How to repair inverters part 3

The image below was taken from a two weeks old china inverter which failed when 600W of load was plugged on it. The power rating of the inverter was 3000W with 6000W surge capacity.





Two stages switching inverter board

Facts about china switching inverters.

All these inverters are two stages high frequency-low frequency inverters.

All these inverters power their second stage using an auxiliary winding on one of the ferrite transformers in the first stage.

They all use mosfets connected in either H-Bridge or Half Bridge for their output.

Only few (about 10 percent) of these inverters uses micro controllers.

The most common PWM IC used is SG3525, SG3524 OR TL494.

Now that we know these about the inverters that we are going to repair, please follow the tutorial below carefully. Please read it until you get the whole concept since its the basis of all two stage inverters seen on our markets.

HOW TWO STAGES HIGH FREQUENCY INVERTER WORKS

Every high frequency inverter consists of:

DC to DC boost stage

low frequency AC stage.

Let's start by talking about the first stage of a high frequency inverter which is the DC to DC boost stage.

At this stage, the input DC battery is switched across a ferrite transformer at frequency between 25kHz. to 60khz. using a pulse width modulation driver IC such as 3524,3525, Tl491 etc. The switching topology used for this stage is mostly push pull using N channel mosfets.

The output high frequency square wave its rectified using fast switching diodes connected to form a bridge rectifier and a filter capacitor of 400V. The resulting output voltage ranges from 270V DC to 330V DC with best value of 310V which is 220V multiplied by the square root of 2 (RMS value).

Feedback is applied to the driver stage using either a separate winding from one of the switching ferrite transformers or using an optocoupler. The feedback ensures that the output voltage of this first stage is always stable.

low frequency AC stage.

This is the final stage of a high frequency inverter and this stage brings the difference between two inverters from different manufacturers. The kind of switching topology employed here will determine the quality, efficiency and durability of the inverter.

At this stage, the boost DC voltage obtained from the first stage is converted to 50Hz or 60Hz. AC which is either:

Pure sine wave
modified sine wave
square wave

using either H bridge topology or Half bridge topology.The switching mosfets used here are high voltage N mosfets or IGBT with minimum voltage of 400V.

The type of wave obtained at this stage which is the inverter output depends on the driving circuit and technique employed.

Note that at each section, there will be op amps or decision making ICs which monitors max current and high or low voltage.

I think i have given a fair idea about the subject. Your comments are welcome.

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