Variable Output AC-DC Converter

Transcription

1 Variable Output AC-DC Converter Mr. Pavankumar.R.Patil Dept. of Electrical & Electronics Engineering. R.V.COLLEGE OF ENGINEERING Bangalore, Karnataka, India. Mr. M.N.Dinesh Dept. of Electrical & Electronics Engineering R.V.COLLEGE OF ENGINEERING Bangalore, Karnataka, India. Abstract: The design of an AC to DC converter, which takes line supply as input and provides a regulated DC output voltage which can be adjusted over a range of voltages is proposed in this project. The main aim is to get a regulated output voltage for any change in line supply. The line and load regulation is expected to be very less. The cascaded connection of rectifier, converter transformer along with output filter is used. To meet the specification the best suited topology is forward converter topology. A two switch forward converter is used to avoid transformer core saturation. To trigger both the MOSFETs at a time through a single pulse transformer the current mode control PWM IC UC3844 is used which avoids using an extra current sensing circuit. The converter transformer has one primary and three secondary windings. The three secondary windings serves three purposes, one used provide power supply to control circuit, other used for voltage feedback to control circuit and the third to feed the load. The output voltage is fed back to control circuit through an opto coupler which provides input to output isolation. Two voltage regulators are provided one at power circuit other at control circuit. Once the output voltage is set for a particular value, the voltage regulator at power circuit senses the variation in output voltage for any change in input and sends proportional voltage to the control circuit this is sensed by PWM IC through regulator at control circuit, then the IC will adjust the duty ratio of the triggering pulse to MOSFETs which leads to maintaining constant voltage at the output. This is achieved with a regulation less than 1% and the required specifications are met. extended design time and cost. There are basically four types of regulators [1]. Linear regulator Buck regulator Boost regulator Inverting regulator (also known as flyback or buck-boost) II. 2 SWITCH FORWARD CONVERTER The forward converter is an example of a buck converter. The Forward converter without transformer core reset is shown in fig.1. From the waveform we can observe the current builds up at each switching cycle. It drives the core into saturation[2]. I.INTRODUCTION The present day power supplies use many converter topologies for DC-DC conversion. Each topology has both common and unique properties, and the experienced designer will choose the topology best suited for the intended application. We will see that some topologies are best used for AC/DC offline converters at lower output powers, whereas others will be better at higher output powers. For applications where several output voltages are required, some topologies will require less number of components, while input or output ripple and noise requirements will also be an important factor. Further, some topologies have inherent limitations that require additional or more complex circuitry, whereas the performance of others can become difficult to analyze in some situations. A poor initial choice can result in performance limitation and perhaps in Fig.1. Without transformer core reset That s why we go for transformer core reset. There are many techniques to achieve this. But here we use the 2switch forward converter method which is more effective compared to the others methods [3]. Here reset is achieved by resetting with RCD clamp. It is shown in fig.2. This method has both advantage and disadvantages. Advantages are Easy to implement. And Q1 peak voltage is equal to Vin ISSN: Page 2164

2 Disadvantages are Additional power MOSFET (Q2) + high side driver is needed. Two numbers of High voltage, low power diodes (D3 & D4) are required. Multiple output voltages are needed with relatively good cross-regulation. Saturable reactor controllers are to be used as auxiliary secondary side regulators. Applications where the complexities of dual feedback loops and/or slope compensation are to be avoided [4]. Fig.2. 2-switch Forward converter Voltage Mode Control III.CONTROL CIRCUIT This was the approach used for the first switching regulator designs and it served the industry well for many years. The basic voltage mode configuration is shown in fig 3. The major characteristics of this design are that there is a single voltage feedback path with pulse width modulation performed by comparing the voltage error signal with a constant ramp waveform. Current limiting must be done separately. It has many advantages. A single feedback loop is easier to analyze. A large amplitude ramp waveform provides good noise margin for a stable modulation process. A low impedance power output provides better cross-regulation for multiple output supplies And the disadvantages include, any change in line or load must first be sensed as an output change and then corrected by the feedback loop. This usually means slow response. The output filter adds two poles to the control loop requiring either a dominant pole low frequency roll-off at the error amplifier or an added zero in the compensation. Compensation is further complicated by the fact that the loop again varies with input voltage. Circuit Topology For our application we choose voltage mode control because there are wide input line and/or output load variations possible. Particularly with low line-light load conditions where the current ramp slope is too shallow for stable PWM operation. High power and/or noisy applications where noise on the current waveform would be difficult to control. PWM IC UC3844 Fig.3. voltage mode controller circuit The two MOSFETs have to be triggered at the same time, for this purpose we need a PWM signal generator. The PWM IC- UC3844 is best suited for our application so the control circuit is designed using the IC. The circuit in fig.4 shows the control circuit which gets the voltage feedback from output and generates the control signal for the MOSFET [5]. The UC3844 is a high performance fixed frequency current mode controller and it is specifically designed for Off Line and DC to DC converter applications offering the designer a cost effective solution with minimal external components. The integrated circuits feature a temperature compensated reference, an oscillator, current sensing comparator, high gain error amplifier and a high current totem pole output ideally suited for driving a power MOSFET. The other features are protective features consisting of input and reference undervoltage lockouts each with cycle by cycle current limiting, hysteresis, a flip flop which blanks the output off every other oscillator cycle a latch for single pulse metering that allows output dead times to be programmed for 50% to 70%. ISSN: Page 2165

