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CHAPTER 5

CONCLUSION AND FUTURE WORK

The present study considered a switching DC-DC buck converter

controlled with Pulse Skipping Modulator with the main objective of

exploring the possibility of applying PSM as whole time mode at low and

medium power applications up to tens of watt. The work also examined the

application of PSM mode in hybrid mode converters apart from using it as a

light load mode.

Study revealed that the regulator has remarkable advantages such

as stability, speed of response, light weight, volume, less expensive, less

complex, regulation over a wide input voltage range.

The converter is stable especially when implemented with

hysteretic control, as it is a variant of on-off control. Response to change in

input voltage and load is good since the inductor current is controlled. The

converter is light with reduced volume due to reduced component count. The

possibility of operation at higher frequency reduces the size of the

components. Less expensive since the controller is simple and board size is

small with reduced BOM.

The converter has some demerits, which are potential enough to

threaten the widespread use of regulator in many applications. If a solution is

not sought after the regulator with all its merits would go unpopular and

would not serve the needy. The demerits include the output voltage ripple and

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high device peak current. It is studied in this thesis and with capacitance and

its parasitic resistance playing a role with a limitation from duty cycle

constancy resulting in peak currents, techniques were mainly concentrated on

inductor current control since inductor current ripple is the mains cause of

voltage ripple.

Study on the exhibition of nonlinear phenomena revealed that the

converter under CCM is chaotic with periodic windows with reduced ripple.

Under discontinuous conduction mode the converter was found to be chaos

free as expected due to order reduction since chaos in a driven system is

observed with the order two or more.

Thesis after studying the converter along with its PWM counterpart

proposed techniques to reduce the ripple and device peak currents through

inductor current control and hybrid regulator with cascaded LDO. In hybrid

mode converters a possibility of avoiding chaos is also proposed after

examining the possibility of operating a converter in DCM with PSM as

hopped mode when circuit parameters correspond to CCM in PWM mode.

5.1 CONTRIBUTION OF THE THESIS

Various operating modes of PSM buck converter classified and

explained. Bifurcation diagrams and Poincare’ sections during chaotic and

aperiodic operation were obtained for the converter under PWM and PSM

control under continuous conduction mode.

A method to plot Bifurcation diagram using PSIM was discussed

and a diagram was obtained for the converter with PSM control under

discontinuous conduction mode and found that the converter is chaos free.

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A modified control technique is proposed, to operate a converter

with discontinuous inductor current through pulse skipping, when the circuit

parameters do not correspond to DCM.

An improved PSM buck converter is proposed that contributes to

improvement of ripple performance in converter with stability and controlled

starting transients.

A hybrid regulator, that combines a PSM converter with linear

regulator, is proposed and studied with SIMPLIS and verified experimentally.

A method to avoid chaos in hybrid PWM/PSM converter through

mode hopping is studied with simulation results.

Found that the PSM converter can be improved so that PSM can be

a whole time control mode.

5.2 FUTURE SCOPE OF THE WORK

Methods discussed herein can be extended to other converter types

like boost, buck boost and similar other derivatives. Digital implementation of

the control is possible using FPGAs. Future work possibilities are discussed

hereunder.

The frequency of operation of the regulator is usually set above the

audio range. Power supplies are required to operate at high efficiency. When

the power supplied is a small fraction of the rated power of the regulator the

switching loss dominates and efficiency is to be improved by reducing the

frequency of operation, which is done naturally by PSM regulator.This would

result in resultant switching frequency falling within the band of audio

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frequencies with input current frequency components falling within audio

frequency range as shown in Figure 5.1.

Figure 5.1 Input current harmonic components – PSM converter

This would cause audio noise and reduce board life due to

mechanical resonance in capacitors and coils (Huber Jovanovic 2011). This

uncontrolled audio noise is a demerit in PSM regulator. This can be avoided

by interleaving the charge and skip control pulses so as to avoid the frequency

components coincide with board component natural frequency with suitable

modification in control.

Interleaving technique divides the current between the phases and

allows high frequency output voltage ripple while keeping the same switching

frequency for each phase. The switching DC-DC power supply has inductor

current ripple leading to output voltage ripple. The interleaved converter

shown in Figure 5.2 (Mummadi Veerachari 2005), operates with control

signals out of phase with ripple currents of different phases staggered with

suitable control signals as shown in Figure 5.3, for a typical two phase buck

converter. This converter can be controlled with PSM signals for ripple

0 .0 0.04 0.08 0.12 0 .16 0.20Frequenc y (MHz)

0 .0

0.1 0

0.2 0

0.3 0

0.4 0

0.5 0

0.6 0

Input Current Spec trum

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reduction, which would also reduce losses of the magnetic core and switches.

(Park and Jian Sun 2008)

Figure 5.2 Interleaved buck converter – PWM

Figure 5.3 Control and output current signals – PWM

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APPENDIX

DATASHEETS

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138

139

140

141

142

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Solder side of PCB for PSM LDO cascade

Component side of dot board for PSM DC/DC converter

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Hardware test