Power supply accepts wide input-voltage range
A SEPIC-based power supply runs from 1.5V to 20V.
Jim Windgassen, Northrop Grumman Undersea Systems, Annapolis, MD; Edited by Martin Rowe and Fran Granville -- EDN, February 17, 2011
The switching power supply in
Figure 1 produces 3.3V dc from
an input voltage of 2.5 to 20V dc with
high efficiency. The circuit operates at
an input voltage as low as 1.5V once it
starts from a minimum of 2.5V dc, allowing
the switcher to fully discharge a
pair of alkaline cell batteries nearing end
of life. The power supply can also run efficiently
off higher input voltages, such
as 12V automotive power. The heart of
the circuit is a SEPIC (single-ended-primary-inductance-converter)-based
switching power supply, which provides
an output voltage greater than or less
than the input voltage (Reference 1).
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The bootstrap circuit allows the converter
to run from very low input voltages
by maintaining the input voltage to
IC2, and it increases efficiency at high
input voltages by eliminating the use
of IC2’s internal linear voltage regulator.
Figure 2 shows the efficiency of the
prototype power supply at both 50- and
500-mA loads. The power supply’s efficiency
is consistent over a range of operating
voltages because of the bootstrapping
circuit.Because the circuit uses a low-threshold-voltage MOSFET, the switch, keeping the gate drive voltage low, reduces the total charge that must go into and out of the MOSFET gate, further improving efficiency. SEPIC controller IC2 normally uses its internal low-dropout capability to generate an operating voltage of 5V from the input. Running IC2 from the bootstrapped output reduces IC2’s operating voltage to approximately 3V, which also limits the drive voltage to Q1’s gate.
the coupled inductor
must have low-resistance windings, and the high current tracks should use
wide copper pours to minimize resistance losses and unwanted inductance.A prototype of the power supply
measures 23×15×3.5 mm (Figure 3).
It uses a custom coupled inductor, but
you can choose from many off-the-shelf
coupled inductors available from
BH Electronics, Coilcraft, and Würth Elektronik. You can download the Linear
Technology LTSpice code for this circuit here.
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Reference |
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Talkback
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The things which make this circuit a little bit interesting are:
1)The use of bootstraping and the way that I did it using the linear regulator. If I had simply bootstrapped the design using two diodes to get the low voltage operation, then the high input voltage eficiency would have suffered. By using the LT3008 regulator instead of a diode, it forces the LTC1871 to run off of it's own output all of the time instead of dropping a high input voltage thru an internal LDO.
2) The switching MOSFET I chose took me awhile to select as it needed to fufill a lot of different requirements. That part is key to the performance of the design.
3) In my application, I did not care about parts cost; physical size and performance over a wide input voltage range were the only things I cared about. There are quite a number of things that could be done to reduce the cost of this circuit at the expense of physical size while still maintaining the performance.
Jim Windgassen - 2011-25-2 11:40:43 PST -
How is this a design idea? It is a standard circuit in the datasheet. How expensive is this solution?
James - 2011-23-2 18:22:09 PST
