Whenever PWM is employed in an inverter for enabling a sine wave output, inverter voltage drop becomes a major issue, especially if the parameters are not calculated correctly.
In this website you might have come across many sine wave and pure sine wave inverter concepts using PWM feeds or SPWM integrations. Although the concept works very nicely and allows the user to get the required sine wave equivalent outputs, they seem to struggle with output voltage drop issues, under load.
In this article we will learn how to correct this through simple understanding and calculations.
First we must realize that output power from an inverter is merely the product of input voltage and current that's being supplied to the transformer.
Therefore here we must make sure that the transformer is correctly rated to process the input supply such that it produces the desired output and is able to sustain the load without any drop.
From the following discussion we'll try to analyze through simple calculations the method to get rid of this issue, by configuring the parameters correctly.
Analyzing Output Voltage in Square Wave Inverters
In a square wave inverter circuit we will typically find the waveform as shown below across the power devices, which deliver the current and voltage to the relevant transformer winding as per the mosfet conduction rate using this square wave:
Here we can see that the peak voltage is 12V, and the duty cycle is 50% (equal ON/OFF time of the waveform).
To proceed with the analysis We first need to find the average voltage induced across the relevant transformer winding.
Supposing we are using a center tap 12-0-12V /5 amp trafo, and assuming 12V @ 50% duty cycle is applied to one of the 12V winding, then the average voltage induced within that winding and MOSFET drain can be calculated as given below:
12 x 50% = 6V
This also becomes the average voltage across the gates of the power devices, since the 50% duty cycle are applied to the MOSFET gates from the oscillator
For the two halves of the trafo winding we get, 6V + 6V = 12V (combining both the halves of the center tap trafo.
Multiplying this 12V with the full current capacity 5 amp gives us 60 watt
Now since the transformer actual wattage is also 12 x 5 = 60 watts, implies that the power induced at the primary of the trafo is full, and therefore the output will be also full, allowing the output to run without any drop in voltage under load.
This 60 watt is equal to the actual wattage rating of the transfomer, i.e. 12V x 5 amp = 60 watts. therefore the output from the trafo works with maximum force and does not drop the output voltage, even when a maximum load of 60 watt is connected.
Analyzing a PWM based Inverter Output Voltage
Now suppose we apply a PWM chopping across the gates of the power mosfets, say at a rate of 50% duty cycle on the gates of the mosfets ( which are already running with a 50% duty cycle from the main oscillator, as discussed above)
This again implies that the previously calculated 6V average is now impacted additionally by this PWM feed with 50% duty cycle, reducing the average voltage value across the mosfet gates to:
6V x 50% = 3V (although the peak is still 12V)
Combining this 3V average for both the halves of the winding we get
3 + 3 = 6V
Multiplying this 6V with 5 amp gives us 30 watts.
Well, this is 50% less than what the transformer is rated to handle.
Therefore when measured at the output, although the output might show a full 310V (due to the 12V peaks), but under load this might quickly drop to 150V, since the average supply at the primary is 50% less than the rated value.
To rectify this issue we have to tackle two parameters simultaneously:
1) We must make sure that the transformer winding matches the average voltage value delivered by the source using the PWM chopping,
2) and the current of the winding must be accordingly specified such that the output AC does not drop under load.
Let's consider our above example where the introduction of a 50% PWM caused the input to the winding to be reduced to 3V, to reinforce and tackle this situation we must ensure that the winding of the trafo must be correspondingly rated at 3V. Therefore in this situation the transformer must be rated at 3-0-3V
Current Specs for the Transformer
Considering th above 3-0-3V trafo selection, ans considering that the output from the trafo is intended to work with 60 watts load and a sustained 220V, we may need the primary of the trafo to be rated at 60 / 3 = 20 amps, yes that's 20 amps which the trafo will need to be to ensure that the 220V is sustained when a full load of 60 watt is attached to the output.
Remember in such situation if the output voltage is measured without a load, one might see a abnormal increase in the output voltage value which might appear to be exceeding 600V. This might happen because although the average value induced across the mosfets is 3V, the peak is always 12V.
