Introduction To Dc Dc Converter Engineering Essay

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ABSTRACT

A bidirectional dc-dc converter is used for dc-dc power conversion applications. The power converter includes two full bridge converters This Bidirectional dc-dc converter is best for electrical vehicle applications. A bidirectional, isolated topology is proposed in consideration of the differing fuel cell characteristics from traditional chemical-power battery and safety requirements. The topology proposed in the paper has advantages of simple circuit with soft switching implementation without additional devices, high efficiency and simple control.

INTRODUCTION TO DC-DC CONVERTER:

DC-DC converters are devices which change one level of direct current/ voltage to another (either higher or lower) level. They are primarily of use in battery-powered appliances and machines which possess numerous sub circuits, each requiring different levels of voltages. A DC-DC converter enables such equipment to be powered by batteries of a single level of voltage, preventing the need to use numerous batteries with varying voltages to power each individual component.

 1.1. BUCK-BOOST CONVERTER

Fig. 1: schematic for buck-boost converter

With continuous conduction for the Buck-Boost converter Vx =Vin when the transistor is ON. When the transistor is OFF the inductor in trying to maintain the current in the same direction reverses its polarity as a result of which the diode is forward biased and Vx =Vo. For zero net current change over a period the average voltage across the inductor is zero.

Fig. 9: Waveforms for buck-boost converter

Vin ton + Vo toff = 0

which gives the voltage ratio

and the corresponding current

Since the duty ratio "D" is between 0 and 1 the output voltage can vary between lower or higher than the input voltage in magnitude. The negative sign indicates a reversal of sense of the output voltage.

 CONVERTER COMPARISON

The voltage ratios achievable by the DC-DC converters is summarised in Fig. 10.We can notice that only the buck converter shows a linear relationship between the control (duty ratio) and output voltage. The buck-boost can reduce or increase the voltage ratio with unit gain for a duty ratio of 50%.

Fig. 10: Comparison of Voltage ratio

 

1.3 BI-DIRECTIONAL DC-TO-DC CONVERTER

A DC-DC converter which can be operated alternately as a step-up converter in a first direction of energy flow and as a step-down converter in a second direction of energy flow is disclosed. Potential isolation between the low-voltage side and the high-voltage side of the converter is achieved by a magnetic compound unit, which has not only a transformer function but also an energy storage function. The converter operates as a push-pull converter in both directions of energy flow.

The DC-DC converter can be used for example in motor vehicles with an electric drive fed by fuel cells.

A bi-directional converter for converting voltage bi-directionally between a high voltage bus and a low voltage bus, comprising a switching converter connected across the high voltage bus, the switching converter comprising first and second switching modules connected in series across the high voltage bus, a switched node disposed between the switching modules being coupled to an inductor, the inductor connected to a first capacitor, the connection between the inductor and the first capacitor comprising a mid-voltage bus, the first and second switching modules being controllable so that the switching converter can be operated as a buck converter or a boost converter depending upon the direction of conversion from the high voltage bus to the low voltage bus or vice versa; the mid-voltage bus being coupled to a first full bridge switching circuit comprising two pairs of series connected switches with switched nodes between each of the pairs of switches being connected across a first winding of a transformer having a preset turns ratio; and a second full bridge switching circuit comprising two pairs of series connected switches with switched nodes between each of the pairs of switches being connected across a second winding of the transformer, the second full bridge switching circuit being coupled to a second capacitor comprising a low voltage node.

1.3.3 WORKING OF DC-DC Converters

In its simplest form, a DC-DC converter simply uses resistors as needed to break up the flow of incoming energy - this is called linear conversion. However, linear conversion is a wasteful process which unnecessarily dissipates energy and can lead to overheating. A more complex, but more efficient, manner of DC-DC conversion is switched-mode conversion, which operates by storing power, switching off the flow of current, and restoring it as needed to provide a steadily modulated flow of electricity corresponding to the circuit's requirements. This is far less wasteful than linear conversion, saving up to 95% of otherwise wasted energy.

