AC/DC CRITICAL CONDUCTION MODE BUCK-BOOST CONVERTER WITH UNITY POWER FACTOR

The buck-boost converter operating in critical conduction mode (CRM) is commonly utilized in various applications because of many advantages like protection against short circuit, minimum component count, low operating duct-cycle, and low voltage on MOSFETs. However, its input power factor (PF) is not high while operating in constant ontime control. To attain unity PF for universal input voltage range, a new control scheme of variable on-time control (VOTC) is proposed in this paper. The VOTC can be implemented by modulating the turn-on time of the buck-boost switch. The working principle and performance comparison of the converter is discussed with both types of control scheme. The input PF the converter is high in case of VOTC than the COTC. Simulation results are presented to verify the effectiveness of the proposed control strategy).


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 more power loss, voltage distortion and EMI compatibility issues etc.
In order to meet relevant harmonic standard and reducing input current distortion, power factor correction (PFC) converter has been widely applied (García et al., 2003;Singh et al., 2011;Memon et al., 2017;Memon et al., 2019). Generally, conventional power converter topologies, such as boost, buck-boost and buck converters, can be used to achieve low cost single-stage PFC, and each converter topology has its own characteristics.
The traditional boost PFC converter, with advantages of low input current ripple, high efficiency and inherent current shaping ability, is a good choice for PFC application.
However, it cannot maintain high efficiency at universal input voltage. Buck converter can maintain high efficiency at all input voltages. However, there is no input current when the output voltage is less than input voltage . The traditional buckboost topology, with advantages of inherent current shaping ability, low cost, step-down and step-up voltage conversion, is a good choice compared with flyback, CUK and SEPIC converters. It is used in many applications such as wind energy control, Adaptive control applications, and power amplifier applications etc. However, when the on-time is constant, the power factor (PF) of buck-boost PFC converter is low.
For modifying the performance of buck/boost converter, various researchers have proposed various techniques and control schemes. In Ghanem, Al-Haddad, and Roy, (1996), a new control mechanism is presented to increase the PF near to unity for a cascaded buckboost converter for the high-power application in continuous conduction mode (CCM).
Comparative analysis between single stage buck converter and the single buck-boost converter in discontinuous conduction mode is given in Moschopoulos and Zheng (2006).
The work in Wei et al. (2008)  buck-boost PFC topology for improving the efficiency. In Jayahar and Ranihemamalini (2011), inductor average current control strategy is proposed for improving PF of CCM buck-boost converter. The work in Jayahar, Ranihemamalini, and Rathnakannan (2016) has given the solution to improve PF for CCM buck converter. The bridgeless buck-boost converter with switched capacitor for low power applications is put forward in Saifullah et al. (2017) for reducing the conduction losses and improving the efficiency.
In this paper, an improved control scheme for buck-boost converter operating under critical conduction mode (CRM) is proposed to realize unity PF.
This paper is divided into six sections. In section 2, the operation states of CRM buckboost PFC converter are analyzed with traditional control. The proposed control scheme is introduced in section 3 to realize unity PF. Then the comparative analysis is discussed in section 4 in terms of input PF. In section 5, the effectiveness of proposed topology is evaluated by simulation results. Finally, some conclusions are drawn in section 6.  The instantaneous and rectified input voltage during half line cycle can be given as (1) Whereas ''Vpk'' represent the input voltage amplitude, θ represent the input voltage angle.