But there's nothing to be worried about if you happen to see this high voltage without a load, because it would quickly settle down to 220V as soon as a load gets hooked up.
Having said this if users find it rattling to see such increased level of voltages without load, this can be corrected by additionally applying an output voltage regulator circuit which I have already discussed in one of my earlier posts, you may effectively apply the same with this concept also.
Alternatively, the raised voltage display can be neutralized by connecting a 0.45uF/600V capacitor across the output or any similarly rated capacitor, which would also help to filter out the PWMs into a smoothly varying sine waveform.
The High Current Issue
In the above discussed example we saw that the with a 50% PWM chopping, we are forced to employ a 3-0-3V trafo for a 12V supply, forcing the user to go for a 20 amp transformer just to get 60 watts, which looks quite unreasonable.
If 3V calls for 20 amps to get 60 watts, implies that 6V would require 10 amps to generate 60 watts, and this value looks quite manageable....... or to make it even better a 9V would allow yo to work with a 6.66 amp trafo, which looks even more reasonable.
The above statement tells us that if the average voltage induction on the trafo winding is increased, the current requirement is decreased, and since the average voltage is dependent on the PWM ON time, simply implies that to achieve higher average voltages on the trafo primary, you just have too increase the PWM ON time, that's another alternative and effective way to correctly reinforce the output voltage drop issue in PWM based inverters.
If you have any specified queries or doubts regarding the topic, you can always make use of the comment box below and jot in your opinions.
Chria says
Hi Swagatam. I removed the feedback from the Circuit. On each side, the gate voltage was 5v (that is, the voltage from either pin 11 or pin 14 was 5v each.)
Here’s my question.
1) Is the voltage on either pin 11 or 14 when measured as you directed me using a voltmeter, supposed to be 12v each?
If yes then please drop a link to a driver circuit using sg3524 as the ic for me which would produce 12v on either sides. I’d then make use of your driver circuit.
Thanks.
Swagatam says
Hi Chria,
They will be 12V, but since the outputs of the IC are oscillating at 50% duty cycle, the average output voltage measured across these pinouts will be 6V. It could be also 5V because of 1 V lost in the deadtime setting.
You can confirm the 12V by adding a diode on one of the output pins followed by a 10uF capacitor across the diode’s cathode and the ground line. Measure the voltage across this capacitor and you will find it to be 12V.
Chris says
You mentioned adding a diode and a capacitor.
Can i include this diode and capacitor in my actual inverter circuit? Would it improve the output of my inverter?
Swagatam says
No, this is only to confirm that you are getting a 12V peak at the outputs of the IC….please do not do this with the mosfet gates, otherwise the mosfets will blow of instantly.
Chris says
Thanks a lot for your quick response. Without the feedback, i was able to get 5v across the outputs. From your explanation above, it means 5+5=10v. So to harness the full power from my driver, am i adviced to use a 10-0-10 transformer while still using a 12volt battery?
Swagatam says
You are right, if the gate voltages are around 10V, then you must use a transformer rated at slightly less primary voltage than 10V, a 9-0-9V will be much suitable.
Chris says
Hi. Please how do i measure the voltage across the gates of my mosfet?
I’m using an sg3524 ic as my driver.
Thanks.
Swagatam says
It can be measured using a voltmeter, touch the positive probe on the gate, and the negative probe on the source.
Chris says
Thanks a lot Swagatam.
I measured the gate voltage it and i got about 4.67v maximum on both sides. And by adjusting the variable resistor that controls the feedback, the gate voltage varied but i got 4.67v maximum.
My question here is.
Is there anyway i can increase this gate voltage to about 6v or slightly higher because, i experience a slight voltage drop when i apply a load across the transformer’s output.
I’m using a 12-0-12v transformer.
I’m using sg3524 ic as my driver.
