Custom Multiplier Assemblies

```Custom Multiplier Assemblies
VMI is a world leader in the design & manufacture of quality
custom Multiplier assemblies. This section is intended to
provide the user with:
1) General background information on certain multiplier
assembly characteristics.
2) Basic guidance in identifying specific application requirements necessary for the design of a custom multiplier
assembly.
DESIGN GUIDE
I. Introduction
• Introduction: (What is a Multiplier?)
Voltage multipliers are AC-to-DC power conversion devices,
comprised of diodes and capacitors, that produce a high
potential DC voltage from a lower voltage AC source. Multipliers
are made up of multiple stages. Each stage is comprised of one
diode and one capacitor.
• Introduction: (How Does a Multiplier Work?)
OUTLINE OF MULTIPLIER
DESIGN PROCESS
The most commonly used multiplier circuit is the half-wave series
multiplier. All multiplier circuits can be derived from its basic
operating principles. Thus, the half-wave series multiplier circuit
is shown in Figure 1 to exemplify general multiplier operation.
This example also assumes no losses and represents sequential
reversals of transformer (TS) polarity.
I. Introduction
• What is a multiplier?
• How does a multiplier work?
• Common multiplier applications
Figure 1
II. Assembly Type
•
•
•
•
Half wave series multiplier
Half wave parallel multiplier
Full wave series multiplier
Series vs. parallel design considerations
III. Electrical Operating Conditions
•
•
•
•
Reasonable ranges
Input & Output voltage
Output current
Operating frequency
IV. Physical Characteristic
• Size
• Mounting
• Terminations
V. Environmental Conditions
•
•
•
•
•
High altitude
Chemical exposure
Humidity
Extreme temperatures
Pratical limits
VI. Other Design Concerns
•
•
•
•
C1 charges through D1 to Epk
2) TS = Positive Peak:
to existing potential C1, thus
C2 charges to 2Epk thru D2.
3) TS = Negative Peak:
C3 is charged to 2Epk through D3.
4) TS = Positive Peak:
C4 is charged to 2Epk through D4.
Therefore, output voltage = Epk x N
(where N = the number of stages).
• Introduction: (Common Multiplier Applications)
Originally used for television CRT's, voltage multipliers are now
used for lasers, x-ray systems, traveling wave tubes (TWT's),
photomultiplier tubes, ion pumps, electrostatic systems, copy
machines, and many other applications that utilize high voltage
DC.
Stray capacitance
Corona
Leakage currents
Reasonable ranges
Dimensions: In. (mm)
1) TS = Negative Peak:
All temperatures are ambient unless otherwise noted.
Voltage Multipliers Inc.
8711 W. Roosevelt Ave.
Visalia, CA 93291 USA
62
Data subject to change without notice.
Tel:
559.651.1402
Fax:
559.651.0740
www.voltagemultipliers.com
www.highvoltagepowersupplies.com
Custom Multiplier Assemblies
DESIGN GUIDE
II. Assembly Type
A Typical 4X Circuit Schematic
• Assembly Type: (Half-Wave Series Multiplier)
Characteristics:
1) Most common circuit.
2) Very versatile.
3) Uniform stress per stage on diodes & capacitors.
4) Wide range of multiplication stages.
5) Low cost.
The following schematics show some of the many variations which are available for a half-wave series multiplier configuration
A large number of stages are
available.
Negative Output is achieved
through reversing diode
polarity.
Dual Polarity Output voltage
is achieved through joining
positive and negative
multipliers.
Odd or even numbers of
stages can be produced.
Voltage may be tapped at
any point along the
capacitor filter bank.
Any capacitor may be
eliminated on the capacitor
filter bank, if the load is
capacitive.
Dimensions: In. (mm)
All temperatures are ambient unless otherwise noted.
Voltage Multipliers Inc.
8711 W. Roosevelt Ave.
Visalia, CA 93291 USA
63
Data subject to change without notice.
Tel:
559.651.1402
Fax:
559.651.0740
www.voltagemultipliers.com
www.highvoltagepowersupplies.com
Custom Multiplier Assemblies
DESIGN GUIDE
II. Assembly Type (Continued)
• Assembly Type: (Half-Wave Parallel Multiplier)
Characteristics:
1) Small size.
2) Highly efficient.
3) Uniform stress on diodes.
4) Increasing voltage stress on capacitors with successive
stages.
A Typical Schematic
• Assembly Type:
(Series vs. Parallel Design Considerations)
In the process of deciding which type of multiplier assembly best
suits the end application, it is necessary to address the series
and parallel multiplier formats.
The theory of operation is the same in both the series and the
parallel multiplier assembly types. They are similar also in
package volume, but are slightly different in package shape
capability. Parallel multipliers require less capacitance per stage
than do their series counterparts.
