Circuit provides universal ac-input-voltage adapter
Use a TRIAC and optocoupler to automatically change between doubler and bridge configurations.
JB Castro-Miguens, Cesinel, Madrid, Spain, and C Castro-Miguens, University of Vigo, Vigo, Spain; Edited by Paul Rako and Fran Granville -- EDN, August 25, 2011
The input-rectifier stage of an
offline power supply converts an
ac-line voltage to a dc voltage, which
powers a dc/dc converter. If you configure
the rectifier section as a full-wave
bridge for a universal mains input of 92
to 276V rms and 47 to 63 Hz, you must
design the dc/dc converter to operate
over a wide range of input voltages. This
approach increases the cost of components
and reduces the efficiency of the
dc/dc converter.
For 115V-rms operation, if you configure
the rectifier stage as a simple
bridge, the output-voltage ripple ranges
from 113 to 192V, given a ±20% tolerance
on the ac input voltage (Table
1). Note that the voltage ripple corresponds
to an effective output capacitance
of 115 μF, the series equivalent of
two 330-μF capacitors, and to a load of
250W. However, if you configure the
rectifier stage as a voltage doubler for
115V-rms operation, the output voltage
ranges from 200 to 372V—much closer
to the 215 to 387V voltage ripple for
230V-rms operation.
This Design Idea proposes a rectifier
stage that automatically configures
itself as a voltage doubler for 115V-rms±20% line operation. Figures 1
and 2 show the proposed rectifier stage.
The STMicroelectronics
snubberless BTB16-600BW TRIAC
(triode alternating current) is suitable
for a 250W load (Figure 1).
For 115V-rms±20% line operation,
the Reference terminal of the TL431
programmable shunt regulator has a
voltage lower than its 2.5V internal
reference (Figure 2). In this situation,
the MOSFET is on, and the IL4216 optocoupler continuously fires the
TRIAC.For 230V-rms±20% line operation, the Reference terminal has a voltage higher than 2.5V, and the MOSFET and the TRIAC are off. C1 is necessary to prevent the rectifier from starting in doubler mode when you turn on the supply with a low rectified line voltage.
This approach prevents the rectifier
stage from changing from doubler mode
to bridge mode during start-up, which
would create a voltage drop in the bulk
output capacitors that would differ
from the voltage in steady-state operation.
This difference would cause one
of the capacitors to have an abnormally high voltage and the other to have an
abnormally low voltage.A practical value of C1 is 1 μF, which introduces an approximately 8-msec time delay for starting in doubler mode. You can use a 15V zener diode to clamp the gate voltage of the MOSFET.
Talkback
-
Power factor requirements prevents the use of this circuit today.
Bruce B - 2011-6-9 13:30:47 PDT -
Seems like a lot of parts for a simple function. Why not just a relay, with its 240vac coil across the AC line, and its normally closed contact in place of the triac? At 120vac, the relay won't pull in, its contact is closed, and you have a doubler. At 240vac, the relay pulls in and you have a bridge.
Lee Hart - 2011-2-9 10:08:38 PDT -
The effective capacitance will be 165µF, not 115µF.
Nortel "Universal" power supplies for the MSS 7K product line used the same basic circuit, but with a relay instead of a triac, and it was for the same reason of DC-DC converter efficiency.
Jacques Fortin - 2011-29-8 11:40:31 PDT