Thank you
Swagatam says
Hi Chris, what you are seeing is the average DC on the gates because the feedback is consistently trying to suppress the mosfet conduction. You can try increasing the feedback threshold and then measure the gate voltage, or you can temporarily remove the feedback and then check the gate voltage and you will find it to be 12V, because the output from the SG3524 will be always equal to its Vcc supply Dc value
yusuf says
good morning sir,this is really interested.my question is: can i combine both pwm and output voltage correctio to further strengthen the inverter against voltage drop?
Swagatam says
Thanks yusuf,
you can prevent over voltage, but you cannot prevent a voltage drop due to over load, no matter what correction circuit you use. If your inverter voltage is dropping below a minimum level, it means either your battery is discharged below the required level, or the load has exceeded the maximum specified level.
With a constant PWM from the inverter there’s no need of an extra voltage correction circuit, according to me.
ACHA says
HI,, Cottage country, I have an off grid 1500 watt inverter ( 110 volts 60 Hz), with plenty of battery power over 1000 amps available, yes, lots of power available.
I have connected a small bar fridge, 150 watts, and when the fridge starts the lights ( 3 LED light bulbs 10 watt each) dim for a split second and then all is normal, no problems there, and it’s expected because of a small voltage drop due to the fridge compressor start up surge.
I can leave with that.
Here is what I don’t quite get it yet:
I also have a small 15 watt LTE 4 G Router ( runs on a little SMPS 90 to 240 VAC to 12 volts DC) Big deal..lol
Here is the problem: if and when I plug my laptop power supply ( also SMPS 90 to 240 VAC), the router reboots as if the voltage dropped below a preset value, and it happens whether the fridge is running or not, and the fridge is not affected. Switch mode power supply, seams not to like the inverter or vise versa..lol
My suggestion to fix this to install this device at the output of the inverter, an EMI filter DFC-2715R rated to 250 VAC at 15 Amps AC, that may be sufficient to keep the voltage value steady for a few milliseconds, when an SMPS is connected, because it’s the only time that other devices powered by SMPS will reboot, whatever is running on SMPS will resart, regardless how small of its power consumption.
Even my small LEDTV 45 watts reboots because it has a built in SMPS.
My system is not overloaded, I am way below the inverter power, and not all items are plugged at the same time, believe me, it’s these crazy SMPS that are creating havoc.
Please give me your opinion on my idea, and do not answer back with a lot of questions that you can extrapolate from this text. The inverter is properly grounded and as long as I don’t have any SMPS units plugged it, all is fine..and still is, except some will reboot as I explained above.
A friend has a 10 KVA inverter ( also off grid ) and the same happens..lol
I think I will install that EMI filter at the output of the inverter, what do you think?
ed
Swagatam says
Yes please install the EMI with some additional large capacitors, that might possibly solve the problem.
Chinedu says
Thanks, Mr Swagatam, for your generosity in passing knowledge. Please, my question is: does the automatic voltage regulator for an arduino sinewave inverter you discussed about in your other post for controlling high output voltages in the case for a charged battery control the output voltage drop when load is applied?
Swagatam says
Chinedu, the voltage regulator circuit will only prevent a high voltage output, but will not prevent a low voltage, because the low voltage would indicate over load situation and lack of battery power to tackle the load, which cannot be compensated by any means.
Peter Ferreira says
Hi, thank you first of all for all the information. My question. I build a small inverter with a 4047ic as the multivibrator, 2 x IRF3205 mosfets. So when i apply 12v 7.2ah battery i see that my outputs on the ic(pin 10 & 11) is around 6.5v. So i used a 12-0-12v transformer. When i apply a 48w Lcd tv the 220ac voltage drop to 180ac. My output is a 50% duty cycle at around 51hz, Square wave.
Q. Should my transformer be 6-0-6, or if i put 24v dc with a 7812 voltage regulator to protect the ic can i then keep the 12-0-12 transformer and will that improve my inverter output.
Q. Please can you recommend what i should do to juice up the watts.
Q. if i understand you correctly from the above diagram a full bridge is much more efficient than my push pull circuit. If that is the case then i will try to build the circuit. Just 2 questions.