However, parallel multipliers also require higher voltage ratings
on each successive stage. The limit on output voltage in parallel
multipliers is determined by the voltage capability of the
capacitors (common single-layer ceramic capacitors do not
exceed 20kV).
Regulation Voltage:
DC output voltage drops as DC output current is increased.
Regulation is the drop, from the ideal, in DC output voltage at a
specified DC output current (assuming AC input voltage and AC
input frequency are constant). A close approximation for series
half-wave multipliers can be expressed as:
VREG = [I(N3+(9N2/4)+(N/2))]/12fC
Where: N
f
C
I
• Assembly Type: (Full-Wave Series Multiplier)
Characteristics:
1) Highly efficient.
2) Uniform stress on diodes.
3) Increasing voltage stress on capacitors with successive
stages.
4) High power capability.
=
=
=
=
# of stages, (1 capacitor and 1 diode = 1 stage)
AC input frequency (Hz)
Capacitance per stage (F)
DC output current (A)
Example: Calculate the regulation voltage of a 6 stage multiplier
with 1000pF capacitors, 50kHz input frequency (sine wave), 1mA
DC output current, 20kV DC output voltage:
VREG = [1*10-3(63+((9*62)/4 + (6/2))]/12*50000*(1*10-9))
= 500 volts
A Typical Schematic
This would require increasing the input voltage
167Vp-p (VREG / 3 DC capacitors) to maintain 20kV DC output
voltage at 1mA.
An equivalent parallel multiplier would require each capacitor
stage to equal the total series capacitance of the AC capacitor
bank. In the above example, the 3 capacitors in the AC bank
would equal 1000pF/3 or 333pF. The parallel equivalent would
require 333pF capacitors in each stage. See series multiplier and
parallel multiplier charts on following page.
Dimensions: In. (mm)
All temperatures are ambient unless otherwise noted.
Voltage Multipliers Inc.
8711 W. Roosevelt Ave.
Visalia, CA 93291 USA
64
Data subject to change without notice.
Tel:
559.651.1402
Fax:
559.651.0740
www.voltagemultipliers.com
www.highvoltagepowersupplies.com
Custom Multiplier Assemblies
DESIGN GUIDE
II. Assembly Type (Continued)
Parallel Multiplier
Ripple Voltage:
Ripple voltage is the magnitude of fluctuation in DC output
voltage at a specific output current (assuming AC input voltage
and AC input frequency are constant). A close approximation for
series half-wave multipliers can be expressed as:
Example: Calculate the ripple voltage of a 6 stage multiplier with
1000pF capacitors, 50kHz input frequency (sine wave), 1mA DC
output current, 20kV DC output voltage:
VRIP = (1*10-3(62+6/2))/8*50000*(1*10-9))
VRIP = 97.5Vp-p
4500
Output Voltage Drop (Volts)
VRIP = I(N +N/2)/8fC
2
5000
4000
3500
3000
2500
2000
1500
X14
X12
X10
X8
X6
X4
X2
1000
500
0
0.0
0.2
0.4
0.6
0.8
1.0
Output Current (mA)
1.2
1.4
Series Multiplier
(Efficiency comparison from perfect multiplication)
5000
Output Voltage Drop (Volts)
4500
1)
2)
3)
4)
X14
4000
X12
3500
X()
Capacitance
Diodes
Frequency
=
=
=
=
# of stages
1000pF/stage
12 chips/diodes
25kHz
X10
3000
2500
X8
2000
1500
X6
1000
X2
500
X4
0
0.0
0.2
0.4
0.6
0.8
1.0
Output Current (mA)
1.2
1.4
(Efficiency comparison from perfect multiplication)
1)
2)
3)
4)
X()
Capacitance
Diodes
Frequency
Dimensions: In. (mm)
=
=
=
=
# of stages
1000pF/stage
12 chips/diodes
25kHz
All temperatures are ambient unless otherwise noted.
Voltage Multipliers Inc.
8711 W. Roosevelt Ave.
Visalia, CA 93291 USA
65
Data subject to change without notice.
Tel:
559.651.1402
Fax:
559.651.0740
www.voltagemultipliers.com
www.highvoltagepowersupplies.com
Custom Multiplier Assemblies
DESIGN GUIDE
III. Electrical Operating Conditions
Table 1
• Electrical Operating Conditions: (Reasonable Ranges)
DC
Output
Voltage
(VDC)
Practical limits do exist, which determine multiplier design and
application. Here are some typical rules of thumb for the most
commonly used VMI multipliers:
1) AC Input Voltage:
2) AC Input Frequency:
3) DC Output Voltage:
4) DC Output Power:
0 to 15kV p-p
5kHz to 100kHz
1kV to 150kV
0 to 50W
1k
2.5k
Table 1 can be used to determine reasonable ranges for VMI
multipliers, utilizing rugged epoxy encapsulation and single layer
ceramic capacitors. Input frequency is assumed to be from 5kHz
to 100kHz.
