A number of situations could benefit
from the high efficiency of a switched
power supply, save for the supply’s
intrinsically high noise level. For
example, an optical communications
application using a large number of laser
diodes could employ a switched power
supply to avoid the need for heat-removal
techniques.
However, the laser diodes require a
noiseless environment. A hasty decision
to use a switcher would not be rewarding
unless much attention is paid to noise
reduction. Furthermore, RFI and EMI
could cause problems during compliance
testing.
The technique illustrated in the figure
uses an ultra-low-noise regulator, a
type of switching IC designed to reduce
conducted and radiated electromagnetic
interference.1 It also employs a relatively
new type of adjustable resistor called a
Rejustor, which can complement the
ultra-low-noise switcher.
The regulator, in this case an LT1533,
reduces noise and EMI by allowing users
to control the output voltage and current
slew rates and to program the device
to optimize the switcher’s harmonic
content versus efficiency. The result is
as much as a 40-dB reduction in high-frequency
harmonic power with only minor
losses in efficiency. The Rejustor is nonvolatile,
requiring no power to hold its
adjustment, and can be readjusted many
times bidirectionally to very high precision
using only electrical signals.2
The adjustments, which can be made
after assembly, consider not only natural
component variations (i.e., inductance
and capacitance) but
also board specifics like layout and type
of load. For simplicity, the components
and pins of the adjustment circuitry
(dashed box) that aren’t essential for this
discussion are omitted.
R2 and R4 define the output voltage of
the switch regulator U1. The output voltage
is fixed. When the load is connected,
the current through R1 being converted
into voltage by instrumentation amplifier
G1 is the measure of the regulator’s
efficiency. R1 may be an integral part (a
sense resistor) of an electronic circuit
breaker, hot-swap controller, or any
current-monitoring circuitry.
G1’s output, which represents the
controller’s efficiency, is conveyed to the
Microbridge Rejustor Calibration Tool
(RCT).3 U2, an LTC1968 rms-to-dc
converter, measures the regulator’s
noise value. The converter
acquires the regulator noise at In1
and passes the noise value to the
RCT through buffer A1, which
acts as an interface between the
converter and the RCT.
Properly selecting the differential
gain (G1), as well as R5 and R6, ensures
that the appropriately weighted coefficients
can be applied to measure either
regulator efficiency or noise. These
coefficients help to optimize the RCT
performance. A PC-driven calibration
procedure sets the dual Rejustor’s resistor
values, which adjust the feedback
to the regulator. (Linear Technology
advises that the resistor values be equal
for most cases.) The two control loops
are combined internally to provide a
smooth transition from current slew
control to voltage slew control. The
user must choose the correct balance
between efficiency and noise.
See associated figure
REFERENCES:
1.
http://www.linear.com/pc/
2.
http://www.mbridgetech.com/pdfs/microbridge_032307.pdf
3.
http://www.mbridgetech.com