Nailing accurate and lossless current sensing in high-current converters

Other Parts Discussed in Post: LM27403

It’s clear that high efficiency and small size are key benchmarks for DC/DC converter solutions. As a systems engineer, I’m keenly aware that higher efficiency is the blueprint for reduced power loss, lower component temperatures, as well as more useable power for a given airflow and ambient temperature environment. However, compressing the solution into a small PCB form factor is another challenge.

As more demanding efficiency and power density targets come to fruition, it’s highly valuable to use parasitic circuit resistance(s) in the power train for lossless current sensing. In that sense, inductor DCR current sensing helps meet performance goals related to overcurrent protection, multi-phase current sharing, current-mode control, or load current telemetry.

To put this in perspective, take a look below at the two main regulator topologies, buck and boost, using the inductor DCR for continuous current sensing. By closely emulating the inductor admittance with a low-pass RC sense network in parallel with the inductor, a voltage image proportional to the inductor current is derived. The inductor DCR is normally specified at ±8% tolerance and sometimes even lower, leading to acceptable sensing accuracy.

Inductor DCR current sensing in (a) buck, and (b) boost synchronous regulators

 Inductor DCR current sensing in (a) buck, and (b) boost synchronous regulators

Forgoing shunts and current transformers, but instead leveraging parasitic circuit elements for lossless current sensing, is advantageous in terms of power density and cost. In a buck or boost converter, merely sensing the voltage across the low-side MOSFET’s RDS(ON) is quite inaccurate. For example, the RDS(ON) at 25°C is not tightly specified and depends on applied gate voltage. As well, sensing is only available when the low-side MOSFET is conducting. No sensing occurs during the high-side MOSFET on-time. Inductor DCR current sensing, on the other hand, offers continuous sensing.

Of course, the circuit for inductor DCR sensing must accommodate the DCR’s known variation with temperature. The temperature coefficient of resistance (TCR) of copper is 3930ppm/°C, implying a 20% variation for a 50°C temperature swing.

This problem is conveniently solved using a low-cost BJT connected as a thermal diode and located remotely at the inductor. With that in mind, I recently wrote an article that delves into this topic in more depth.

"Tim Hegarty’s article on “Inductor DCR Current Sensing With Temperature Compensation: An Accurate, Lossless Approach For POL Regulators” offers engineers an easy to understand, practical solution for implementing accurate and lossless current sensing in their point-of-load regulator applications—a subject that will be of interest to the large number of engineers who must implement power management at the board level," said David Morrison, Editor of the How2Power Today newsletter. Tim explains the underlying concepts necessary to understand inductor DCR current sensing techniques, simply and clearly. After explaining  the drawbacks of existing methods, he then presents a practical solution complete with experimental results that demonstrate for readers that the solution is ready for use in the real world."

Below is an LM27403 buck converter that uses the inductor DCR current sensing with a remote BJT for thermal compensation. The DVBE of the BJT is directly proportional to temperature in Kelvin. Also available is a system-level thermal shutdown feature with programmable setpoint for increased reliability. Watch a video demonstration of the LM27403 regulator solution here.

LM27403 buck regulator with temperature-compensated inductor DCR current sensing

 LM27403 buck regulator with temperature-compensated inductor DCR current sensing

What other factors affect current sensing performance in buck or boost converters? Feel free to leave your comments on other issues you face while designing high-current power solutions.