Abstract:
Constant current regulator ( 10 ) for supplying electrical power corresponding to a predetermined output power, comprising a plurality of modules ( 13 ) electrically connectable for simultaneous operation to jointly provide the output power, the modules are configured for providing a module output power contributing to the output power of the regulator. Each module comprises a transformer ( 135 ) for providing galvanic insulation as required by local standards, and a microcontroller ( 231 ) for controlling operation of the module. The constant current regulator comprises a data communication network ( 17 ) configured to be connected to the microcontrollers ( 231 ) of the modules ( 13 ), wherein the microcontrollers ( 231 ) are operable to exchange data over the data communication network ( 17 ) so as to make the constant current regulator ( 10 ) modular, meaning that one or more of such modules ( 13 ) can be added to or removed from the constant current regulator.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation of U.S. patent application Ser. No. 14/394,910 filed on Oct. 16, 2014 that is a National Phase entry of PCT/EP2013/063421 filed on Jun. 26, 2013 that claims priority to EP Application No. 12173866.0, filed on Jun. 27, 2012. 
    
    
     TECHNICAL FIELD 
     The present invention relates to constant current regulators (CCRs), in particular regulators used for supplying airfield lights with electrical power. 
     BACKGROUND 
     CCRs of the above kind are known from e.g. US 2010/283400. A CCR&#39;s goal is to regulate a current output level around a working point which can e.g. be set by an operator or an airport control tower. The current output level should be as constant as possible near to a pre-set working point in order to ensure that the airfield lights emit at constant luminous intensity. Since it is desirable to use different intensities depending on the circumstances (e.g. day or night, good or low visibility), the working point can change and the CCR must be able to adapt the current output level accordingly. 
     Airfield lights are typically arranged in long series connections. The length and number of lights in such series and hence the power to be delivered to it can differ between airports, between the type of lights and their use. CCR manufacturers therefore need to provide a wide range of CCRs, each corresponding to a particular input line voltage and frequency, output current and equipment power. This leads to a large number of different CCR product items, which increases stocks, manufacturing resources and cost. 
     CCR reliability is another aspect, since it is an important safety issue. It is known to have complete backup CCR units installed at airports, which requires a huge additional space and leads to a significant cost increase. 
     An attempt to tackle the reliability issue is described in US 2010/026207 by providing a twin voltage conversion unit connected in cascade to a common transformer operating at grid frequency (50/60 Hz). The idea here is to add a redundant (non-operating) voltage conversion unit, which is easily replaceable. In the event of a failure, the operating and non-operating voltage conversion units can be switched automatically, such that the broken part can be replaced later during a maintenance service. 
     The same document describes that it is not excluded to use the two voltage conversion units contemporarily for the operation of the CCR, but fails to teach how the co-operation is to be carried out. Additionally, the document describes a common control unit for controlling operation of the voltage conversion units and for activating switches configured to activate and deactivate one or the other voltage conversion unit. 
     The system of US 2010/026207 is more reliable, but at an increased expense per CCR unit, since each CCR unit will now have an additional voltage conversion unit identical to the one already present. 
     Hu Wei et al., in “Input-series output-parallel AC/AC converter”, 2010 5th IEEE Conference on industrial electronics and applications, 1 Jun. 2010, pages 1018-1022 describes a two-level AC/AC converter with a high frequency link, which can be used in applications with high input voltage and relatively low output voltage. Such a circuit is not applicable to airfield lighting installations due to its limited power. Furthermore, a disadvantage is that it is not reliable, since operation of both levels is required for obtaining a desired output. 
     EP 1063758 describes a power supply device comprising a plurality of resonance type switching converters arranged in parallel. Such devices allow for supplying a large power to a load. However, the described devices feature a DC output voltage, which is not workable for an airfield lighting installation, since airfield lighting installations must always be provided with AC current for corrosion protection of the cables and connectors, and in order to be able to use transformers to connect the lights to the supply line for lightning protection, independently of the type of lights (incandescent or LEDs). 