3 Power Circuit Calculations: Specifications: Input Voltage- 230+/-15% AC IV.DESIGN Output Voltage- Variable in the range 50V- DC Output Current- 0-2A Output Power- 360W Transformer Turns ratio calculation Vo = η*vmin*dmax*n N = 2.61 Where: Vout is the output voltage η is the targeted efficiency Vmin is the min. input voltage Dmax is the max duty cycle of UC3844 N is the transformer turn ratio Maximum duty cycle at high input line DCmin (Based on the previous equation) Vo = η*vmax*dmin*n Dmin = 0.31 Vmax is the max. Input voltage [6]. Magnetizing inductor value. A minimal magnetizing current is needed to reverse the voltage across the winding which is used for resetting properly the core. (Enough energy must be stored so to charge the capacitance) ( ILmag_pk = 10% Ip_pk) Lmag=Vmin/10%Im/Ton Lmag= (170*.5)/(.1*80000*.94) Lmag= 11.3mH LC Output Filter Crossover frequency (fc) selection Arbitrarily selected to 10 khz. fc > 10 khz requires noiseless layout due to switching noise (difficult). Crossover at higher frequency is not recommended Cout estimation If we consider a ΔVout = 250 mv the following equation can be written on the basis of Cout, fc :[7]. C= I(dt/dv) dt= D/ fc =125uf Where: fc crossover frequency Inductor Vin = L(di/dt) L = 12.5mH MOSFET With a 2-switch forward converter max voltage on power MOSFET is limited to the input voltage. Peak primary current Ip = (1.56*Po)/(Vdcmin) = 4A Maximum off voltage stress Vm = 2.5*Vdcmax = 500V Usually a derating factor is applied on drain to source breakdown voltage (BVDSS) equal to 15%. If we select a 500-V power MOSFET type, the derated max voltage should be 425 V (500 V x 0.85) [8]. IRF650pb has been selected Part Number- IRF650 ISSN: Page 2166

4 BVDSS-500V RDS(on)-.27 ohm ID-21 Secondary Diodes D1 and D2 sustain same Peak Inverse Voltage (PIV): PIV= NV/(1-kD) = 870V Where kd is derating factor of the diodes (40%). PIV is high so IR720P diode can be selected. Photograph.2. Waveform generated at pin6 of IC UC3844 V. RESULTS 400W AC-DC variable output forward converter was fabricated on general purpose printed circuit board as per the design. Where control circuit was fabricated on separate general purpose printed circuit board as shown in photograph 1. The output waveforms of control circuit are captured at pin6 of IC UC3844, where time period was 12.5 microseconds which shows that the frequency is 80 khz. This frequency is set by appropriately choosing the values of RT & CT at pin4. This is shown in photograph 2. Photograph.3. Gate to source waveform Using this circuit we can vary output voltage from 50V to smoothly. Hence we can get any output voltage between 50V to. As an example the output waveforms for a regulated output of 50V and 125V is shown in photographs 5 and 6 respectively. Photograph.1. Prototype of Variable output AC-DC converter The waveforms captured here shows the gate to source pulse has amplitude of 12V and a frequency of 80 khz. Shown in photograph 3. Photograph.4. Output Voltage waveform (50V) Voltage division=5v Probe multiplication factor=10 ISSN: Page 2167

5 Amplitude=1 div,output Voltage=5*10=50V Table.2. Line regulation-output voltage set to Photograph.5. Output Voltage waveform (125V) Input Voltage 200V 220V 240V 260V Output Voltage Voltage division=5v Probe multiplication factor=10 Amplitude=2.5 div, Output Voltage=5*10*2.5=125V Load Regulation The table 3. below shows line regulation for various values of load current and adjusting the output voltage to 180.0V. Table.3. Load regulation-output voltage set to 180.0V Line Regulation The table below shows line regulation for various values of input voltage and adjusting the output voltage to and. Here input voltage is varied from to 260V in steps of 20V by adjusting output voltage at in Table 1. below. We can observe the output voltage is constant irrespective of input voltage and the line regulation observed is 0. And input voltage is varied from to 260V in steps of 20V by adjusting output voltage at in Table 2. below. We can observe the output voltage is constant irrespective of input voltage and the line regulation observed is 0. Table.1. Line regulation-output voltage set to Input Voltage Output Voltage Load current Output Voltage No load 180.0V 0.5A 179.8V 1.0A 179.5V 1.5A 179.3V 2.0A(Full load) 179.0V Here input voltage is kept constant at 230V and output voltage is set at 180.0V. From Table 3. we can observe the output voltage is 180.0V at no load and is goes on decreasing as the load increases. At full load of 2.0A the output voltage is 179.0V. By this we can observe that load regulation is less than 0.56%. 200V 220V 240V 260V VI. CONCLUSION A variable output AC-DC converter was fabricated on general purpose printed circuit board as per the design. For variation of input voltage from 170V to 270V, the output can be regulated for any voltage between 50V to as per the design. ISSN: Page 2168

6 This converter was loaded up to 2A effectively. The regulation observed was less than 0.56%. REFERENCES [1]. Abraham I. Pressman, Keith Billings, Taylor Morey, Switching Power Supply Design Third Edition [2]. Ned Mohan, Tore M. Undeland, William P.Robbins, Power electronics converters, applications and design, John Wiley and sons, Third edition, [3]. C. Basso, Switch Mode Power Supplies: SPICE Simulations and Practical Designs, McGraw Hill, [4]. Muhammad H.Rashid, Power electronics: Circuits, Devices & applications, Prentice Hall India, third edition, [5]. Techser Power Solutions Pvt Ltd design documents [6]. Philip T. Krein, ECE369 Power Electronics Laboratory. Urbana, Illinois: University of Illinois, 1999,106. [7]. Philip T. Krein, Elements of Power Electronics. New York: Oxford University Press, 1998, , [8]. Robert W Ericson and Maksimovic, Fundamentals of power electronics, Lucent technologies Inc, Second edition, ISSN: Page 2169