Digital Implementation of Paralleling DC_DC conv
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This document summarizes the implementation of a digital current sharing controller for multiple power modules using a PIC16F876A microcontroller. The digital controller provides accurate current sharing of less than 1% error using a software-based automatic master-slave architecture. It offers advantages over analog controllers like easy implementation of complex algorithms, soft-starting, and fault protection through software updates instead of hardware changes. An experimental setup evaluated the digital controller alongside an analog UCC29002 controller, demonstrating balanced current sharing between two DC-DC converter modules.
![DA148 5
Analog current share controller UCC29002 is capable of
adjusting the voltage for small voltage difference of 0.2V
between the slave and master module.
VII. CONCLUSION:
From the above test results we can say that the low cost digital
load share control card can replace the analog load share
control card with accuracy in its current sharing and various
advantages of it compared to analog load share controller as
discussed earlier. The usage of these digital load share
controller as proven advantageous over some of the previous
Digital controller because of its parallel processing capability
and due to its hybrid quality; of using both digital and analog
components. By choosing Hybrid load share controller we
have reduced the burden on the µC and maintained sufficient
computational speed and performance without opting for High
end processors such as DSP.
VIII. ACKNOWLEDGMENT
The current work was carried out at M/S Chirra Electronics
Power labs Pvt Ltd, Bangalore. We wish to acknowledge their
support and guidance throughout the tenure of this work.
IX. REFERENCES
[1]T.S.Anandhit, S.P.Natarajan and T.Anitha, “UC3907 ASIC and
TMS320F2407A DSP based Control of Paralleled Buck DC-DC
Converters”, Indicon 2005 Conference, Chennai, India, 11- 13 Dec. 2005
[2] Siew-Chong Tan, Yuk-Ming Lai and Kevin Yan-Chun Wong, “An
Alternative configuration for Digitally controlled parallel connected DC-
DC Power Converters”, ECTI transactions on Electrical eng, electronics
and communications vol.4, no.1 February 2006
[3]Texas Instrument, “UCC29002 Load Share Controller” , 2007
[4] GUO Guoyong and SHI Bingxue, “Design of multi-phase dc-dc converter
with master-slave current sharing control”, IEEE, 2002.](https://image.slidesharecdn.com/0d082e9f-dcfe-4323-9e7c-312c8759b9f3-160319153953/85/Digital-Implementation-of-Paralleling-DC_DC-conv-5-320.jpg)
Digital Implementation of Paralleling DC_DC conv
- 1. DA148 1 Abstract— This paper describes implementation of digitally controlled current sharing in multiple power modules to be used in high current requirement applications. This leads to better system performance and system cost reduction. The Digital Load share controller is developed using low cost PIC16F876A µC with appropriate analog interface. The performance of the Digital Load share controller is presented in this paper along with Analog Load share controller UCC29002. Index Terms— Analog Load Share Controller, Digital Load share controller, Isolated buck DC-DC converter, paralleled power modules. I. INTRODUCTION wide range of mission critical applications such as Satellites, telecommunication, servers, etc., requires either or both (n+1) redundancy and high load current requirement. In order to satisfy the above needs we need to provide Multiple Paralleled Power modules. But whenever the converter modules are in parallel, one power module will take the full load and the other module becomes idle. In order to overcome this problem the only solution is current sharing among the parallel power modules. The current sharing equalizes the load current and thermal stresses among paralleled power modules which lead to improvement in overall system performance. The current sharing also provides an improvement in terms of electrical component reliability; for example, Mean Time between Failure (MTBF) roughly doubles with every 10°C decrease in operating temperature. Enforcing proper load sharing can also result in using power supplies with lower nominal ratings because of the reduced current in each. This technique, in turn, can translate into an overall lower system cost. There are several typical Current sharing methods, such as - Automatic Master/ Slave Architecture Droop Architecture Democratic Architecture Current mode control Master/ slave Architecture Dhananjay P, M.TECH – Power Electronics, 4th Semester, The Oxford College of Engineering, Bangalore, dhananjay474@gmail.com, Phone no: 9480011359. Jayakumar, M.TECH, Assistant Professor, EEE Dept, The Oxford College of Engineering, Bangalore, njktry@yahoo.co.in. Among various methods specified Automatic Master/ Slave architecture is more efficient and hence implemented in this work. II. WORKING OF PIC16F876A BASED DIGITAL LOAD SHARE CONTROLLER: The basic working of Digital Load share control card is based on Automatic master/ slave architecture. Where the power module carrying highest current will be the master and the remaining modules becomes the slave. The slave power module adjusts its regulated output voltage in terms of mV according to command received from master module until balanced current sharing occurs. The Fig 2.1 shows the block diagram of two paralleled 24V/15A DC-DC converter along with two PIC16F876A µC based Digital Load share controller connected at the load side. For reasons of simplicity, only two converters are paralleled in this work. The functional blocks of PIC16F876A µC based Digital load share control Fig 2.1 Digital load share control card connected to multiple 24V/15A DC – DC converters card is shown in the Fig 2.2 and its working are as follows: A. Current Sense resistor (Rsense): The Current sense resistor connected in positive output rail of the DC/DC converter is of value 2mΩ. Rsense resistor provides a maximum of 30mV, as an input to the Differential Current sense amplifier at full load of 15A. The advantage of Rsense resistor is, power dissipation is very less of 0.45W at full load. Digital Implementation of Current Sharing in Multiple Power Modules A. Dhananjay P and B. Jayakumar A