1.3.2 BIDIRECTIONAL DC-DC CONVERTERS TOPOLOGIES

There are many circuit topologies for bidirectional dc-dc converter. Some of them are

Non isolated (Without transformer):

Full bridge bidirectional dc-dc converter (shown in fig)

Half bridge bidirectional dc-dc converter

II. Isolated (with transformer):

Full bridge bidirectional dc-dc converter ( shown in fig)

Half bridge bidirectional dc-dc converter

1.3 NON-ISOLATED BIDIRECTIONAL DC-DC CONVERTER:

Fig2: Full bridge bidirectional dc-dc converter

Fig 17 shows a basic circuit diagram of a full bridge bidirectional DC-DC converter.

It has interleaved operation for both boost and buck modes

It has smaller passive components

It has less battery ripple current

1.3.2.2 ISOLATED BIDIRECTIONAL DC-DC CONVERTER (PROPOSED CONVERTER):

Fig18: lv-side "current source" and hv-side "voltage source"

Fig 18 shows the circuit diagram of an Isolated DC-DC converter. This converter has the following features

Simple voltage clamp circuit implementation

Simple transformer winding structure and low turns ratio

High choke ripple frequency (2fs)

Start up problem will be present in this circuit

1.4 SEMICONDUCTOR SWITCHING:

Semi conductor switching are of two types. They are

1. Hard Switching

2. Soft Switching

1.4.2 SOFT SWITCHING

More recently, new power conversion topologies have been developed which dramatically reduce the power dissipated by With "soft switching" techniques, reduction in wasted power will often improve the efficiency of a unit by more than 2%. While this does not sound significant, it can account for a saving of more than 20 W in a 1000 W power supply. This 20 W is power that would have been dissipated by the main power transistors, the most critical and most heavily stressed semi-conductors in any switch mode power supply. Reducing the power here lowers their junction temperature, giving increased thermal operating margins and, hence, a longer life for the power supply. Not only does a "soft switching" power supply generate significantly less electrical noise, it achieves greater efficiency, longer mean time between failures (MTBF), and higher immunity to the effects of other equipment operating nearby.

It is desirable for power converters to have high efficiencies and high power densities. Packaging and cost limitations require that the converter have a small physical size and weight. Power density and electrical performance are dependent on the switching frequency as it determines the values of the reactive components in the converter. Thus, high frequency operation of the converter is highly desired. However, operation at high frequency results in higher switching losses and higher switching stresses caused by the circuit parasitics (stray inductance, junction capacitance).

The circuit topology of the proposed bidirectional isolated converter is shown in Fig. According to the power flow directions, there are two operation modes for the proposed converter. When power flows from the low-voltage side (LVS) to the high-voltage side (HVS), the circuit operates in boost mode to draw energy from the battery. In the other power flow direction, the circuit operates in buck mode to recharge the battery from the high-voltage dc bus. Based on the symbols and signal polarities introduced in Fig. 2, the theoretical waveforms of the two operation modes are shown in Fig. (a) and (b), respectively.

Fig42: Theoretical waveform under (a) boost and (b) buck operation

Boost Mode (Discharging Mode) Operation

When the dc bus voltage in the HVS is not at the desired high level, such as during a cold start, the power drawn from the low-voltage battery flows into the high-voltage dc bus. During this mode, the proposed converter is operated as a current-fed circuit to boost the HVS bus voltage. The LVS switches Q1, Q4 and Q2, Q3 operate at asymmetrical duty ratios and 1- which require a short overlapping conduction interval. Referring to the equivalent circuits for the boost mode operation in Fig. 43, the detailed operating principle can be explained as follows.