     US 2002/0074862 describes a parallel power source system comprising a plurality of power supply units, so designed that even when one power supply unit fails, the other power supply unit can ensure power supply. The described system, however, generates DC outputs from AC input supply voltages and cannot therefore be used for supplying airfield lighting installations. Each power supply unit comprises a high frequency transformer supplied with a high frequency pulse current to its primary coil. 
     OVERVIEW 
     It is an aim of the present invention to provide a constant current regulator which overcomes the above drawbacks. In particular, it is an aim to provide a CCR which does not only address the reliability issue, but also the economical issue. It is an aim to provide a CCR which has at least a same, and possibly better reliability. It is also an aim of the invention to provide a CCR which is more economical to manufacture. Yet another aim of the present invention is to provide a CCR which has an increased lifetime and operates more efficiently. 
     In the view of the present inventors, system reliability can be increased beyond the above proposed solutions, while obtaining an economy in manufacturing costs by building the CCR modularly with a plurality of identical modules, each module being in fact a CCR of its own. 
     According to the invention there is therefore provided a constant current regulator as set out in the appended claims. The CCR comprises a plurality of advantageously identically rated modules, each module being configured to provide a module output current and voltage which contributes to the CCR&#39;s output power. According to the invention, each module is in fact a CCR of its own, having its proper transformer and microcontroller. The CCR is provided with a data communication network operable to exchange data between the microcontrollers. The data communication is so organised that one or more modules can advantageously be added to or removed from the CCR without affecting the ability of the other modules to communicate with each other and operate simultaneously to jointly provide the output power. As a result, a CCR is obtained which is modular to full extent, both at the input stage, and the output stage, as well as at the control level. 
     Whereas in US 2010/026207, modularity is restricted to the voltage conversion unit (i.e. the input stage only), in the present invention modularity applies to all the components of the CCR. An advantage of such modularity is that CCRs with different rated outputs can be assembled based on one and a same CCR module. This drastically decreases the number of different parts that manufacturers need to keep in stock or produce. By way of example, CCRs of all the existing power ratings can be made by using one and a same (small) transformer, thereby obviating the need of having different transformer sizes, from smaller ones to large ones. Since larger quantities of the same transformer can be produced, an economy of scale can be obtained. As an additional advantage, the full modularity enables to achieve increased reliability. Indeed, the CCRs described in US 2010/026207 will fail if the central controller fails, or in case the common output stage fails. This is not the case in CCRs of the present invention, since these stages are included in the modular design. The other documents discussed above do not describe such a modularity either. 
     Advantageous aspects are set out in the dependent claims. 
     According to another aspect of the invention, there is provided a module for use in a CCR according to the invention, as set out in the appended claims. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Aspects of the invention will now be described in more detail with reference to the appended drawings, wherein: 
         FIG. 1  represents an overall diagram of a CCR according to the invention; 
         FIG. 2  represents a diagram of a power module of a CCR according to the invention; 
         FIG. 3  represents input and output connection schemes of the power modules, for the exemplary case of a CCR comprising two power modules. 
         FIG. 3A  represents a connection scheme wherein the power modules&#39; outputs are connected in series.  FIG. 3B  represents a connection scheme wherein the power modules&#39; outputs are connected in parallel. 
     
    
    
     DESCRIPTION 
     It will be convenient to note that current and voltage levels indicated in the present description refer to root mean square (rms) values thereof. Therefore 1 A refers to a 1 A rms current and 1 V refers to a 1 V rms voltage. 
     It will be likewise convenient to note that the term ‘rated power’, ‘rated current’, or ‘rated voltage’ refers to the maximum operating power, current or voltage respectively. 
