FIELD OF INVENTION

The present invention relates to power supplies modules for distribution automation system and more particularly to provision of power supply in remote input output devices such that more than one power supplies modules may be effectively connected together.

BACKGROUND OF THE INVENTION

Several Distribution Automation (DA) products would need input/output (10) extension, flexible local IO replacement or additions to offer scalability. In case of remote IOs, meaning self- contained 10 device connecting physical, digital/analog I/O to other DA products, the remote 10 devices need to be capable of supplying required power to connected DA products. Required power is supplied through use of a power supply module. The power rating of the individual power supply module is usually limited by the available height, board space and by the limitation on volumetric heat dissipation. For example, a power supply module in remote 10 device may have a capability to supply maximum 7.5W as an example but the need may be to have capability to supply 15W. In such scenario, two power supply modules may be connected together in parallel. Thus, to supply higher power several parallel connected power supplies may be used. In addition to higher currents, the parallel modules can also offer redundancy, an important factor to achieve reliable, uninterrupted operation and extended life expectancy in the systems.

Independent power supply modules when connected in a parallel configuration may result in each of the independent power supply modules sharing unequal loads including sinking current instead of sourcing due to differences in operating point or output V-I characteristic, even when they are of similar kind. This may result in uneven distribution of electrical and thermal stress, and consequently possible decrease in life of overloaded power supply module. To avoid the problem of unbalanced sharing of load current, it is common to include circuitry for proportionally distributing load current among parallel connected power supply modules depending on their individual capacity. This technique is referred to as "load sharing". Load sharing is typically found in power supplies wherein more than one power module service a single load or has a common point such as on a common output voltage bus which supplies power to multiple loads. Further, the common output voltage bus need to be provided in such a manner that independent power supply module are capable to indicate their respective health status in spite having multiple independent power supply modules connected in parallel. Each power supply module need to be decoupled from any effect from having voltage output from power supply combination be available at the output of the each power supply modules in the combination including that of the faulty supply.

It is common for power supplies to use high frequency switching as a means for efficiently converting the input source to the desired DC voltage level. Power supplies may be designed to operate around large range of high frequency that may also generate noise capable to create interference. The noise in power supply modules connected in parallel may result from cross talk/interference due to the coupling at the input/output and due to interaction from load sharing control loops of various modules used for proportionally distributing load current among parallel connected power supply modules. Such spreading of noise from a module across the entire system may lead to stability issues in any of the power supply modules, eventually risking the stability of the entire system. The task of controlling noise and improving stability is specifically challenging when there is no common input voltage source to the converter power supplies and/or there is a need for power supply modules to support wide input voltage range.

The noise and stability may be improved by effective decoupling by reducing noise and cross talk in the system for reliable operation. Further, decoupling is also required for reliable health indication and communication in the system.

SUMMARY OF THE INVENTION

In one aspect of the invention, the invention provides a power supply module comprising of a means to connect to an input power source (through suitable terminals or bus connection), a regulating means for regulating output DC voltage, a rectification module generating output DC voltage from high frequency switched voltage, a transformer having a primary side connected with the regulating means and the power input power source (one or more), and the secondary side connected to the rectification module, a voltage feedback means (eg voltage divider) for providing voltage feedback signal to the regulating means to regulate the output DC voltage, a connecting means to connect the power supply module with at least one other power supply module (eg through bus connection when power supply modules are stacked together) and a load share control module for controlling load sharing for the power supply module. The power supply module in addition has a filter amplifier connected in the secondary side of the transformer for reducing noise in the power supply module (specially that in the voltage control loop in the power supply module).

In a preferred embodiment, the filter amplifier attenuates high frequency noise produced/appearing due to switching in a power supply module (system when multiple power supply modules are connected together). The filter amplifier at the secondary side of the power supply module together with voltage error amplifier in the primary side of the power supply improve the signal to noise in the power supply and improve stability.

In a preferred embodiment, the modular power supply module has a health check module to infer its health status based on comparison of the DC output voltage and a signal derived from an equivalent rectification module connected at the secondary side of the power supply module.