OPERATING PRINCIPLE OF THE CONVERTER
There are two switching cycles when buck-boost converter works in critical  The instantaneous and rectified input voltage during half line cycle can be given as: Whereas ''V pk '' represent the input voltage amplitude, θ represent the input voltage angle.
There are two switching cycles when buck-boost converter works in critical conduction mode (CRM). In case of first switching cycle, switch (Q b-b ) is ON, the inductor is charged as shown in Figure 2 and the value is given as Figure 1. Power circuit of a buck-boost converter.
The instantaneous and rectified input voltage during half line cycle can be given as (1) Whereas ''Vpk'' represent the input voltage amplitude, θ represent the input voltage angle.
There are two switching cycles when buck-boost converter works in critical conduction mode (CRM). In case of first switching cycle, switch (Qb-b) is ON, the inductor is charged as shown in Figure 2 and the value is given as Figure 2. the Operation of converter during switching pattern 1. (3) During second switching cycle, (Qb-b) is OFF; the inductor will discharge through load and output capacitor as indicated in Figure 3.
The expression for discharge time is (4) Also (5) on off t t t = +  The inductor and switch current waveforms are shown in Figure 4. From (4) and (5), following relation is obtained The duty-cycle of buck-boost switch is expressed as With traditional control the input current of buck-boost converter is given as The expression of average input power is derived as The inductor and switch current waveforms are shown in Figure 4. The inductor and switch current waveforms are shown in Figure 4. From (4) and (5), following relation is obtained (6) The duty-cycle of buck-boost switch is expressed as With traditional control the input current of buck-boost converter is given as The expression of average input power is derived as From (4) and (5), following relation is obtained: The duty-cycle of buck-boost switch is expressed as: With traditional control the input current of buck-boost converter is given as: The expression of average input power is derived as: The value of t on is calculated from '' (9)'' by assuming 100% efficiency: The input PF with traditional control scheme can be got by joining (1)  The table of input PF with traditional control is drawn in Table 1 with the help of equation (11) and the specification of the converter. It indicates low PF at high input voltage. Based on (9) and (12), Figure 5 is drawn. It indicates the comparison of measured current harmonic with IEC Class C limits. It can be observed that the 5th and 7th harmonic for converter is unable to meet the limit value. Specially, the 5th harmonic cannot meet the standard for universal input voltage range, while 7th harmonic at high input voltage. Based on (9) and (12), Figure 5 is drawn. It indicates the comparison of measured current harmonic with IEC Class C limits. It can be observed that the 5th and 7th harmonic for converter is unable to meet the limit value. Specially, the 5th harmonic cannot meet the standard for universal input voltage range, while 7th harmonic at high input voltage.

Proposed variable on-time control scheme to improve input PF
To achieve unity PF, the variation rule for ton should be

PROPOSED VARIABLE ON-TIME CONTROL SCHEME TO IMPROVE INPUT PF
To achieve unity PF, the variation rule for t on should be: By substituting (13) into (8), we can get average input current with VOTC as: It shows shape of average input current is purely sinusoidal at all input voltage. Thus, unity PF can be realized. From (1) and (15), the average input power is expressed as:

COMPARATIVE ANALYSIS
From (14), the input PF curve with proposed control scheme is drawn in Table 2, which also includes the PF values with traditional control scheme of Table. It can be concluded that the PF of the converter with proposed control is higher as compared to COTC. The percentage improvement of PF increases as the input rms voltage is increased.

SIMULATION VERIFICATION
For verifying the effectiveness of VOTC strategy, simulations are carried out. The input voltage range is 90-264VAC, and the output is 24V. For ensuring the current to be in CRM, L6561 IC is used. All the components in the circuit are selected as idea. The Simulation results in Figure 9 and Figure 10 shows that vin, and iin, for proposed converter with COTC and VOTC at 110VAC input, respectively. The input waveform shows that with VOTC the input current is sinusoidal as compared with COTC. Hence, the near unity PF can be realized by using proposed control scheme. Figure 9. v in , and i in , with COTC. Figure 10. v in , and i in , with VOTC.

CONCLUSION
A variable on-time control scheme and the implementation circuit are proposed in this paper to make the shape of average input current purely sinusoidal for the CRM buckboost PFC converter. The analysis and simulation results are given. Compared with that of the COT control: 1. Input current meets the harmonic standard.

PF is high
3. THD is low