       FIG. 1  depicts a schematic diagram of a CCR  10  according to the invention, with power line input  11  for connection to an electrical power network and power output  12  for connection to a series connection  9  of lights. Series connection  9  is typically built up of series circuit transformers  92 , the primary coils of which connected in the series  9  and the secondary coils of which connected to the lights  91 . 
     CCR  10  is formed out of modular units  13 - 1 ,  13 - 2  through  13 - n,  which will be referred to hereinafter as power modules  13 . Power modules  13  are connected to the power line input  11  through input power connections  14 , forming a wired interface between the power line input  11  and the line input  131  of each power module  13  (see  FIG. 2 ). The power modules are connected to the power output  12  of CCR  10  through output power connections  15 . 
     Input power connections  14  provide for a parallel electrical connection of the modules  13 - 1  through  13 - n  to the power line input  11 , as is schematically represented in  FIG. 3 . 
     Referring to  FIG. 2 , it will be convenient to note that each power module  13  is in fact a constant current regulator of its own. Power module  13  hence is operable to provide an output power (current and voltage), which can advantageously be directly supplied to the power output  12  of the CCR  10  and hence to the series circuit  9 , without any additional galvanic insulation or level correction. 
     All power modules  13 - 1  to  13 - n  are advantageously substantially identical, in that they have identical maximum operating (rated) power, current and voltage. As with every CCR, each power module is also configured to output a power and to generate a current corresponding to pre-set levels. The output voltage is related to the current and to the output resistance and can change between zero and a maximum (rated) voltage. 
     To do so, a power module  13  advantageously comprises input stage circuits  132 - 134  and output stage circuits  136 - 138  galvanically separated, as prescribed by local standards, such as FAA (Federal Aviation Administration) or IEC (International Electrotechnical Commission) standards, by transformer  135 . Transformer  135  may for example provide galvanic insulation higher than or equal to 23 kV. 
     Input stage circuits may comprise a low pass filter  132  for filtering the line input  131  of power module  13 . 
     Input stage circuits comprise a voltage conversion circuit, schematically represented by blocks  133 - 134  in  FIG. 2 . Voltage conversion circuit  133 - 134  is advantageously configured for generating a regulated voltage widely independent of the input voltage level and of the frequency of the input voltage (e.g. mains frequency 50 or 60 Hz). Voltage conversion circuit advantageously comprises a rectifier block  133  followed by a power inverter block  134 . Rectifier block  133  can be based on a diode bridge circuit, which is operable to transform an AC (alternating current) signal (such as an advantageously filtered line input, at grid frequency) to a DC (direct current) signal. Power inverter block  134  is operable to transform the DC signal from the rectifier  133  to an AC signal. It therefore advantageously comprises a LLC resonant converter circuit, possibly preceded by a power factor corrector (PFC) circuit, advantageously an interleaving PFC. 
     The regulated voltage output by voltage conversion circuit  133 - 134  is advantageously a high frequency AC voltage, such as of at least 1 kHz, advantageously at least 10 kHz, advantageously at least 20 kHz, advantageously at least 25 kHz, advantageously at least 30 kHz. The high frequency AC voltage can be lower than or equal to 100 kHz. The obtention of such high frequency signals can be accomplished by a high frequency LLC quasi resonant converter power circuit. One such LLC quasi resonant converter power circuit is described in an internet paper titled “Phase Shifted Full Bridge LLC Resonant Converter” by Martin Zhang and Sober Hu, and downloadable at the following web page, the contents of which being incorporated herein by reference: http://blog.dianyuan.com/blog/u/42/1150776421.pdf. 
     An advantage of a high frequency voltage output in the input stage is that size and weight, and hence cost of transformer  135  can be significantly reduced. By way of example, a high frequency transformer working at about 40 kHz is 10 to 20 times smaller in weight and size compared to its 50 Hz counterpart. 
     Additionally, at the indicated high frequency voltages, the power losses due to the high frequencies remain acceptable, such that an optimal balance between size reduction and power loss is achieved. 
