PWM Inverter circuit with charger using a single transformer. The circuit is based on SG3524 IC.
Inverters are electronic devices which converts battery power (DC) to alternating current (AC) which is clean enough to power our useful appliances.
Inverters can be expensive but going through this tutorial can give you enough ideas to build your own solar inverter at home without spending much.
Let’s go through the designing and construction of 100W to 6000W single phase PWM inverter or UPS with charger using a single transformer, since two transformer systems requires much money and also much space.
The PWM inverter circuit is made up of three sections:
1. Oscillator section
2. Power driver stage
3. Change over and charging stage
THE OSCILLATOR STAGE
FOR PWM INVERTER CIRCUIT WITH CHARGER
Credits to Nick_Zouein of instructables.com who provided the driver stage. which was later updated by me base on testing.
This is the heart of the solar inverter design. The main inverting work is done by this section using pulse width modulation IC (SG3524 or KA3524) or similar. Below is a 12V inverter circuit but few components can be changed or added to work for 24V, 48V and 96V systems whiles the concept still remains unchanged.
In this design the oscillator section is powered by a nine volts regulator IC (LM7809) when switch S1 is closed. The output frequency of the inverter is determined by R22, R23 and U12 (104 fixed cap). Theoretically, total resistance values should be higher but practically these values works best without humming in inductive loads such as fans. The output of the inverter/UPS is regulated using U3 which is 10K variable resistor or pot. This ensure that the output is always stable or within accepted range when loaded.
POWER DRIVE STAGE FOR PWM INVERTER CIRCUIT WITH CHARGER
This stage switches the transformer on/off 50 times is a second. That is 50Hz frequency base on the output from pin 11 and 14 of SG3524.
The main components used here are N-Channel mosfets connected in parallel to deliver the required current to the transformer as shown in the circuit. In my designs I always assume that each pair will switch 20A of DC current. So if I want to switch 1000W using 12V DC supply, I calculate the max current to switch, which is 1000/12 = 83.3A. I then divide 83.3 by 20A which is my assumed current for each pair of mosfets and get 4pairs as the number of Fets to use.
Another great function of this section is to act as a half bridge rectifier during charging. During inverter charging, pin 10 of SG3524 receives positive signal from optocoupler U17 and shut down the SG causing the mosfets to turn off. The internal body diode of the mosfets then acts as rectifier to achieve DC battery charging.
PWM Inverter circuit with charger
INVERTER CHARGING
Battery charging is controlled automatically by LM358. It is configured such that the output pin 1 goes high when the battery voltage drops from a set value using U19. The high output turns Q10 on, but since Q10 and Q11 are connected to form an AND gate, when optocoupler U16 senses the presence of 220V input, Q11 also turns on and relay U14 switches and charging begins. When the battery is full adjust U19 until the relay switches and the full indicator turns on to set battery full.
Relay U15 provides output from the inverter as well as your nation grid for the inverter to work as UPS. Output is filtered using 335 by 400V capacitor. In some cases without filter some inductive loads will not run. The system is protected from power surge using NTC 8 ohms or better. You can add NTC in parallel when handling higher wattage. this will cause the NTC not to over heat.
How to wind PWM Inverter Transformer
The transformer secondary coil must be Thick enough to handle the high current at the DC side else undue voltage drop will occur when loaded with little load. A center tap transformer is to be used for this project with the following specifications:
SECONDARY: 12-0-12
24-0-24
48-0-48
PRIMARY: 0-220-250
0-110-140 for US
NB: use 0-200-250 secondary for places with low line voltages else charging won’t occur.
AWG for primary should be 11 and below depending on the wattage. (You can double AWG or use higher voltage design for high output power)
NB. You can lower or increase charging current by reducing the secondary turns and voltage. E.g. To lower 12-0-12 charging current…make it 11-0-11 and vice versa.
Feedback winding: this should be a separate winding on the same transformer in the range of 12V to 16v. Winding gauge should be smaller…AWG…..18 to 28.