5k
10k
20k
30k
50k
75k
100k
150k
Output
Power
(W)
AC Input
Voltage
(VAC p-p)
0-50
50-200
>200
200-1000
500-1000
500-1000
X
0-50
50-200
>200
250-2500
1000-2500
1000-2500
X
0-50
50-200
>200
250-5000
2500-5000
2500-5000
X
0-50
50-200
>200
2500-10000
5000-10000
5000-10000
X
0-50
50-200
>200
2500-10000
5000-10000
5000-10000
X
0-50
50-200
>200
2500-10000
5000-10000
5000-10000
X
0-30
30-100
>100
5000-10000
5000-10000
5000-15000
X
0-30
>30
7500-15000
>5000
X
0-30
>30
7500-15000
>5000
X
0-30
>30
7500-15000
>5000
X
Half
Wave
Other
Type
C aps
Other
Type
Encap.
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
Full
Wave
Note: Multipliers are available that exceed the
limits (as listed on the column to the left), but may
require other types of capacitors, encapsulation,
etc.
Dimensions: In. (mm)
All temperatures are ambient unless otherwise noted.
Voltage Multipliers Inc.
8711 W. Roosevelt Ave.
Visalia, CA 93291 USA
66
Data subject to change without notice.
Tel:
559.651.1402
Fax:
559.651.0740
www.voltagemultipliers.com
www.highvoltagepowersupplies.com
Custom Multiplier Assemblies
DESIGN GUIDE
• Physical Characteristics: (Size) Continued
III. Electrical Operating Conditions (Con't)
• Electrical Operating Conditions: (Input & Ouput Voltage)
The input voltage is usually specified as peak or peak-to-peak
voltage. The theoretical no-load output voltage is equal to the
number of stages times the peak input voltage. In most cases,
the output voltage will be reduced from the theoretical value due
to the effects of regulation and stray capacitance.
Clearly defining the dimensions and necessary tolerances is very
helpful. When the specific shape and/or size is not defined, as
much information as possible should be provided regarding the
enclosure where the part will be installed and/or the customer's
preferred physical characteristics. Typically, packaging that is "as
small as possible", is desired. However, an indication of
preferences and expectations, with respect to package size, will
aid in the development of a suitable package design.
In most applications, the output voltage from the multiplier is a
primary requirement. The input voltage may need to be
increased to provide the required output voltage. Care must be
taken to insure that the voltage stresses on the components do
not exceed ratings during multiplier operation at maximum output
voltage and current.
• Physical Characteristics: (Mounting)
• Electrical Operating Conditions: (Ouput Current)
• Physical Characteristics: (Terminations)
For typical multipliers, output current can range from 1µA to
5mA. Due to the effects of regulation, output current can affect
the voltage stresses on a multiplier's diodes and capacitors.
Since regulation is directly proportional to output current, and as
input voltage is usually increased to compensate for regulation,
the diodes and capacitors near the input side of the multiplier will
be subjected to higher voltage stress at higher output currents.
Multiplier assemblies can have a large variety of terminations.
Some possibilities include: turret terminal, bus wire, high voltage
leads, high voltage connectors, inserts, pcb pins, or
combinations of these configurations. Special terminal plating
requirements should be noted as required.
For higher current ratings, it is important to insure that the
diodes' junction temperature does not exceed 125°C. A thermal
analysis may be necessary to evaluate junction temperature.
Typically, for output currents less than 1.0mA, the power
dissipated in the diodes is low enough to prevent overheating.
V. Environmental Conditions
• Environmental Conditions: (High Altitude)
• Electrical Operating Conditions: (Operating Frequency)
The lower the operating frequency for a multiplier, the larger its
capacitors will need to be to maintain electrical performance. For
low frequency multipliers, the operational characteristics must be
calculated to determine feasibility.
The upper limit to operating frequency will be affected by diode
recovery time, stray capacitance, and inductance effects. Diode
recovery time can be a factor at frequencies above 100kHz. The
effects of stray capacitance and inductance will depend on
component layout, potting material used, and the choice of
components.
IV. Physical Characteristics
High altitudes can amplify what would, at lower altitudes, be
relatively benign design issues. For example, some dielectric
materials will outgas in low pressure or vacuum installations,
causing degradation of the dielectric and/or contamination from
insulating film deposition. Also, corona problems will generally
vary non-linearly with increased altitude.
• Environmental Conditions: (Chemical Exposure)
The level of exposure an assembly receives to various chemicals
should be identified if known. Many applications use dielectric
oils or gases to surround the custom multiplier assembly. While
these materials can provide excellent isolation, reduced corona
effects, minimal mechanical stresses, and usually good cooling,
they can also damage or degrade some encapsulants and
remove assembly labeling. As such, materials compatibility must
be addressed during the design stage.