     The LLC resonant converter circuit is hence connected to the primary side  235  of transformer  135 . The secondary side  335  of transformer  135  is connected to the output stage circuits  136 - 138 , which can comprise: a rectifier diode bridge circuit  136  and a transistor H-bridge inverter circuit  137 , to provide a regulated module output  139  (current and voltage). H-bridge inverter circuit  137  is advantageously configured for working at a high frequency, such as the ones indicated hereinabove in relation to the voltage conversion circuit  133 - 134 . A low pass filter  138  can be provided between the bridge circuit  137  and module output  139 . The regulated output  139  is typically, though not necessarily, a low frequency pure sine wave AC output, at grid frequency of typically 50 or 60 Hz. Regulated outputs of other shapes are possible as well, such as pulsed DC. 
     The output low pass filter  138  is advantageously operable to filter out any signal components of the above indicated high frequency (i.e. frequency of operation of the rectifier and transistor H-bridge). Operating at such high frequencies is advantageous, since it allows for making the output low pass filter  138  very small and economical compared to output filters used in prior art CCRs, which typically operate at frequencies of a few hundred Hz. 
     Referring to  FIG. 3A , the outputs  139  of power modules  13 - 1  to  13 - n  can be connected in series to realize a regulated output current of CCR  10  being maximally the rated output current of one power module, but achieving a higher CCR output power compared to each single power modules, since the regulated CCR output voltage is the sum of the module output voltages. By way of example, considering power modules with 5 kW rated power (on resistive load) with a rated output current of 6.6 A. The rated output voltage of each power module will then be 757 V. Connecting the power modules&#39; outputs in series allows for obtaining a CCR output current of maximum the rated output current of a power module (6.6 A), but at variable power depending on the number of power modules connected in series. Connecting four such power modules in series allows for obtaining a CCR having a rated output power of 20 kW 6.6 A×(4×757 V). 
     Alternatively, as represented in  FIG. 3B , the outputs  139  of power modules can be connected in parallel to obtain a CCR output current being the sum of the power module outputs. By way of example, connecting three of the above power modules in parallel allows for obtaining a maximum rated CCR output current of about 20 A at a voltage level of 757 V. 
     It is also possible to combine series and parallel connections of power modules, such as a series connection of two groups of two 5 kW modules in parallel, thereby obtaining a 20 kW CCR with a maximum output current of 12 A. 
     The electric connection of the power modules  13 - 1  to  13 - n  in the above described series or parallel connections, is advantageously a hard-wired connection. 
     Hence, according to the invention, all the power module outputs are configured to contribute to achieving the CCR (global) output  12 . There are different ways for obtaining a co-operation between the power modules  13  to achieve the predetermined CCR power output  12 , which is set externally, such as by an airport control tower, or an operator. 
     An advantageous way for obtaining such a co-operation between power modules is based on a masterless (or multi-master) data communication protocol. This means that none of the power modules would act as a master module determining the contribution level of each power module to the CCR output, but rather each power module is situated at a same level in the power module hierarchy and each power module self-determines its proper module output level  139  based on the predetermined output of the whole CCR  10  (output of all power modules  13 ) and on the output set by the other power modules. This is achieved by letting the power modules  13  almost constantly communicate with one another to obtain an efficient co-operation. 
     An alternative way for obtaining co-operation and communication between the modules is based on a master-slave data communication protocol. In such a protocol, the master advantageously can change automatically in case a power module  13  is added to or removed from the CCR  10 . By way of example, when a power module  13  is added to the CCR, it automatically receives an address for data communication, which can be based on a serial number or unique identifier of any of the power module&#39;s components. The address can therefore be hardware implemented. The address is compared to the addresses of the other power modules, and the protocol can describe that e.g. the power module having the lowest or highest address acts as master governing the data communication, the other modules being slaves. Such a procedure ensures that in case of failure of the current master, the CCR can continue to operate by automatically providing a new master. 