In another aspect a modular power supply system having a plurality of power supply modules coupled at the output using a common output voltage bus and common current share bus is provided. The power supply modules can be connected together in parallel by simply stacking them together without any additional settings or use of additional devices/signals.

In another embodiment the modular power supply system having power supply modules connected in parallel is provided with independent and different input power sources.

In yet another embodiment the modular power supply system has some of the power supply modules provided with an independent and different input power sources and some other power supply modules are provided with a common input power source.

In still yet another embodiment the modular power supply system has at least one power supply modules provided with backup battery input sources.

BRIEF DESCRIPTION OF THE DRAWINGS These and other features, aspects, and advantages of the present invention will become better understoc when the following detailed description is read with reference to the accompanying drawings in whi( like characters represent like parts throughout the drawings, wherein:

Fig. 1 is a circuit diagram illustrating an embodiment of individual power supply module ;

Fig. 2 is a circuit diagram illustrating an embodiment of two power supply modules connected in parallel;

Fig. 3 is a circuit diagram illustrating an embodiment of two power supply modules with noise reduction capability connected in parallel;

Fig. 4 is circuit diagram illustrating an embodiment of health check module in two power supply modules connected in parallel;

Fig. 5 is a block diagram illustrating Standalone mode of operation;

Fig. 6 is a block diagram illustrating multiple power supply mode of operation;

Fig. 7 is a block diagram illustrating multiple power supply mode of operation having a backup battery for input source for one power supply module.

DETAILED DESCRIPTION OF THE INVENTION

The invention describes a technique to control noise and improve stability for power supply modules that are connectable in parallel for its use in distribution automation system. These power supply modules are modular and are mountable in DIN rail and may have a common input voltage source or individual input source and support a wide range as input voltage.

Fig.l illustrates an exemplary power supply module. Referring to Fig. 1, the power supply module 100 includes an independent input power source 1 10, a regulating means 120, a rectification module 140 generating output DC voltage 160, a transformer 130 having a primary side connected with the regulating means 120 and the input power source 110, and a secondary side connected to the rectification module 140, a voltage feedback means 150, voltage divider in the illustration, for providing voltage feedback signal to the regulating means 120 to regulate the output DC voltage 160 in the voltage control loop. The regulating means in the illustration is designed with use of a switching element (switch) which is driven by a PWM (Pulse width modulation) based control.

Critical applications where high power and reliable powering solution is desired, more than one power supply modules are connected in parallel. User is provided with the flexibility to connect number of power supplies depending on his requirement including the future needs, where if any application demands more power, user has a flexibility to simply add more power supply modules.

Some additional design considerations that are provided in the invention for high power and reliable powering solution are: a) All power supply modules may be identical without any segregation of master/slave type for controlling equal loading and are capable of working in "standalone mode" or in "multiple power supply mode" without any need of any selection/external device/extra connection or any fine setting of output voltage. b) All power supplies may have independent input voltage source i.e. input voltage sources of each parallel connected power supplies (i.e. in "multiple power supply mode") can be different. For example, power supply module 1 (PSM1) can have 220V AC as input source and power supply module 2 (PSM2) can have 1 lOVDC as input source. The range may be wide, as an example the voltage for the power supply module may be in the range of 85-264VAC or 88-375VDC. Further, the input voltage sources for a parallel connected power supplies can be a power harvesting system which drains power from the HV power line or be an alternative energy system like photovoltaic system or from a backup battery. c) Parallel connected power supplies i.e. in "multiple power supply mode" can continue to work, even if any power supply has no input source or any of the power supply from the parallel connected power supply failed. In this mode, all the participating healthy power supplies share load current equally over the complete range of total load [i.e.(Power capacity of one power supply) x (No of power supplies connected together)]. The load sharing to be good even at light loads and have better load regulation. d) In "multiple power supply mode", if any power supply module will fail or input source of any power supply module will be off, other power supply modules continue to work normally and so system will continue to work normally. Also the not working power supply needs to communicate its health condition to the system. It is desired to have the main output voltage control loop & health information for each module be easily separated (decoupled) from the load share control mechanism. Each power supply module individually indicates and communicates its health status in efficient manner.

