17 CHAPTER 2 DESIGN AND MODELING OF POSITIVE BUCK BOOST CONVERTER WITH CASCADED BUCK BOOST CONVERTER 2.1 GENERAL Designing an efficient DC to DC buck-boost converter is very much important for many real-time battery powered applications. A positive buck-boost converter is one which has the capability of switching its operating modes according to the variation in the input supply. In this chapter, a positive buck-boost converter and cascaded buck boost converter had been designed with a variable battery supply. The drawbacks associated with buck-boost converter, the different conventional solutions and the demerits of such conventional methods are presented in this section. The reasons behind the selection of a positive buck-boost converter for application, instead of buck-boost converter are clearly presented here. 2.2 BUCK BOOST CONVERTER The buck boost converter is a type of DC-DC converter that has an output voltage magnitude, which is either greater or less than the input voltage magnitude. It is a switch mode power supply with a circuit topology similar to the boost converter and the buck converter. The output voltage is adjustable, based on the duty cycle of the switching transistor. One possible
18 drawback of this converter is that the switch does not have a terminal on the ground; this complicates the driving circuitry. Also, the polarity of the output voltage is opposite the input voltage. Figure 2.1 shows the schematic diagram of a buck boost converter (49). given below The basic operation of buck boost converter is simple which is When in the On-state, the switch S is in the ON condition. This time the input voltage source is directly connected to the inductor L. This results in energy accumulating in L. In this stage, the capacitor supplies energy to the output load. While in the Off-state, the switch S is in the OFF condition. This time the inductor is connected to the output load and capacitor; so energy is transferred from L to C and R. The output voltage can vary continuously from 0 to (for an ideal converter). The output voltage ranges for a buck and boost converter are respectively 0 to V i and V i to. But the polarity of the output voltage is opposite to that of the input voltage. Figure 2.1 Buck Boost Converter schematic Diagram
19 2.2.1 Drawbacks of Using Buck Boost Converter The various issues associated with buck boost converter, which prevents its use for specific applications are mentioned below (51). The biggest problem associated with buck-boost converter is that the output of such a converter is inverted. Of course, it can be inverted, but it requires a transformer, which adds to the cost and space. The output voltage polarity is opposite to that of the input voltage, and these types of converters are also known as inverting regulators. The efficiency of the converter is very low. It produces high output transients. 2.3 EXISTING SOLUTIONS TO DEAL WITH THE BUCK BOOST CONVERTER The above mentioned disadvantages of a normal buck-boost converter have been solved by using different methods. Some of these solutions had been analyzed and the drawbacks of such solutions are described below. 2.3.1 Problem Existing in a SEPIC Converter A very popular buck boost topology that requires more components but produces a non-inverting output is the Single Ended Primary Inductance Converter (SEPIC) shown in Figure 2.2. SEPIC, a popular buck boost circuit has limited efficiency and requires either a transformer or two inductors (2).
20 Thus, including a transformer or two inductors would occupy more space and hence, increase the size and the cost. The use of these components would add to the losses, and thereby degrade the efficiency of the converter. Figure 2.2 SEPIC Converter 2.3.2 Drawbacks of a Cascaded Buck Boost Converter The Cascaded buck boost topology applies two dc dc converters cascaded together as shown in Figure 2.3. Hence, the loss of the whole single converter is actually doubled in this case, resulting in poor efficiency (66). The number of external components, such as inductors, decoupling capacitors, and the compensation networks needed for both controllers in this case, is more. Due to more components, more space is occupied, which results in higher cost. The sub harmonic problem is another issue, which prevents utilizing cascaded converters (52-54). Figure 2.3 Cascaded Boost Buck Converter
21 2.3.3 Boost Converter Cascaded to a Low-Dropout (LDO) Voltage Regulator This is another cascaded topology, composed of two different converter circuits. The first one is a boost converter, followed by an LDO voltage regulator as shown in Figure 2.4. Here, the varying input voltage from the battery is stepped up; using the boost converter, and then the output of the boost converter is regulated using the LDO to obtain a voltage in the middle range of the varying battery voltage. The biggest disadvantage of an LDO circuit is lower efficiency, due to the fact that it is a cascaded network of two converters, which in turn, double the number of components and losses used in the proposed converter stage. The cost is another concern here. Thus, this topological control is not much desired (81-83). Figure 2.4 Cascaded Boost Converter and LDO regulator 2.4 POSITIVE BUCK BOOST CONVERTER Proposing a battery powered application is very difficult; by using the above mentioned buck boost converters. But introducing a positive buck boost converter can solve most of the problems associated with the other buck boost converters. The proposed positive buck boost converter consists of only two switches and two diodes; thus, it reduces the total cost when compared
