herunterladen
Application Note 118
AN118-1
an118fb
March 2008
transformers originally intended for LCD display backlight
service are readily available. These transformers are mul-
tiply sourced, well proven and competitively priced.
Figure 1’
s resonant Royer topology achieves 100µV
P-P
noise at 250V output by minimizing high frequency
harmonic in the power drive stage. The self oscillating
resonant Royer circuitry is composed of Q2, Q3, C1, T1
and L1. Current flow through L1 causes the T1, Q2, Q3,
C1 circuitry to oscillate in resonant fashion, supplying sine
Figure 1. Current Fed Resonant Royer Converter Produces
High Voltage Output. A1 Biases Q1 Current Sink, Enforcing
Output Voltage Stabilizing Feedback Loop. A1’s 0.001µF–1kΩ
Network Phase Leads Output Filter, Optimizing Transient
Response. D5-D6, Low Leakage Clamps, Protect A1
Introduction
Photomultipliers (PMT), avalanche photodiodes (APD),
ultrasonic transducers, capacitance microphones, radia
-
tion detectors and similar devices require high voltage,
low current bias. Additionally, the high voltage must be
pristinely free of noise; well under a millivolt is a common
requirement with a few hundred microvolts sometimes
necessary. Normally, switching regulator configurations
cannot achieve this performance level without employing
special techniques. One aid to achieving low noise is that
load currents rarely exceed 5mA. This freedom permits
output filtering methods that are usually impractical.
This publication describes a variety of circuits featuring
outputs from 200V to 1000V with output noise below 100µV
measured in a 100MHz bandwidth. Special techniques
enable this performance, most notably power stages
optimized to minimize high frequency harmonic content.
Although sophisticated, all examples presented utilize
standard, commercially available magnetics—no custom
components are required. This provision is intended to
assist the user in quickly arriving at a produceable design.
Circuits and their descriptions are presented beginning
with the next ink.
BEFORE PROCEEDING ANY FURTHER, THE READER
IS WARNED THAT CAUTION MUST BE USED IN THE
CONSTRUCTION, TESTING AND USE OF THE TEXT’S
CIRCUITS. HIGH VOLTAGE, LETHAL POTENTIALS ARE
PRESENT IN THESE CIRCUITS. EXTREME CAUTION
MUST BE USED IN WORKING WITH, AND MAKING
CONNECTIONS TO, THESE CIRCUITS. REPEAT: THESE
CIRCUITS CONTAIN DANGEROUS, HIGH VOLTAGE
POTENTIALS. USE CAUTION.
Resonant Royer Based Converters
The resonant Royer topology is well suited to low noise
operation due to its sinosoidal power delivery
1
. Addition-
ally, the resonant Royer is particularly attractive because
High Voltage, Low Noise, DC/DC Converters
A Kilovolt with 100 Microvolts of Noise
Jim Williams
L, LT, LTC and LTM are registered trademarks of Linear Technology Corporation.
All other trademarks are the property of their respective owners.
AN118 F01
1µF, 400V
1µF, 400V
–
+
LT1635
0.001µF
400V
1k
250V
OUT
1M*
1k
OUTPUT
ADJUST
499Ω*
430k
430k
10k
L1
250µH
IRLRO24
Q1
A1
Q3Q2
820Ω
5V
T1
10k
D1-D4
10 6
5V
D5 D6
5 4 1
2
3
0.1µF
0.22µF
x2
5V
C1
V
REF
= 0.2V
DANGER! Lethal Potentials Present — See Text
= ZDT1048 DUAL
L1 = CTX250-4, COILTRONICS
T1 = 210605R, COILTRONICS
1µF = WIMA MKS-4
0.22µF = WIMA MKP-2
D1-D4 = TOSHIBA DUAL DIODE 1SS306.
CONNECT EACH UNIT IN SERIES.
D5-D6 = 2N4393
* = 1% METAL FILM RESISTOR
Note 1.
This publication sacrifices academic completeness for focus
on the title subject. As such, operating details of the various switching
regulator architectures utilized are not covered. Readers desiring
background tutorial are directed to the References. Resonant Royer theory
appears in Reference 1.
