A novel buck-boost converter

proposed. Output voltage is positive and the voltage stresses on the power switches and the diodes are low. Suggested topology is based on conventional boost converter. Proposed converter can provide a large step down voltage conversion ratio. Control of converter can be done with a simple I-type (Integrator) controller.


Introduction
*Voltage bucking/boosting is required in many applications such as car electronics (Luo and Ye, 2004;Zhu and Luo, 2007a;Zhu and Luo, 2007b), fuel cell systems (Sahu and Rincón-Mora, 2004;Ren et al., 2008;Changchien et al., 2010;Liu et al., 2010) and digital devices like notebooks and cell phones. Some topologies are suggested for buck-boost converter using KY converter (Hwu and Yau, 2008;Hwu et al., 2009a;Hwu et al., 2009b). In Liao et al. (2012) a non-inverting buck-boost converter for fuel cell systems was proposed. Ismail et al. (2008) put two switched capacitor cell into the basic converter and obtained a series of DC-DC converters but input and output are not common grounded. Miao et al. (2016) proposed a buck-boost topology with high step-down gain, common ground between input and output and low voltage stresses on switches and diodes. This paper introduces a new buck-boost converter. Suggested converter can provide a wide range of output voltages. Its control can be done with a simple I-type controller. However, its uses more switches so switching and conduction losses increase. Also, output terminal and input terminal have no common ground.
Converter's operating principles; steady-state analysis, small-signal model and controller design problem are studied in this paper. Finally, the Simulink ® simulation is done.

Suggested topology
Suggested topology is shown in Fig. 1.

Fig. 1: Suggested topology
Switches S1, S2, and S3 are turned on and off simultaneously. To derive the relationship between input and output voltages, these assumptions are made: a) Inductor (capacitor) is very large so the current in (voltage across) it is constant. b) Circuit is operating in steady state (i.e. voltages and currents are periodic). c) For duty ratio of D, switches S1, S2 and S3 are close for time DT and open for (1-D) T. d) Switches and diodes are ideal.
When switches S1, S2 and S3 are closed, the diodes are off and circuit is as shown in Fig. 2.

Fig. 2:
Circuit with switches S1, S2 and S3 closed and diodes D1, D2 and D3 off Output voltage ( ) must be positive otherwise diode D3 can't be reverse biased. Inductor voltage ( ) for 0 < < can be calculated as (Eq. 1): When switches S1, S2 and S3 are opened, the diodes are closed and circuit is as shown in Fig. 3. (2) Average voltage across inductor must be zero for periodic operation. Eq. 1 and 2 are combined to get (Eq. 3): Voltage conversion ratio (M) vs. duty ratio (D) is shown in Fig. 4.
To determine the boundary between continuous and discontinuous current, minimum inductor current (IL,min) is set to zero. This leads to (Eq. 7): so, converter works in CCM if (Eq. 8):

Voltage stresses
Voltage stress on different components of the circuit is the most important criteria to choose the appropriate devices. When switches S1,S2 and S3 are closed diodes D1 and D2 are reverse biased with voltage equal to -Vs volts and D3 is reverse biased with -Vo volts. When diodes D1, D2 and D3 are forward biased switches S1, S2 and S3 must tolerate Vs, Vs and Vo volts, respectively.

Dynamic of converter
When switches S1, S2 and S3 are closed (0 < < ) circuit's Eq. can be written as (Eq. 9): When Diodes D1, D2 and D3 are forward biased ( < < ) circuit's Eq. can be written as (Eq. 10): Applying State Space Averaging (SSA) to these Eq. 11 leads to: Simulink diagram is shown in Fig. 5. Output voltage is shown in Fig. 6  Assume output load changes from 50 Ω to 18.75 Ω at t= 20 ms. As shown in Fig. 7, output voltage changes.
To avoid such changes, a close loop control system must be designed. For the aforementioned values control to output transfer function is calculated as (Eq. 15): Pole-zero and Bode diagram of Eq. 15 is shown in Fig. 8 and 9, respectively. Using Routh-Hurwitz table 0 < K I < 0.249 stabilize the system. Using MATLAB's control system toolbox K I = 0.11 is selected to have no overshoot. Testing the performance of close loop system is done with the aid of following scenario: Input voltage source changes from 100 V to 75 V at t=100ms, output load changes from 50 Ω to 18.75 Ω at t=200 ms and finally, control system reference signal changes form 200 V to 250 V at t= 300 ms. Table 1, summarize the aforementioned scenario. Response of close loop system to the test scenario is shown in Fig. 10.  As shown in Fig. 11 proposed topology can provide a high step down gain.

Conclusion
Voltage bucking/boosting has many applications. A novel buck-boost topology has been proposed in this paper. Steady state, dynamical behavior and control of proposed converter has been studied. Control of suggested topology can be done with a simple I type controller. Proposed topology can provide a high step down gain and can be used for applications which load's voltage must change in a large range.