22 with other converters. Its output is positive and it operates as both a buck and a boost converter. The circuit topology of such a positive buck boost converter is shown in Figure 2.5 (87). Figure 2.5 Circuit diagram of the Positive Buck Boost Converter In the buck boost operating mode, always two switches Q 1 and Q 2, and two diodes D 1 and D 2, are switching in the circuit. A positive buck boost converter can operate as a buck converter by controlling switch Q 1 and diode D 1, when Q 2 is OFF and D 2 is conducting. It can also work as a boost converter by controlling switch Q 2 and diode D 2, while Q 1 is ON and D 1 is not conducting. When the voltage of the battery is more than the output reference voltage, the converter operates as a buck converter. As soon as the voltage of the battery drops to a value less than the output reference voltage, the converter should switch to the boost mode. The added advantage of the converter is that the output of such a converter is always positive. The different modes of operation with the direction of current flows are neatly sketched in Figure 2.6. When the positive buck-boost converter works in the buck mode, switch Q 2 is always in the OFF condition and only switch Q 1 conducts, while in the boost mode, switch Q 1 is always in the ON condition and only switch Q 2 controls the circuit. In the buck-boost mode, both switches Q 1 and Q 2 conduct. The dotted line of Figure 2.6 indicates the direction of the current flow when the controlling switch of each mode is in
23 the ON state, where as the continuous line indicates the direction of the current when the controlling switch is in the OFF state (98-99). Figure 2.6 Current flow directions of (a) Buck, (b) Boost and (c) Buck Boost Operating Modes of a Positive Buck Boost Converter
24 2.5 HARDWARE DESCRIPTION The hardware of a positive buck boost converter is designed, based on the parameters listed in Table 2.1. The converter operates at 100 khz switching frequency. Two n-type MOSFET switches and two Schottky barrier diodes are used for real time buck boost converter configuration. Table 2.1 Hardware Specifications for Positive Buck Boost Converter Parameter Value Digital Signal Processor (DSP) 320 F 2812 n-type MOSFET IRF540 Schottky barrier diode 1N5817 Switching frequency 100KHz Output resistance 30 Output filter capacitance 400µF Magnetizing inductance 110µH Input voltage 8V 15V Output voltage 12V The MOSFET switches and diodes are IRF540 and 1N5817, respectively. A controller has been implemented using a Texas Instruments digital signal processor (DSP) (320F2812). The output voltage reference is set to 12 V, and input voltage varies from 15 to 8 V (66). The overall configuration of the converter and controller is shown in Figure 2.7.
25 Figure 2.7 Hardware circuit configuration The operating modes are dependent on the mode selection signals, applied from the DSP. G1 and G2 are the buck pulse and boost pulse, sequentially. MD_SEL0, MD_SEL1, and MD_SEL2 determine the operation modes: Figure 2.8 presents the experimental setup, composed of the DSP controller, gate driver, gating logic, and converter (86, 88). Figure 2.8 Experimental Setup of PBBC 2.6 COMPARISON OF THE NUMBER OF COMPONENTS OF THE PBBC AND CBBC From the above discussion, it is clear that the number of components in the positive buck boost converter is less compared to the cascaded buck boost converter, as shown in Table 2.1.
26 Table 2.2 Components Comparison between the PBBC and CBBC Description PBBC CBBC n-type MOSFET 02 04 Schottky barrier diode 02 Nil Capacitor 02 01 Magnetizing inductance 01 02 2.7 OPEN LOOP RESPONSE OF CBBC AND PBBC 2.7.1 Case i: Buck Mode - Open Loop Response of CBBC The transient response in the output voltage is analyzed by applying an input voltage of 6V and an output voltage of 4.35V is obtained. The ripple content in this case is 14% and the time taken for the output voltage to reach the steady state value is 0.014sec, as shown in Figure 2.9. Output voltage (V) Input voltage (V) Input voltage = 6V Output voltage = 4.35V Figure 2.9 Input and Output Voltage Waveform of Buck Operation CBBC Time (S)
27 2.7.2 Case ii: Boost Mode - Open Loop Response of CBBC For an input voltage of 3.5V an output voltage of 4.56V is obtained and the transient response in the output voltage is analyzed. The ripple content is 15% and the time taken for the output voltage to reach the steady state value is 0.0154 sec for this case as shown in Figure 2.10. Output voltage (V) Input voltage (V) Input voltage = 3.5V Output voltage = 4.56V Figure 2.10 Input and Output Voltage Waveform of Boost Operation CBBC Time (S) 2.7.3 Case iii: Buck Mode - Open Loop Response of PBBC When an input voltage of 6V is applied an output voltage of 4.59V is obtained. The transient response in the output voltage is analyzed. The ripple content in this case is 12% and the time taken for the output voltage to reach the steady state value is 0.012 sec, as shown in Figure 2.11.
28 Output voltage (V) Input voltage (V) Input voltage = 6V Output voltage = 4.59 V Figure 2.11 Input and Output Voltage Waveform of Buck Operation PBBC Time (S) 2.7.4 Case iv: Boost Mode - Open Loop Response of PBBC For an input voltage of 3.5V an output voltage of 4.76V is obtained and the transient response in the output voltage is analyzed. The ripple content is 10% and the time taken for the output voltage to reach the steady state value is 0.0024 sec for this case as shown in Figure 2.12. Input voltage (V) Input voltage = 3.5V Output voltage (V) Output voltage = 4.76V Figure 2.12 Input and Output Voltage Waveform of Boost Operation PBBC Time (S)
29 Based on the analysis, the results show that the PBBC gives better transient response compared to that of CBBC. 2.8 SUMMARY The disadvantages of some conventional buck-boost converters are analyzed in this chapter. The need for a novel PBBC and its designing are discussed. The circuit diagram for the proposed model is compared with the cascaded buck boost converter and its hardware circuit is presented and discussed.