DN175 - Off-Line Low Noise Power Supply Does Not Require Filtering to Meet FCC Emission Requirements

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Off-Line Low Noise Power Supply Does Not Require Filtering
to Meet FCC Emission Requirements – Design Note 175
Jim Williams
Introduction
Off-line power supplies require input filtering components to meet FCC emission requirements. Additionally, board layout is usually quite critical, requiring
considerable experimentation even for experienced
off-line supply designers. These considerations derive
from the wideband harmonic energy generated by the
fast switching of traditional off-line supplies. A new
device, the LT®1533 low noise switching regulator,
eliminates these issues by continuous, closed-loop
control of voltage and current switching times.1 Additionally, the device’s push-pull output drive eliminates the flyback interval of conventional approaches.
This further reduces harmonics and smooths input
1 In depth coverage of this device, its use and performance verification appears
in LTC Application Note 70, “A Monolithic Switching Regulator with 100μV Output
Noise,” by Jim Williams.
L, LT, LTC, LTM, Linear Technology and the Linear logo are registered trademarks
of Linear Technology Corporation. All other trademarks are the property of their
respective owners.
T1
!!
GER
!!
DAN LTAGE
O
V
H
HIG
0.1μF
AC LINE
+
HV
100μF
400V
1N4005
1.6k
1W
0.001μF
250V
0.001μF
250V
1.6k
1W
HV
HV
+
10μF
5k
0.5W
360k
Q1
MPSA42
Q2
12V
D
Q5
Q6
IRF840 IRF840
S 1N5818 1N5818 S
Q3
0.002μF
12V
Q4
1k
Q7
12V
Q8
COL
COL
470Ω
VIN
VC
LT1533
4V
330Ω
SHDN
+
ISLEW
10M
VSLEW
RCK
CCK
4N28
+
E
1μF
HV
1μF
22k
75k
3.9k 3300pF
15k
L2
22nH
+
15μF
12V
12V
0.48V
470Ω
+
C2
1/2 LM393
–
SCREENED AREA CONTAINS LETHAL HIGH VOLTAGES!
USE CAUTION IN CONSTRUCTION AND TESTING!
0.15μF
BAT-85
470k
+
240k
43k
1μF
0.8Ω
4V
LT1431
COL RTOP
C1
1/2 LM393
+
1V
L1 = COILTRONICS UP-4
L2 = COILCRAFT B07T
NPN = 2N3904 UNLESS OTHERWISE NOTED
PNP = 2N3906
T1 = COILTRONICS CTX02-13978-X3
10k
0.002μF
1V
FB
12V
+
510Ω
D
1k
10k
1N759A
12V
3k
360k
12V
1k
4.7μF 8.2k
+
220μF
–
100k
5V
2A
L1
10μH
RMID
REF
0.48V
2.5V
POWER LIMIT
CURRENT LIMIT
= 20CJQ045(I.R.) UNLESS OTHERWISE NOTED
= AC(HOT) RETURN
= 1N4148
= OUTPUT COMMON
FGND
SGND
Figure 1. 10W Off-Line Power Supply Passes FCC Emission Requirements Without Filter Components
3/98/175_conv
DN175 F01
current drain characteristics. Although intended for
DC/DC conversion, the LT1533 adapts nicely to off-line
service, while eliminating emission, filtering, layout and
noise concerns.
Circuitry Details
Figure 1 shows the supply. Q5 and Q6 drive T1, with a
rectifier filter, the LT1431 and the optocoupler closing
an isolated loop back to the LT1533. The LT1533 drives
Q5 and Q6 in cascode fashion to achieve high voltage
switching capability. It also continuously controls their
current and voltage switching times, using the resistors
at the ISLEW and VSLEW pins to set transition rates. FET
current information is directly available, although FET
voltage status is derived via the 360k–10k dividers and
routed to the gates via the NPN-PNP followers. The
source wave shapes, and hence the voltage slewing
information at the LT1533 collector terminals, are nearly
identical in shape to the drain waveforms.
Q1, Q2 and associated components provide a bootstrapped bias supply, with start-up transistor Q1 turning
off once T1 begins supplying power to Q2. The resistor
string at Q2’s emitter furnishes various “housekeeping”
bias potentials. The LT1533’s internal 1A current limit is
too high for effective overcurrent protection. Instead,
current is sensed via the 0.8Ω shunt at the LT1533’s
emitter pin (E). C1, monitoring this point, goes low
when current limit is exceeded. This pulls the VC pin
low and also accelerates voltage slew rate, resulting in
fast limiting while minimizing instantaneous FET stress.
Prolonged short-circuit conditions result in C2 going
low, putting the circuit into shutdown. Once this occurs, the C1–C2 loop oscillates in a controlled manner,
sampling current for about a millisecond every second
or so. This action forms a power limit, preventing FET
heating and eliminating heat sink requirements.
Performance Characteristics
Figure 2 shows waveforms for the power supply. Trace
A is one FET source; traces B and C are its gate and
drain waveforms, respectively. FET current is trace D.
The cascoded drive maintains waveshape fidelity, even
as the LT1533 tightly regulates voltage and current
transition rates. The wideband harmonic activity typical
of off-line supply waveforms is entirely absent. Power
delivery to T1 (center screen, trace C) is particularly
noteworthy. The waveshapes are smoothly controlled,
and no high frequency content is observable.
Figure 3, a 30MHz wide spectral plot, shows circuit
emissions well below FCC requirements. This data
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Linear Technology Corporation
was taken with no input filtering LC components and
a nominally nonoptimal layout.
Output noise is composed of fundamental ripple residue,
with essentially no wideband components. Typically,
the low frequency ripple is below 50mV. If additional
ripple attenuation is desired a 100μH –100μF LC section
permits <100μV output noise. Figure 4 shows this in a
100MHz bandpass. Ripple and noise are so low that the
oscilloscope requires a 40dB low noise preamplifier to
even register a display (see footnote 1).
A = 10V/DIV
B = 10V/DIV
C = 200V/DIV
D = 0.5A/DIV
10μs/DIV
DN175 F02
Figure 2. Waveforms for One of the Power Supplies’
FETs Show No Wideband Harmonic Activity. LT1533
Provides Continuous Control of Voltage and Current
Slewing. Result is Smoothly Controlled Waveshapes
for FET Source (A), Gate (B) and Drain (C). FET
Current is Trace D
AREA UNDER HORIZONTAL
LINE INDICATES ACCEPTABLE
FCC EMISSION LIMITS
Figure 3. 30MHz Wide Spectral Plot Shows Circuit
Emissions Well Below FCC Requirements Despite
Lack of Traditional Filter Components
100μV/DIV
20μs/DIV
DN175 F04
Figure 4. Power Supply Output Noise Below 100μV
(100MHz Measurement Bandwidth) is Obtainable
Using Additional Output LC Section. Without LC
Section Wideband Harmonic is Still Absent, Although
Fundamental Ripple is 50mV
For applications help,
call (408) 432-1900
dn175f_conv LT/TP 0398 370K • PRINTED IN THE USA
1630 McCarthy Blvd., Milpitas, CA 95035-7417
(408) 432-1900
●
FAX: (408) 434-0507 ● www.linear.com
© LINEAR TECHNOLOGY CORPORATION 1998
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