- 2. DA148 2 B. Differential Current Sense Amplifier: Its used to measure voltage across current sense resistor (Rsense) which will be in terms of mV. The gain of the Differential currrent sense amplifier is 100. Hence, the output of the differential amplifier will be, Vcso = Gain × (Vin+ - Vin-) 2.1 The maximum output of current sense amplifier is 3V at full load of 15A. The current sense amplifiers Noise roll off is designed for 3.5Hz. Fig 2.2 Functional Blocks of Digital Load Share Control Card. C. Load Share Unity Gain Amplifier: It’s a unity gain buffer amplifier to provide isolation between the Load share bus voltage and the output of Current sense amplifier. The amplifiers feedback consists of diode which is used to select the particular power module as ‘master’ and remaining modules as ‘slave’. D. Buffer: It’s a unity gain buffer, which is used to ensure that the Load Share bus is not loaded by the internal impedance of the Digital control card and to provide the Load Share bus voltage to one of the analog pin of µC. E. Load Share Bus: The Load share bus is used to carry the Master modules current sense voltage (Vcso) to the Buffer of the slave Power modules digital control card. F. Programmable Current Sink: The output voltage of the R-C filter is used to adjust the output voltage of the slave power module to balance the load current. This is done by the adjust amplifier and its companion NPN transistor. The adjust amplifier provides signal conditioning for the error signal and its output drives an NPN transistor which is configured as a programmable current sink. A resistor from its emitter to ground and the error voltage defines the current, IADJ, which flows through the ADJ pin of the digital Load–share controller and the positive output terminal. The IADJ current causes a voltage drop across the resistor which requires the power supply to increase its output voltage. The resulting higher power supply output voltage increases the output current of that particular module until the output current levels equal out among the units. At that point load sharing has been established. G. Algorithm: The following operations have been performed in the program for current sharing in multiple power modules. 1. The analog signal of the differential current sense amplifier output and Load share bus voltage has been converted to digital form using the inbuilt “Analog to digital converter” 2. Comparison of the “Load share bus digital value” and “Current sense amplifier digital value” is done. Here, the Output may be… (I) If “Load share bus digital value” is greater than the “Current sense amplifier digital value” by 100mV, the µC increments the error voltage from zero. (II) If the “current sense amplifier digital value” becomes greater than or equal to “Load share bus digital value” by 100mV, the adjust voltage stops incrementing and retains the previous value. Hence during this condition balanced current sharing is achieved. (III)If the “Load share bus digital value” becomes less than or equal to the “Current sense amplifier digital value” by 10mV then the error voltage starts decreasing and continues to decrease until the above condition becomes false or error voltage reaches to zero. This condition occurs during false operation of the system i.e. when the load share bus shorts with the ground or supply or when the other Power module Turn – off. In order to provide the stability to the digital controller we have provided a “Hysteresis gap” of 90mV between the increment and decrement conditions. After performing one of the above three conditions, the samples of Current sense amplifier output value and the Load share bus value are again obtained and checked for any of the three conditions discussed above. The detailed working of the µC is shown in the fig 2.3.The averages of 16 samples of individual current sense amplifier value and load share bus value are calculated separately and further used for computation purposes. This is to eliminate the effect of noise on the analog input or wrong conversion. In order to convert the Digital adjust voltage value to DC form, we use the combination of PWM generator and RC filter instead of DAC, mainly to reduce the system cost. System configuration required in order to use the Digital Load share control card is – “Power module should have Remote sense capability or the Power module should have adjust input or both”. III. BRIEF REVIEW OF CONVENTIONAL ANALOG LOAD SHARE CONTROLLER:
