Typical Application: A High Voltage Isolated, Low Noise, Dc-Dc Converter; Circuit Description - Stanford Research Systems SIM954 Operation And Service Manual

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3.7 Typical Application: a High Voltage Isolated, Low Noise, DC-DC Converter

3.7.1 Circuit Description

+-6V, 100mA,100uV ripple, 20kV isolation voltage
DC/DC converter with SIM954 sinusoidal driver.
SIM954 3" RG58
Td=5n Z0=50
T1
V1
SINE(0 6 500kHz)
Rser=50
SRS DS345 Signal Generator
or equivalent
Figure 3.8: Schematic of the 20 kV isolation, sinusoidal drive ultra-low ripple DC-DC
converter.
The ability of the SIM954 to drive significant power into a transformer
can be used to provide isolated power to circuits under unusual
circumstances for which no easy commercial solutions exists. In the
following we describe a 5 V, 100 mA isolated DC-DC converter with
20 kV isolation. Remarkably, the circuit exhibits less than 50 V
output ripple and noise.
In order to achieve 20 kV isolation voltage with minimal e ort, Dear-
born 392250 20 kVDC, 150 C UL 3239 Style high voltage wire is used
to build a 1:1 isolation transformer on a Fair-Rite 2843009902 dual-
aperture core. This large broadband noise suppression core has two
0.250" holes which can accept two turns of the Dearborn high volt-
age wire. A singe loop of wire is used for the primary and a second,
isolated loop for the secondary winding. The windings have enough
inductance to operate this transformer between 250 kHz and 1 MHz.
Toward the lower end of this range, this transformer is limited by its
low winding inductance, and above 1 MHz the core losses in the Type
43 material of this core will dominate and limit performance. Other
core materials and larger cores, which allow for higher inductance,
can extend the frequency range of this design considerably.
The primary winding can be driven directly by the SIM954 through
a 50 coaxial cable. The cable lengths should not exceed 3 to avoid
losses due to mismatched termination. Since neither the SIM954 nor
the transformer load are matched to the 50
coax will have a complex impedance.
100nH beads
on HV cable
close to core
L1
L2
20kV isolation transformer
one turn each of Dearborn 392250
on Fair-Rite 2843009902 core.
A cable that is physically very short compared to the wavelength
SIM954 300 MHz Dual Inverting Driver Amplifier
cable impedance, the
C1
C5-C8 Ceramic Chip
600Ohm/100MHz
L5
1nF
1206 SMT beads
10µH
D1
L3
1N4148 C5
C3
10µF
10µF
C4
C6
D2
10µF
10µF
L4
1N4148
L6
C2 10µH
3 turns on Fair-Rite 6611
1nF
TYPE 43 multi aperture bead
All physical construction on PCB with ground plane using RF design rules.
All current loops have to be minimized, wires should be twisted where possible.
Magnetic shielding of the core may increase performance.
Application notes
rms
C9, C10 Low ESR Tantalum or
L7
organic polymer caps
10µH
R1
C7 C9
10µF 100µ
56
R2
C8 C10
56
10µF
100µ
Simulated Load
L8
10µH