FIELD OF INVENTION
This invention relates to a high frequency inverter for supplying an alternating current to a load from a storage source of direct current with a grid charger based on forward converter.
PRIOR ART
In today's world there is wide use of computers, data processors, controllers and other house load such as TV, music system, tube lights, fans, fridge, washing machine, dryer, mixer, grinder, microwave oven, geyser, air conditioner etc. For safe and reliable operation of equipments and appliances, these systems are required to be supplied with continuous quality power during their operation. Due to inadequate capacity and increasing load demands, commercially-supplied power is often subject to complete failure of the power signals or a reduction in the magnitude of the available voltage during peak demand periods. In cases where various customers subject the power system to sudden electrical loads, transients are generated in the system affecting the magnitude and phase of the supplied power signal. Since the storage of data in a computer system is predicated on the continuous operation of the computer, power interruptions can adversely affect the integrity of the stored data. When the power drops abruptly, the computer terminates operation probably with a high possibility of component damage.
In order to overcome this situation, computers operating in a data processing system environment have been supplied with uninterruptible power supplies. Such uninterruptible power supplies have been designed where the primary commercial power source and the reserve power source are connected in parallel. Both the primary power source and the reserve power source are continuously operated and contribute towards energizing the load.
An inverter known in the prior art was to covert the DC from battery to AC by using push-pull or H-bridge converter and then to amplify the AC signal to higher voltage through a suitable low frequency 50Hz/60Hz transformer which used to be quite bulky and heavy adding to cost, weight and volume of the equipment. Charging sections in such inverters are generally based on phase controlled SCR/TRIAC operation which results in high ripple in the charging current. The output of the inverter can be square wave or quasi sine wave or sine wave. Square and quasi square wave inverters have very high harmonic distortion which results in the heating and shorter life of the connected equipments or appliances and humming noise from the equipments or appliances.
Further, reference may be made to US Patent 4517470 directing to a High Frequency Inverter. In this regard, the first transformer gives the first portion/phase/half cycle and second transformer gives the second portion/phase/half cycle of AC output supply. Analog pulse width modulator, large numbers of logic gates, two numbers center tapped transformers and other bulky components are used for DC to sinusoidal AC output generation which makes the system complicated and less reliable. Further, 72V battery is used for energy storage purpose.
Technology on stepped square wave output signal is disclosed in US Patent 5790391 which provides a Standby Power System. This system includes stepped square wave output signal at line frequencies wherein the total harmonic distortion (THD) is very high which may result in noise, heating, premature failure or damage of connected equipment, r.m.s and peak output voltage control through digital signal are also provided. However, the said power system is associated with some distinct disadvantages i.e. the controller is being used in low DC and there is no protection for output over current. Therefore, it gives rise to need of a system which can provide output over current protection as well as direct measurement of output current.
Now, some prior art reference may be listed herein below:
JP2005073305, US2005146906, US5841650, JP59021286 and US5032767 are related to Electronic Ballast High Frequency Inverter, which are only suitable for ballast application.
JP3036961, JP4274189 and JP 9260045 disclose High Frequency Inverter for Induction of Heating effective for heater/cooker.
JP4087570 High Frequency Inverter suitable only for the addition of multiple parallel HF Inverter.
JP60125173 is High Frequency Inverter to reduce circulating current for 3-phase, which requires a High Frequency Inverter for single phase.
JP9271175, JP9070174, JP2001078463, JP168246, JP9271175 and JP 9070174 are for specific solutions for voltage, current sensing and MOSFET drives etc.
CN2509773Y proposes High-Frequency switch Inverter for vehicle application and EP1250030 is for cathode heating.
JP3173359 High frequency PWM Inverter Device and JP61161820 PWM Signal Generating Circuit of High Frequency Inverter generates sine wave by using analog technique which causes problem in result by slight variation in the passive components and offset introduced by other discrete / analog components.
The following patents deals with technology based on resonant/ phase shift/ zero voltage high frequency inverters-JP9260045, JP9065656, JP2005094913, JP6111929, EP0372792, JP 199769, GB2196803, AU527438B, CN1489272, CA2353422, US 6519168 and US2002008981. Resonant/ phase shift/ zero voltage high frequency inverters are very complex and require bulky components which results in higher cost of system and is difficult to produce in volume. Hence, there is requirement of a system which is simple in construction and cost effective.
