In this post we learn how to correctly calculate inverter parameters with associated stages such as battery and transformer, by calculating the matching the parameters correctly.

Contents

## Introduction

Making an inverter all by yourself can be definitely lot of fun. However if the results are not satisfactory can completely spoil the whole purpose of the project.

Installing and configuring the various inverter parameter like the battery and the transformer to with the actual assembled circuit needs special care and attention for deriving optimal results from the assembly.

The article discusses how to calculate and match a battery and transformer wit the relevant circuit and also enlightens regarding the possible faults that might be encountered and the respective troubleshooting procedures.

The article enlightens the many newcomers with some of the important clues, which might be helpful while configuring an inverter circuit with the battery and the transformer, so that efficient and optimal results can be achieved.

## Calculating Transformer and Battery Specs

While making an inverter, two calculations must be broadly taken into account, viz. the transformer and the battery ratings.

1) The transformer must be rated higher than the maximum load that is expected to be used with the inverter. For example if the intended load is 200 watts, then the transformer must be rated at minimum 300 watts. This will ensure a smooth running of the inverter and less heat generating from the transformer.

The voltage rating of the transformer must be slightly lower than the battery voltage for square wave inverters.

However, for concepts involving PWM or SPWM, it should be equal to the average voltage applied at the gates of the MOSFETs. This can be measured by measuring the average DC voltage applied at the gate of the MOSFETs from the oscillator stage. So, suppose your battery voltage is 12 V, but because of PWM your average switching voltage from the oscillator shows 7.5 V DC, that implies your transformer must be a 7.5-0-7.5 V and not 12-0-12 V.

2) And the battery Ah must be rated 10 times more than the maximum current rating of the load. For example if the battery is 12V rated and the load 200 watts, then dividing 200 with 12 gives us 16 amps. Therefore the battery Ah must be 10 times of this amp rating, that is 160 Ah. This will ensure your battery runs with a healthy 0.1C discharge rate and provides a back up of around 8 hours.

## Calculating MOSFET Rating

Calculating MOSFET for an inverter is actually quite simple. One has to take into account the fact that MOSFETs are nothing but electronic switches, and must be rated just like we rate our mechanical switches. Meaning the MOSFET's voltage and current ratings must be adequately selected so that even at the maximum specified load, the MOSFET working is well within its breakdown level.

To ensure the above condition, you can refer to the datasheet of the mosfet and check the Drain-Source Voltage and the Continuous Drain Current parameters of the device, such that both these values are well above the load's maximum consumption values, or are selected with appreciable margins.

Suppose if the load is rated at 200 watts, then dividing this with the battery voltage 12V we get 16 amps. Therefore the MOSFET could be selected with voltage ratings anywhere between 24V to 36V as its Drain-Source Voltage (**Vdss**), and 24 amp to 30 amp as its Continuous Drain Current (**Id**).

Take the example of the MOSFET in the image above, here the maximum tolerable voltage Vdss of the specified MOSFET is 75V, and maximum tolerable current Id is 209 amps, when operated with proper heatsink. It means this MOSFET can be safely used for all applications where the load wattage is not more than 14000 watts.

Although in real life the load handling capacity may be even lower than this.

As a rule of thumb, make sure the **Vdds x Id** value is at least 30% higher than the max load wattage, and the MOSFETs are adequately heatsinked.

This takes care of the MOSFETs, and ensures a perfect working of the devices even at full load conditions, but do not forget to mount them on appropriately dimensioned heatsinks.

After procuring all the necessary components as explained above, it would be important to get them checked for compatibility with one another.

Only the battery, which is one the most crucial member, hopefully will not require any prior checking, because the printed rating and the charged voltage conditions should be sufficient to prove its reliability. It is assumed here that the condition of the battery is good and it’s relatively new and “healthy.”

### Checking the transformer

The transformer, which is the most important component of the inverter, surely needs a thorough technical assessment. It may be done as follows:

The rating of the transformer can be best checked in the reverse order, i.e. by connecting its higher voltage winding to the AC mains input and checking the opposite winding for the specified outputs. If the current ratings of the lower voltage section are within the maximum limits of a regular multi-tester (DMM), then it may be checked by switching ON the above AC and connecting the meter (set at, say AC 20 Amp) across the relevant winding.

Hold the meter prods connected across the winding terminals for a couple of seconds to get the readings directly on the meter. If the reading matches with the specified transformer current, or at least is close to it, means your transformer is OK.

Lower readings would mean a bad or a wrongly rated transformer winding. The assembled circuit broadly needs to be checked for proper oscillation outputs across the bases of the power transistors or the MOSFETs.

This may be done by connecting the circuit to the battery, but without including the transformer initially. The checking should be done using some good frequency meter or if possible using an oscilloscope. If the above gadgets are not there with you, a crude testing can be performed using a pair of ordinary headphones.

