Current detection: past, present and future

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What you will learn:

  • Common methods used to measure current.
  • A new approach to current sensing: GaNSense technology.

For any electrical control system, voltage and current input signals are essential for the system to operate as designed and in a safe manner. In power supplies and motor drives, for example, current is measured for purposes of control (such as peak current mode or average current mode control in power converters), protection (such as protection against short-circuit overcurrents) and data logging.

Shunt Resistors

One of the simplest methods of measuring current is to add a shunt series resistor in the current path of interest and then measure the voltage across it as a representation of that current – knowing the relationship voltage = current × resistance, (V = I × R), and assuming a stable or linear resistance.

This popular method is simple and accurate. It can be used for DC or AC current measurements and is flexible for a variety of applications. Although not inherently isolated, some vendors offer isolated amplifiers or isolated ADCs for this approach. Vendors like Allegro, Broadcom, ON Semi, Skyworks, Texas Instruments and many more offer solutions in this area.

However, this method of current sensing introduces an additional power loss element, particularly at high current, which can be a significant limiting factor in high-power, high-efficiency systems. It also adds stray source inductance into the power path and gate loop of low-side switches where it is most unwanted, causing delayed turn-off and voltage spikes.

These resistors are also not standard off-the-shelf components. To carry the full rated load current and meet typical 1% resistance over temperature accuracy, very low on-state resistance (RDS(ON)) Highly optimized FETs and shunt resistors should be used. To avoid heavy losses, a variety of low RDS(ON) FETs with large thermal package options are available from current sense resistor vendors such as Bourns, Ohmite, Susumu, Vishay and others.

Pushing for low power loss and low voltage drop across these resistors then requires excellent op amps to gain and properly condition the signal for use. Additionally, to meet efficiency and thermal requirements with this additional series resistance, designers must also use a lower RDS(ON) and more expensive components to reduce the other power loss in the power path to compensate (e.g. using even lower RDS(ON) power MOSFETs).

What started as a simple measurement now requires several additional components that must be carefully selected, designed, and powered correctly, all while consuming PCB space and cost.

Current transformers

Another common method of current measurement that simplifies the system when isolation is required, or with high-power, high-efficiency systems, is to use a current transformer (CT). The gain can be obtained with the transformation ratio of the transformer, the insulation is integrated and the bidirectional current can be measured. Additionally, no bias power is required for the CT whether used in high side (floating reference) or low side (ground referenced) configuration.

The disadvantages, however, are that direct current cannot be measured; duty cycle limitations can prevent transformer saturation; and these CTs are also usually large components – most often toroids like those in Figure 1, which may limit their adoption in high-density systems. LEM, Renco, Wurth and many other magnetic product suppliers offer dedicated products focused on this approach.

Hall effect sensors

For even higher power applications, or where DC information is required, designers often look for sensing methods that minimize lossy passing elements added to the system (such as shunt resistors or CT primary windings ), and instead rely on measuring the field effects of the system.

Considered “contactless”, “lossless” sensors, methods such as Hall effect current sensors (Fig.2) work on the principle that for a given conductor (such as a copper trace on a printed circuit board) through which a current passes, a proportional magnetic field is created around the current-carrying conductor. By measuring such a magnetic field, one can obtain information about the value of the current which produced it. The sensing element often runs the PCB copper through the sensing element housing, and some others place the sensor above the copper trace and sense by proximity only.

Hundreds and thousands of amperes can be measured with little loss because resistance and inductance are not added to the system. However, the method becomes inaccurate at low current, can be made inaccurate by external fields or mechanical steering, and often requires a zero current offset and a hub to amplify the signal for a narrow range of current of interest. In addition, a dedicated bias power supply is required for such a measurement integrated circuit. Allegro, Koshin, Melexis and many other vendors offer products that focus on this approach.

Anisotropic magnetoresistive sensing (AMR)

The AMR IC is capable of detecting both AC and DC signals and offers a high bandwidth solution up to 1.5 MHz, with an isolated output. It works by flowing current through the device through a low resistance “U-bend” in the lead frame, where it generates a forward or reverse magnetic field which is detected by two differential current sensors.

This approach is attractive in that it is quite compact, resistant to external fields and noise, and has low offset error while responding very quickly (

The Future: Introducing “GaNSense” Technology

With the continuous search for higher efficiency, GaN-based power converters have become more popular in the market. Additionally, with high levels of integration used in monolithic GaN power ICs, power converters could be pushed to higher frequency with a minimal number of external components, allowing much smaller systems.

Breakthrough products such as the latest fast mobile phone chargers have been able to achieve power densities even 3 times higher than previous best-in-class solutions. But efforts to extract the full capabilities of GaN continued, and in late 2021 GaNSense technology offering deeper integration and expanded functionality was launched.

In its simplest form, GaNSense is a lossless current sensing circuit, eliminating the shunt current sense resistor and its associated headaches. (Fig.3). It improves overall system efficiency with lower total series resistance, while increasing robustness with fast 30-100 ns internal short-circuit protection.

This approach is “localized” in that it is detected and applied locally to current and power items of interest. The localized control and response time allow this method to be fast and efficient, while minimizing corruption of control signals by system noise, long traces, etc., compared to other methods. Therefore, it becomes possible to use this method not only for overcurrent safety and short-circuit protection, but also for cycle-by-cycle current limiting and current-mode control and regulation.

Such a self-contained power supply may indeed represent the future of power stage sensing and protection, enabling greater reliability, pushing performance boundaries, and freeing up the main system controller to focus. on more complex control algorithms and responsibilities. This “lossless” current sensing in the current sense (CS) block is implemented via a popular parallel current mirror design technique, and optimized for use with high-speed GaN FETs.

As seen in Figure 4, in block CS, the main power FET device is connected to the common drain and gate connections. For the sake of simplicity, we will also show that the source is connected in common. Using well-matched devices and a high-resistance sensing FET (maybe >1000-1500X higher RDS(ON) than the main power FET), a small portion of the load current branches to the sense FET and can be accurately measured by various techniques. RDS(ON) and tempered effects are also canceled naturally.

Due to the close matching, the current is based only on the ratio of the resistances of the devices. As FET sense has much higher RDS(ON) than the main power FET, the loss of this approach is negligible, especially compared to alternatives such as shunt current sense resistors in the main power path. The current limit is always adjustable and programmable, with the Current Sense (CS) pin and the programming resistor.

All of these benefits translate into system improvements. For example, as seen in Figure 5designers leveraged GaNSense technology to create a 120W AC/DC cell phone charger that was 70% smaller, 65% lighter, and nearly 6% more efficient than the previous production solution.

Summary

In motor drives and power supplies, voltage and current will continue to be signals that must be monitored and measured for accurate and reliable real-time control, protection, and data logging. With a variety of applications and requirements, multiple current sensing approaches help achieve design goals in the most cost-effective and size-efficient way.

A technology and approach has been discussed here with the advent of near-ideal, lossless “GaNSense” current sensing, fully integrated into the power device, which is fast, accurate, and can serve the critical functions of control and protection with zero size and effectiveness. impact on costs. Although not an exhaustive review, some of the most common methods used in the industry today have been discussed and summarized in the table above.

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