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DESIGN AND DEVELOPMENT OF PROTOTYPE HIGH VOLATGE
POWER SUPPLY WITH PSM TECHNIQUE FOR RF AMPLIFIER
S.K.Thakur,Santwana K.*,A.De,Y. Kumar,V.Lilhare,A. Bera,S.S.Pal,V.K.Khare,T.P.Tiwari,S. Saha
Variable Energy Cyclotron centre, Kolkata, India
Abstract
CONTROL ALGORITHM OF PSM
The prototype high voltage power supply with Pulse
Step Modulation (PSM) technique is designed and
developed for RF amplifier. The power supply consists of
several switching power modules connected in series
through freewheeling diodes. Each power module is
phase shifted from others and the total output voltage is
obtained by modulating their delay between on time and
pulse width resulting in higher ripple frequency at output
stage. A floating point Digital Signal Processor (DSP)
based controller TMS320F28335 manufactured by Texas
Instruments is used for the implementation of digital
control algorithm. This paper discusses PSM technology,
control algorithm, experimental results and simulation
results showing the expected performance of the system.
According to the principle of PSM, the control system
of the power supply is such that each module should on
and off for equal time. This will ensure equal loading of
all modules.
In the system, 4 modules are connected in series
through freewheeling diode. Each module has a period T
of 200µs and duty cycle of 0.8. As control signal of the
PSM is based on stage rotation [2], therefore the second
IGBT turned on T/4 later than the first one, the third is
turned on T/4 later than second one and forth is turned on
T/4 later on than third one as shown in Fig. 1. Based on
this principle total output voltage (Eq. 1), ripple
frequency (Eq. 2) at output stage can be calculated.
INTRODUCTON
RF amplifier such as Klystron, Magnetron, Inductive
Output Tube (IOT), Vacuum Tubes require power supplies
with special demands like very high output voltage, high
output power, high accuracy of output voltage, low
amount of stored energy, fast turn on and turn off etc.
Due to development of solid state device available for
both higher voltage and higher switching frequency leads
to replacement of more traditional power system based on
vacuum tubes (tetrodes). Some standard power supply
techniques are compared and improvement by introducing
the PSM technique is also discussed in [1].
The advantages of PSM technique are modular structured
(consists of a number of identical low voltage switching
modules), solid state design using IGBTs, very low stored
energy, low ripple voltage at output stage, very fast turn
on and turn off capability, extremely high reliability due
to built in redundancy, good efficiency etc [2]. Due to
very low stored energy, it automatically replaces
additional requirement of crowbar protection. With the
introduction of latest digital technologies in PSM system,
performance of modular power supply can be improved
further. As compared to analog controller digital
controller is more flexible, reliable and faster.
For the validation of PSM technique a prototype high
voltage power supply with this novel technique is
designed and developed. This power supply is consists of
four switching power supply with total output voltage of 2.4kV/2A. This power supply is tested on resistive load.
With the experience of this, our main goal is to design &
develop -40kV/5A, which will have 60 modules with 12
pulse rectification mode.
___________________________________________
*santwana@vecc.gov.in
Vo = NVs ton /T
(1)
fo = Nfs
(2)
Where Vo, N, Vs, fo and fs are output voltage, number
of working modules, input voltage of each module, ripple
frequency and switching frequency respectively. ton is On
time of IGBT for each module and td is the delay between
on time of each module.
4Vs
3Vs
4
3
2
Vs
1
T/4
T
Figure 1: Output voltage waveform with 4 modules.
Therefore, ripple voltage Vs with frequency 4fs needs to be
filtered out by using low pass filter.
SYSTEM DESCIPTION
The prototype power supply consists of multi secondary
transformer, four switching modules, the DSP controller,
optical fibers for logic signals, voltage feedback
measurement circuit. The full schematic of the power
supply is shown in Fig. 2. The modules are fed by the
multi secondary windings of transformer. Each module
consists of a diode rectifier bridge, a capacitor bank, an
IGBT switch, a freewheeling diode, resistance
capacitance snubber circuit and a gare driver circuit. For
the detection of faulty module, it has voltage to frequency
(v/f) converter which transmit signal to the DSP
controller through optical fiber for voltage measurement.
built 12- bit A/D converter of DSP. Reference voltage is
also fed to one of the ADC channels of the DSP.
SIMULATION RESULTS
Simulation of the power modules with phase shifted
PWM along with output filter in close loop is done in
MATLAB. Fig. 4 shows the simulink model of power
converter based on PSM control algorithm.
