Designing an output choke for a switch-mode power supply that must operate over large load variations can be problematic. If you design the inductor for the maximum load the power converter will see, the inductance will be below the critical inductance required at light load. This inadequate inductance will cause the voltage on the output capacitor to peak up, resulting in increased ripple on the output.
If, on the other hand, you design for the much larger value of inductance required at light load, your inductor will be overspecified for the nominal load and will be too large physically.
One solution is to add a bleeder resistor on the output of the supply to keep a minimum amount of current flowing at all times. This "fix" is not very efficient and often is an unacceptable solution, especially for battery-powered circuits.
A better solution is to use a swinging choke, which exhibits a large inductance at light load and a progressively smaller inductance as the load increases. The size of the swinging choke is considerably smaller than an overspecified conventional choke. And a swinging choke is more efficient than a bleeder resistor.
A new series of gapped ferrite toroids from Ferrite Specialties Inc (Conshohocken, PA) makes swinging-choke-design easy. You wind together a gapped ferrite toroid and an ungapped ferrite having the same form factor. The ungapped ferrite provides high inductance at light load but saturates at less than full load current. The gapped ferrite core provides much lower inductance at light load but doesn't saturate, providing a working inductance at full load. The result of the series combination of the two inductors is a single compact structure that doesn't require a coil bobbin and is easy to construct and mount. Fig 1a offers a schematic representation of the swinging choke; Fig 1b shows the mechanical construction.
As a design example, assume you want a 12.5-µH choke that operates at a nominal load current of 2A. This specification requires 20 turns of #18 AWG wire on a gapped Ferrite Specialties PGTX221406A36 core. This core has a gap lG0.0138 cm, a cross-sectional area AE0.259 cm, and an AL36 mH/1000T. At the nominal operating current of 2A, this core sees a flux density of
At this value for flux density, the core is operating far from saturation at nominal current and has plenty of design margin.
To design the high-inductance portion of the choke for the lightly loaded condition, use a standard ungapped ferrite core, a Philips 846T250-3C85. This core has an AL1220, µ=2000, and a magnetic path length lC5.42 cm. For the same number of turns, N, the inductance of this core is
This core fully saturates at approximately B=5 kG. The current at this flux density is
The total inductance of the core is the series combination of the gapped and ungapped inductors. At low current, the ungapped core dominates, producing a very large inductance. At 0.54A, however, the ungapped core saturates, reducing its inductance (essentially) to zero. The ungapped core then dominates, producing a working inductance at the full load current of 2A. The ZIP file attached to EDN BBS /DI_SIG #1692 contains a copy of this write-up. (DI #1692)