Fig.2 illustrates a power supply system 200 with two power supply modules 100 connected in parallel with load sharing circuitry. In the parallel system, the figure illustrates a load sharing circuitry having load share control module 210 which is responsible for load sharing control, a common output voltage bus 230 from parallel coupling of output voltages of all the independent power supply modules and a common current share bus (SHARE signal) 220 which is used as a reference for load sharing.

The respective load share control module 210 in each of the power supply modules uses the common current share signal to have active load share control. The operating principle of a load share mechanism is to measure the output current of each individual module and to be able to modify the output voltage of the units, until all participating power supplies deliver equal output currents. Thus, all the power supply modules actively make their load current equal to the common current share signal by appropriately controlling their respective regulating means 120.

The interaction of output voltage control/feedback loop & load sharing control loop are done in such a way that, the power supply module can work in standalone mode and in multiple power supply mode without any need of selection/external device/extra connection.

Further, the independent power supply modules may have a common input source or have independent input sources and each of the power supply module may have input filter & protection, health status indication & communication, output protection (not shown in the figure).

In "multiple power supply mode", all connected power supplies equally share total load current.

Normally, each of the power supply modules work below their maximum load capacity. When required power is more than the defined capacity of the single power supply or the power supply system in use, user only needs to put together (side by side) additional required number of power supplies & to stack them together.

The parallel connected power supplies convert their independent input voltage to a defined DC output voltage. Each of the modules will continue to provide independent filtering, protection & health indication/communication. Apart from two terminals of the DC output voltage, the parallel connected power supplies share only one extra terminal (i.e. SHARE signal 220) between them, which indicates the maximum current supplied by any of the connected power supply. Each of the parallel connected power supplies try to make their shared load current equal to the SHARE signal (as in common current share bus), by using active current sharing technique over the complete range of load current even though their respective input sources or voltage ratings maybe different.

Any noise, if significant in magnitude and phase in any of the power supply module, will interfere in the power supply system and can negatively impact the system leading to undesired situations; one such situation being oscillations in power supply output. The source of noise in the voltage control loop in a power supply module 100 may be due to the following as examples are:

• 50/όΟΗζ line frequency

• Variation in load (transient/low frequency)

• Switching of power supply module (high frequency)

The Fourier components of noise undergo gain changes and phase shifts in filter and amplifier components in the system. If for any one of these Fourier components, the gain and phase shifts result in positive rather than negative feedback, the power supply module will have oscillations. So even if the initial source of noise signal dies out, the circuit will continue to oscillate at that particular frequency component. A common way to improve the stability of the system is to increase the phase margin. Hence to achieve stability in the voltage control loop, it is required to have enough phase margin at gain cross over frequency (i.e. frequency at which open loop gain is unity).

Further, taking the case of parallel connected power supply modules, when there are different sources/ratings of input voltages used for parallel connected power supply modules, the regulating means 120 in each module may switch at different duty cycles from one another. Therefore, disturbances due to switching maybe different in each of the parallel connected modules. It may happen that disturbance/noise due to individual input sources 110 in each of the modules or due to switching operation in modules, will reflect on the share bus 220. So each parallel connected module will react to this disturbance, in a different way (because they are operating at different duty cycles) making crosstalk between different control loops prone to unstable operation.

Fig. 3 illustrates an exemplary power supply system 300 having two power supply modules connected in parallel effectively decoupled from each other to achieve better stability during operation in standalone mode and in multiple power supply mode. The power supply system has common voltage bus and common current share bus for providing the power to the load by having each of the power supply modules sharing load equally. A filter amplifier 310 (voltage error amplifier) is connected on the secondary side of the power supply module to condition the voltage feedback in the voltage control loop. The load share control module 210 is designed for being slow acting i.e. poor response to high frequency changes, which is acceptable as it responds to equalize the mean (average) output current. The filter amplifier 310 and the voltage error amplifier in the regulating means 120, both provide filtering and gain to help achieve stability over the complete input voltage range. The filter amplifier 310 on secondary side of the power supply module performs the following function:

• Attenuates the noise due to switching and prevent them to go to primary side voltage error amplifier in the regulating means 120.