Connect the headphone jack to the bases of the relevant power transistors; you should get a strong humming sound in the headphones, confirming a sound functioning of the oscillator stages.

The above confirmations should be enough to prompt you to configure all the sections together. Connect the transformer to the relevant transistor or the power devices terminals; make sure the power devices are correctly integrated with the oscillator stage.

### Installing the Final Inverter Set up

Finally the battery may be connected to the power inputs of the above configuration, again do not forget to include an appropriately rated FUSE in series with the battery positive. The output of the transformer now may be attached with the specified maximum load and the power may be switched ON.

If everything’s is wired up correctly, the load should start operating at its full fledged power, if not, then something’s wrong with the circuit stage. Since the oscillator section was appropriately checked before the final installations, surely the fault may lie with the power device stage.

If the fault is associated with low power outputs, the base resistors may be tweaked for possible faults, or may be reduced by adding parallel resistors to their existing base resistors.

The results may be checked as discussed above, if the results are positive and if you find improvements in the power outputs, the resistors may be further modified as desired, until the expected power output is delivered.

However, this may lead to further heating of the devices and due care must be observed to keep them under check by either including cooling fans or increasing the heatsink dimensions.

However if the fault is accompanied with blowing of the fuse would mean a definite short circuit somewhere in the power stage.

### Troubleshooting the Inverter Connections

The problem may also indicate a wrongly connected power device, a blown-of power device due to a possible shorting between the power device’s output terminals or the any of the terminals that needs to be perfectly kept aloof of each other.

Having explained a few of the above possibilities while configuring an inverter optimally, a thorough knowledge regarding electronic becomes an absolute necessity from the part of the individual who may be involved with the construction, without which the proceeding with the project may somehow get jeopardized.

## Have Questions? Please Comment below to Solve your Queries! Comments must be Related to the above Topic!!

Who’s so ever that is having difficulties in building inverter should let me know just chat me up learn froe the one you don’t know

+2348144714713

I am having difficulties selecting MOSFET or knowing the number of MOSFETs to be parallel in my SMPS based on safe area operation.

Please provide the specifications of the SMPS.

Sir,

How could I read the amps of the transformer of the lower voltage?According to the article, Will it be in series with a load?

Or parallel as it’s winding terminal?

Ayodele, The load and the ammeter should be in series with the transformer supply.

hello sir, do you have any article of 12v source and ups with backup battery that you can recommend is for an access control project

Hello Marcelo, I have a couple of UPS based articles on this website which you can refer to:

Simple Online UPS Circuit

4 Simple Uninterruptible Power Supply (UPS) Circuits Explored

Dear Sir

Please sir, how can I know original lead acid battery to buy? Or which type of it is original?

2. How do I test if a lead acid battery is OK in case of buying a fairly used or used battery?

Thank you sir

Hi Godfrey, you can do an online search and read the reviews to know which battery is the best and which are from a reputed brand.

It can be difficult to test a battery quickly, you will need at least some hours to discharge the battery through a circuit and then analyze its backup time and current.

https://www.homemade-circuits.com/precise-battery-capacity-tester-circuit-backup-time-tester/

Hello sir, that’s a very superb article you have got here, thanks to your website am now understanding many things difficult before, pls I have gone through the article and I understand what you have said but I need to ask a question pls, After calculating the mosfet/transistor vdss…… and others how are am I to calculate the number of mosfets/ transistors needed for the the intended inverter watt, say for example am to use 16 mosfets for 1000 watts, how am I to calculate and know that am to use exactly 16 mosfets for 1k Watts. Thanks for answering.… Read more »

Thank you Anthony, the VDDS and ID indicate the maximum tolerable power the mosfet can handle, but we need to provide a 50% margin so that the mosfet can work with normal heatsinking. The product of the VDDS and ID and 50% will determine the total wattage the mosfet can handle….you can divide the inverter wattage with this result to get the total number of mosfets required to be added in parallel

Hi swagatam

Really appreciate ur work,keep up gud work excellent information on inverter discrete comonents

Thank you Suresh for your kind feedback!

Hi, I am researching inverters as I would like to build one sometime soon, one thing on my mind is choosing the right transformer, I would like to buy one instead of making it myself. Your article stated that I should put a load threw the mosfet and measure the voltage before I could choose the right transformer is that correct?

Hi, yes you can do that, measure the voltage across the load, or the drain/source terminals, this will be your transformers primary voltage rating

Dear sir,

please sir, in respect to checking the transformer for the specified secondary current.

1. What alternate method can be used to measure the current if AC current range on the DMM is below the value specified?

2. And again, for DMM that doesn’t have AC current test, can the secondary of the transformer be rectified with full bridge or half bridge, and then setting the DMM to DC current range to measure the current? Will the result be the same if it was to be measured using AC ammeter.

