10.07.2015

Dcdc converter load regulation

Stretching the power budget is a common challenge for design engineers developing battery-powered products. Processor chips have improved in performance roughly in harmony with Gordon Moore’s 1965 prediction that the number of transistors on a chip doubles every 24 months. This presents engineers designing portable devices with a dilemma: They want to offer customers faster products with more functionality and brighter screens, yet those same customers demand longer battery life from the enhanced device. Clever silicon design has resulted in chips running at lower voltages and demanding less power while still offering high performance. The key reason for this efficient operation is the use of MOSFETs in modern switching converters, replacing the rectifying diode of old designs. When a MOSFET is used as a synchronous switch, the current normally flows in reverse (source to drain), allowing the integrated body diode to conduct current during the dead time. Unfortunately the efficiency curve for a switching converter is not flat; performance at low or high loads is worse than for medium loads. The conduction losses caused by the current ripple become dominant because the overlap loss scales down with the load current, while the power dissipated by current ripple normally remains constant. Fortunately, some modern switching converters employ technology to limit inefficiencies at low and very low loads. When turning on, the switch voltage is first brought to zero before the gate voltage is applied, which results in a virtually zero-loss transition. Figure 4a and b compares the voltage waveforms for a soft-switched MOSFET compared with a hard switched version. Figure 6: In discontinuous mode the inductor fully discharges and then the inductor voltage rests at zero. The discontinuous mode offers improved light load efficiencies, but generates more noise because of the constantly varying switching frequency.
Intersil says its ISL8016 high-efficiency synchronous buck regulator can be configured for discontinuous or forced continuous operation at light load. The biggest challenge of a portable product is managing the power resources while meeting consumer expectations of a big screen, lots of features, and long battery life. The solution is to look for converter modules that extend battery life by utilizing features such as soft switching, discontinuous operation, and skip modes, as they are more efficient than conventional modules during low load operation. Home Journals Books Conferences News About Us Jobs Energy and Power Engineering Vol. Consumers expect more functionality and longer run times from each new product iteration, but, because battery technology improves only very slowly, this is proving increasingly difficult. However, efficiency is not constant across the output current range, tailing off markedly at low loads.
For example, Lithium ion (Li ion) cells, the mainstay power source for high-demand portable devices such as cell phones and digital cameras, have improved in energy density by only about 7 percent a year (the improvement is mainly due to incremental engineering refinements). However, it is still incumbent on the engineer to ensure that his design makes the most of the advantage endowed by low power consumption silicon by choosing the best power management chips. A poor choice of module might leave consumers thanking the designer for warming their hands on a cold day, but a few hours later cursing him as the display on their smartphone winks out. Engineers now have a wide selection of choices when deciding on a power management module for a portable design. In those older products, the diode’s dissipated power equaled the forward voltage drop multiplied by the current, compromising performance. That is particularly a challenge for portable equipment designers because battery-powered devices spend a lot of time in standby or sleep modes to save power when not in use.
In the high load region, the dominant power losses are due to high conduction losses induced by the load current. A new cycle is then initiated when the output voltage drops below the regulating threshold. If EMI considerations are more important than battery life, the chip can run in forced continuous operation to reduce noise during light load conditions. The operating frequency is kept constant, but some switching operations are skipped eliminating the losses normally incurred with every cycle.
This device provides efficient power conversion over a 100:1 load range by entering a hysteretic or sleep mode to maintain efficiency. This is a popular chip for powering microprocessors and FPGAs, among other devices, in portable products. Because of the very low-channel resistance for power MOSFETs, the standard forward drop of the rectifying diode can be reduced to just a few millivolts, boosting peak efficiencies to well above 90 percent. At these low currents, the switching converter is relatively inefficient and can negate some of the power savings the designer was hoping to achieve in standby or sleep operation.
While operating in this mode, the switching frequency is proportional to the load current³, so it will vary over time.