US Patent 6798676 provides an Inverter for Changing Direct Current to Alternating Current.
JP11122948 relates to High-Frequency Inverter Device having a first AC-side terminal at stable potential in a DC and a second AC-side terminal at fluctuating potential and supplying power through cables connected to these first and second AC-side terminals. The cables consist of cable conductors and housing conductors arranged concentrically so as to surround the peripheries of the cable conductors. The housing conductors are joined with the first AC-side terminal, the cable conductors are joined with the AC-side terminal, and high-frequency power is supplied by these
housing conductors and cable conductors. However, this device is limited only for cable conductors, AC/DC terminals and housing parts. Further, in conventional inverters the size and loss of the transformer is very large.
It is therefore a principal objective of this invention to provide a D.C. to A.C. inverter for generating A.C. power signals from a D.C. source at the time of power failure of the A.C. line source.
It is another objective of this invention to provide an inverter using semiconductor field-effect transistors (MOSFET) for switching the output of a D.C. source at a high frequency rate.
Further object of this invention is to provide an inverter which is simple in construction from production and service point of view and low in cost.
Also the objective of this invention is so as to have complete isolation of battery and output by using an isolated control feedback and control circuitry. The battery terminals are completely isolated from the input and output AC power and hence it is relatively safe.
It is an objective of this invention so as to operate the system with a low DC voltage ranging from 10V DC to 16V DC. This low DC voltage has been transformed to high DC bus voltage by using isolated DC-DC boost converter in push-pull configuration with soft start feature to limit the current at start-up of the boost operations.
It is further objective of this invention to operate the same system with the DC voltage ranging from 20V DC to 32V DC by substituting few magnetic components and semiconductor device ratings whilst keeping the system design intact.
Another object of this invention is to propose a high frequency inverter, which is quite cost effective by eliminating use of 50Hz transformer. 50Hz transformer is quite heavy and the material cost of the core and windings are high. The present invention makes use of a 35 KHz ferrite based transformer, which results in the reduced size, weight and cost of the system.
Further object of the present invention is to propose a high frequency inverter with a stable DC boost output through frequency and phase compensation under varying load conditions and nature.
Still another object of the present invention is to propose a high frequency inverter to eliminate the conventional battery charger having SCR/TRIAC and bulky 50Hz transformer with high frequency SMPS charger. The conventional chargers have very high peak currents (even if their average values are less), resulting in increased battery stress. Therefore, this invention also aims at providing forward converter based pure DC current battery charger for longer life of the batteries wherein requirement of water topping of batteries would be relatively lesser because of pure DC current charging.
Yet another object of the present invention is to propose a high frequency inverter to efficiently utilize the features of low cost microcontroller for producing PWM for generating pure sine wave output, complex operations and complete monitoring of the overall inverter functionalities.
Yet another further object of the present invention is to propose a high frequency inverter so as to provide the possibility of future up-gradations of system by utilizing the micro-controller with digital flash memory and EEPROM.
Still another object of the present invention is to propose a high frequency inverter which provides a communication with PC for monitoring, setting and data logging of various system parameters for analysis of system performance and power quality etc.
Yet further object of the present invention is to propose a high frequency inverter, so as to provide a user interface through GUI software for configuring the system as per requirement like setting of battery low cut, boost and float voltage, charging current setting, input mains voltage and frequency window setting for bypass and charger operation, output voltage setting and events logging like number of events of mains failures, mains low/high, overload, short circuit, fuse blown, battery low etc.
SUMMARY OF THE INVENTION
These and other objectives of the invention are accomplished by providing an inverter which includes a pulse-width modulated circuit for the generation of pure sine wave output. The controller maintains an output frequency of 50 Hz / 60Hz ±0.1 Hz and regulated RMS output voltage at all loads. There is implementation of pulse by pulse current limiting scheme resulting in efficient overload as well as short circuit protection. An SMPS based battery charger is provided, using integrated power switch and with built in controls for the SMPS. The inverter consists of a DC-DC converter which converts the battery voltage to a higher voltage. Output of converter is fed to the MOSFET derived H-bridge converter which converts the signal into pure sine wave. Microcontroller and H-bridge operate with an auxiliary supply. This auxiliary supply takes power from one of the bias winding of isolated DC-DC boost converter.
