DESIGN AND ANALYSIS OF TWO STAGE BOOST CONVERTER WITH IMPROVED CONTROL

: In this article, a high-gain two-stage boost converter with improved control is proposed. In order the increase the regulation quality and power density a common control strategy introduced. In the proposed control method, each boost stage is controlled by the same controller. This method provides easy control and low cost and allows the use of lower voltage capacitors at the output of first boost stage. The converter is designed in two stages to increase from 12 VDC input voltage to 42 VDC and 150 VDC. Detailed theoretical analysis of the proposed converter is carried out and validated with a prototype having 100 W output power. Using the results obtained from the application circuit, the proposed boost converter with improved control is compared with the conventional two-stage boost converter operated under the same conditions.


INTRODUCTION
Nowadays, with the development of technology, the need for energy has increased.Thus, the importance of electrical energy and renewable energy sources has increased and efficient conversion of energy obtained from renewable energy sources has gained importance (Diaz-Saldierna et al., 2021; Bodur et al., 2021).PWM DC-DC converters are widely used in the energy conversion applications such as solar energy systems, electric vehicles, battery charging, defense industry etc. High efficiency operation of these converters is a requirement for the power electronics field (Bodur and Yıldırmaz, 2017;Zhao and Lee, 2003;Navamani et al., 2022).
The importance of converters working with high efficiency and power density has increased and many studies are being carried out today.Increasing power efficiency and density provides advantages in terms of cost and sizes.As the switching frequency increases, the filter sizes get smaller however the switching losses become dominant (Zhang et al., 2022).
In general, the output voltages of the renewable energy sources are relatively low so that the converters used in this field have high voltage gain.In addition, DC-DC converters used in the renewable field must have low input current ripple.As a result, inrush current ripples can damage the devices.
The conventional boost converter is used to increase the DC voltage applied to the input and obtain high voltage at the output.Circuit structure consists of a coil that stores energy in a magnetic field, a semiconductor switching element that switches at high frequency, and a fast diode.In addition, boost converter is widely used in industry due to its natural compatibility with power factor correction circuits (Ozdenturk and Akkaya, 2020;).
The duty cycle D is increased to obtain high output voltage in conventional DC-DC boost converters.However, increasing duty cycle D too much causes high current ripple, lower efficiency and difficulty in control (Bodur et al., 2020;Bodur et al., 2013).
The boost converter has nonlinear voltage as shown in Figure 1.The voltage gain increases from 0 to the optimum point Dopt.Higher duty cycle than Dopt results in less output power and lower efficiency (Upadhyay and Kumar, 2019).In the conventional boost converter, as the duty cycle increases, the switching element conducts for a longer period, while the conduction time of the diode becomes shorter.Thus, inductor current ripple increases.Today, this drawback resulted the duty cycle limit feature to appear in boost converter controllers (Zhang and Park, 2022;Tarzamni et al., 2023).Two-stage circuits have been proposed to solve the duty cycle problem in the boost converter.Although two-stage circuits have high regulation quality, efficiency is low because the power is processed twice.These circuits have high dynamic response time, high cost and complex circuit structure.(Isilay, Haci and Abdulkerim, 2022).
In order to achieve high voltage-gain and overcome duty cycle limitations, a two-stage boost converter proposed in this paper.Also, a common stage control is used to provide simplified circuit structure, reduced costs, fast transient response and higher power density.
The converter is obtained by using two-series converter in a cascade structure.The gain of the converter improved.Since power is processed twice, power losses increased.As a matter of fact, the gain of the circuit increased at the expense of decreasing power efficiency.(Navamani et al., 2018) Circuit structure is given in Figure 2.

Control Method
The conventional two-stage boost converter uses two separate controllers for each stage as shown in Figure 3.The controls are independent of each other and each stage is controlled according to its own output voltage as proposed (Gragger et  In this paper control for Stage 1 and Stage 2 are combined so that single controller is used instead of two separate controllers.In proposed control method, feedback received from output voltage of the Stage 2 and the voltage gain ratio of the two converter stages are same, so the duty cycles gets equal.This method simplifies the control and provides lower cost by reducing the number of controllers and components.Furthermore it provides fast response thanks to single feedback loop.Suggested control structure diagram is given in the Figure 4 below.

Operating Principles and Analysis of Proposed Converter
The proposed two stage boost converter is given in Figure 5 below.Following equations are valid for the converter.

Converter Design Considerations
The design parameters of the converter are given in Table 1.

Duty Cycle Consideration
The output voltages are obtained as below where  1 and  2 are duty cycle for stage 1 and stage 2 respectively.
Following equations are valid while determining of the duty cycle.

Inductor Design
The proposed converter is operated with critical conduction mode.Waveforms are given in Figure 10.

Figure 10: Inductor voltagecurrent waveforms when the converter was operating in boundary conduction mode.
Following equations are valid for Stage 1.
Following equations are valid for Stage 2. The boost inductance of Stage 2 L2(critical) is obtained in (19).
Input power is obtained below assuming that the efficiency of Stage The converter operates with max.duty cycle D when input voltage gets minimum.Thus, max.duty cycle D is calculated as below.
Ripple current of the inductance is equal to its peak value in critical conduction mode boost converter.Thus, peak value of the inductance for Stage 1 is calculated as below.
∆ 1 =  1() = 2  = 12  (24) Then, the critical inductance value of Stage 1  1() is calculated as below.Using the equations 26-32, stage 2 critical inductance  2() is calculated.First, the power in stage 1 is calculated with equation 26.Using stage 1 output power,  1 current calculated with equation 27.The ripple value of the current is equal to twice the output current of stage 1 in equation 28.Since the control is common, the duty is taken equal in equation 29 and average current of  2 inductance is equal to stage 1 output current in equation 30.Then the value of,  2 inductance is given in equation 31.
Since DCM operation provides zero soft switching turn on for the switches and soft switching turn-off for the boost diodes and easy control, the prototype is operated in discontinuous conduction mode (DCM (Koc et al., 2022).

