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# Dc to dc converter with controlled active clamping topology

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Dc to dc converter with controlled active clamping topology

1.INTRODUCTION

DEPARTMENT OF ELECTRICAL AND ELECTRONICS                                                      Page 1
Dc to dc converter with controlled active clamping topology

In the field of electrical solar power conversion efficiency is the most important topic. In
most applications a special DC-to-DC converter is necessary to couple the solar cells, operating
at low DC-voltages, to the inverter's DClink. These converters with rather low input respectively
high output voltage ratings have, due to the high current ratings, a relatively low efficiency.

Conventional solutions for efficiency optimized solar equipment use large over
dimensioning of all power semiconductors, reduced switching frequency, and mostly hard
switching topologies leading to bigger size and weight of the converters. In this paper a special
design for the voltage adapting DC-to-DC converter is presented where an additional converter is
used to feed back most occurring snubber losses. This helps to increase the overall efficiency of
the first stage for about 2.5 % compared to conventional solutions. Nevertheless, the switching
frequency can be raised to a serviceable range (up to several 100 kHz) without appreciable losses
in the main snubber.
In the field of solar application it also makes sense to operate several phase-
shifted converter stages in parallels haring the load to achieve acceptable over-all efficiency.
The presented solution is designed to adapt a 12 V or24 V battery to a 400 V DC-link, well-
suited to the European 230 V power grid. The conversion ratio, depending on the input voltage
range from 10.. 14.4 V(20..28.8 V) to output voltage range 380..400 V therefore varies from 1/26
up to 1/40 at 12 V.The total power to be managed on the output of the DC to-DC converter is
1kW. In case of a single stage inverter, this leads to about 100 A input current (at 12 V), causing
peak values in the power switches of up to 200 A! The resulting component stress is very hard
and also the design is difficult to handle. So it is suitable to select a topology which is also well
suited for paralleling the power switches leading to a scalable solution with the advantage of a
more optimal design in each stage. Here a structure was used where several converter stages can
easily be paralleled due to its current source characteristics.

DEPARTMENT OF ELECTRICAL AND ELECTRONICS                                                            Page 2
Dc to dc converter with controlled active clamping topology

Here a structure was used where several converter stages can easily be paralleled due to its
current source    characteristics. The principal energy flow of the first stage (DC-to-DC
converter for voltage level adaptation) of a conventional solar inverter topology is given in
figure
The different sections are marked:
1 converter for snubber energy recuperation
2 main converter (consists of a DC-to-AC inverter, the main transformer and a rectifier)

During normal operation both converters work simultaneously. Therefore synergies
can be used to optimize the system structure leading to an effective design. The main goal of this
investigation is to find a topology which overcomes the inconvenient system arrangement and to
find a simple and robust solution. The chosen structure based on the well known push-pull
converter [3] is given in Fig

Due to the leakage inductance of the main transformer, a snubber circuit is required to
protect the primary switches from over voltage. The energy, stored in the leakage inductor of the
transformer

DEPARTMENT OF ELECTRICAL AND ELECTRONICS                                                           Page 3
Dc to dc converter with controlled active clamping topology

W=(L.i.i)/2

times twice the switching frequency has to be handled       by the snubber

Ps=L.i.i.f

Due to the rather high primary current (for 12 Operation about 200 A in the peak
can be assumed), this leads to excessive additional losses of the converter which perceivably
lowers its efficiency. During normal operation(without any transformer losses) the voltage across
Cs is twice the input voltage UIN (resulting from the push-pull concept). This also leads to
snubber losses without any damping effect

In this paper a possible solution for such a specific solar converter is presented. The
presented solution is designed to adapt a 12 V or24 V battery to a 400 V DC-link, well-suited to
the European 230 V power grid. The conversion ratio, depending on the input voltage range from
10.. 14.4 V(20..28.8 V) to output voltage range 380..400 V therefore varies from 1/26 up to 1/40
at 12 V.The total power to be managed on the output of the DC to-DC converter is 1 kW.

DEPARTMENT OF ELECTRICAL AND ELECTRONICS                                                          Page 4
Dc to dc converter with controlled active clamping topology

2.PROPOSED TOPOLOGY

To overcome       this problem such as large over dimensioning of all power
semiconductors switching frequency, and mostly hard switching topologies leading to bigger
size and weight of the converter. In this paper a special design for the voltage adapting DC-to-
DC converter is presented where an additional converter is used to feed back most occurring
snubber losses       an additional, specially controlled DC-to-DC converter is used to feed back the
energy stored in the leakage inductor of the transformer.

Fig. 2.1. Principal solar inverter with active clamping topology: 1-energyrecuperation, 2-main converter section

DEPARTMENT OF ELECTRICAL AND ELECTRONICS                                                                      Page 5
Dc to dc converter with controlled active clamping topology

The converter proposed here uses the well-known push pull converter in the up-link
and a buck converter for energy recuperation. A special control circuit was implemented to
optimize the system behavior. Input voltage was measured and current was estimated to control
the snubber voltage. The usage of a dynamically controlled snubber voltage leads to a
remarkable efficiency improvement. A buck-converter was added to control the energy flow
from the storage capacitor of the snubber to the input section.
The big advantage of this topology is that adaptive control of this converter helps
minimizing system losses while the maximum voltage across the main switches can exactly be
defined. The result is an optimized converter structure with minimized component count leading
to an easy-to-handle and robust design, well suited for paralleling.

