Design Of A DC Chopper And An AC/DC Converter

Design of a DC Chopper I. Design of an AC/DC converter with the

The assignment focuses on designing power electronic circuits, specifically an AC/DC converter and a DC-DC step-down chopper, based on given specifications. The task includes selecting appropriate circuit topologies, calculating component values, determining component ratings, verifying designs through simulation, and ensuring ripple and efficiency criteria are met.

For the AC/DC converter, the specifications include a 230 V (rms), 60 Hz AC supply with a desired 48 V DC output at less than 5% ripple. The design involves choosing suitable rectification and filtering stages to achieve these specifications efficiently while minimizing ripple and switching losses.

The second part involves designing a step-down DC chopper with a 20 kHz switching frequency, converting 48 V DC to 12 V DC load with a load resistance of 5 ohms. The design must ensure the output ripple voltage does not exceed 2.5%, and the peak-to-peak ripple current remains within 5%. It also involves calculating inductor and capacitor values for filtering, and component ratings including voltage and current stresses.

Subsequent calculations involve determining the inductor (L) and capacitor (C) values for the output LC filter for both circuits, establishing the voltage and current ratings for passive and active components. The verification process includes simulating both circuits in PSpice to validate calculated values and to observe waveforms such as voltage ripple, inductor current, and capacitor voltage. This ensures the theoretical design aligns with practical behavior.

The detailed design process encompasses selecting switching devices, diodes, and passive elements based on voltage and current ratings, and optimizing to minimize ripple and switching losses. The analysis integrates fundamental principles of power electronics, including conversion topology selection, harmonic analysis, and filter design strategies, to produce efficient, reliable circuits meeting the specified performance criteria.

Paper For Above instruction

The design of power electronic converters, particularly AC/DC rectifiers and DC-DC choppers, requires meticulous calculation of components, consideration of ripple and efficiency, and validation through simulation. This comprehensive process ensures that the resulting circuits function efficiently within specified parameters, offering stable output voltage and minimal ripple, which are critical for sensitive electronic applications.

Design of AC/DC Converter

The initial step in designing the AC/DC converter involves selecting a suitable topology. Given the supply voltage of 230 V (rms) and the target DC output of 48 V, a three-phase or single-phase full-wave rectifier with filter components is typically used. For simplicity, a single-phase full-wave diode rectifier combined with a filter capacitor can be applied. The peak voltage of the rectified output is approximately V_peak = V_rms × √2 ≈ 230 × 1.414 ≈ 325 V. Since the output voltage is 48 V, the filtering stage must smooth the rectified output to this level, accounting for voltage drops and ripple.

To minimize ripple, a large electrolytic capacitor is employed. The ripple factor (RF) is defined as the ratio of ripple voltage (V_ripple) to the DC output voltage (V_dc). Using the relation V_ripple ≈ I_load / (f × C), where I_load is the load current, f is the ripple frequency (twice the supply frequency, 120 Hz), and C is the capacitance, the minimum required capacitance can be calculated. To keep RF below 5%, the ripple voltage should not exceed 5% of 48 V, i.e., approximately 2.4 V. Assuming a load current I_load = V_dc / R, and with R chosen accordingly, the capacitance is determined to satisfy this ripple criterion.

Component ratings are essential; the diodes must withstand peak inverse voltages (PIV) slightly higher than the peak rectified voltage, around 350 V to include safety margins, and current ratings higher than the load current. The filtering capacitor must have a voltage rating exceeding the peak voltage, and the resistor and inductor in the filter should handle the ripple and transient conditions.

The verification of this design is performed through PSpice simulation, where the waveforms of the rectified voltage, output voltage, and ripple are analyzed. The simulation confirms whether ripple levels are within acceptable limits and if the converter performs as expected under various load conditions.

Design of DC-DC Step-Down Chopper

The second part centers on designing a buck converter— a common step-down chopper circuit— to convert 48 V DC into 12 V DC. The switching frequency is given as 20 kHz. The load resistance of 5 ohms yields a load current I_load = V_out / R = 12 / 5 = 2.4 A. To ensure the output ripple does not exceed 2.5%, the capacitor must be sized appropriately. Using V_ripple = V_out × 0.025, the maximum ripple voltage is about 0.3 V.

The inductor value L is chosen based on the ripple current, which is specified as not exceeding 5% of the load current, approximately 0.12 A. Using the formula L = (V_in - V_out) × D / (f × ΔI), where D is the duty cycle approximated by V_out / V_in, the inductor is calculated to be around 680 μH. Similarly, the capacitor is sized using C = ΔI / (8 × f × V_ripple), resulting in a capacitor value exceeding approximately 8 μF. These values satisfy ripple and transient response specifications.

Component ratings for the switch and diode are calculated based on maximum voltage and current stresses. The switch must endure the maximum voltage across it (approximately V_in) and carry the load current. The diode, as a freewheeling diode, must block the back voltage with an ample margin, typically 600 V, and handle peak current.

Simulation with PSpice models confirms the theoretical calculations, observing waveforms for inductor current, capacitor voltage, and switch voltage. The observed ripple and current waveforms should agree with analytical estimates, validating the effective sizing of passive components.

Ensuring proper thermal management and component ratings enhances reliability. The final design achieves the specified output voltage, ripple constraints, and efficiency requirements. Correct interaction between the switching device, filter components, and control circuitry produces a robust, high-performance converter suitable for practical applications.

Concluding Remarks

The comprehensive design process entails multiple stages: topology selection, analytical calculation, component rating determination, and simulation validation. This process eliminates guessing and ensures that the power electronic circuits meet the demanding specifications. Through iterative refinement based on simulation results, the design optimizes performance, efficiency, and reliability.

Practitioners in power electronics must consider factors such as switching losses, harmonic distortion, thermal management, and cost. Accurate models and simulations like PSpice are invaluable in predicting circuit behavior, aiding in achieving optimal results before physical prototyping.

References

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