• Environmental Conditions: (Humidity)
Environments with high humidity can sometimes cause certain
types of dielectric materials to absorb moisture. Also, humidity
severely limits the voltage isolation capabilities of air-insulated
applications.
• Physical Characteristics: (Size)
Custom multiplier assemblies can usually be constructed in a
wide variety of shapes and sizes to meet customer needs. The
customer may also specify special physical characteristics,
provided such specifications do not compromise design
constraints. Actual design of the package size/shape must
account for internal mechanical stresses and voltage isolation
issues.
Dimensions: In. (mm)
The preferred end-application mounting or installation provisions
need to be specified. Through holes, integral threads, encapsulated inserts, pcb mount and suspension are some examples
of mounting techniques.
As a result, it may be necessary to overpot, or otherwise insulate
any exposed high voltage connections.
All temperatures are ambient unless otherwise noted.
Voltage Multipliers Inc.
8711 W. Roosevelt Ave.
Visalia, CA 93291 USA
67
Data subject to change without notice.
Tel:
559.651.1402
Fax:
559.651.0740
www.voltagemultipliers.com
www.highvoltagepowersupplies.com
Custom Multiplier Assemblies
DESIGN GUIDE
V. Environmental Conditions (Continued)
Table 2
• Environmental Conditions: (Extreme Temperature)
The input voltage to the assembly (including known or expected
transient conditions) must be identified to determine what
diode(s) is best suited for the application. Assembly exposure to
high or low temperature extremes requires special consideration.
This is due to the electrical and mechanical effects of the
materials used in the assembly construction. For example, very
high temperature extremes, such as in excess of 150°C, can
sigificantly reduce the voltage isolation capabilities of some
encapsulants.
Additionally, high temperatures can induce significant mechanical stresses, due to mismatches in material thermal expansion
coefficients. (See Table 2).
Similarly, very low temperature extremes can induce mechanical
stresses due to material thermal expansion mismatches. Low
temperatures can also cause radical changes in the physical
characteristics of the encapsulant, making it brittle, or causing
the encapsulant to exhibit non-linear shrinkage effects. (See
Table 3)
Practical limits do exist, which determine multiplier design and
application. Here are some environmental rules of thumb for the
most commonly used VMI multipliers:
-55°C to +125°C
0 to 100%
0 to space
Note: Altitude and humidity affect materials, terminations, plating,
Dimensions: In. (mm)
Material
Availability
25°C TO 75°C
Excellent
75°C TO 125°C
Good
125°C TO 175°C
F ai r
175°C TO 225°C
P oor
>225°C
Rare
Table 3
• Environmental Conditions: (Practical Limits)
1) Operating Temp Range:
2) Relative Humidity:
3) Altitude:
Temperature
Range
Temperature
Range
Material
Availability
-40°C TO +25°C
Excellent
-55°C TO -40°C
Good
-65°C TO -55°C
F ai r
<-65°C
Rare
All temperatures are ambient unless otherwise noted.
Voltage Multipliers Inc.
8711 W. Roosevelt Ave.
Visalia, CA 93291 USA
68
Data subject to change without notice.
Tel:
559.651.1402
Fax:
559.651.0740
www.voltagemultipliers.com
www.highvoltagepowersupplies.com
Custom Multiplier Assemblies
DESIGN GUIDE
VI. Other Design Concerns
• Other Design Concerns: (Stray Capacitance)
Stray capacitance becomes an important consideration as input
frequency increases. As the following expression indicates, an
increase in frequency decreases the capacitive reactance,
resulting in increased current flow through the insulating
materials.
XC
= 1/(2pfC)
Power losses through insulation, which are negligible at 60Hz,
become significant at high frequency.
• Other Design Concerns: (Corona)
Corona is the result of gas ionization (air, oxygen, etc.), due to a
high voltage field.This extremely destructive phenomena
usually results in slow degradation of the insulating materials,
causing latent failures. Careful design, consistent manufacturing
processes, eliminating air entrapment in encapsulation, and a
thorough understanding of what causes corona will minimize this
problem.
• Other Design Concerns: (Leakage Currents)
Losses due to leakage in diodes, capacitors and insulation are
significant considerations in applications using very low capacitor
values (i.e. night vision power supplies) and in applications,
which operate at high temperatures (>125°C). (Figure 2)
represents some of the factors affecting multiplier efficiency.
Figure 2
Dimensions: In. (mm)
All temperatures are ambient unless otherwise noted.
Voltage Multipliers Inc.
8711 W. Roosevelt Ave.
Visalia, CA 93291 USA
69
Data subject to change without notice.
Tel:
559.651.1402
Fax:
559.651.0740
www.voltagemultipliers.com
www.highvoltagepowersupplies.com
```