International Research Journal of Applied and Basic Sciences 2014 Available online at www.irjabs.com ISSN 2251-838X / Vol, 8 (9): 1289-1296 Science Explorer Publications Designing buck chopper converter by sliding mode technique Sepideh Fazel 1, Seyedeh neda Ashraf 2 Tabriz Islamic Azad University Corresponding author email : fazel.sepideh@gmail.com ABSTRACT: In many industrial applications, it is needed to convert a DC source with a variable voltage. The purpose of DC-DC converter is providing DC output voltage set to a resistance with a variable load from a swinging DC input voltage. Choppers are DC-DC converters used for transmission of electric power from a DC source to another DC source that a load might be passive. Choppers have two separate operation mode that have very different properties: continuous conduction mode and discontinuous conduction mode. In practice, a converter may operate in two modes. So, control of converter is designed for both operations controlled in order to create a desired level, in which input voltage and output load may fluctuate. DC-DC transformers in the switch mode, apply one or more switches for DC transmission from a desired level to another one. In DC-DC converter with a given DC voltage the average output voltage is controlled by controlling on and off times in the switch. Buck converter produces the average output voltage which is less than input DC voltage. Keywords: Buck converter, Sliding mode, DC supply source INTRODUCTION Choppers are widely used in the regulated DC power supply and DC control of motor speed.the method of sliding mode was introduced for some systems with variable structure. Chopper converters have originally the variable structure because of switching characteristics, so applying such a control is proper for chopper converter. This is especially relevant to the chopper converter working in continuous conductive mode, because the converter has controllable, continuous and measurable modes in this case. The taxonomic driver circuit should have the ability to turn on and off switch quickly between control signals and power switch. In this paper has been provided a method for designing buck chopper converter in the method of sliding mode. Design of power driver circuit, driver characteristics and a comparison of actual and theoretical mode. Choppers are used for controlling motor in the electrical cars, lifter clutches in mining, etc. Their characteristics include precise control of acceleration, high efficiency and fast dynamic response. Choppers are also used in DC engine. Braking in order to restore energy which makes energy storage in the transport systems with large stop. DC choppers can be used in the variable regulators for converting DC voltage (generally not consolidated) into DC output voltage (consolidated).stabilize is usually performed using pulse width modulation at a fixed frequency and switching element is usually BJT, MOSFET or power IGBT. (Rashid2002) Cases and method of block diagram design of buck converter In figure 1 block diagram design of buck converter is shown. Input transformer performs a task related to matching of ac voltage level and the transformer isolation from network. Rectifier Bridge has been used for converting output ac power of transformer into DC power needed for converter. Capacitor for filtering is also used for reducing the output voltage ripple in the rectifier. In order to start, C f is designed so that voltage ripple is limited to 5 volt in Chopper input (V r ).
Figure1. block diagram design of buck converter By this selection, it is considered that used control method should be able to adjust the output optimally. Meanwhile, with respect to voltage drop in the controlled switch of buck converter, input voltage of converter can be considered between 13 to18 volts. In such condition and with respect to the voltage drop in the Rectifier Bridge diodes, the secondary voltage of transformer The converter inductor is designed so that it will be always in the continuous mode under the different conditions. In the border of continues and discontinuous conduction area, L b can be calculated by the following equation: D is duty cycle of converter, R l and f s are load resistance and switching frequency, respectively. It is observed that L b becomes maximum per the high amount of load resistance and the least amount of D. we consider the maximum (max) value of load resistance as R L = 10 Ω. The lowest value of D occurs when the output voltage of converter is Minimum and its input voltage is Maximum. If Minimum output voltage is considered as V (min) o =5 v On the other hand, we know the relation of the output ripple voltage in converter (V or ) and capacitor c is expressed based on the following relationship: if we plan to limit the ripple rate of output voltage in the converter up to 1%. In the other word, value of capacitor will be calculated so that V or is equal to 50mm. The above equation clearly indicates that the highest rate of ripple in the output voltage of converter occurs per Maximum value of output voltage. In the above equation the amount of converter inductor is considered 470 mh. In this obvious that the Minimum value of D increases C capacitor. Therefore, It is clear that for designed value, the output ripple voltage in the converter will be always less than 1% of output. 1290