Q. At the primary there is no center tap
Q. The IRF9540’s in your diagram not all the gates seems to be connected.
Thank you very much for all your input.
Peter Ferreira
South Africa
Swagatam says
Hi, your first assumption regarding the transformer, battery is absolutely correct.
To increase the current you will have to increase the wire thickness of the transformer, and accordingly increase the battery Ah rating.
Full bridge is definitely more efficient, and it will require a 2 wire primary between the MOSFETs, a center tap is not required.
The gates of the p channel MOSFETs are connected in parallel with each other, just like their drain and the source terminals.
Patrick says
Good day Sir;
Thank you for your invaluable Tech. guides,i appreciate.This voltage drop address is a very important issue.I just watched a video on Youtube on which the host uses two 555 ic to create pulses ,one astable and the other Monostable.He then passes the astable pulses to the monostable ic and with the help of variable resistor to generate duty circle of any percentage.Can you open-up on this.There was no circuit—only schematic he gave.
From your analyses of voltage drop due to low duty circle from 12v to 3v on transit of sq wave to pwm,is it good if i use a 48v transformer so that it narrows down to 12v which is the normal dc voltage,or do i have to rewind my 12v transformer to 6v or 3v for proper matching?.
I have some transformers in which the input and output voltage is written but lacks the current and power rating,like—(1) 220/15-0-15…..(2) 120-012 (3) 6.8-0-6.8. Number (2) and (3) presumably from 1200kva or 800kva ups. Can one use a guessed current for calculation?
What do you suggest on these issues above.
Thank You.
Swagatam says
Thank you Patrick,
It was probably me who ws the first person to have implemented the IC 555 based SPWM in an inverter, here’s the diagram:
https://www.homemade-circuits.com/wp-content/uploads/2013/10/3kvainvertercircuit.png
for the detailed explanation you can refer to the last two concepts under this post:
https://www.homemade-circuits.com/modified-sine-wave-inverter-circuit-2/
For the transformer selection, the rule of thumb is, its primary should be equal to the average DC value which is measured at the gates of the driver MOSFETs
Colin Lesley says
I would like some advice please.
I bought a sma sunny boy 2 and 4 400 watt panels.
Installing myself complete because of covid problem no sparkies.
All was well until inverter stopped working most mornings between 0830 to 10 ish.
I realized the the rice mill a few hundred yards from my farm was running.
Hence our bad voltage drop.
The equipment supplier said talk to your electricity supplier.
I know before I start this will be a battle, it took 4 years just get electricity here.
So is there other options I can persue.
Look forward to any help you may have.
Thank you.
Swagatam says
The easiest option is to buy a 500 watt inverter and hook it up with your solar panel and use it during the low voltage periods. Or if you are well versed with building inverters and basic electronic concepts you can select an appropriate design from this website and do it yourself.
Endel Neiva says
Dear Swagatam,
I would like to know what to do and how to implement an EFFICIENT DRIVER circuit,
with BJT trasistors, because I am not able to implement this with IR´S
because all the examples that I found here on the internet, teach only how to avoid with the IR´S
for the “H” BRIDGE system, and the system I’m thinking of creating is of the type: PUSH-PULL
In this way I found nothing teaching how to dodge with the IR´S, for this system that I will use
Could you help me ? if so, the information would be the following:
– SG3525 PWM = 5V 60hz square wave, I want to increase this voltage level and obviously current tb to about 10 to 12V voltage and current enough to generate 20 IRF640 mosfet´s, 10 per ARM that I will use on my inverter… ..
I hope you can help me with my problem….
Swagatam says
Dear Endel, if you want a fail proof efficient N channel based full bridge topology, then you must go for a specialized IC based design such as using IRS.
Alternatively you can go for P and N channel combination as shown below. But this may require many p channel MOSFETs, although the design is very easy implement.
