MODIFIED VARIABLE ON-TIME CONTROL SCHEME TO REALIZE HIGH POWER FACTOR FOR AC/DC INTEGRATED BUCK-BOOST CONVERTER

In today’s modern era low power factor (PF) is major issue in the field of power electronics which has made our life, simpler, easier and comfortable. However, with this comfort and easiness this technology brings power quality issues because it is centered on solid-state devices. These issues introduce harmonic contained current or distorted current which has several drawbacks like high power loss, voltage distortion and EMI compatibility issues etc. The conventional boundary conduction mode (BCM) integrated buck-boost converter (BBC) operating with constant on-time control (COT) control scheme have low PF with high total harmonic distortion (THD) because of harmonic contained input current waveform. So, in order to make the input current waveform as a sinusoidal by changing the on-time of only buck switch, a modified variable-on-time (VOT) control scheme for Integrated BBC is proposed in this paper. The VOT control scheme can achieve high PF with low THD by utilizing the input and output voltage to modulate the on-time of only buck switch. The theoretical analysis is given, and the simulation results confirm the advantages of the proposed control scheme. The object of the research paper is to propose the control scheme to realize unity PF for CRM buck converter by only modulating the on-time of buck switch. The research methodology is based on: Input PF analysis of buck converter with traditional control scheme; Introduction of proposed control scheme; comparative analysis and simulation results to show the effectiveness of proposed control scheme.


INTRODUCTION
Power electronic technology is employed in various sorts of modern equipment's which has made our life, simpler, easier and comfortable. However, with this comfort and easiness this technology brings power quality issues because it is centered on solid-state devices.
These issues introduce harmonic contained current or distorted current which has several drawbacks like high power loss, voltage distortion and EMI compatibility issues etc.
The DC/DC converters or choppers such as buck, boost and buck-boost etc. are normally employed for PFC application.
Each converter has its own advantages and disadvantages. The boost converter is good selection for PFC due to various advantages like high PF, less current ripples and high efficiency , However, its efficiency is low at low input line due to use of more duty-cycle when the input is 90Vrms and the output is 400 V . The buck-boost converter can step up or step down the input voltage and its characteristics are better as compared to SEPIC, Flyback and CUK converter. However, its efficiency is low, voltage and current stress is more compared to boost and buck converter because the energy of output is charged from inductor (Chen & Maksimović, 2010;Hwang & Park, 2012).
The buck converter now days have attracted the attention of the many researchers (Mahdavi & Farzanehfard, 2010;Liu et al., 2014;Wu et al., 2011;Wu et al., 2012;Memon et al., 2016;Memon et al., 2018aMemon et al., , 2018bMemon et al., 2019aMemon et al., , 2019bMemon et al., , 2019cMemon et al., , 2019dLiu et al., 2020). It can maintain high efficiency at all input voltages. It also offers other advantages like cost reduction, low output voltage, protection against inrush current, low stress on the switch and life time improvement. The input PF of buck converter in low because of dead zone in the average input current that is present until input voltage is more than output voltage. It causes the input current to contain large harmonic disturbances.
This paper is divided into six sections. In section 2, the operation states of BCM IBBC are analyzed with traditional constant on-time control scheme (COTCS). The introduced VOTCS is discussed in section 3. Then the comparative analysis is discussed in section 4 in terms of PF. In section 5, the effectiveness of proposed topology is evaluated by simulation results. Finally, some conclusions are drawn in section 6.

RESEARCH METHODOLOGY
The research methodology is based on: 1. Mathematical analysis of the operating principle of the traditional control scheme for BCM IBBC with the help of MATHCAD software.
2. Analysis of input PF of BCM integrated BBC.
3. Introducing the proposed control scheme to obtain high PF with low THD.
4. Comparative analysis of the converter for COT control scheme and VOT control scheme strategy in terms of input PF and THD.
5. Developing the simulation model of the converter with traditional and proposed control scheme with the help of MATLAB software.
6. Confirming the results. Figure 1 shows the main circuit of the IBBC. It is a buck converter in series with a boost converter with a common inductor. It works in buck mode with the boost switch opened when the instantaneous input line voltage is higher than the boundary voltage and otherwise in boost mode with buck switch closed. The boundary voltage is set a little higher than the output voltage. The converter operates in BCM and its working principle can be analyzed in two cases. The input voltage and rectified input voltage is expressed as

CONVENTIONAL COTCS FOR BCM IBBC
where V rms is the rms value The IBBC is working in buck mode until input voltage is more than boundary voltage.
Thus Q buck is operating while Q boost is off When buck switch is on, the inductor voltage is expressed as where So its peak value is where t on is the on-time of the switch.
While Q buck is off, the inductor is discharged through load.
For each switching cycle, the value of off-time is calculated from inductor's volt-second balance (6) From (3) and (5), the following relation is obtained (7) In addition (8) Substituting (6) into (7) The value of average input current for buck converter is got from (10) Substituting (3) and (8) into (9) (11) The IBBC is working in boost mode until input voltage is less than boundary voltage. Thus

Q boost is operating while Q buck is off
When boost switch is on, the inductor voltage is given as Same as (6), for each switching cycle, the value of off-time is calculated from inductor's volt-second balance (13) Substituting (13) into (8), we get (14) The value of average input current for boost converter is determined as (15) Combining (12) and (14), we get (16) Based on above analysis, the average input current of the IBBC with COTCS is as following. (17) Based on (1) and (16), The input power of the IBBC is expressed as (18) Now t on can be calculated by assuming the efficiency of IBBC as 100% (19) And input power factor can be calculated as (20) Based on (16-20) and specification, the curve of the input PF can be drawn and is given in Figure 2 from which it can be observed that the input PF is not unity for the wide range of the input voltage.

PROPOSED VOTCS FOR BCM IBBC
For obtaining unity PF for the buck converter, the on-time of Q buck in (11) should vary as (21) where k on1 is a constant Substituting (21) into (11), the average input current of the buck converter is The average input current of the IBBC with VOTCS is (24) Because of the power balance of the input and output, the input current for the unity PF can be expressed as (25) Based on (25-26), the input current at can be written as (26) From (26), it can be concluded that From (27-28) it can be summarized as k on1 = k on2 = k on Hence, the input current of the IBBC with VOTCS is Equation (29) demonstrates that input current of IBBC with VOTCS is pure sinusoidal.
Thus unity PF can be realized by using proposed control scheme. From (14), the input PF curve with proposed control scheme is shown in Figure 3 which also includes the PF values with traditional control scheme of Figure 2. It can be concluded that the PF of the converter with proposed control is higher as compared to COTC. The values of input PF for COTCS & VOTCS along with percentage improvement is written in Table   1. It can conclude that improvement at low line is more as compared to high line.

SIMULATION RESULTS
In order to verify the effectiveness of VOT control scheme, simulations results are given.
The range of input voltage is 90-264 V rms . The output voltage is selected as 90 V. The control IC 6561 will ensure the current to be in BCM. Ideal components are used in simulation   COT control scheme and VOT control scheme at 220VAC inputs, respectively. It can be conducted that current is almost sinusoidal in case of VOTCS as compared to COTCS.

CONCLUSIONS
In this paper, integrated BBC is presented and analyzed. It is composed of buck and boost converter. With COT control scheme, the input PF of the IBBC is low. In order to attain high input PF, VOT control scheme is proposed. Simulation results are presented for the verification of the analysis.