Introduction to PWM Inverters.
Now a days most of the inverters available in the market utilizes the PWM(Pulse Width Modulation) technology.The inverters based on PWM technology are superior in many factors compared to other inverters designed using conventional technologies. The PWM based inverters generally use MOSFETs in the output switching stage.In such cases the inverters are generally termed as PWM MOSFET inverters.The inverters based on PWM technology has a lot of protection and control circuits compared to the traditional inverters.
What is PWM technology? & Why PWM is used?.
PWM or Pulse width Modulation is used to keep the output voltage of the inverter at the rated voltage(110V AC / 220V AC) (depending on the country) irrespective of the output load.In a conventional inverter the output voltage changes according to the changes in the load.To nullify effect caused by the changing loads,the PWM inverter correct the output voltage according to the value of the load connected at the output.This is accomplished by changing the width of the switching frequency generated by the oscillator section. The AC voltage at the output depend on the width of the switching pulse.The process is achieved by feed backing a part of the inverter output to the PWM controller section (PWM controller IC).Based on this feedback voltage the PWM controller will make necessary corrections in the pulse width of the switching pulse generated at oscillator section.This change in the pulse width of the switching pulse will cancel the changes in the output voltage and the inverter output will stay constant irrespective of the load variations.
Some of the circuits used in the PWM inverter are explained below.
Battery charging current sensor circuit.
This circuit senses the charging current of the battery in order to keep the charging current with in the rated value.A charging current lower or higher than the allowed range can reduce battery life.
Battery voltage sensing circuit.
This circuit senses the battery voltage in order to charge the battery when the battery is exhausted and to start trickle charging the battery when it is fully charged.
AC mains sensing circuit.
This circuit senses the availability of AC mains.When AC mains is available the inverter will be in charging mode.When AC mains is not available the inverter will be in battery mode.
Low battery cut off circuit.
This circuit shut downs the inverter when battery is exhausted.The inverter will be started only after the battery is sufficiently.
Soft start circuit.
This circuit delays the charging of the battery 8 – 10 seconds after the power is resumed.This is generally used to protect the MOSFETS from high currents generated as a result of immediate charging.This mechanism is also termed as Mains Delay
Change over circuit.
Changeover circuit switches the inverter operation between charging mode and battery mode according to the availability of mains voltage.
Overload sensing circuit.
Over load monitors the output current of the inverter .If the output current increases above the rated value ,it will send a signal to the Shut down circuit indicating that there is an overload.
Shut down circuit.
Shutdown circuit closely monitors the inverter and shuts it down when any abnormality occurs.The out put from the various sensor circuits are the input to the inverter.
PWM Controller circuit .
PWM controller circuit regulate the output voltage of the inverter.Usually a single PWM IC such as KA3525 or LM494 are used for the purpose.The entire circuits needed for the PWM operation will be incorporated in such ICs.
Battery charging circuit.
Battery charging circuit controls the charging process of the battery.The output of AC mains sensing circuit and Battery voltage sensor circuits are the inputs for the battery charging circuit.
Oscillator circuit generates the switching frequency.Generally the oscillator circuit will be incorporated in the PWM IC itself.
Driver circuit drives the output section of the inverter according to the switching frequency.Transistors or Specially designed driver ICs are employed in the driver circuit.The driver circuit is some what similar to a preamplifier.
Output section drives the load.It consists of a step up transformer for stepping up the battery voltage to the line voltage and an array of switching MOSFET devices for driving the primary of the step up transformer.The output voltage will be available at the secondary of the step up transformer.
***Some of the circuits given above are used in non-PWM inverters too.But the PWM circuit will be exclusive to PWM inverters only.***
Block diagram of an elementary PWM Inverter.