     A master-slave protocol, wherein the master can automatically be re-assigned can also be referred to as a masterless protocol. 
     To implement any of the above protocols, CCR  10  advantageously comprises a communication network  17  linked to all the power modules  13 . Communication network  17  can be implemented with any topology, and advantageously as a communication ring. It can be implemented e.g. as a CAN-bus. 
     In order to allow for an exchange and proper interpretation of the information, each power module  13  is provided with a microprocessor (microcontroller)  231  linked to communication network  17  over a data communication port  233 . Microcontroller  231  may be operable to control the input stage circuits  132 - 134 . Possibly, power module  13  comprises a second microprocessor (microcontroller)  232  operable to control the output stage circuits  136 - 138 . The output stage microprocessor  232  may communicate with the input stage microprocessor  231  and with the communication network  17  either directly, or, as shown in  FIG. 2 , through the input stage microprocessor  231 . It is possible to integrate the output stage microcontroller  232  in the input stage microcontroller  231 . Galvanic insulation between the two microprocessors  231 - 232  can be obtained through opto-isolators  234  and/or optical transmission of data. 
     In case the master-slave data communication protocol discussed above is implemented, the address allotted to each power module can be based on an electronic signature of the microcontrollers chip, which is a unique identifier. 
     Communication network  17  can further be linked to a remote control interface  18  provided in CCR  10  and configured for establishing remote connection with any remote control entity, such as an airport control tower. Communication network  17  can be linked to a man-machine interface  19  provided in CCR  10  and allowing an operator to control CCR operation, e.g. during maintenance service. 
     Communication network  17  is advantageously implemented with a dedicated shared data structure which contains information relevant to the different pieces of equipment. The power modules  13  and other components of the CCR  10 , such as the remote control interface  18 , are allowed to manage the shared data structure under some priority rules. 
     Hence, a modularity is obtained, wherein, advantageously, CCR  10  is allowed to operate even when one or more power modules are out of service. It is even possible to provide one or more redundant power modules  13  which can promptly be put in service in the event of failure of another power module. These features can be implemented in a software program which is run on the microcontroller  231 . 
     Advantageously, the implemented data structure allows each piece of equipment, and in particular the power modules  13 , understand how the CCR  10  is assembled, such as understanding how many power modules are present in the CCR and how they are connected. Advantageously, the implemented data structure allows for adding power modules or other pieces of equipment without requiring reprogramming the CCR  10 . Advantageously, the implemented data structure allows for optimizing the global power efficiency of the CCR by adapting the outputs of individual power modules. This will lead to energy savings and hence environment-friendly operation. Advantageously, the implemented data structure allows for reconfiguring the system in case of a fault, such as when a power module goes down, the remaining power modules could provide the missing output power. 
     An example co-operation between power modules  13 - 1  to  13 - n  can be as follows. An airport control tower (not shown) sets a predetermined brightness level (e.g. between 1 and 5), which is converted to a current output level of CCR  10 , which arrives through remote control interface  18  on the communication network  17 . The new set-point is read by all the microcontrollers  231  of power modules  13 - 1  to  13 - n.  Depending on the data communication (network) protocol, the microcontrollers  231  may either begin communicating with one another over the communication network  17  such that each microcontroller  231  establishes an output level  139  for its proper power module  13 , or the microcontroller  231  of the master power module may determine the output levels  139  for each (slave) power module upon reading the new brightness level. Considering by way of example a CCR having three identical power modules with rated output current of 6.6 A at 757 V connected in parallel, then the maximum (rated) global output of the CCR as explained above is 20 A at 757 V. Now, let the CCR output current level be set at 15 A, then the microcontrollers  231  and possibly  232  will communicate with one another to eventually set the output level of each power module to 5 A. When the control tower would set the CCR output current level at 7 A, possibly the system could decide to operate two power modules at an output current level of 3.5 A, and to keep one power module inactive, since it is more efficient to operate a power module at levels close to rated output level. Such control algorithms can advantageously be software-implemented in the microcontrollers  231  of power modules  13  according to the invention. 
