Every charge profile is only as good as the power stage that delivers its current, yet the analog power chain is the part of a charger most often treated as a detail. The topology determines whether a NiMH charger runs cool at 90 percent efficiency or dissipates its own weight in heat, whether it holds a precise constant current for valid -delta-V detection, and whether it can step a wide range of input supplies down to a multi-cell pack. This paper compares linear and switching regulators, develops the buck converter as the standard NiMH charger stage, explains why current-mode control is the natural match for a current-controlled chemistry, and maps the efficiency-heat-size-EMI trade-offs across charger classes from single-AA USB chargers to multi-cell pack units.
A linear regulator drops the difference between input and battery voltage across a pass transistor, delivering a clean, ripple-free current with almost no electromagnetic interference and a tiny component count; for a single cell from a modest supply this is cheap and effective, and its absence of current ripple makes -delta-V measurement especially clean (Paper 6). It is the classic choice in low-cost, low-current NiMH chargers.
Its weakness is power dissipation equal to current times voltage difference: charging a multi-cell pack from a high-voltage adapter, or fast-charging at 1C, turns the linear stage into a heater that needs bulky copper or a heatsink, lowers overall efficiency and adds heat to the very cells being charged - self-defeating in a thermally sensitive fast charger.
A buck converter switches an inductor to step voltage down with high efficiency - commonly 85 to 95 percent - so little power is wasted as heat, enabling compact fast chargers and packs across a wide input range; a synchronous-buck replaces the freewheel diode with a second MOSFET to gain a few more efficiency points, worthwhile at high charge currents. Switching is the standard topology wherever current, cell count or enclosure heat makes linear dissipation untenable.
The cost is design complexity: inductor and capacitor selection, loop compensation, MOSFET gate drive, and conducted/radiated EMI that must be filtered and laid out carefully; switching ripple also appears as current and voltage noise that the sensing chain must reject, a direct consequence the termination firmware has to accommodate.
NiMH requires a controlled current with voltage only sensed for termination (Paper 15); current-mode control regulates peak or average inductor current directly against the state machine's command, yielding an inherent current limit, fast response to input and load steps, and a stable, well-defined charge current through fast, top-off and trickle levels. The inner current loop simplifies delivering the exact current the profile assumes - a prerequisite for interpretable -delta-V and dT/dt.
The controller (a dedicated charge IC or MCU plus gate driver) commands the current setpoint; the power stage enforces it cycle by cycle. Multi-stage and pulse profiles (Papers 2, 13) become changes of setpoint and duty, and pulse maintenance a low-average-current modulation the buck produces efficiently - capabilities a linear stage provides only by dissipating.
Topology choice follows the input-to-battery voltage relationship: a buck fits when the adapter always exceeds the maximum pack voltage; wide or overlapping ranges may need buck-boost; USB inputs impose current-limit and enumeration constraints that interact with the charge current. Inductor saturation must exceed the fast-charge current with margin, input/output capacitance must control ripple that disturbs sensing, and thermal design of the power stage is coordinated with the cell thermal management of Paper 28 so converter heat is not mistaken for cell temperature.
At multi-cell scale, efficiency directly sets enclosure temperature: a 30 W charger at 85 percent dumps 5 W of heat internally, while 93 percent dumps nearer 2 W - often the difference between needing forced airflow and not.
Switching noise and battery sensing share a board, so layout places the power loop compactly, uses Kelvin-style sensing at the cell rather than across connector drops, and synchronises voltage sampling away from switch edges (or filters to below the ripple frequency); this co-design is what lets a switching charger achieve termination robustness comparable to a linear one. The first figure contrasts linear and buck power flow and dissipation; the second maps topology choice across current and cell-count requirements.
Failure modes - inductor saturation, thermal shutdown of the switcher, input dropout under solar/USB variation - must degrade the charge profile safely (hold or reduce current) rather than corrupt termination.
Dedicated charge-management ICs (next paper) integrate the switcher control with termination, but the external power-stage design remains the designer's responsibility and sets achievable efficiency and noise. Weijiang supplies the cell-side voltage/current characteristics and current-ripple sensitivity that help tune the power stage and sensing for clean termination. The next paper opens the charge-management IC itself to show how the control and termination architecture is integrated in silicon.
Weijiang Power designs and manufactures nickel-metal hydride cells, matched packs and charging-ready configurations for consumer, industrial, medical and mobility customers, and supports partners with charge-protocol guidance, IEC 61951-2 performance files, IEC 62133-1 safety evidence and charger co-validation. Share your cell format, charge rate, thermal envelope and cycle target and our engineers will specify a cell-and-charge combination that protects both runtime and service life. Review the range on the products page.