Table 1. Calculated values for the buck chopper converter Characteristics values Unit The range of input voltage variations 13 vi 18 Volt The range of output voltage variations(reference) 5 v o 10 Volt The range of allowable load resistance changes 1 R l 2.5 Ω Capacitor for filtering c f c f =20 MF Reducer transformer inductor L L=470 MF Reducer transformer capacitor C C=470 Transformer winding ratio n 1 / n 2 = 16 Switching frequency of the converter f s=5 KH Design of driver circuit of the power switch The taxonomic driver circuit should have the ability to turn on and off switch quickly between control signals and power switch. It also performs the task of providing the necessary isolation between the electrical parts and power part. At least there are two main methods for isolating in order to drive converter: use of eptocoplers and use of the isolated transformer. In fact the purpose of designing the IGBT driver circuit is determining the value of the gate resistance.the value of the gate resistance should be designed based on a reconciliation of the following two parameters: whatever the gate resistance is smaller, the input capacitors of switch are charged earlier and time for its turn on is reduced. So,the rate of switching losses is reduced by reducing the gate resistance. An important issue that limits the low gate resistance is EMI phenomenon. Snubber circuits are elements used for limiting voltage variations and current of the switch. By means of snubber circuits, switching losses may be reduced but snubber losses should be considered in this case- which is added to the total system losses. Usually a high power resistance (5 20 V) with a small ohms at high switching frequencies is considered as the gate resistance, because the leakage inductance between the driver circuit to the switch gate and also the input capacitor forms a RLC that this circuit will cause the gate current fluctuations. If the gate resistance is removed (ideally), it is possible that fluctuations are not dampedand the input voltage of device (gate) causes an increase in the gate voltage up to double of supply. The operation related to turning off and on of a semiconductor switch is called "driving". The purpose of designing and manufacturing a driving circuit is that we can perform the isolation act and giving current for IGBT switches that is done by 7667 and 6N137 ICs. Figure 2. The overall schematic of the driver circuit Driver feature It should have the ability to charge and discharge the input capacitor quickly. It should have the ability for giving and taking current ( I source, I sink respectively), The current should be as possible as high so that switch turns on and off quickly. In order to turn on IGBT completely, should be applied a minimum voltage equal to +15v to its gate-source. The output of control circuit (comparator) is not able to provide taken current from the MOSFET gate when switch turns on and off. In addition, the circuit MOSFET should have capacity for providing current in both directions. For setting up MOSFET, the output voltage of control circuit should be applied to gate-source of switch and due to impossibility of this work; an isolation power source should be used for this work. Because of severity variations in voltage and current in the power circuit and also due to some parasitic elements of the circuit switch, some noises leak from the power circuit to the control circuit and this impairs their performance. So, the output of control circuit should be isolated from power. We use the optical isolation (IC6 N137 ) here.its task is isolation IGBT driver is available as a provided ICs at market that IC7667 is one of the ICs. Its task is driving the switch and also amplifiering the current. If R G is high, a period of time for charging and discharging the switch is longer and it will turn on and off later (switching losses will rise), when R G is very low, the problem of EMI (Electromagnetic 1291
interference) may be occurred. R G (EMI) should be selected within 4.7 ~100Ω. Note that it is possible that voltage fluctuations is not damped and is collected at the gate and increases voltage at the gate extremely (greater than +V cc ), so we will use 1N4004 diode in this part so that voltage at the gate becomes always less than +15. After operation of 1N4004 diode, we will attached a 12 KΩ resistance with a LED to the ground and LED starts with a pulse. Finally we measure a voltage connected to the input of switch that is about 15V (figure 3). Figure 3. Total output of driver circuit: Voltage = 5 V, Effective frequency = 1.5khz Driver circuit is used for strengthening the current and isolation; by giving a pulse about 0-6 Volt we will obtain a voltage