     By so doing, advantageously, the CCR output level can attain the set (predetermined) level within a few milliseconds. 
     The power modules  13 - 1  to  13 - n  connected in series and/or in parallel in CCR  10  are advantageously synchronized to provide current and voltage outputs  139  which are advantageously in-phase with the line input. This allows for sinking a maximal output power at same time instants when the input power from the line input is maximal. Advantageously, each module provides for a proper synchronization between the output signal (at output  139 ) and the input signal (at input  131 ). To do so, each module may comprise a phase detector, possibly coupled to the input filter  132  and each microcontroller  231  may read a line phase timing from the phase detector. 
     A possible way of obtaining synchronization between the output signal (at output  139 ) and the input signal (at input  131 ) is by reading/detecting the zero crossings of the input signal by the microcontroller  231 . A signal synchronous with the detected zero crossings is constructed by the microcontroller  231  (or, as the case may be, the output stage microcontroller  232 ). Such a signal is advantageously a square wave at the input line frequency, e.g. 50 Hz or 60 Hz. This signal can be used by the output stage circuits to generate an output sine wave with the same zero crossings. 
     CCR  10  advantageously comprises an output measuring unit  16  configured for measuring parameters of the power output  12  of CCR  10 , such as voltage and current levels, phase information and frequency, and other parameters relevant to module operation such as load inductance and resistance to earth. Output measuring unit  16  is advantageously linked to the communication network  17 , allowing the measured information to be fed back to the power modules  13 , so that they may adapt their outputs  139 . 
     Output measuring unit  16  may be configured for measuring an output parameter representative of phase information of the output signal. Output measuring unit  16  may comprise circuitry, such as a microprocessor, to interpret the measured output parameter in order to calculate the resistive part and the reactive part of the load. This information can be sent over the communication network  17  to microcontrollers  231  which could adapt the output parameters, such as the shape of the output power wave, to the different kinds of field (light) circuits (resistive, inductive or capacitive). 
     Advantageously, output measuring unit  16  is configured to measure one or more parameters of the CCR power output  12  at time intervals smaller than or equal to 100 μs, such as about 50 μs. The time interval between measurements can be smaller than or equal to 5 μs, advantageously between 1 μs and 3 μs. 
     Advantageously, the modules  13  are configured for reading phase-related data fed by measuring unit  16  and for using the phase-related data for phase synchronization of the output. This phase synchronization based on “external” data (with regard to each module) can be provided in addition, or in alternative to the “internal” phase synchronization between the line input and the module output as described above. In case of addition, the phase synchronization based on the “external” data can allow for improving the accuracy of the “internal” synchronization. 
     It will be convenient to note that since the CCR output is a low frequency AC signal, typically of 50 or 60 Hz, synchronization time is not critical. 
     Advantageously, the power modules  231  are provided with a software program, which when run on the microcontrollers  231 , is configured to operate at least the input stages of the power modules, and advantageously both the input and the output stages. By so doing, an enhanced system control is obtained compared to the prior art, which increases flexibility and reliability. This flexibility, which is in part based on a software implementation of some functions, allows the CCR to operate in different modes according to customer requirements. A “green mode” can be implemented by switching a module completely off in case of low power output of the CCR is desired. In case speed is required, all the modules can be kept always on to ensure fast reaction times. Other modes can be implemented by updating the software program without any change in the hardware. 
     Advantageously, the microcontroller can be programmed to implement one or more digital P/PI/PID feedback loops. Such digital control loops have the advantage that the signals can be digitally filtered before being applied in the control (feedback) loop, which enables to work reliably even in a highly noisy environment. To this end, the power module  13  advantageously comprises one or more analog to digital converters (ADC) to provide for (one-way) communication from the different blocks in both the input stage (blocks  132 - 134 ) and the output stage (blocks  136 - 138 ) to the microcontroller(s)  231  (and  232 ). The ADCs and any possible DACs are represented by blocks  236  in  FIG. 2 . 
