Stages of an Inverter

As the name suggests DC to AC inverter is an electronic device which is able to "invert" a DC

potential normally derived from a lead-acid battery into a stepped-up AC potential. The output from
an inverter are normally quite comparable to the voltage that is found in our domestic AC Mains
outlets.

Repairing sophisticated inverters are not easy due to their many involved complex stages and
requires expertise in the field. Inverters which provide sine wave outputs or the ones which
use PWM technology to generate modified sine wave can be difficult to diagnose and troubleshoot
for the folks who are relatively new to electronics.

However, simpler inverter designs that involve basic operating principles can be repaired even by a
person who is not specifically an expert with electronics.

Before we move into the fault finding details it would be important to discuss how does an inverter
work and the different stages normally an inverter may comprise:

An inverter in its most basic form may be divided into three fundamental stages viz. oscillator, driver
and the transformer output stage.

Oscillator:
This stage is basically responsible for the generation of oscillating pulses either through an IC circuit
or a transistorized circuit.

These oscillations are basically the productions of alternate battery positive and negative (ground)
voltage peaks with a particular specified frequency (number of positive peaks per second.) Such
oscillations are generally in the form of square pillars and are termed as square waves, and the
inverters operating with such oscillators are called square wave inverters.

The above generated square wave pulses though are too weak and can never be utilized to drive
high current output transformers. Therefore these pulses are fed to the next amplifier stage for the
required task.

For info on Inverter oscillators you can also refer to the complete tutorial which explains how to
design an Inverter from the scratch

Booster or Amplifier (Driver):

Here the received oscillating frequency is suitably amplified to high current levels using either power
transistors or Mosfets.

Though the boosted response is an AC, it is still at the battery supply voltage level and therefore
cannot be used to operate electrical appliances which work at higher voltage AC potentials.

The amplified voltage is therefore finally applied to the output transformer secondary winding.

Output Power Transformer:

We all know how a transformer works; in AC/DC power supplies it is normally used to step-down the
applied input mains AC to the lower specified AC levels through magnetic induction of its two
windings.
In inverters a transformer is used for similar purpose but with just opposite orientation, i.e. here the
low level AC from the above discussed electronic stages is applied to the secondary windings
resulting in an induced stepped up voltage across the primary winding of the transformer.

This voltage is finally utilized for powering the various household electrical gadgets like lights, fans,
mixers, soldering irons etc.

Basic Principle of Operation of an Inverter

The above diagram shows the most fundamental design of an inverter, the working principle
becomes the back bone for all conventional inverter designs, from the simplest to the most
sophisticated ones.

The functioning of the shown design may be understood from the following points:
1) The positive from the battery powers the oscillator IC (Vcc pin), and also the center tap of the
transformer.

2) The oscillator IC when powered starts producing alternately switching Hi/lo pulses across its
output pins PinA and PinB, at some given frequency rate, mostly at 50Hz, or 60Hz depending as per
the country specs.

3) These pinouts can be seen connected with the relevant power devices #1, and #2, which could be
mosfets or power BJTs.

3) At any instant when PinA is high, and PinB is low, the Power Device#1 is in the conducting mode,
while Power Device#2 is held switched OFF.

4) This situation connects the upper tap of the transformer to ground via the power device#1, which
in turn causes the battery positive to pass through upper half of the transformer, energizing this
section of the transformer.

5) Identically, in the next instant when the pinB is high and PinA is low, the lower primary winding of
the transformer becomes activated.

6) This cycle repeats continuously causing a push-pull high current conduction across the two halves
of the transformer winding.

7) The above action within the transformer secondary causes an equivalent amount of voltage and
current to switch across the secondary by means of magnetic induction, resulting in the production of
the required 220V or the 120V AC across the secondary winding of the transformer, as indicated in
the diagram.

DC to AC Inverter, Repairing Tips

In the above explanation a couple of things become very critical for obtaining correct results from an
inverter.

1) First, the generation of the oscillations, due to which the power MOSFETs are switched ON/OFF,
initiating the process of electromagnetic voltage induction across the primary/secondary winding of
the transformer. Since the MOSFETs switch the primary of the transformer in a push-pull manner,
this induces an alternating 220V or 120V AC across the secondary of the transformer.

2) The second important factor is the frequency of the oscillations, which is fixed as per the country’s
specifications, for example countries that supply 230 V, generally have a working frequency of 50
Hz, in other countries where 120 V is specified mostly work at 60 Hz frequency.

3) Sophisticated electronic gadgets like TV sets, DVD players, computers etc. are never
recommended to be operated with square wave inverters. The sharp rise and fall of the square
waves are just not suitable for such applications.

4) However there are ways through more complex electronic circuits for modifying the square
waves so that they become more favorable with the above discussed electronic equipment.

Inverters using further complex circuits are able to produce waveforms almost identical to the
waveforms available at our domestic mains AC outlets.

How to Repair an Inverter

Once you get well versed with the different stages normally incorporated in an inverter unit as
explained above, troubleshooting becomes relatively easy. The following tips will illustrate how to
repair DC to AC inverter:

Inverter is “Dead”:
If your inverter is dead, do preliminary investigations such as checking battery voltage and
connections, checking for a blown fuse, lose connections etc. If all these are OK, open the inverter
outer cover and do the following steps:

1) Locate the oscillator section; disconnect its output from its MOSFET stage and using a frequency
meter confirm whether or not it is generating the required frequency. Normally, for a 220V inverter
this frequency will be 50 Hz, and for 120V inverter this will be 60 Hz. If your meter reads no
frequency or a stable DC, it may indicate a possible fault with this oscillator stage. Check its IC and
the associated components for the remedy.

2) In case you find the oscillator stage working fine, go for the next stage i.e. the current amplifier
stage (power MOSFET). Isolate the MOSFETS from the transformer and check each device using a
digital multimeter. Remember that you may have to completely remove the MOSFET or the BJT from
the board while testing them with your DMM. If you find a particular device to be faulty, replace it
with a new one, and check the response by switching ON the inverter. Preferably connect a high
wattage DC bulb in series with the battery while testing the response, just to be on the safer side and
prevent any undue damage to the battery

3) Occasionally, transformers can also become the major cause for a malfunction. You can check for
an open winding or a loose internal connection in the associated transformer. If you find it to be
suspicious, immediately change it with a new one.

Although it won't be that easy to learn everything about how to repair DC to AC inverter from this
chapter itself, but definitely things will start "cooking" as you delve into the procedure through
relentless practice, and some trial and error.

Still have doubts...feel free to post your specific questions here.