2.1BLOCK DIAGRAM

Buck converter                        controller

Solar panel                   Dc to ac               transformer                  Ac to dc
converter                                           converter

DEPARTMENT OF ELECTRICAL AND ELECTRONICS                                                           Page 6
Dc to dc converter with controlled active clamping topology

2.1.1 SOLAR PANELS

Solar panels use light energy (photons) from the sun to generate electricity
through the photovoltaic effect. The structural (load carrying) member of a module can either be
the top layer or the back layer. The majority of modules use wafer-based crystalline silicon cells
or thin-film cells based on cadmium telluride or silicon. The conducting wires that take the
current off the panels may contain silver, copper or other conductive (but generally not
magnetic) transition metals.

The cells must be connected electrically to one another and to the rest of the
system. Cells must also be protected from mechanical damage and moisture. Most solar panels
are rigid, but semi-flexible ones are available, based on thin-film cells.

Photovoltaic tiles take the place of ordinary roof tiles and can be perfectly blended
to fit the look of the outside of your house. Solar Energy has several advantages to the
environment since it is clean, renewable (unlike gas, oil and coal) and sustainable. Governments
are beginning to offer grants to assist in paying for photovoltaic roof tiles, which should help
making solar power more widely available in the near future.

DEPARTMENT OF ELECTRICAL AND ELECTRONICS                                                            Page 7
Dc to dc converter with controlled active clamping topology

2.1.2MAIN CONVERTWER UNIT

Fig2.4 main converter

This unit consists of a DC-to-AC inverter, the main transformer and a rectifier.
The dc ac converter convert dc voltage from panels into ac voltage suitable for the transformer
in which amplification of voltage takes place. after this ac is converted to dc

2.1.3.CONTROLCIRCUIT

The control circuit was implemented to optimize the system behavior. Input
voltage was measured and current was estimated to control the snubber voltage. The usage of a
dynamically controlled snubber voltage leads to a remarkable         efficiency improvement.

DEPARTMENT OF ELECTRICAL AND ELECTRONICS                                                           Page 8
Dc to dc converter with controlled active clamping topology

2.1.4ENERGY RECUPERATION CONVERTER

To overcome this problem an additional, specially controlled DC-to-DC converter is used
to feed back the energy stored in the leakage inductor of the transformer. A buck-converter was
added to control the energy flow from the storage capacitor of the            snubber to the input
section. The big advantage of this topology is that adaptive control of this converter helps
minimizing system losses while the maximum voltage across the main switches can exactly be
defined.

Fig2.5Improved solution: active snubber circuit for energy recuperation (buck-converter formed by CS, SFB, DFB,
LFB)

DEPARTMENT OF ELECTRICAL AND ELECTRONICS                                                                 Page 9
Dc to dc converter with controlled active clamping topology

3.WORKING OF DC TO DC CONVERTER

The low voltage from the battery to the main convetr section the input voltage is
about 12v to 24v.aphotovoltaic generator is a current source depending oo solar radiation
intensity and temperature.simple charge controllers and dc to dc converters with maximum
power tracking are used to interface solar panels with batteries.
From main converter the energy recupperation converter feedback the energy
stored int the leakage indctance of transformer.this converter is controlled by control circuit to
opimise system behaviour.in the main converter a step transformer with convertion ratio 1/12
or 1/24.so with tranformer5 the o/p is about 400V.
The o/p is taken from the rectifier of the main transformer.so o/p voltage range is

DEPARTMENT OF ELECTRICAL AND ELECTRONICS                                                          Page 10
Dc to dc converter with controlled active clamping topology

4.CONVETRE DESIGHN AND SIMULATIONS

4.1CONVERTER DESIGN

To ensure a simple design an industrial standard buck voltage regulator was
used as basics of the snubber converter. Figure shows the principal realization. In this                design a
simple PI-controller is used to specify a desired upper rail in the snubber voltage Us.
The usage of a fixed upper limit leads to a very simple design, where standard
components can be used and additional measurement equipment can be avoided. To fulfill the
requirements given above the optimal components have to be chosen
Due to the fact that the charging requires much less power (depending on a battery
preserving charging method) it is possible to use cheaper components in the downlink path

Fig. 4. 1. Principle realization of the inverter stage

DEPARTMENT OF ELECTRICAL AND ELECTRONICS                                                                  Page 11
Dc to dc converter with controlled active clamping topology

4.2CONVERTER DESIGN-SIMULATION

To explain the operation principle the proposed converter structure was
simulated in PSPICE based on circuit level model. A 1 kW converter was modeled and
compared to conventional solutions.
The technical characteristics of the simulation model are:
Input voltage: 12V
Output voltage:400v
Max. output power:1000w
Switching frequency: 100 kHz
The simulation results are given in Table 1. One can seethe significant improvement of the
inverter's efficiency.