Although the LVS switches subject to higher voltage stress, this is an advantage because the battery voltage is low. Because the overlapping interval for the LVS switches Q1, Q4 and Q2, Q3 is very short, the LVS transformer current flows through only one LVS switch at most time. Thus, the conduction losses for Q1, Q4 and Q2, Q4 can be greatly reduced to improve the conversion efficiency. Moreover, the LVS circuit produces a relatively ripple free battery current that is desirable for the low voltage battery. The voltage transfer ratio Mboost for the boost mode operation for the proposed dc-dc converter can be derived from the volt-second balance condition across the inductor L1 represented by (7). The current stresses of the inductor windings can be also determined as (6).The inductances of the power inductor L1 can be determined for their given peak-to-peak current ripples, ΔI1

Where λ (%) is the ripple percentage of the inductor currents IL1

B. Buck Mode (Charging Mode) Operation

Different from the traditional electric vehicle driving system, the fuel cell powered system needs an additional energy storage device to absorb the feedback power from the electric machine. This energy storage device may be a lead-acid battery as shown in Fig44 . The proposed circuit works in buck mode to recharge the battery from high-voltage dc bus. During this mode, the proposed converter is operated as an asymmetrical half bridge circuit with synchronous rectification current doubler to recharge the battery from high-voltage dc bus.

The HVS switches Q5, Q8 and Q6, Q7 operate at asymmetrical duty ratios and 1- which require short and well-defined dead time between the conduction intervals. Referring to the equivalent circuits in Fig. , the detailed operating principle of this mode can be explained as follows.

Fig44: modes of operation in buck mode

While the LVS switches, Q1, Q4 and Q2, Q3, share unequal voltage and current stresses, the HVS switches, Q5, Q8 and Q6, Q7, share equal voltage stresses as (8). Then the current stresses of the HVS switches can be found as

DESIGN CONSIDERATIONS FOR KEY COMPONENTS

To verify the feasibility of the proposed scheme, a 2-kW laboratory prototype operated at 20 kHz was built. The simulation and experimental results will be shown and discussed in the next section. The LVS of the design example was connected to a 12-V lead-acid battery whose terminal voltage could swing from 10-15 V. The nominal voltage on the HVS dc bus was designed to 300 V, with an operating range from 150-400 V. The design considerations

Based on (5), the turn-ratio selection of transformer can be calculated as (15). The HVS device ratings can then be calculated using (8)-(10) as follows:

B. Power Inductors

Let the peak-to-peak current ripples be 20% of the inductor currents under full power. The current rating and the inductance of the power inductor L1 can be determined using (6)- (7) as follows:

Because of the ripple cancellation on the battery current, a larger ripple current in inductor L1 and can be allowed in practical applications. Thus, the inductance and the size of the inductors L1 might be smaller.

To verify the theoretical operating principles, a 2-kW design example was simulated by using MATLAB. There is a good agreement between the simulation results and theoretical analysis. In this research, a 2-kW laboratory prototype was implemented and tested to evaluate the performance of the proposed bidirectional isolated dc-dc converter.. The ripple cancellation between two inductor currents can be observed. This is desirable for a low-voltage battery.

7.1 BOOST OPERATION FOR BIDIRECTIONAL DC-DC CONVERTER

CONVENTIONAL CIRCUIT FOR BOOST MODE

7.3 RESULTANT WAVE FORM:

7.3.1 BOOST OPERATION

Input and Output waveform:

Fig 48

CONVENTIONAL CIRCUIT FOR BUCK MODE

Fig47

7.4 RESULTANT WAVEFORM FOR BUCK OPERATION :

Input and Output voltage waveform

Fig 49

Proposed Bidirectional DC-DC converter

Input and Output Voltage Waveforms:

Fig50

Fig 51

Inductor Current Waveforms:

Fig52

CONCLUSION

A soft-switched isolated bidirectional dc-dc converter has been implemented in this paper. The operation, analysis, features and design consideration were illustrated. Simulation and experimental results for the 45W, 20 kHz prototype was shown as per principle. It is shown that ZVS in either direction of power flow is achieved with no lossy components involved As results, advantages of the new circuit including ZVS with full load range, decreased device count, high efficiency (measured more than 94% at rated power), and low cost as well as less control and accessory power needs, make the proposed converter very promising for medium power applications with high power density.

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