about 0-15 Volt in the output and switch will be turned on. Designing process of standard sliding mode Figure4.Buck chopper converter based on SMVC Figure 4 shows the regarded buck chopper converter. First, sliding line is calculated by measuring the output voltage and current in the capacitor. Then using a HYSTERESIS controller, S W switch turns on and off so that the capacitor response always within -K<S<K.Meanwhile, the output voltage measurement is done based on a simple resistance voltage divider circuit.for measuring the current in the power electronic circuits several methods have been considered. In this study, we will use Hall Effect current sensors because of their high precision, establishing the isolation between input and output, the possibility of DC current measurement, etc. Designing process Buck chopper converter design principles are fairly straightforward. Assume f s = 200 KHz, R L =6 Ω, 13 v <V i < 30 v and the Maximum value of output ripple voltage has been selected about 50 mv. According to this fact that converter works in the continuous conductive area, so a Minimum value of L can be expressed as follow (L cri represents a critical value for L), L > L cri = T sv i D 1 D 2I L where D is the duty cycle of switch, T s is the switching periodicity, I L is DC from the current passing from the circuit inductor. If Vod = 12 v is selected, rang of D changes can be stated as follow: D = V od V i 0.4 < D < 0.9231 According to equality between inductor current and output current in DC mode (I L = 2 A ), L cri can be calculated in both modes from the above equation. 1292
Dاگر = 0.4 L cri = 3.9μH Dاگر = 0.9231 L cri = 1.15μH It is clear that L cri = 309 μh should be selected as the critical value of inductor. In practice, L may be larger than critical value in order to limit the slope of the rise and fall of current in the power switch. For example, L= 100 μh will be used here. The following equation can be used to select the minimum value for needed capacitance; where V o represents the output voltage ripple (peak to peak). D = 0.4 C min = 4.5μF اگر Therefore in practice, capacitance used in output can be selected about 4.5 μf. According to the characteristics of electronic circuits needed to create V ref, usually the gain of sampling net should be less than the output voltage. For example, if V ref = 3.3 v then β can be calculated simply. β = V ref = 3.3 V od 12 = 0.275 Width of the required hysteresis cycle can be also calculated. (In this case it is assumed that the input voltage has a fixed value equal to V sd = 24 kω. Although such a selection causes switching frequency changes in the converter, totality of subject is maintained) In the above equation f sd is a desired switching frequency and is assumed about 200 KHz.Designing Hysteresis cycle controller is fairly straightforward. It can be implemented as follow: the Figure 5. Circuit schematic of Hysteresis cycle controller (R P.U. is pull-up resistor related to comparator and is equal to 3.3kΩ using LM339). THE RESULTS FROM SIMULATION OF THE BUCK CHOPPER CONVERTER USING SMVC METHOD In this part, we will discuss about the mode equations simulation of the buck chopper converter with sliding mode method in MATLAB/ Simulink setting. The simulation is carried in the case of u=1and the values of Vi =48 v. Rl=10 C=0.000005 L=0.0001 Vref=3.3 V B=0.4 Figure 6. The mode equations simulation of the buck chopper converter in the control method of sliding mode 1293
Figure 7. The output waveform in three dimensions A: u=1, B: u=0 If the results from the above simulation overlap per the various Vis and according to u=1 and u=0, it will be shown that steady-state error will be equal to zero (Mahdavi, 2005). Comparison of actual and theoretical mode In the actual mode, we have found the value of output voltage and duty cycle at different frequency then have compare the value of output voltage with theoretical mode according to the Table. Table 2.The value of output voltage in the actual and theoretical modes at a frequency equal to 1/07 KHz Figure8. Diagram for the comparison of output voltage in the actual and theoretical modes at a frequency equal to 1/07 KHz Figure 9.The simulation of Buck transformer 1294
Figure10. buck converter waveforms a step waveform inductor voltage diode current source current THE EXPERIMENTAL RESULTS Figure 11. A: Control circuit B: buck converter and driver 1295
Figure12. A: Output waveform of control circuit (Capacitor), B: Total waveform of circuit REFERENCES Mahdavi J, Emadi A, Toliyat HA.2005. Application of State Space Averaging method to Sliding Mode Control of PWM dc/dc Converters in Proc. IEEE Conf..Industry Applications Society (IAS) vol.2,oct,199 Ned M.2002.Power Electronics:Converters,Applications,and Design. Rashid Muhammad H.2002.Power electronics: circuits, devices and applications. Siew-Chong T, Lai YM, Martin Cheung KH, Chi K. Tse, MARCH.2005.On the Practical Design of a Sliding Mode Voltage Controlled Buck converter IEEE TRANSACTIONS ON POWER ELECTRONICS, VOL. 20, NO. 2, 1296