advertisement

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 Data Sheet Download

www.linear.com

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

DN175 - Off-Line Low Noise Power Supply Does ... - Linear Technology

USE CAUTION IN CONSTRUCTION AND TESTING! 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.

111KB Sizes 2 Downloads 280 Views

Recommend Documents

DN1037 - Power Supply Sequencing - Linear Technology
in position 2. Multiple LTC2937s can share sequence position information, so that sequence position N happens at the same time for all LTC2937 chips, and.

High Voltage, Low Noise, DC/DC Converters - Linear Technology
Application Note 118. AN118-4 an118fb. Low Noise Switching Regulator Driven Resonant. Royer Converters. Figure 8 examplifies the aforementioned “special ...

High Voltage, Low Noise, DC/DC Converters - Linear Technology
can provide surprisingly good performance, particularly at low output current. ...... minimal ripple current may not create a significant field, but such an inductor ...

Low Noise, Precision Op Amp Drives High ... - Linear Technology
performance. The best way to create a differential signal while maintaining linearity is by using the ... Page 3 ... call (978) 656-3757 higher above its bandwidth ...

Low Noise Amplifiers for Small and Large Area ... - Linear Technology
Introduction. Photodiodes can be broken into two categories: large area photodiodes with their attendant high capacitance. (30pF to 3000pF) and smaller area ...

Using the LTC6900 Low Power SOT-23 Oscillator ... - Linear Technology
The LTC6900 master oscillator is controlled by the ratio of the voltage between V+ and the SET pin and the current,. IRES, entering the SET pin. As long as IRES ...

Low Power Filter, Headphone Driver Revisited - Linear Technology
Driver Revisited. Design Note ... 230µA, although data sheet supply maximum values suggest that ... ment headphone drivers is a rational enterprise, given the ...

Low Power, Precision Op Amp Simplifies Driving ... - Linear Technology
data sheet SNR, THD and offset performance with very low power dissipation if the input .... A simple driver for the LTC2372‑18 18‑bit, 500ksps,. 8‑channel SAR ...

Design Note 1034: Low Power, Precision Op Amp ... - Linear Technology
L, LT, LTC, LTM, Linear Technology and the Linear logo are registered trademarks of Linear ... data sheet SNR, THD and offset performance with very low power ...

Versatile Industrial Power Supply Takes High ... - Linear Technology
Master/slave channels are enabled via the master's enable pin and regulate to the master's feedback network. Output current can be increased to 3A or 4A by ...

Single-Channel Power Supply Monitor with ... - Linear Technology
from the monitoring and control features of a power sup- ply manager, but most power supply manager ICs have more than one channel. In an application that ...

Designing Power Supply Parameters in Five ... - Linear Technology
a power supply in five simple steps: (1) entering supply specifications and selecting .... ponent selections to compare with an alternative design for optimum results. ... sources of a powerful local PC, without being limited by shared internet and .

Single-Channel Power Supply Monitor with ... - Linear Technology
relying on the IDAC to be accurate, we have very good knowledge ... produced by Linear Technology®. ..... sures, so sensitivity is in degrees Centigrade per volt.

AN139 - Power Supply Layout and EMI - Linear Technology
Application Note 139. AN139-2 an139fa. The green line is the hot loop in the top layer. AC current ...... [1] http://www.conformity.com/past/0102reflections.html. [2] ...

LT1534 Ultralow Noise Switching Regulator ... - Linear Technology
noise performance—noisy switching regulators call for filtering, shielding and .... data sheets and Linear Technology's Application Note 70. 5μs/DIV. VOUT.

AN159 - Measuring 2nV/√Hz Noise and 120dB ... - Linear Technology
MEASURING OUTPUT VOLTAGE NOISE. Being Quiet is Nothing New. The subject of noise has been broached before. Linear. Technology Application Note 83, ...

Supercapacitor-Based Power Backup System ... - Linear Technology
PowerPath™ controller that simplifies the design of backup systems. ... Backup Power Application. Figure 1 ... Designing a power backup system is easy with the.

Linduino for Power System Management - Linear Technology
erences selection on the menu bar as shown in Figure 3. Finding Preferences ... libraries have a predefined main() that calls setup() and an infinite loop calling ...

Wireless Power User Guide - Linear Technology
Figure 9 shows a typical charge profile with this wireless power configuration. Actual data will vary with component tolerance and specific setup. Demo Board ...

A low-power highly linear cascoded multiple-gated ... - koasas - kaist
[email protected]; [email protected]). B. Kim is with RF Products, System LSI Business, Semiconductor, Samsung. Electronics, Korea. B. Ko is with Integrant ...

DN562 - 60V Low IQ, Dual Output Synchronous ... - Linear Technology
150°C—meets the needs of industrial, automotive, telecom, military, aerospace and distributed systems. The LTC3892's low IQ is indispensable in battery-.

Low EMI LED Driver Features 2A, 40V, Integrated ... - Linear Technology
Introduction. The breadth of LED applications has grown to encom- pass everything from general lighting to automotive, industrial and test equipment, sign ...

Battery Backup Regulator is Glitch-Free and Low ... - Linear Technology
for battery backup applications. It eliminates ... DROPOUT, are useful for monitoring the status of the ... Figure 2 shows a typical application of the LT1579 with.