ENDEL NEIVA says
I do not want anything with FULL BRIDGE dear friend, but with push pull, because I do not like this topology of FULL BRIDGE, I prefer PUSH PULL, because here in Brazil my country, the power is 110vac and not 220vac, we do not have much energy … so I prefer the push pull system, would you have any examples with IR drivers for the push pull system please dear friend
Swagatam says
Hi, a full bridge is specifically a push-pull design. There’s no better push-pull design than a full bridge. A center tap version is also push pull, but it uses two winding at the primary and is less efficient than full bridge.
qwerty says
hello. I builded a pure sine wave inverter which based on spwm signals. power supply is 18.5V Dc and the rms voltage of the primary of transformer is approximately 9V. The transformer has 15-0-15-400 on primary and 0-230V on secondery windings. Im usin 15-0 port of the transformer. The power of the transformer is 100VA/321VA . Without any load i can see the 220V RMS voltage on the output.When i plug the load (25W bulb) the voltage decreasing about 90VRMS. Why this is happening ?? I have also tried another loads but it is the same. Need help here !
Swagatam says
Hello, if the transformer primary is 15-0-15V then this voltage is the RMS of the transformer primary, not 9V.
Your inverter output is dropping because the power supply which are using instead of a battery does not have sufficient current. Use a 10 amp SMPs or a battery for the power supply and then you will find the output behaving consistently.
Gordon Reeves says
I have issues with my Magnavox Inverter on a solar PV system. When my water pump kicks in (when a tap is turned on) the voltage drops off and appliances such as oven, washing machine (anything that has a circuit board controller) cutout and either need resetting or if the washer is in a cycle it will restart the cycle from scratch. HELP?!
Swagatam says
You will need an additional 100 Ah battery working in parallel with the PV system, this battery will compensate the extra power needed during the pump switch ON, and could be disconnected as soon as the pump has started.
Shakir Nasrat says
Hi
Pleas if you can find copy in arbic , that is we wont to understand it.
Swagatam says
Hi, I think Google has English to Arabic translation, which you can use!!
rashantha vagiranath says
I made 500w modifisinwave inverter. I want to run my 150w refrigerator.but I can”t it.because volatage drop 160v . I use 200ah two batteries connect parrell.please help me solve this problem. forgive me my poor english.
Swagatam says
A correctly built 500 watt inverer will never drop its voltage when loaded with 150 watt. Your battery rating is OK no doubt, may be your transformer is not OK. It could be rated at much lower wattage than 500 watts,you must get it checked from a professional trafo designer.
For the battery even a single 200Ah should be more than enough to handle the 150 watt, even with the initial surge.
You may also want to confirm if your refrigerator is actually consuming 150 watt or much more than that??
moha says
Hello,
I thankful to you to encourage the people to do things better and better,
I need your help, Pls suggest its my request,
Question 01. If i want the output is 5000w pure sine wave inverter require but i have only 500w Transformer 10 nos in my hand, can i use the transformers parallel or series in single circuit to get output 5000w ?
Question 02: can i use single oscillator and driver circuit to drive multiple output mosfet drive possible, pls explain. Note : One oscillator and driver to drive output point mosfet and 10 nos separated 500w transformers .
thanks and regards
Moha
Swagatam says
Hello Moha, yes you can connect the primary wires in parallel but not the output secondary wires…you may terminate the outputs to different sockets and to separate loads.
yes you can use a single oscillator circuit for driving different mosfets configured with different transformers….but make sure to connect all their negative lines in common.
tinuke says
In my thinking, for 12v input to avoid voltage drop, why can’t we increase the voltage and make the trafo 24v-0-24v so that pwm will be 6v.
Miebaka says
nice article, I really appreciate it, I will like you to do a tutorial on ups/inverter transformer. Am really impressed with it’s charging and regulating future, and will really like you to feature it.
Thanks again for all you support.
Swagatam says
Thanks Miebaka, according to me there’s no difference between an inverter transformer and an UPS transformer, a transformer is designed for transforming a given voltage from one level to another through mutual induction, which is the same in inverers and UPSs
Miebaka says
Ok, but if you run mains voltage into another turns in the secondary side of the transformer (with correct amount of turns) wouldn’t it no affect the primary side of the transformer which is linked the mosfets and stuff?