• Amplify the noise due to 50/60Hz line frequency & Variation in load

Variation affecting the load share control loop is being taken care only by low gain provided by voltage error amplifier on primary 120. The usage of filter amplifier 310 on the secondary side and voltage error amplifier in the PWM based module on the primary side of the regulating means 120, where both the components have different frequency response, provides for high phase margin in power supply module. By using this approach, stability has been achieved over the wide input voltage range (eg. for universal range i.e. 85-264VAC/ 88-375VDC) and/or different input power sources including AC input sources for parallel connected power supply modules with equal load sharing.

Thus, with use of additional filter amplifier component 310, the effect of noise and thereby stability of power supply module and of the system is improved. Another aspect related to distribution automation system products is to have proper health indicators and health communication for the distribution automation product. For parallel power supply modules, it is desired that the health status indication and communication for individual power supply module is decoupled from that of others. A well-known approach to solve this problem is through the use of decoupling series diodes for each of the independent power supply modules. However these diodes can cause power loss (max power loss = Output current X Diode voltage drop, approxl Volt) which has a negative impact on the system.

Fig. 4 illustrates a technique for health indication and communication with a circuit diagram depicting secondary side of a power supply module having rectification module 140 connected in parallel with an additional set of components to have an equivalent rectification module 420. The circuit further has a voltage divider 430 provided with the equivalent rectification module 420. A health check module 440, associated with each power supply module consists of a comparator to evaluate health of the respective power supply module based on comparison of the signal from the voltage divider 430 with the common output voltage bus 230 to provide a health status signal 450. . The equivalent rectification module 420 consists of elements similar to the elements in the rectification module 140, however these elements have very low power rating (as they are required only for signal output) and hence consume little power. Since the equivalent rectification module 420 is not directly associated with the common output voltage bus 230, the power supply modules are decoupled with each other for health indication. This technique for health indication and communication is efficient as it does not affect the load current or output voltage from any of the modules involved in supplying power. The health check module has LED indication to indicate health status and a communication module to communicate health status signal in the distribution automation system.

It may be noted that each power supply module provides for output DC voltage, SHARE signal and health signal. As an embodiment, the power supply module function in a standalone mode is as depicted in Fig. 5. Standalone mode is utilized when the required power is within the defined capacity of a single power supply. As another embodiment, two power supply modules are connected in parallel, though the required power maybe within the defined capacity of a single power supply. In normal condition, when both input voltage sources are available, all connected modules share load current equally. When there is any problem in one of the power supply modules including the input voltage source, the healthy power supply module will continue to work normally by providing the required load current. Here, the power supply modules are working in the multiple power supply mode though the requirement of power is within the capacity of one power supply to increase reliability of the power supply system.

As another embodiment, multiple power supply modules may be connected together having multiple combinations of input voltage source to improve reliability in a distribution automation system. The multiple power supply modules may share a common input voltage source or have independent input voltage sources. Further, as illustrated in Fig. 6 and Fig. 7, the multiple power supply modules may have input voltage source derived from input voltage sources that are less in number compared with the number of power supply modules connected together. Here, as an

example illustrated in Fig. 6, two input voltage sources (610 and 620) are used to provide input voltage to three power supply modules 100 connected together. Fig. 7, illustrates again a case with two input voltage sources (710 and 720) for three power supply modules (100) but depicting use of a battery backup supply 710 to supply to a power supply module.

While only certain features of the invention have been illustrated and described herein, many modifications and changes will occur to those skilled in the art. It is, therefore, to be understood that the appended claims are intended to cover all such modifications and changes as fall within the true spirit of the invention.