The isolated closed feedback loop circuit controls the output voltage of this converter. The control circuit with error amplifier monitors the DC bus and controls the duty cycle of the MOSFET drives (PWM_A & PWM_B) to maintain a constant DC bus output of DC-DC boost converter. The control circuit reduces the duty cycle if DC bus tries to increase due to either increase in input battery voltage or reduction in output load of the DC-DC boost converter. On the contrary, the control circuit increases the duty cycle if DC bus tries to decrease due to either decrease in the input battery voltage or increase in output load. Frequency and phase compensation has been provided in the feedback loop by an integrating element of error amplifier which results in the stability of the feedback loop under varying load conditions and nature.
The opto isolation is required to maintain isolation between the input low DC voltage and the output live power stages (High voltage DC bus and AC power section). The output from opto isolator is fed to error amplifier via potential divider to control the output of the DC-DC boost converter at required
level 380VDC. Further, a LC filter circuit is provided to remove most of the high frequency component from H-bridge output.
The inverter of the present invention is also provided with a low cost microcontroller for producing PWM for generating pure sine wave output, complex operations and complete monitoring of the overall inverter functionalities.
There is a communication between the PC and the inverter for monitoring, setting and data logging of various system parameters for analysis of system performance and power quality etc. There is also a user interface through a software for configuring the system as per requirement like setting of battery low cut, boost and float voltage, charging current setting, input mains voltage and frequency window setting for bypass and charger.
Further, the present system can also be converted into any output voltage/frequency configuration like 220VAC/230VAC/240VAC/50Hz/60Hz or 110VAC/60Hz by substituting some magnetic, passive components and switching devices.
BRIEF DESCRIPTION OF DRAWINGS
Further objects and advantages of this invention will be more apparent from the ensuing description when read in conjunction with the accompanying drawings and wherein:
FIG. 1 is a block diagram of inverter according to the present invention FIG. 2 is a circuit diagram of DC-DC boost converter with feedback
FIG.3 is a circuit diagram of DC-DC Boost Controller Supply (Low Power Stage) according to the present invention.
FIG. 4 is a circuit diagram of push-pull drives according to the present invention. FIG. 5 is a circuit diagram of Snubber circuit according to the present invention.
FIG. 6 is a circuit diagram of Inrush Current Sensing in DC-DC Boost Stage according to the present invention.
FIG. 7 is a circuit diagram of Auxiliary supply and proportional battery sense circuit according to the present invention.
FIG. 8 is a circuit diagram of Auxiliary supply for microcontroller, H-bridge drivers, signal conditioning & sensing circuitry (High DC & AC power stage).
FIG. 9 is a circuit diagram of H-Bridge according to the present invention.
FIG. 10 is a circuit diagram of H- Bridge Driver according to the present invention.
FIG. 11 is a circuit diagram of Output AC Sensing circuit according to the present invention.
FIG. 12 is a circuit diagram of LC Filter circuit according to the present invention.
FIG. 13 is a circuit diagram of Input AC Sensing circuit according to the present invention FIG. 14 is a circuit diagram of Zero Cross Sensing circuit according to the present invention.
FIG. 15 is a circuit diagram of Load Current Sensing circuit according to the present invention. FIG. 16 is a circuit diagram of Transfer Switch for load according to the present invention. FIG. 17 is a circuit diagram of Transfer Switch (For Charger) according to the present invention. FIG. 18 is a circuit diagram of Fan Drive circuit according to the present invention.
DETAIL DESCRIPTION OF THE INVENTION WITH REFERENCE TO THE ACCOMPANYING DRAWINGS;
Reference may be made to fig. 1 indicating the overall view of the functionality of the system i.e. High Frequency Inverter with grid charger. There is a DC-DC boost converter with isolated feedback which controls the output voltage of this converter. Micro-controller and H- bridge converter operate with an auxiliary supply. The microcontroller monitors and controls the operation of the inverter and charger though circuits like input voltage sensing, output voltage sensing, isolated battery voltage sensing, zero-cross sensing, current sensing, temperature sensing, dc bus sensing etc. Micro-controller monitors and controls the operation of SMPS charger. The H-bridge converter converts the high DC bus voltage into sine wave AC power through sine modulated PWM generated by micro-controller. The output of the H-bridge converter after filtering through LC filter can be transferred to load by transfer switch in case of the failure or bad quality of grid power.