TABLE I.

EFFICIENCY COMPARSON (THEORETICAL IMPROVEMENT)

DEPARTMENT OF ELECTRICAL AND ELECTRONICS                                                         Page 12
Dc to dc converter with controlled active clamping topology

4.3CONVERTER DESIGN-REALIZATION

To verify the basic simulation results an advancedmodel was derived to
check the full dynamic behavior. Inthis 'practical' model the linearized current sink was
replaced by a switching regulator, with a transfer power ofmax. 10 A, and the results are
compared to a conventionalsnubber solution.

TABLE II.

EFFICIENCY COMPARSON

DEPARTMENT OF ELECTRICAL AND ELECTRONICS                                                          Page 13
Dc to dc converter with controlled active clamping topology

Table II shows the results when a state-of-the-art efficiency with auxiliary SMPS-
circuit for energy recovery was used. In all simulation models the duty-cycle was recalculated
and fixed during simulation.The comparison clarifies the improvement resulting from the altered
topology. It should be mentioned that anM2 rectifier (Fig. 6) can further reduce the losses, as
only one diode and no series connection of two diodes is necessary in the output stage. A
drawback is the center tapped secondary winding

Fig. 4.2Output stage with M2 rectifier

To explain the efficiency improvement of the new system more in detail a further
simulation step was performed. Here, the practical operation condition of the converter has been
taken into considerations. Figure 7shows the energy flow for quasi-sinusoidal input conditions.

DEPARTMENT OF ELECTRICAL AND ELECTRONICS                                                              Page 14
Dc to dc converter with controlled active clamping topology

Fig. 4.3 Load current and converter efficiency with conventional snubber solution

Fig.4.4Load current and converter efficiency with auxiliary solution estimated

DEPARTMENT OF ELECTRICAL AND ELECTRONICS                                                                 Page 15
Dc to dc converter with controlled active clamping topology

From the minimum input voltage and the required output power the maximum
average primary current can be estimated at about 110 A. The maximum voltage across the
primary transistors SI respectively S2 is twice the maximum battery voltage plus a margin
defined by the auxiliary converter. To keep the transfer current within an acceptable range it
makes sense to use about twice or more than the input voltage range. So they have to withstand
at least 40 V (12 V system) or 100 V (24 System) respectively In our test application two
IXFK120N20 have beenused. Care has to be taken when dimensioning the Shottkydiodes D3 and
D4 and the storage capacitor. Here excessive pulse currents will occur.
The blocking voltage is here slightly higher (max. 2 UIN + Us). As diodes the60 V
type MBR6060 was used. In conjunction with practical current ratings of the auxiliary converter
further simulation results concerning the snubber voltage show an optimum of about four times
of the input voltage. So in our case (12 V input voltage)the snubber rail will be at 50 V. This
leads to a maximum current in the auxiliary converter of 10 A. As a result the over-all efficiency
of the converter is maximized. In the appendix simple models (without losses) are given for
controller design.

DEPARTMENT OF ELECTRICAL AND ELECTRONICS                                                          Page 16
Dc to dc converter with controlled active clamping topology

5.ADVANTAGES OF DC TO DC CONVERTER

1.Big advantage of this topology is the adaptive control of this converter helps minimizing
system losses

2. The proposed topology is well suited in high current, low voltage applications, when the
design deals with high efficiency, small size, and high switching frequency.

3. The simple structure with a minimum of semiconductors and a minimum of heat
dissipation due to its feasibility to feed back most parts of the energy stored in the transformer's
leakage inductor.

DEPARTMENT OF ELECTRICAL AND ELECTRONICS                                                           Page 17
Dc to dc converter with controlled active clamping topology

6.CONCLUSION

The topology proposed here leads to several major advantages in the field of
battery-buffered solar converters. One of them is the simple structure with a minimum of
semiconductors and a minimum of heat dissipation due to its feasibility to feed back most parts of
the energy stored in the transformer's leakage inductor. The proposed topology is well suited in
high current, lowvoltage applications, when the design deals with high efficiency, small size, and
high switching frequency. Due to the current output characteristics of the structure, several
converters can easily be paralleled to form a solar battery array.
It is also possible to use a flyback converter with transformer to recuperate the
energy charged in the capacitor Cs of the overvoltage snubber. In this case energy can be fed into
the output. No potential barrier is necessary in this case as the input side of the flyback converter
(the voltage across the snubber capacitor CS)has to be controlled

DEPARTMENT OF ELECTRICAL AND ELECTRONICS                                                           Page 18
Dc to dc converter with controlled active clamping topology

7.REFERNCES

[1] Karl H ,Felix” DC-to-DC Solar Converter with ControlledActive Clamping System”-IEEE
power electronics & motioncontrolconfernce2006,EPEPEM2006,12 th international
Vol3,pages124-127

[2] M. Jain, M. Daniele, P.K. Jain: "A bidirectional DC-DCconverter topology for low power
application" IEEE Transactions on Power Electronics, Volume: 15 Issue: 4 , July 2000, pp.:
595-607