Considering SG3225 inverter circuit and automatic change over…
Swagatam says
yes it will affect, the mosfet side will need to be simultaneously cut off while this is changeover is implemented.
Miebaka says
Thanks a lot, really appreciate all the work you have put to your listeners.
Swagatam says
Thanks Miebaka
Farhan says
Hello Sir i am Farhan
I have a 100 watt inverter
I also have a AC led tube light of 18 watt
I want to connect them like qhen i press the Light switch on
Then it will run on main Ac current
And when the ac line gone
Light will be connected with the inverter line
Thanx
Swagatam says
Hi Farhan,
you can do it by wiring a relay appropriately…. simply connect the coil of a DPDT relay or two SPDT relays with a DC supply obtained from mains input, then connect the N/C contacts of the relay with the inverter AC, N/O contacts with the mains AC, and the poles with the LED
NvD says
Pattern of flashing can be according to your wish sir…only thing is that it should look attractive…
Swagatam says
OK, let me try!
Anas Ahmad says
Thanks but please check your mail..I attached a picture..
Swagatam says
your sent design is a PIC based so I could not interpret its functioning details, and therefore could not solve your query.
Anas Ahmad says
Building a non center tap transformer for pure sine wave.. And battery is 24v.. Therefore the required primary volt is 14v..and secondary 240v.. What if the primary volt is less than 14 or more than 14? Will there be error or something? Using TPV formula
Swagatam says
for 24V supply, the trafo primary must be also around 24V…unless PWM is being is used…it cannot be 14V
slightly higher supply will not affect the trafo performance but if it's significantly high then the trafo will begin heating up, and the 220v output will also proportionately get high…for the low voltages the transformer will not heat up, but the output will drop proportionately
Evans mworeh says
Hi,with the example above (for 60w using 3v transformer will require a 20 amp transformer which is cost) but using a 9v transformer will call for a less amp transformer hence much cheaper ,Will the use of a 20v transformer for a 24v be ok for a pwm?
Swagatam says
Hi, You will have to calculate it as explained in the above article
Evans mworeh says
Hello sir, Iam working on a project to build an inverter charger with pwm using a non centre tap transformer. I have tried an 18-0-18 transformer it’s working but with charging it’s very slow. I had to increase the transformer voltage to 20-0-20 and connected a bridge diode to test charging speed and it’s just wow,now am yet to connect it to the mosfets and try out if it will still deliver/mosfets might go boom? Pliz help me know if this idea is ok? Or what changes needed? I will really appreciate.
Swagatam says
Hi Evans, without seeing the schematic or understanding the working of the circuit it can be difficult to suggest anything. Are you using the same transformer for inverter and battery charging both?
Evans says
Yes sir,am using the same transformer for inverter and charging.
NvD says
Sir,do you have simple circuit of led cube without arduino or ic programming
Swagatam says
NVD, if it's a solid cube that you want, then that can be simply done by connecting the LEDs in series/parallel. MCU won't be required
NvD says
sir,i want the leds to be flashing and running
Swagatam says
OK,I'll try to design it, but the sequencing will be as per the convenience of the design, with minimum complexity…
NvD says
ok sir..waiting
Swagatam says
I tried but it looks difficult, and might require much thinking, by the way you can specify the pattern of the flashing/chasing that you want across the grids, if it seems feasible I may try again.
OLUPOT DANIEL says
thanks for the gud work you do to help us
Swagatam says
you are welcome!
Ferdimar International says
okay Thanks.
thumbs up!
Ferdimar International says
Thanks for sharing
so it is recommended to choose a lower voltage high amp transformer the the input volts . eg. if you input is a 12v battery will be good to choose 7-10v for the inverter.
Swagatam says
Thanks, yes that's correct according to my assumptions.
Swagatam says
…it must be implemented as per the calculation presented in the article