Fig 2 is isolated DC-DC boost converter with feedback. The isolated closed feedback loop circuit controls the output voltage of this converter. The control circuit with error amplifier monitors the DC bus and controls the duty cycle of the MOSFET drives (PWM_A & PWM_B) to maintain a constant DC bus output of DC-DC boost converter.
The control circuit reduces the duty cycle if DC bus tries to increase due to either increase in input battery voltage or reduction in output load of the DC-DC boost converter. On the contrary, the control circuit increases the duty cycle if DC bus tries to decrease due to either decrease in the input battery voltage or increase in output load.
Frequency and phase compensation has been provided in the feedback loop by an integrating element of error amplifier which results in the stability of the feedback loop under varying load conditions and nature.
The opto isolation is required to maintain isolation between the input low DC voltage and the output live power stages (High voltage DC bus and AC power section). The output from opto isolator is fed to error amplifier via potential divider to control the output of the DC-DC boost converter at required level 380Vdc.
Micro-controller monitors the DC bus (output of DC-DC boost converter). The controller has control over the functioning of DC-DC boost converter which can be enabled or disabled by an isolated command signal to the DC-DC boost controller.
Fig 3 is supply section for DC-DC Boost converter section. The DC-DC boost converter section requires a power to operate and this is provided by the block designated a "DC-DC boost controller supply". The diode comes into picture only during the startup of the DC-DC converter.
Thereafter bias winding [T5] supply the power to DC-DC converter and its associated circuitry.
Fig.4 is a circuit diagram of push-pull drives. PWM DC-DC boost controller generates the PWMs (PWM_A & PWM_B) to drive the MOSFETs in push-pull configuration to maintain a constant DC bus output of DC-DC boost converter.
Fig 5 is Snubber circuit. Snubber network minimize the voltage spikes which appear due to the leakage and mutual inductance energy of ferrite transformer during the switching of the power
devices.
Fig 6 is an Inrush Current Sensing in DC-DC Boost Stage. The signals IS1 and IS2 corresponding to the current flowing through the devices are compared with a reference signal corresponding to inrush current to limit the corresponding PWM of DC-DC controller if current exceeds the limit of inrush
current.
Fig 7 & 8 depicts proportional isolated battery sense circuit and an Auxiliary supply (Vjn-Aux). This proportional DC-DC converter will generate an auxiliary supply and an isolated battery voltage sense signal which is proportional to input battery voltage. This proportional signal is then fed to built-in A/D converter of MICRO-CONTROLLER for monitoring of battery conditions. Microcontroller and H- bridge converter operate with an auxiliary supply designated as "Auxiliary Supply". This supply comes through the ANDing operation of two supplies. The auxiliary supply (Vjn-Aux) supplies the power to operate the MICRO-CONTROLLER & H- bridge converter and its associated circuitry only at the startup of the system and off state of the DC-DC boost converter. This auxiliary supply takes power from one of the bias winding [T4] of isolated DC-DC boost converter during on state operation of DC-DC boost and H-bridge converter operation.
Fig 9 is an H-bridge converter which will convert this high DC bus voltage into AC power. The output of the H-bridge converter after filtering can be transferred to load by using transfer switch whenever there is no grid power available or grid power is out of tolerance limit of this proposed system.
Figure 10 Two half bridge drivers have been used in H-bridge configuration. Micro-controller generates a sine modulated PWMs by using a bipolar pulse width modulation technique in H-bridge configuration. Micro-controller controls the regulated 230VAC sine wave output of the H-bridge converter by sensing and comparing closed feedback signal corresponding to output voltage from AC. sensing circuit with internal reference signal.
Fig. 11 depicts the output section of the system. It converts the output AC voltage level to corresponding analog signal in inverter mode. The resistor network prior to the CPU ensures that the signal falls between the operating ranges of the CPU. This analog signal is then fed to the in-built Analog to Digital converter of the CPU which samples these analog signals at a fast sampling rate to obtain the equivalent digital signal. The CPU compares this signal with the internal reference signal corresponding to constant output voltage. The CPU modulates PWMs of the H-bridge converter according the output feedback error to maintain the regulated output voltage.
