Balancing inductor and transformer: the magnetic energy-transfer element in active cell balancers

01

Forward transformer between one cell and a 14-cell stack: 3.5:1 turns, at least 190 µH primary, leakage at or below 1%

In TI's switch-matrix scheme the EMB1499Q controller drives a bidirectional forward converter that exchanges energy between a single cell and the stack it belongs to, at a fixed 250 kHz and a stack voltage of up to 60 V; the EMB1428 switch-matrix driver routes the inductor current to the cell being charged or discharged, up to 7 cells per device. [1] [2] The datasheet ties the transformer to the cell count: the turns ratio follows n = 0.25 × number of cells, giving 3.5:1 for a 14-cell stack, and the minimum primary inductance is 190 µH for 14 cells, scaled to 95 µH for 7. [1]

TI recommends leakage inductance of no more than 1% of the winding inductance, and for a 5 A design a primary DC winding resistance below a typical 60 mΩ and a secondary below a typical 12 mΩ, scaled for constant DC loss at other balancing currents. [1] The primary and secondary sense resistors are chosen with a ratio close to the turns ratio; the datasheet example pairs a 10 mΩ secondary resistor with about 35 mΩ on the primary for 3.5:1. [1]

On the cell side the converter's output inductor carries the balancing current itself. TI suggests an inductance that gives a peak-to-peak ripple of 20% to 40% of the steady-state balancing current, an inductor that does not saturate at peak current, and minimal DC winding resistance. [1] Each balancing burst runs at constant current for less than 8 s before the controller ramps the inductor current down, which takes about 300 µs. [1]

TI's design rules ask for primary MOSFETs and the clamp capacitor rated to at least 2.5 times the maximum stack voltage. [1] Editorial note: state the insulation required between the transformer windings against the stack voltage, not the cell voltage.

02

A 1:1 coupled flyback transformer of 3.4 µH, rated 10 A peak, for six-cell bidirectional balancers

Coilcraft's MA5421-AL was developed for Linear Technology's LTC3300-1 bidirectional cell balancer. It is a ferrite-core, two-winding part with a 1:1 turns ratio, 3.4 µH ±15% primary inductance at 0 A and at least 2.8 µH at its 10 A peak current, which Coilcraft defines as the peak primary current drawn at minimum input voltage. [3]

Leakage inductance is at most 0.15 µH, measured on the paralleled primary windings with the secondary shorted, and the DC resistance is 0.009 Ω on the primary and 0.011 Ω on the secondary. [3] Editorial calculation: 0.15 µH leakage against 3.4 µH nominal inductance is about 4.4%, so the 1% limit TI sets for its forward transformer is not a rule to carry over to flyback parts; state the limit for the design in hand.

03

One shared 33 µH inductor with a 1 µH / 1 µF resonant branch for direct cell-to-cell transfer

A TI patent describes a single-inductor approach in which each cell connects through a pair of back-to-back MOSFETs to one larger shared inductor, and a small series inductor, a capacitor and an extra switch form a resonant branch that reverses the current direction in the main inductor. In the embodiment given, the main inductor is 33 µH, the resonant inductor 1 µH and the capacitor 1 µF. [4]

Because energy goes from cell to cell without a transformer as a buffer, the patent states balancing efficiency of up to 85% or more, and it describes the arrangement as a low-cost option using one switch pair per channel and one larger inductor per group of cells. [4] The patent adds that the values it lists may be exact or approximate and relate to specific implementations. [4]

A multi-inductor scheme modelled in Scientific Reports gives a sense of how far model values can sit from the parts above: the authors used three 10 mH, 10 Ω inductors at 10 kHz and 50% duty on a pack of four 12.8 V, 40 Ah cells, with a maximum balancing current of 13.76 A, and tested it in MATLAB/Simulink and on an OPAL-RT 5700 hardware-in-the-loop simulator rather than with wound parts. [5] Its single-layer version balanced in 108 s in simulation and 110 s in HIL testing. [5]

04

Data for a balancing inductor or transformer enquiry

Paste it into your enquiry below or into an email to a maker, then replace every bracket.

Balancer: [controller IC or reference design; topology: forward, flyback, shared inductor]
Stack: [number of series cells; maximum stack voltage; cell chemistry and voltage window]
Current: [balancing current; peak current; RMS current; burst duration]
Magnetics: [turns ratio; primary inductance at 0 A and at peak current; leakage limit; DCR per winding]
Insulation: [isolation voltage between windings; creepage]
Environment: [ambient range; part temperature limit; mounting; weight]

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