In this study, a temperature-based current sharing strategy rather than equal sharing for loads was applied to promote the reliability of uninterruptible power systems (UPS). According to the temperature of each power supply module in a UPS, it would be better to reduce the output current ratio for a hotter supply module in the UPS. In this design, we implemented our regulation circuits by the UMC 0.25-μm CMOS technology with an input range from 3 V to 4.2 V and the regulated output at 1.1 V. The rated output current was 100 mA for each phase. We also employed a current-mode error-correction circuit to improve the current sharing performance based on the averaged current of each phase at the same temperature. According to our simulation results, the current sharing error can be restricted within ± 5% for the supply modules at the same temperature in our system.
For heavy-load power supply systems, a system with multiple power supply modules connected parallelly may provide several advantages [
Recently, a simple current sharing method was proposed for multiphase voltage control to achieve good current sharing performance [
The thermal effect in UPS arises from the fact that the temperature of the power supply module on the top of a power shelf is usually higher than the bottom one. Because of the thermal convection, the generated heat from all the power supplies would accumulate at the top in the shelf. Therefore, the top module suffers higher temperature than the others. If all the power modules share the same current output, the module suffering higher temperature would exhibit poor reliability. According to general phenomena, the failure rate of a power device would get doubled for every 10˚C increase in temperature. Therefore, the reliability of the whole power system would degrade 50% in life time even with only one power supply getting its temperature raised by 10˚C. To solve this problem, we proposed a method by detecting the temperature of each power module in each phase to redistribute the current ratio. With this scheme, the hot power module supply would share a lower current to cool down itself with an aim to achieve a better balance of the failure rate for each module in the UPS.
The circuit structure was revised from the basic circuit proposed in [
For conventional designs, the average current sharing method and the droop current sharing method are usually employed.
N-phase current sharing method purposed by [
In our study, only 2 phases were employed. Unlike the conventional sensing of the load current through a small resistor, in our circuit, the current information at the load was directly sensed at each phase. After comparison, our circuit would turn on the phase with the lower current if the two modules were at the same temperature. The advantage of this sharing method is that its response would be faster. In the meanwhile, the disadvantage of this scheme is that it would generate an error between the sharing currents of the two phases due to the turn-on times of the two phases were different. For this issue, the output current of each module will revise when the error accumulated over the ripple in the load current. The criterion to change the sharing ratio will be discussed in the later section.
Taking the circuit in
mine which phase provides lower current. By a control scheme to turn on the suitable module, a reliable current sharing can be achieved. Our design of the control scheme will be described in the next section.
In the design of the current error correction circuit, we have to know how much error will generate at each switching cycle. According to the error, the turn-on time of the power supply module having lower current phase need to be extended. And the turn-on time of the module having higher output current should be reduced.
In our circuit, the error of between the currents of the two phases in each switching cycle was estimated. After the accumulated current error was higher than a criterion, the controller would trigger the correction circuit to change the turn-on times for both phases.
In our study, when one phase supplied more current than the other one, its current would be reduced by decreasing its turn-on time. For this purpose, we designed an auxiliary current source to help the decreasing process faster. The auxiliary current source is marked by the block in
As seen in
The simulation results of this circuit without and with the correction circuit can be seen in
figures, it can be found that the current sharing performance is improved significantly with the help of the auxiliary circuit. According to this simulation results, the current sharing error can be restricted within ± 5% in our system. Due to the correction scheme, the auxiliary circuit can reduce output noises which can be caused by large current drop in each phase without the correction scheme.
In our study, the conventional design of the power supply module to equally share the load current is not ideal for practical operations. In practical applications,
the reliability of an electronic device, especially for a power device, is closely related to the thermal effect. Since the temperatures of the power modules are different in a power case due to the non-uniform distribution of heats accumulated by all the power modules in the case. Therefore, every power module suffers different temperature even they share the load equally. In our study, we would like to propose a new scheme of current sharing control depending on itself temperature of a power module. In the control circuit, the load of a power supply module having higher temperature was reduced by decreasing its turn-on time such that the reliability of the whole system can be extended significantly. Therefore, the current sharing scheme was no longer just 50% for both phases. Instead, the sharing ratio of each module would change depending on the temperature of itself.
To achieve this object, a circuit sensing the temperature of each phase was proposed as shown in
To redistribute the current sharing ratio for each phase of different temperature, an addition control signal relevant to the temperature information of each phase was required.
To achieve this target, the current difference affected by the temperature was utilized at the input of the current comparator in
In order to change the ratio between ISENSE1 and ISENSE2 and according to the temperature difference between the two power modules, we added two additional currents, IDISCH1 and IDISCH2, as in
In this study, we designed our circuit based on UMC 0.25 μm 2P3M CMOS technology. As seen in
20 mA to the middle load, 100 mA.
temperature of the phase offering larger current would increase faster than that of the other one, the ultimate result may be the case that the temperatures of
both modules approach to almost the same to each other. In this case, the output current would be equivalently shared by both modules. Therefore, it can be expected that the reliability of the multiphase power supply can be improved significantly.
In this paper, we propose a new current correction circuit to improve the current sharing performance. In addition, the temperature effect is also considered by a temperature sensing circuit to redistribute the current ratio for the power modules. In our circuit, the hotter power supply would share lower ratio for the load current for better thermal balance. Since the unbalanced current sharing is affected by the temperature difference, the whole system may arise another feedback mechanism between the load sharing ratio and the temperature difference. It is of theoretical interests to investigate on the interaction between the current ratio and the temperature difference for practical systems. In our primary figuring, since the temperature changes very slow, it was expected that the ultimate condition is the temperature difference reduced to 0. Nevertheless, the temperature-based sharing is beneficial for the lifetime of a power system. For heavy load applications, it is promising in our study that the parallel connected multiple phase power supply with temperature-base sharing can be feasible.
The technical support from UMC Inc. is appreciated. And the financial support of Department of Electronic and Information Engineering, Xiamen City University is also acknowledged.
The authors declare no conflicts of interest regarding the publication of this paper.
Zhang, L.K., Wu, H.C., Cheng, C., Liu, D., Shiau, M., Lai, W.D. and Zheng, Z.W. (2018) A Temperature-Based Hysteresis Buck Converter for Dynamic Current Sharing. Circuits and Systems, 9, 213-223. https://doi.org/10.4236/cs.2018.912019