A very simple yet highly sophisticated modified sine wave inverter circuit is presented in the following post. The use of the PWM IC TL494 not only makes the design extremely economical with its parts count but also highly efficient and accurate.
Using TL494 for the Design
The IC TL494 is a specialized PWM IC and is designed ideally to suit all types of circuits which require precise PWM based outputs.
The chip has all the required features in-built for generating accurate PWMs which become customizable as per the users application specs.
Here we discuss a versatile PWM based modified sine wave inverter circuit which incorporates the IC TL494 for the required advanced PWM processing.
Referring to the figure above, the various pinout functions of the IC for implementing the PWM inverter operations may be understood with the following points:
Pinout Function of the IC TL494
Pin#10 and pin#9 are the two outputs of the IC which are arranged to work in tandem or in a totem pole configuration, meaning both the pinouts will never become positive together rather will oscillate alternately from positive to zero voltage, that is when pin#10 is positive, pin#9 will read zero volts and vice versa.
The IC is enabled to produce the above totem pole output by linking pin#13 with pin#14 which is the reference voltage output pin of the IC set at +5V.
Thus as long as pin#13 is rigged with this +5V reference it allows the IC to produce alternately switching outputs, however if pin#13 is grounded the outputs of the IC is forced to switch in a parallel mode (single ended mode), meaning both the outputs pin10/9 will begin switching together and not alternately.
Pin12 of the IC is the supply pin of the IC which can be seen connected to the battery via a dropping 10 ohm resistors which filters out any possible spike or a switch ON surge for the IC.
Pin#7 is the main ground of the IC while pin#4 and pin#16 are grounded for some specified purposes.
Pin#4 is the DTC or the dead time control pinout of the IC which determines the dead time or the gap between the switch ON periods of the two outputs of the IC.
By default it must be connected to ground so that the IC generates a minimum period for the "dead time", however for achieving higher dead time periods, this pinout can be supplied with an external varying voltage from 0 to 3.3V which allows a linearly controllable dead time from 0 to 100%.
Pin#5 and pin#6 are the frequency pinouts of the IC which must be connected with an external Rt, Ct (resistor, capacitor) network for setting up the required frequency across the output pinouts of the IC.
Either of the two can be altered for adjusting the required frequency, in the proposed PWM modified inverter circuit we employ a variable resistor for enabling the same. It may be adjusted for achieving a 50Hz or 60Hz frequency on pins9/10 of the IC as per the requirements, by the user.
The IC TL 494 features a twin opamp network internally set as error amplifiers, which are positioned to correct and dimension the output switching duty cycles or the PWMs as per the application specs, such that the output produces accurate PWMs and ensures a perfect RMS customization for the output stage.
Error Amplifier Function
The inputs of the error amplifiers are configured across pin15 and pin16 for one of the error amps and pin1 and pin2 for the second error amplifier.
Normally only one error amplifier is used for the featured automatic PWM setting, and the other error amp is kept dormant.
As can be seen in the diagram, the error amp with the inputs at pin15 and pin16 is rendered inactive by grounding the non-inverting pin16 and by connecting the inverting pin15 to +5V with pin14.
So internally the error amp associated with the above pins remain inactive.
However, the error amp having the pin1 and pin2 as the inputs are effectively used here for the PWM correction implementation.
The figure shows that pin1 which is the non-inverting input of the error amp is connected to the 5V reference pin#14, via an adjustable potential divider using a pot.
The inverting input is connected with pin3 (feedback pin) of the IC which is actually the output of the error amps, and enables a feedback loop to form for pin1 of the IC.
The above pin1/2/3 configuration allows the output PWMs to be set accurately by adjusting the pin#1 pot.
This concludes the main pinout implementation n guide for the discussed modified sine wave inverter using the IC TL494.
Output Power Stage of the Inverter
Now for the output power stage we can visualize a couple of mosfets being used, driven by a buffer BJT push pull stage.
The BJT stage ensures ideal switching platform for the mosfets by providing the mosfets with minimum stray inductance issues and quick discharge of the internal capacitance of the fets. The series gate resistors prevent any transients trying to make its way into the fet thus ensuring the operations to be entirely safe and efficient.
The mosfet drains are connected with a power transformer which could be an ordinary iron cored transformer having a primary configuration of 9-0-9V if the inverter battery is rated at 12V, and the secondary could be 220V or 120V as per the user's country specs.
The power of the inverter is basically determined by the transformer wattage and the battery AH capacity, one can alter these parameters as per individual choice.
Using Ferrite Transformer
For making a compact PWM sine wave inverter, the iron core transformer can be replaced with a ferrite core transformer. The winding details for the same may be seen below:
By using super enamelled copper wire:
Primary: Wind 5 x 5 turns center tap, using 4 mm (two 2 mm strands wound in parallel)
Secondary: Wind 200 to 300 turns of 0.5 mm
Core: any suitable EE core which would be capable of accommodating these winding comfortably.
TL494 Full Bridge Inverter Circuit
The following design can be used for making full bridge or H-bridge inverter circuit with IC TL 494.
As can be seen, a combination of p channel and n channel mosfets are used for creating the full bridge network, which makes things rather simple and avoids the complex bootstrap capacitor network, which normally become necessary for full bridge inverters having only n channel mosfet.
However incorporating p channel mosfets on the high side and n channel at the low side makes the design prone to shoot-through issue.
To avoid shoot-through a sufficient dead time must be ensured with the IC TL 494, and thus prevent any possibility of this situation.
The IC 4093 gates are use for guaranteeing perfect isolation of the two sides of the full bridge conduction, and correct switching of the transformer primary.