     It will be convenient to note that communication from the microcontroller to the input/output stage blocks is typically digital. Since the electronic circuits in blocks  133 ,  134 ,  136 , and/or  137  can be controlled by pulse with modulation (PWM), signals going out from the microcontroller in principle do not require any analog conversion. Alternatively, digital to analog converters (DAC) may be provided for two-way communication with the microcontroller  231  (and  232 ) as circumstances may require. 
     One such PID feedback loop can be implemented between the power inverter block  134  and the rectifier block  133 . Microcontroller  231  may be programmed to read and analyse actual signals from either blocks  133  and  134  digitally in order to provide for necessary control. By way of example, in case the power inverter block  134  is a LLC resonant converter circuit, a signal connection with the microcontroller can be configured to feed the actual bus voltage of the resonant bridge to the microcontroller. The microcontroller can be programmed to compare it digitally with the target bus voltage. Based on the difference, the microcontroller can be programmed to determine and/or adapt a pulse width modulation (PWM) scheme (duty cycle, frequency) for being applied to the rectifier  133  in order to compensate for the difference. By so doing, a satisfactory power factor can be obtained in every condition. Additionally, the microcontroller can be programmed to control a bus voltage of any of the output stage circuits (blocks  136 ,  137  or  138 ) by adapting the PWM scheme applied to rectifier  133 . 
     Advantageously, the microcontroller  231  can be programmed to apply a PWM signal to each of the two branches of the LLC resonant converter bridge in block  134 , which signal can be different between the two branches. This can be obtained by implementing a second PID feedback loop in the microcontroller  231 , in addition to the one discussed above. The first PID feedback loop is used for low load and controls the phase shift between the two PWM signals provided to the two branches of the LLC resonant bridge  134 . For higher loads, if the target voltage has not been reached at 180° phase shift (i.e. maximum output of the first PID feedback loop), the second PID feedback loop comes into action in order to further increase the output voltage. It reduces the PWM signal frequency of both branches until the desired bus voltage is attained (is read by the microcontroller  231 ), maintaining a 180° phase shift. 
     Additional digital PID feedback loops can also be implemented in the software program configured to run on microcontroller  231 / 232  in order to control the bus voltage of any one of the output stage circuits  136 - 138 . To this end, use can be made of pulse width modulation in the output stage bridges  136  and/or  137  as is known in the art. The signal connections between bridges  136 / 137  and the microcontroller  231 / 232  can be provided through ADCs similar to the input stage. 
     Advantageously, a set of different PWM signal schemes can be stored in the microcontroller and applied by it in function of the output waveform, from a DC output to a high frequency AC output. 
     Power modules  13  according to the invention are advantageously built such that they can be mounted into standard racks. CCRs of different rated outputs according to the invention can be assembled utilising a same rack size, simply by adding one or more power modules to the rack. Whereas previously a manufacturer would have needed different transformers and different voltage conversion boards for CCRs of different output ratings, according to the present invention all kinds of CCRs can be assembled based on one and the same power module, which drastically cuts stocks and economizes manufacturing. Furthermore, operating the transformers within a power module at high frequency further allows for reducing size and weight of the power modules, such that CCRs according to the invention have at least similar and advantageously decreased dimensions and weight than prior art CCRs. 
     Power modules  13  advantageously have a power output rating smaller than or equal to 10 kVA, advantageously smaller than or equal to 8 kVA, advantageously smaller than or equal to 6 kVA. The power output rating of power modules  13  is advantageously at least 1 kVA, advantageously at least 2 kVA. Indicated rating values are on resistive load. Constant current regulators according to the invention advantageously comprise between one and ten power modules, advantageously between one and six.