents or very high AC currents, there is a small number of current sensing methods that can be implemented as small circuits in a PCB. Let’s take a look at these methods and how they can work together to provide accurate current measurements.
Current Sense Methods
Typically, there are three broad approaches to current sensing techniques used in power electronics. Current is not normally measured directly. Instead, it can be converted to a voltage, and the voltage can be measured by comparing that voltage with a stable reference voltage (such as in an ADC). In other words, current sensing is essentially voltage sensing in disguise, and this current-to-voltage conversion is accomplished by dropping the current over a precision resistance.
Current Sense Resistors
A current sense resistor provides current-to-voltage conversion action, most notably in current-mode controlled power regulators. An example is shown below, where R8 is the current sense resistor.
Usage: DC or AC
Components that are typically used for current sense resistors will have high precision and very low voltage. The idea is that the current does not drop very much power across the current sense resistor, and the voltage developed across that current sense resistor is V = IR (Ohm’s law).
When the current is high, the voltage might be too high to feed into a regulator feedback pin or an ADC. In that case, the voltage across the current sense resistor can be stepped down to produce a lower voltage, which can then be read with an ADC, amplifier, comparator, etc.
Current transformers are a type of transformer that is specifically rated for high currents. The reason for this is that they are typically used to step down an AC current, specifically at a harmonic frequency. They are often used to take measurements of currents on AC main lines or to monitor currents on circuits that require AC mains power.
Once the AC current is stepped down (or up), the peak of the AC current waveform can be measured. The easiest way to do this is to rectify the current with Schottky rectifier diodes, and then drop the current across a precision resistor. The voltage across the resistor can then be measured with an amplifier or ADC.
Current Sense Amplifiers
If you want a pre-packaged solution to measure currents quickly and accurately, you can use a current sense amplifier. These can be built from discrete components, essentially by dropping the current across a precision resistor and comparing with a reference voltage. However, there are pre-packaged current sense amplifier ICs that can be used with a large range of currents.
Current sense amplifiers essentially convert a current to a voltage, and the output voltage is proportional to the current flowing into the circuit. These components take the precision resistor route to convert a current into a small voltage, and that voltage is amplified and output to another part of the system. For example, the output can be used in an ADC for a direct measurement.
An example current sense amplifier circuit with an H-bridge DC/DC converter is shown below.
Usage: DC or AC
In this circuit, the measurement required for current sensing is dropped over an external resistor and internal feedback is used to set the gain for the output voltage. This is typical for many current-sensing systems. These components can operate from DC up to very high frequencies, but be sure to check the phase margin/bandwidth rating on the amplifier to ensure it will work with your signal.
Mix and Match
Sometimes, you need to mix multiple methods to bring your current down to a reasonable level and measure it accurately. This means there will be a requirement to mix and match the current sense methods listed here in order to take a measurement. For example, with measuring current in an AC mains-powered system, you might have the following signal path:
Supply current from AC mains
Step down the current with a current transformer
Convert the current to a voltage with a current sense resistor
Further adjust the output voltage with a voltage divider
Feed the voltage into a current sense amplifier
This is very common in large power systems that require measurement and compensation via a feedback loop where a large amount of current is present. There are not many ICs that can support the large voltages and currents involved in these systems, so you may need to build a system from discrete components.
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