Design of a MOSFET driven Quasi Sine Wave Analog Inverter

Inverter systems are standby power systems that take over when the mains power fails. The aim here is to present an article that will show one type of analog system that was built here. We start with an Analog Inverter system, and will progress to more advanced systems such as digital and High Frequency systems which utilize microcontrollers to control them.

This Analog Inverter design is presented for the DIY and electronic enthusiasts among you. The design method, when started in 1996, was the 'hard knocks method', with no way of verifying or simulating any circuit until it was actually built, either in breadboard or on general purpose board. In those days information was also quite difficult to come by and the original IRF MOSFET databook  - bought second hand off the street - that was used for many years is still here. Since then, we have progressed to digital and high frequency UPS and Inverter systems, with sinusoidal outputs, and today simulation is done  - using mostly LTspiceIV, though sometimes other packages -  as much as possible before building any new circuit.

This entire circuit is drawn using KiCAD, which is a free, open source schematic and PCB drawing tool, which I have been using now for about 3-4 years or so. It's fast and easy to learn. It has a built in auto-router also, though I prefer to use another free autorouter that accepts the KiCAD generated net file.

Here is the analog inverter with explanations on each part.

Features

1. Quasi Sine Wave or Modified Square Wave Inverter system
2. Output Voltage Feedback and Correction ability with adjustable output voltage (by potentiometer)
3. Automatic Switchover between Mains and Inverter on power fail and back
4. MOSFET driven.
5. Automatic Bulk and Float charging for battery, with adjustable charger cut-in (to bulk) and cut-out (to float) voltages
6. Battery voltage sense and Battery low trip to prevent Deep Discharge.
7. The same PCBs - with minor component variations - can work up to 36 volts. It has been tested up to 24 volts since there weren't anybody we found who wanted to store three big 12volt truck batteries for a standby power system
8. The system is capable of fast Constant Voltage charging.
9. Short Circuit/Overload protection (adjustable via pot)
10. Manual Reset button as well as Automatic trip reset on Mains presence (Power On Reset).
11. LM/SG3524 based Controller
12. Minor modifications, an extra transformer and a couple of big power diodes can make this a double conversion isolated online ups for computers - Quasi Sine Wave only.

Parts of the Inverter

The system will be described here in modules or sections, not necessarily in the order given below:

1. Mains Sense Section
2. Controller Section
3. Feedback Section
4. Power Drive Section
5. Charger Control Section
6. Battery low Level Sense Section
7. Power Supply Section
8. Inverter Power Transformer

The entire inverter circuit with component values, the PCB layouts, unassembled as well as assembled kits will be available here in a few days.