- 3. DA148 3 Among various available Analog Load share control IC’s in market we have chosen UCC29002 IC made by Texas instrument. The Analog control cards are designed using large number of on-chip op-amp’s and transistors to establish Automatic Master/Slave architecture as shown in Fig 3.1. Here the UCC29002 is capable of current sharing with 1% current share error. It is also capable of current sharing only for range of output voltage 4.5V to 35V. The IC is capable of fault analysis and protection i.e. when the load share bus is shorted to ground or supply. The UCC29002 load share control IC requires minimum external components as shown in fig 3.2. IV. ADVANTAGES OF DIGITAL LOAD SHARE CONTROL CARD: A complex control algorithm is difficult to implement with analog hardware components but can be easily implemental in firmware. Soft starting, slope compensation, interleaving, and fault protection can also be easily implemented without any external components. Modifying the control methodology for different field applications requires only software revision, and does not require hardware modification. The fan out can be increased. Accurate current sharing of less than 1% current share error at full load may be achieved. V. EXPERIMENTAL SETUP: The specifications of the Isolated buck DC – DC converters used in this prototype are shown in Table 5.1. The Digital load share controller designed is evaluated by placing Digital control card in one power module and the UCC29002 based Analog control card in another power module as shown in the Fig5.1. Before evaluating the performance of the Digital Load share control card, we set one power module as the Master and the other as the slave. In the Fig5.2 the power module on the right side is the master, taking the full load and the power module on the left side is the slave, taking no load. The master module Output voltage is greater than the slave power module by 0.1V. When the digital control card power supply of +12V, ±5V is made on, the current sharing take place and finally the balanced current sharing is achieved as shown in Fig 5.3. Fig2.3: Flow Chart of working of PIC16F876A in Digital Load Share control Card. Since Load share bus voltage is greater than the current sense amplifier output voltage in the slave module. The microcontroller makes error voltage to increase in terms of 20mV. Hence there will be a voltage drop at the resistor
- 4. DA148 4 present at the emitter side of the programmable current sink which makes the output voltage of the slave module to rise by driving the current. Fig 3.1: Functional Blocks of UCC29002 Analog Load share control IC Fig 3.2: Typical UCC29002 Load share controller connection in between the power supply and the common power bus. Fig 5.1: Proposed Parallel DC – DC converter with Digital Load share control card. Table 5.1: Specifications of Isolated DC-DC converter. The output current of the slave module increases until uniform current sharing occurs in both the module. When the balanced current sharing occurs the Load share bus amplifier output voltage will be almost equal to current sense amplifier output and under this condition output voltage on both the power module will be of 24.2V. Fig 5.2: The paralleled Power modules output Voltage and current display before current sharing. Fig 5.3: Balanced current sharing of two paralleled 24V/15A DC – DC converters. VI. EXPERIMENTAL RESULTS AND DISCUSSIONS: The test results is taken for both PIC16F876A µC based and UCC29002 based load share controller as shown in the table 6.1 and 6.2. The digital load share controller exhibits efficient operation under wide power ranges and it’s capable of generating 5V as an Table 6.1: Test results of UCC29002 based Load share control of two Paralleled DC-DC converter modules Table 6.1: Test results of PIC16F876A µC based Load share control of two Paralleled DC-DC converter modules. Input to the programmable current sink and in this work the maximum voltage input given to the programmable current sink by µC is 2V. The analog load share controller UCC29002 is capable of generating only 3V as error voltage to programmable current sink used in the IC. When the current sense amplifier output voltage is greater than 5V we can just place the potential divider at the input of the µC, hence it’s capable for higher current application greater than 50A. The Digital current share control card is capable of adjusting the voltage of the slave module for large voltage difference between the slave and master module of 0.5V accurately. But
- 5. DA148 5 Analog current share controller UCC29002 is capable of adjusting the voltage for small voltage difference of 0.2V between the slave and master module. VII. CONCLUSION: From the above test results we can say that the low cost digital load share control card can replace the analog load share control card with accuracy in its current sharing and various advantages of it compared to analog load share controller as discussed earlier. The usage of these digital load share controller as proven advantageous over some of the previous Digital controller because of its parallel processing capability and due to its hybrid quality; of using both digital and analog components. By choosing Hybrid load share controller we have reduced the burden on the µC and maintained sufficient computational speed and performance without opting for High end processors such as DSP. VIII. ACKNOWLEDGMENT The current work was carried out at M/S Chirra Electronics Power labs Pvt Ltd, Bangalore. We wish to acknowledge their support and guidance throughout the tenure of this work. IX. REFERENCES [1]T.S.Anandhit, S.P.Natarajan and T.Anitha, “UC3907 ASIC and TMS320F2407A DSP based Control of Paralleled Buck DC-DC Converters”, Indicon 2005 Conference, Chennai, India, 11- 13 Dec. 2005 [2] Siew-Chong Tan, Yuk-Ming Lai and Kevin Yan-Chun Wong, “An Alternative configuration for Digitally controlled parallel connected DC- DC Power Converters”, ECTI transactions on Electrical eng, electronics and communications vol.4, no.1 February 2006 [3]Texas Instrument, “UCC29002 Load Share Controller” , 2007 [4] GUO Guoyong and SHI Bingxue, “Design of multi-phase dc-dc converter with master-slave current sharing control”, IEEE, 2002.