Figure 2: Schematic of prototype power supply.
The schematic layout of one module is shown in Fig. 3.
The AC voltage is rectified by three phase rectifier. The
pulsating DC voltage is filtered by the electrolytic
capacitor bank. For electronic switch elements with high
voltage and frequency upto 20 kHz IGBT can be used as
it also offer high reliability, fast switching with low
losses. The optimum choice in this application is one
module with single IGBT G4PH50UD of rating
1200V/24A. The output of one module is about 600V/2A
Figure 4: Simulink model of the power converter.
The output voltage at final stage without filter is shown
in Fig. 5. We have ripple voltage of magnitude equal to
one module voltage as discussed earlier.
(500v/div, 50µs/div)
Figure 5: Output voltage without filter
To maintain output voltage constant with the variation
in load and line voltage, discrete proportional integral (PI)
controller is designed. The parameters of PI controller are
obtained through iteration in simulink/MATLAB. Step
change in load is carried out and response of the power
converter is checked with controller, shown in Fig. 6.
Figure 3: Layout of one Power Module.
Each module has two states of operation, when IGBT is
on output voltage Vs and when IGBT is off output voltage
is zero. The freewheeling diode provides path for the
current of other switching module if IGBT of this module
is not turned on. Phase shifted PWM (Pulse Width
Modulation) signals for the switch is generated by DSP
and transmitted through optical fibers. Optical fiber
insures the isolation between high voltage module and
low voltage control circuit. For the regulation of output
voltage, feedback voltage is sensed by the voltage
transducer, and then input into the controller through in
Time( 50ms/div)
Figure 6: Output voltage (upper) with load disturbance
(lower)
TEST RESULTS
To test the performance of the system prototype high
voltage power supply is developed and tested at resistive
load. The parameters of the power supply are shown in
Table 1.
Table 1: Specifications of power supply
Parameters
increase input voltage to maintain output voltage
constant.
Ch1
Ch2
Value
Input Voltage (each module)
750V
Output Voltage (each module)
600V
Maximum output current
2A
Output Vp-p ripple
0.01%
Switching frequency (each module)
5 kHz
Load regulation ( 50% increase in load current)
< 1%
Line regulation
<5%
As shown in Fig. 7 for each module shifted gate pulses
for are generated by PWM module of DSP. Each module
has switching frequency of 5 kHz, duty ratio is 80% and
delay is 50 µs. The output ripple voltage of magnitude
equal to one module voltage with ripple frequency equal
to 4 x 5 kHz = 20 kHz. This ripple voltage is attenuated
using low pass LC filter at final output stage.
Figure 9: Change at output with variation in input voltage.
Ch1: input voltage to the one module, Ch2: final output
voltage
Load transient test is performed to see how fast this
power supply responds to the change in the load. Figure
shows the respond of the power supply in terms of output
voltage. Load current is doubled at point 1 and it is
restored back to pervious value at point 2 as shown in Fig.
10. The response time of the converter is found less than
50ms with some overshoot.
2
1
Figure 10: Output voltage with load disturbance.
CONCLUSION
Figure 7: Shifted PWM for 4 modules from DSP.
Ch2
Ch1
Figure 8: Output voltage waveform with change in line
voltage. Ch1: input voltage to the one module, Ch2: final
output voltage.
The PI controller is implemented on DSP for the output
voltage regulation of the power supply. Input voltages of
all four modules are varied 10 % and measurement is
taken for the calculation of line regulation. From Fig. 8
and Fig. 9 it can be seen that pulse width decreases as we
Prototype high voltage power supply of rating 2.4kV/2A is designed and developed. Preliminary test on
this power supply is carried out and it showed results in
accordance with the results calculated and simulated. The
main advantages of this power supply are small size of
output filter, low stored energy, modular design etc. DSP
based digital controller used for this power supply has
many advantages over conventional analog controller
such as flexible, reconfigurable, less affected by noise and
accurate. Further, development of -40kV/5A power
supply based on this novel PSM technology is in progress
and investigation is also going on for improvement of
dynamic behaviour of the power converter.
REFERENCES
[1] J. Alex, W. Schminke, “A High Voltage Power Supply for
Negative Ion NBI based on PSM Technology,” 17 th
Symposium on Fusion Engineering, San Diego, CA 1997.
[2] J. Alex, W. Schminke, “Fast Switching Modular High
Voltage DC/AC power supplies for RF- Amplifiers and
other applications,” 16th Symposium on Fusion
Engineering, Champaign IL, 1995