Fig. 12 is the LC Filter. LC filter circuit is provided to remove the desired high frequency component from H-bridge output.
Fig 13 & 14 depicts the input section of the system. Figure 13 senses the input voltage levels and the corresponding zero cross of the product. It converts the input AC voltage level to corresponding analog signal. The resistor network prior to the CPU ensures that the signal falls between the operating ranges of the CPU. This analog signal is then fed to the in-built Analog to Digital converter of the CPU which samples these analog signals at a fast sampling rate to obtain the equivalent digital signal.
Fig 14 generates the zero cross signal corresponding to input grid supply. Microcontroller senses this zero cross signal to detect phase and frequency of the input grid supply.
Fig (15) depicts the load current sense section of the system. It senses the load current levels of the single phase inverter in the inverter mode. The op-amp has been used here as a non-inverting amplifier. The dual RC network before the CPU provides a second order filter. This analog signal is then fed to the in-built Analog to Digital converter of the CPU which samples this analog signal at a fast sampling rate to obtain the equivalent digital signal. The CPU then records these levels to ensure that this parameter of the single phase inverter/UPS is within permissible range. If the received values fall outside the tolerance band, then CPU generate warning of overload to reduce the load though display and specific buzzer beep. The CPU checks this parameter for 'No Load', 'Full Load' and 'Overload' conditions. A pulse by pulse current limiting circuit has been used to limit the inrush current through MOSFET devices of H-bridge converter in case if there is an increase in the output current over 300% due to either short in the output of system or load characteristics or transient load. In this condition CPU interrupted when load current exceeds 300%. The CPU limits the current pulse within few microseconds by disabling the PWMs. CPU counts these events. If these reach a reference number, then the CPU shuts down the system. The CPU retries to start the system. If these overload or short circuit condition persist for 8 numbers of times, then the CPU will shut down the inverter (DC-DC boost & H-bridge converter) operation permanently. User can restart by resetting the system manually though the user interface panel of the system.
Fig 16 is a circuit diagram of transfer switching circuit of the present invention. Micro-controller periodically senses the grid supply voltage and frequency. If grid supply is available and within the specified limits, then micro-controller first disables the DC-DC boost, bridge converter and then switches the position of the transfer switch to by-pass the grid supply to run the output load. In this case both line out and neutral out from H-bridge converter remain disconnected from the grid supply line and neutral.
If micro-controller detects that the grid supply has failed or outside specified limits, then it disconnects the charger and switches the position of the transfer switch to run the load from H-bridge output.
Fig 17 is a circuit diagram of charger on/off relay switch. During the presence of grid supply, microcontroller enables the SMPS charger by the relay switch. Micro-controller operates relay at zero voltage to enhance the life of the relay. SMPS charger works in forward converter mode and charges the battery in constant current and constant voltage mode. Micro-controller monitors the battery voltage periodically. As the battery reaches in charged condition, micro-controller switches the charger from boost cum absorption state to float state by a float active signal. Charger has built-in over current and under/over input AC voltage protection. Micro-controller also switches off the charger by disconnecting the grid supply through relay switch if grid supply fails or is out of specified range.
Fig 18 is a circuit diagram of Fan Drive circuit. The micro-controller monitors the temperature of the system through temperature sensor inside the system and operates the fan if temperature rises above the predefined temperature. Micro-controller also shuts down the fan if temperature falls below the predefined temperature.
The present invention provides DC to Sinusoidal AC generation by using a DC-DC boost and H-bridge DC-AC converter. Sinusoidal AC output generation and control are taken care by a microcontroller which makes the system reliable. In the present invention single ferrite transformer is being used without any centre tap winding. The present invention is suitable for battery voltage ranging from 10VDC to 16VDC or 20VDC to 32VDC. Further, the present system can be converted into any output voltage/frequency configuration like 220VAC/23 OVAC/240VAC/50Hz/60Hz or 110VAC/60Hz by substituting some magnetic, passive components and switching devices.
According to the present invention a controller is provided in high DC side for the measurement of output current and is being used to generate the sinusoidal output waveform with THD less than 5% at any load (0% - 100%) on battery working range condition. Current, voltage (r.m.s and peak) and zero cross and phase signal sense are also provided.
The battery terminals of the inverter are completely isolated from the input and output AC power and hence it is relatively safe.
