Use a boost regulator beyond its rated voltage
Use an external FET in a common-gate cascode configuration.
Kevin Tompsett, Analog Devices, Fort Collins, CO; Edited by Paul Rako and Fran Granville -- EDN, January 5, 2012
Inexpensive boost regulators with on-chip FET power switches work well for low-voltage boost converters, SEPICs (single-ended primary-inductance converters), and flyback converters. For higher voltages, designers typically use a more expensive approach comprising a controller with an external FET or a high-voltage boost regulator.
The internal FET then turns on, the
VX node (Figure 3) is driven to ground,
and the gate of the high-voltage FET
remains constant, turning on the high-voltage
FET. The lower FET acts like a
low-resistance gate driver, and the FET
quickly turns on, resulting in low turn-on
loss. When the internal FET turns
off, the inductor current pulls up the SW
node until the external FET turns off.
The highest voltage the internal FET
will see is the gate voltage minus the
threshold voltage of the external FET.
The turn-off transition is slower because
it is proportional to the peak current in
the inductor, but, with a correctly sized
FET, there is generally plenty of drive
current for a fast transition and low loss,
even at low load. The entire BOM cost
is less than $2 (1000).
You can design and simulate this
circuit using the ADIsimPower design
tool, which is available at www.analog.com/adisimpower. The tool
lets you design boost, SEPIC, and
SEPIC-Cuk converters using this
technique and allows input voltages
of 1.8 to 90V and output voltages of
1.2 to 90V. Figure 4 shows good agreement
between the ADIsimPower tool
and the measured results, in terms of
efficiency.
Talkback
-
Suggested a Cascode MOSFET solution in reallity reduces efficiency due to up to 0.3 Ohm resistance of lower MOSFET used as switch.
The design uses an external MOSFET and does not look it will be cost effective compare with controller without power transistors.
Vladimir Doubovis - 2012-13-1 14:04:54 PST