Control Design
In this work, conventional two stage boost converter and proposed two stage boost converter are operated under same conditions and compared with each other.In conventional two stage boost converter, output voltages of the two sateges are controlled separately.The voltage feedback of each stage is measured and substracted from a reference voltage to produce an error voltage.The gate signal is produced by comparing the error voltage with the saw signal.The control structure is shown in Figure 11.

PROTOTYPE RESULTS
Prototype results of a two stage boost converter with separate and common control are examined in this section.Theoretical analyses have been verified with a DC-DC two stage boost converter application having input voltage of 12 VDC, output voltage of 150 VDC, and output power of 100 W.
Prototype design parameters given in Table 2 below.The load is switched on and off.As seen in Figure 16 (a) traditional controlled converter Stage 1 output voltage has voltage overshoot, while the converter using proposed converter doesn't have any voltage overshoot at Stage 1 output voltage.This is the significant advantage of common controlled boost converter.Thus, the lower voltage withstand capacitor can be used at the stage 1 output, resulting in reduced cost and increased power density.When the load is switched off in a separately controlled two stage boost converter, stage 1 tries to regulate itself because the control structure is separate.When the load was switched off, the voltage overshoot is observed in stage 1 voltage since there was no power transfer from stage 1 to stage 2 at this instant.In two-stage control, the feedback signal of stage 2 acts as the feedforward signal for stage 1 and prevents overshoot.The soft start feature in the output voltages of the separate and common controlled boost converter can be seen in Figure 17

Figure 3 :
Figure 3: Traditional Control Method of Two-Stage Boost Converter.

Figure 4 :
Figure 4: Proposed Control Method of Two-Stage Boost Converter.

Figure 5 :
Figure 5: Proposed Two-Stage Boost Converter.The following equations have been used while performing the proposed converter analysis.1.  1 and  2 are the switches of the converter.2.  1 and  2 are the output diodes of the converter.3.  1 and  2 are the output capacitors of each converter stages.4.  1 and  2 are the boost inductances of each converter stages.5.   is the input voltage and   is the output voltage.6. Semiconductor power devices are ideal.7. The input voltage and output voltages are constant over a switching period.8. Converter operates in boundary conduction mode.The proposed converter has 2 operating intervals.The key waveforms for each stage are given in Figure6and Figure7below.

Figure 11 :Figure 12 :
Figure 11: Separate-stage controlled boost converter control circuit.Common stage controlled control circuit is given in Figure12.The voltage feedback received from Stage 2, both stages were controlled with the common gate signal.The voltage feedback sensed from output of the Stage 2 is subtracted from a reference voltage by error amplifier to produce an error and compared with the sawtooth signal to obtain PWM control signals.

Figure 13 aFigure 13 :
Figure13a,b, shows the stage 1 gate-source and drain-source voltages of the separately controlled and common controlled two stage boost converter.Channel 1 signal is the drain-source voltage and measured with the probe at 10x.Channel 2 signal is the gate-source voltage and is kept negative when the switch is on.The reason for this is to prevent the switch from uncontrolled turn-on due to miller capacitance.

Figure 14 a
Figure 14 a,b, shows the gate-source signals of the two circuits.Channel 1 is the gate-source voltage of stage 2 and channel 2 is the gate-source voltage of stage 1.It can be observed that the gate-source voltage of Stage 2 is delayed in separate controlled converter.Figure14bshows that Stage 1 and stage 2 gate signals of the common controlled boost converter are the same because the control is common.The duty cycles of the stages are equal and the gate signals are applied simultaneously.
Figure 14 a,b, shows the gate-source signals of the two circuits.Channel 1 is the gate-source voltage of stage 2 and channel 2 is the gate-source voltage of stage 1.It can be observed that the gate-source voltage of Stage 2 is delayed in separate controlled converter.Figure14bshows that Stage 1 and stage 2 gate signals of the common controlled boost converter are the same because the control is common.The duty cycles of the stages are equal and the gate signals are applied simultaneously.

Figure 14 :
Figure 14: Gate -Source voltages of stage 1 and stage 2 of (a) Separate controlled and (b) Common controlled two stage boost converter.

Figure 15 aFigure 15 :
Figure 15 a,b, shows the gate-source and drain-source voltages of stage 2 of the two different controlled circuit.

Figure 16 :
Figure 16: Output voltages of stage 1 and stage 2 of (a) separate and (b) common controlled boost converter.
(a), (b), respectively.During soft start, two-stage boost converter with separate control was better regulated.

Figure 17 :
Figure 17: Output voltage of (a) separate and (b) common controlled boost converter during soft start process.The number of components and the cost reduction are shown in Figure 18 (a) and (b) below.

Table 1 . Converter Design Parameters.
1 and Stage 2  1 and  2 as 0.95.If  () is chosen as 9 VDC, the maximum input current   is obtained as below.