In one embodiment of the present invention a low DC voltage of 10V DC to 16V DC is used to operate the system. This low DC voltage is transformed to high DC bus voltage by using isolated DC-DC boost converter in push-pull configuration with soft start feature to limit the current at startup of the boost operations.
Further, in other embodiment DC voltage of 20V DC to 32V DC can be used to operate the system by substituting few magnetic components and semiconductor device ratings whilst keeping the system design intact.
Further, in another embodiment the system can also be converted into any output voltage/frequency configuration like 220VAC/230VAC/240VAC/50Hz/60Hz or 110VAC/60Hz by substituting some magnetic, passive components and switching devices.
The high frequency inverter eliminates the conventional battery charger having SCR/TRIAC and bulky 50Hz transformer with high frequency SMPS charger. Actually, the conventional chargers have very high peak currents (even if their average values are less), resulting in increased battery stress. Therefore, this invention also aims at providing forward converter based pure DC current battery charger for longer life of the batteries wherein requirement of water topping of batteries would be relatively lesser because of pure DC current charging.
The frequency inverter also provides the possibility of future up-gradations of system by utilizing the micro-controller with internal digital flash memory and EEPROM.
Further, the inverter provides a communication with PC for monitoring, setting and data logging of various system parameters for analysis of system performance and power quality etc.
Again, it is to be noted that, the high frequency inverter provides a user interface through GUI software for configuring the system as per requirement like setting of battery low cut, boost and float voltage, charging current setting, input mains voltage and frequency window setting for bypass and charger operation, output voltage setting and events logging like number of events of mains failures, mains low/high, overload, short circuit, fuse blown, battery low etc.
It is to be noted that the present invention is susceptible to modifications, adaptations and changes by those skilled in the art. Such variant embodiments employing the concepts and features of this invention are intended to be within the scope of the present invention, which is further set forth under the following claims:-

WE CLAIM
1. A high frequency inverter comprising of a DC-DC boost converter with isolated feedback,
which controls the output voltage of the converter; and micro-controller and H-bridge
converter operating with an auxiliary supply wherein the micro controller monitors and
controls the operation of the inverter and charger.
2. A high frequency inverter as claimed in claim 1 wherein the micro controller monitors and
controls the operation through circuits like input voltage sensing, output voltage sensing,
isolated battery voltage sensing, zero-cross sensing, current sensing, temperature sensing, dc
bus sensing etc.
3. A high frequency inverter as claimed in claim 1 or 2 wherein the H-bridge converter converts
the high DC bus voltage into sine wave AC power through sine modulated PWM generated
by the micro-controller.
4. A high frequency inverter as claimed in any of the preceding claims wherein the H-bridge
converter, output of which after filtering through LC filter can be transferred to load by
transfer switch in case of the failure or bad quality of grid power.
5. A high frequency inverter as claimed in any of the preceding claims comprising of load
current sense section, input sense section and output sense section, which converts the
current, input & output voltage level to corresponding analog signal.
6. A high frequency inverter as claimed in any of the preceding claims, battery terminals of
which are completely isolated from the input and output AC power, which makes it relatively
safe.
7. A high frequency inverter as claimed in any of the preceding claims comprising of push-pull
drives and snubber circuit.
8. A high frequency inverter as claimed in any of the preceding claims eliminates the
conventional battery charger having SCR/TRIAC and bulky 50Hz transformer with high
frequency SMPS charger by providing forward converter based pure DC current battery
charger for longer life of the batteries wherein requirement of water topping of batteries
would be relatively lesser because of pure DC current charging.
9. A high frequency inverter as claimed in any of the preceding claims provides the possibility
of future up-gradations of system by utilizing the micro-controller with internal digital flash
memory and EEPROM.
10. A high frequency inverter as claimed in any of the preceding claims provides a
communication with PC for monitoring, setting and data logging of various system
parameters for analysis of system performance and power quality etc.
11. A high frequency inverter as claimed in any of the preceding claims provides a user interface
through GUI software for configuring the system as per requirement like setting of battery
low cut, boost and float voltage, charging current setting, input mains voltage and frequency
window setting for bypass and charger operation, output voltage setting and events logging
like number of events of mains failures, mains low/high, overload, short circuit, fuse blown,
battery low etc.
12. A high frequency inverter substantially as herein described with reference to the accompanying drawings.