Simulation Results
Firooz says
Hi
I built your circuit and connected it to a 40 Watt 0-7.5-0 transformer and I have 3 issues with this circuit now :
issue1) When I connect my laptop adapter, the output frequency is distort, and a hammering sound starts from Transformer. The re is no effect if I try to compensate with frequency Pot; only the tone of the sound changes.
issue2) The output voltage is changed if I connect different loads, and for every load, ex. lamp or charger, one should proceed to correcting with PWM Pot every time.
issue3) When I try to parallel Mosfet output stages with the same Mosfet configuration, the no- load current increases and Mosfets get hot, and I cannot increase the wattage of the circuit. How can I increase wattage?
Thank You.
Swag says
Hi, please check the frequency that you are getting from the output of the transformer, it must not be high.
secondly I could not understand your transformer rating? what does 0-7.5-0 mean? I could not get it.
The transformer rating, the frequency and the load current must all be compatible with each for the proper functioning of any inverter, make sure these are all correctly selected.
Firooz says
I meant this type of a metal-core transformer : 7.5-0-7.5 in primary and 220 at secondary . Sorry that I made mistake when writing its specifications last time.
Also, in frequency section of the circuit, The only difference with your posted circuit is that I put 33k resistor between pin 6 of TL494 and ground instead of potentiometer and 4.7k in your posted circuit; all other capacitors and resistors are same as your posted circuit.
The laptop AC adapter is ASUS model : ADP-65GD B : input AC100-240V 50-60Hz 1.5 A & output : +19v DC 3.42A.
I do not have frequency meter, but clearly there is a sound difference when I connect this ASUS laptop computer adapter . and I repeat for you my 3 issues here:
issue1) When I connect my laptop adapter, the output frequency is distort, and a hammering sound starts from Transformer. The re is no effect if I try to compensate with frequency Pot; only the tone of the sound changes.
issue2) The output voltage is changed if I connect different loads, and for every load, ex. lamp or charger, one should proceed to correcting with PWM Pot every time.
issue3) When I try to parallel Mosfet output stages with the same Mosfet configuration, the no- load current increases and Mosfets get hot, and I cannot increase the wattage of the circuit. How can I increase wattage?
Swag says
What is your transformers current rating and battery Ah rating?
You must check the results with a 100 watt bulb first, makes sure it illuminates smoothly.
Frequency is very important, you will have to check and make sure it is within 50Hz if the transformer is an iron, however a TL494 may not allow you to reduce the frequency below 1kHz…as per the datasheet.
power can be increased by increasing the transformer, battery and mosfet rating together.
Firooz says
I checked with 3 lamps totally around 100 watts and I gained exact 220V AC across them with no drop -down of voltage while my voltmeter shows 336 VAC at no-load.
The AH of the battery is 65AH ; a new typical car battery, no problem with that.
The transformer is a 40Watt 7.5-0-7.5 to 220 with a iron core size of 6.5cm-3.5sm.
Is it good to make an increase in number of Mosfets with a 100 ohm resistor connected to EACH gate of a single 100 ohm resistor for all mosfets is enough? can you put the suitable circuit showing how to increase the number of mosfets?
or any suitable LC filter at the output to stop the hammering sound when I connect this Laptop adapter to the output?
Thank you.
Swag says
Power handling capacity can be increased by putting mosfets in parallel but here it won’t be useful, because your transformer is 40 watt, single mosfets would be enough to handle that much power.
You can think about increasing mosfets numbers only if your transformer is rated at 300 watts or 500 watts.
I think instead of TL494 you should try some other inverter circuit such as using 4047 or using transistors.
and if possible use a 12-0-12V transformer if the PWM function is not being used, that will give you around 220V output, the hammering sound could be due to high voltage from the transformer.
jEB says
The operating of TL494 is I Khz to 100 kHz so , the frequency you obtain is obviously high and hence its humming
Loki says
Sir can i connect 13,14,15 pin of ic to ground using 10k resistor?
So that i can use pin1 of ic for output correction
Swag says
Loki, if you do that your circuit will stop working and mosfets will burn.
The circuit shown is correctly configured and nothing needs to be changed in it
for feedback control you can connect pin#1 with the output through a suitable resistive potential divided network, as show in the following example:
https://homemade-circuits.com/5v-pwm-solar-battery-charger-circuit/
thatcher says
Hie Sir
As for the feedback, how to calculate resistance of resistor divider network
Swag says
Hi Thatcher, you can take the help of the following software
https://homemade-circuits.com/voltage-divider-calculator-software-potential-divider-calculator/
Loki says
How can i fix the voltage drop issue please help me
Loki says
I dont have 4. 7 ohm resistor can you please tell me the equivalent one
Swag says
use 10 ohm or 20 any resistor below 33 ohm will do
francis says
Hie Sir.
Can you please help me with a complete schematic of the driver stage intergrated with bootstrap for high side switching that will drive h-bridge instead of this half bridge. I intend to use it with tl494 modulations ,the h bridge will be connected to 380vdc.
Swag says
Hi Francis,
you can refer to this article, and try to implement the same in your TL494 circuit
https://homemade-circuits.com/sg3525-full-bridge-inverter-circuit/
ronald bautista says
greetings
I want to wind a transformer using the core area and ferrite core with 12vx2 input and 11v output with the frequency of 1.2khz.
can you please show me how did you compute for the sizes of magnet wire and the number of turns
Swag says
sorry, I have no idea regarding the calculations of the inductor…this design was taken from the datasheet of the IC ready made. You can perhaps give it a try through some trial and error…