Current Sensing Techniques in Power Electronics

Understand current information reveals fault monitoring and protection needs, while current sensing enables closed-loop control, paralleling power supplies, and motor speed-torque management through peak, valley, and average current control.

Examine the subtle difference between current sensing and current measurement, highlighting internal sensing mechanisms like hall sensors or shunt resistors versus external instruments such as probes and oscilloscopes.

Compare voltage sensing and current sensing: voltage sensing is passive and non-loading with a resistor divider, while current sensing is intrusive, sometimes using magnetic-field techniques.

Explore intrusive and non-intrusive current sensing in power electronics. Examine sensor resistor methods, Kelvin shunts, inductor sensing, time constant sensing, mosfet on-state resistance sensing, Rogowski coils, Hall and fluxgate sensors.

Explore how a low-value sense resistor converts current to voltage, enabling accurate, linear current sensing for alternating and direct current applications, with amplification to drive ADCs.

Select sense resistors by balancing value, inductance, tolerance, and tcr with power dissipation for dc, rms, and peak currents, and apply derating for high ambient temperatures.

Design a peak current mode boost PFC sensory system by selecting a current sense resistor to meet 1.3 V to 1.8 V thresholds with margin.

Explore Kelvin sense or four-terminal sensing to eliminate contact and probe resistances and TCR variations in sense resistors, improving measurement accuracy for high-current applications using current injection and voltage sense.

Explore how a PCB copper trace acts as a sense resistor, with resistance set by length, width, and thickness, copper resistivity, and temperature coefficient, highlighting trade-offs.

Understand legacy lead-frame sense resistors inside ICs, where the lead-frame resistor, gain amplifier, and level shifting provide a sensed output voltage with kelvin connections, now largely obsolete.

The on-state resistance of the mosfet serves as a lossless current sense resistor, enabling cycle-by-cycle and peak current control, fault detection, and short-circuit protection without extra components.

Learn how the inductor's dcr acts as a current sense resistor in low-voltage converters, using a coupled secondary to cancel inductive voltage and a current mirror to convert the signal.

Learn how sense-FET devices use a sense pin to mirror current with a 3% accuracy, aided by Kelvin sensing, and compare resistor-based current-to-voltage conversion with a virtual-ground approach.

Introduce indirect current sensing methods and the role of magnetics, exploring energy transfer devices like transformers and energy storage devices like inductors within basic magnetic circuits.

Explore magnetic circuit jargons and how magnetomotive force, reluctance, and magnetic field intensity govern flux density in high-permeability cores, with basics of saturation and b-h behavior for current sensing.

Apply Ampere's law to relate coil current and magnetomotive force to the magnetic field intensity. Show that electromotive force equals N times I, with a toroidal core as an example.

Faraday's law links the voltage induced across a coil with n turns to the rate of change of flux in the magnetic core, with emf e = n dphi/dt.

Explore the analogies between magnetic and electrical domains by equating mmf to emf, flux to current, and reluctance to resistance, and derive the magnetic–electrical relationship using Ampere's and Faraday's laws.

Learn how volt-second balance prevents core saturation and flux walking by keeping the average emf zero in magnetic circuits, ensuring steady-state operation of inductors and transformers.

Understand inductance and its energy storage in power electronics, and how flux equals mmf divided by reluctance. Relate these magnetic concepts to current sensing techniques.

Explore the area-product design method for inductors and transformers, linking core cross sectional area and window area to store energy and relate peak and rms current.

Explore how transformers transfer energy between primary and secondary windings, enabling voltage and current sensing, including turns ratio concepts and potential and current transformer applications.

Understand how a current transformer senses pulsating currents in power electronics, not energy transfer. The secondary output voltage equals R_cs times the secondary current in a one-turn primary, multi-turn setup.

Explore the equivalent model of current transformers, including magnetizing inductance, magnetizing current, parasitic capacitance, and leakage inductance, and how they affect current sensing.

Current transformer sensing isolates primary from secondary circuits, boosting safety. It provides secondary signal levels with bandwidth up to tens of megahertz, yet magnetizing current and design complexity pose challenges.

Learn a step-by-step current transformer design procedure, including turns ratio, magnetizing inductance, core saturation under worst-case conditions, wire sizing, and volt-second balance.

Design a current transformer for a pwm controller by selecting turns ratio and a 0.8 V secondary, ensuring low losses at 200 kHz duty cycles.

Discover how DC current transformers sense DC current using two transformers in phase opposition with dot polarities and mutual coupling, including on-state and off-state currents via oring diodes.

Explore Rogowski coil current sensing, deriving the secondary voltage from the rate of change of primary current with a non-magnetic core, and using an op-amp integrator.

Explore Rogowski coil current sensing, which has no magnetic core and extreme bandwidth for large pulsed currents, while noting placement sensitivity and DC limitation.

Explore the hall effect for current sensing with hall sensors. See how external magnetic fields induce a perpendicular hall voltage that correlates to the primary current.

Explore linear hall sensors, providing a continuous analog output proportional to the magnetic field. Compare PWM hall sensors, which output a PWM signal for BLDC and PMSM applications using interfaces.

Explore the advantages and disadvantages of hall sensors, including galvanic isolation, compact IC integration, and linear output, along with requirements for a supply, flux concentrators, and limited bandwidth.

Learn how flux concentrators boost Hall sensor current measurements by concentrating magnetic flux in a highly permeable core, enabling a closed-loop, low-reluctance path and improved accuracy for various sensor configurations.

Use zero-flux, closed-loop operation with a Hall sensor and a high-turn secondary winding to oppose the primary flux, reducing temperature and part to part variations while boosting bandwidth.

Explore the Fluxgate sensor's closed-loop current sensing, core saturation, and zero-flux operation. Learn how duty cycle and harmonic analysis detect asymmetry, and review topologies for bench-level power measurements.

Explore fluxgate sensing advantages such as closed-loop operation, wide bandwidth, and robust noise immunity, alongside disadvantages like high cost and slower response, for measuring complex AC and DC currents.

Use time constant based current sensing to measure inductor current without a sense resistor by matching L/DCR to RC; capacitor voltage equals I times DCR, enabling direct sensing.