Alternatively, a discontinuous mode provides high efficiency by reducing switching losses at light loads. Such inefficient operation at low loads results in flawed battery life calculation, shorter product operational times, and disappointed consumers. The current limiting as the voltage differential increases can restrict the chip’s usefulness in certain applications. Consequently, a diode is still required to conduct during the interval between the opening of the main switch and the closing of the synchronous rectifier switch (the dead time). V-I overlap losses are proportional to load current, input voltage, and switching frequency. Figure 8 shows how the efficiency at light load is improved by comparing a TPS53353 chip auto-skip mode with one running conventionally. The existing two different DC voltage battery bank of the distributed generation needs to interlink each other using a bi-directional DC-DC converter in order to minimize the unbalance of the output load currents of the three inverters connected to electric grid system.
Through this connection, a current can flow from one system to another or vice versa depending on which systems need the current most. Thus, unbalanced currents of the grid line have been minimized and the reliability and performance of the DER grid connected system has been increased.
A detailed mathematical analysis of the converter under steady state and transient condition are presented.
Mathematical models for boost and buck modes are being derived and the simulink model is constructed in order to simulate the system. Moreover, the model has been validated on the actual operation of the converter, showing that the simulated results in Matlab Simulink are consistent with the experimental ones. Havya Kullan?m? Lehim Nas?l Yap?l?r TDA7377 Amfi Devresi Metal Dedektoru Devresi ICL8038 LM386 Daha fazla yaz?.. Introduction The distributed energy resources (DER) considered in this study is composed of photovoltaics (PV), wind turbines, a fuel cell, batteries and supercapacitors.
The detailed schematic diagram of grid connected DER at the University of Massachusetts Lowell (UMass Lowell) is presented in Figure 1.
A data acquisition system (DAQ) is installed in order to monitor the performance of all energy resources. In other words, the quality, the reliability and the performance of the system are compromised. This bidirectional DC-DC converter would connect the two-battery banks of the distributed energy resources. Through this connection, a current would flow from one system to another depending on which system needed the current the most. Imbalanced currents of the grid line would be minimized, and the reliability and performance of the existing DER grid connected system would increase. The author designed the power and control circuit for this bidirectional DCDC converter and it is suited to this use. The mathematical analysis, simulink model and simulation results are presented here and a prototype was fabricated and tested.
System Design Figure 2 illustrates the power circuit configuration of a bidirectional DC-DC converter that was installed between the two banks of batteries at the renewable energy Figure 1.
A transformer less type was chosen so as to reduce size and weight, and improve economy [1]. Bidirectional power flow was obtained with a current-bidirectional two quadrant realization of the switch [2]. Anti-parallel-connected MOSFETs and diodes were used to construct the currentbidirectional switches. A feedback loop system was used to control of the output voltage of the converter for a PWM.
Reference 1 and Reference 2 on pins 12 and 13 of PIC 16F684 are connected through voltage divider from the 24 V and 48 V battery banks respectively, and they automatically control the pulse width that drives the MOSFET switches. Pin 9 and pin 10 of the PIC are connected to the sensors installed on load sides of the inverter. The Operation The current sensors, installed on the outputs of inverter 1 and inverter 2 (or 3), signals the micro-controller, and are responsible for determining the operation of DC-DC converter in either boost or buck mode.
The micro-controller was programmed so that it could control the operation of the bidirectional DC-DC converter. The control strategies of this converter that are implemented in the PIC 16F684 are the following: 1) When the micro-controller reads that the current output in inverter 2 is higher than the current output in inverter 1, it will enable the PWM1. The microcontroller will drive MOSFET Q2, so that the converter operates as a boost converter. In this case, the 24 V battery bank supplies current to 48 V battery bank, and will supply current to inverter 2; 3) If inverter 1 is not operational while the PV is connected and charging the 24 V battery bank, then the micro-controller will enable PWM2.


If inverter 2 is not operational while the PV connected to it is charging the 48 V battery bank, the micro-controller will enable PWM1.
Steady State Analysis of the Bidirectional DC-DC Converter The voltage and current equations of a DC-DC converter under steady-state conditions can be found by using the two basic principles namely; the principle of inductor voltsecond balance and the principle of capacitor amp-second or charge balance. The principle of inductor volt-second balance states that the average value, or dc component of voltage applied across an ideal inductor winding must be zero and the principle of capacitor amp-second or charge balance, states that the average current that flows through an ideal capacitor must be zero [3].
Thus, to determine the voltages and currents of DC-DC converters operating in periodic steady state, one averages the inductor current and capacitor voltage waveforms over one switching period, and equates the results to zero [4]. In Figure 5 the resistance RL of the inductor in the bidirectional DC-DC converter power circuit is being considered in order to model the circuit. Based on the power circuit shown in Figure 5, the output voltage, and inductor current were derived. Red arrows are in the direction of current in a step down mode while the black arrows indicate the current direction in a step up mode.
Each mode of operation consists of two intervals: the first is when the MOSFET is conducting and the second is when the appropriate diode is conducting. The result is (6) (7) In order to charge the battery E2, the voltage across the capacitor V2 must be adjusted to be greater than voltage E2. Initially, the duty cycle D can be adjusted to make sure that V2 is greater than battery voltage E2 for boost operation. Based on Equation (7), it is expected that the charging current increases as the duty cycle increases. Again, the duty cycle D is initially set so that V1 is greater than battery voltage E1 for buck operation. Dynamic Analysis of the Bidirectional DC-DC Converter A typical configuration of a power circuit for a DC-DC converter, when considering voltage and current parameters, is shown in Figure 5. When the DC-DC converter operates as a boost or buck converter and in a continuous conduction mode, the current in the inductor flows continuously [5]. Red arrows are the direction of current in a step up mode while the black arrows indicate the current direction in a step-down operation.
Based on this power circuit, the output and input voltages, and inductor current are being derived. The Simulink Model Based on the design of the power and control circuits presented in the previous section, an equivalent simulink model of the bidirectional DC-DC converter was constructed and is presented in Figure 10.
In the simulink model, a PID controller is used to implement the control design for the system especially for generating a PWM that drives the MOSFETs and switch relays [9]. All the parameters are based on the actual component values used in the fabrication of the bidirectional DC-DC converter.
While the current flowing to the inductor gives identical results for boost and buck operation but opposite in direction.
It is found that the duty cycle for buck and boost operation must be greater than 50% in order to charge or transfer energy from one battery bank to another.
And as the duty cycle increases, the current flowing through the inductor increases which will also increase the amount of energy transfer from one battery bank to another. It is observed that if the duty cycle is less than 50% for boost and buck operation, then the inductor current tends to be the opposite, meaning that it is not supplying or charging the output.
It is noticed that the voltage change on the high or low side of the converter conforms to the type of mode of operation but the efficiency in buck mode is greater than the efficiency in boost mode operation because the control circuit is connected to the 24 V battery source. It is also observed that the voltage and current change quickly without any overshoot of the values as the mode of operation changes.
Bo, “Transient Current Suppression Scheme for Bi-Directional DC-DC Converters in 42V Automotive Power Systems,” Journal of Power Electronics, Vol.
Kanaan, “Two PWM Techniques for Single-Phase Shunt Active Power Filters Employing a Direct Current Control Strategy,” IET Power Electron, Vol. Rashid, “Power Electronics-Circuits, Devices and Applications,” 3rd Edition, Pearson Education, Upper Saddle River, 2003. Agrawal, “Power Electronics Systems Theory and Design,” Prentice-Hall, Inc., Upper Saddle River, 2011. Salameh, “Design and Installation of a 10.56kWp Grid-Tied PV Power System at University of Massachusetts Lowell,” POWER-Gen Renewables and Fuels Conference, Las Vegas, 6-8 March 2007.



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