Hello Godfrey, you can measure it by applying shunt resistor method. Put a known resistor low value high wattage resistor across the transformer wires, check the AC voltage across it, use the following ohms law to get the current value

I = V/R, where V = voltage reading on the meter, R = value of the resistor

For DC meter, first convert the AC to DC using bridge rectifier and repeat the above process across the output of the bridge.

Thank you very much sir. I really appreciate your time and this great offer of help.

You are welcome Godfrey!

Please how do i know the Ah of the transformer i wind by my self. I used 19 guage copper wire

You can do it by connecting an ammeter across the secondary wires of the transformer for a few seconds and note the current reading on the meter

Pls can I use two different kind of power mosfet in building one Inverter because I don’t have enough mosfet?…i.e irf 1010e and irf4110

yes you can, just make sure their technical specs are similar,…. or the max V and I specs of the required inverter output is 50% lower than the max rating of the MOSFETs

Hi,

Thank you for the post. I am trying to make a pure sign wave inverter using SPWM generated using Arduino. And i am planning to use high frequency PWM capability of Arduino to produce 62kHz PWM signal and use that to simulate 50Hz sine wave. As a result transformer will see 62Khz pulses. In this case for best efficiency do I need to use a high frequency ferrite core transformer calculated/designed for 62KHz or a regular transformer calculated/designed for 50Hz?

Best Regards

Hi, you will need a ferrite core based transformer for handling the 62kHz carrier frequency

Hello sir swagatam, please enlighten me here; which mosfets are more efficient to use in inverters involving high voltage say from 24,36,48,60,72,96 extra?? 1. Mosfets with high RDSon? 2. Mosfets with low Reason? The reason am asking you this is that I’ve tried designing a 24v Inverter using irfz44N and irfp1404 in vain. But if I design a 12v inverter,it functions well,what might be the problem? Am using cd4047 for osciliation from one of your articles sir. Or let me know if there’s something else I need to do when it comes to Inverter with high input voltage? I have… Read more »

Hello Evans,

Higher RDSon will cause more heat dissipation on a MOSFET and vice versa. So lower RDSon is preferable for greater efficiency.

Both the MOSFETs that you have mentioned are suitable for a 24 V inverter, however irfp1404 is better suited due to lower RDSon.

There could some other issue in your inverter, because designing a 24V inverter is as easy as designing a 12 V inverter.

For 5kva output you may have to add many of those MOSFETs in parallel, and use a 20 V transformer for a 24V battery

Thank you for your response. From your opinion,what might be the problem? Coz the circuit I used is from your articles?

Evans, Whether it is from my site or any other, the operating voltage level can never be an issue, as long as the transformer and the MOSFETs are appropriately matched.

Make sure your oscillator IC circuit gets a stabilized voltage, which doesn’t exceed its maximum tolerable limit. And the MOSFets are also protected with external reverse diodes

For a stabilized voltage circuit for the oscillator you can refer to the following article:

500 Watt Inverter Circuit with Battery Charger

Sir my transformer is 17v

then it is OK….

Hello sir swagatam, suppose I decide to use irf064 which has an ID=110 and a vds of 55v. Now i want to build a 5kw/24v inverter and from your explanations above ,multiplying the V and I of the mosfet 55*110=6060w. For my inverter 5000/24=208A so plus 10=218A,so the number of mosfets and each side would be 218/110=2. So if I use 2 mosfets on each side is it ok? Will it run a 5000w load without overheating? Is the vds voltage of 55v ok?

Hello Evans, yes that’s possible, but you will mount them on huge heatsinks with fan cooling….if you want to avoid this, then use at least 5 in parallel.

Thanks sir for your quick response.

My pleasure!

Hello sir swagatam,what is the difference between mosfets in small package and those in large package? For example irf1405 which has a id of 169A and vds of 55V, and irf064 which has a id of 110A and vds of 55V. Now between the two which one can hold 6000W if I used 2 of them on each channel without ;

1. Overheating

2. Blowing up

Evans, the 169 A has more power handling capacity than the 110 A, so it is more favorable.

Sir swagatam,from you explanation above you have said that for concepts involving pwm/spwm that the transformer voltage has to be average the switching voltage. Now my questions are; Let’s take your example above of 7.5v for a 12v battery

If I want to make it an Inverter charger,

1. Will 7.5v charge the 12v battery to it’s maximum charge?

2. How long do you think it will take to fully charge the 12v battery?

Evans, No, 7.5 v cannot be used for charging 12V…so if you want to make a single transformer inverter/charger with pwm that cannot happen. At 10% Ah, it may take 14 hours for the battery to charge

Lead Acid Battery Charger Circuits

Hello sir swagatam,av come across this mosfet irf3713. From the datasheet it has a vds of 30v and an IDs of 260A rds 3.0 . Now my question is;

1. Can this mosfet be used for a 24v Inverter?

2. How many can I use for a 6000w/24v Inverter?

Hello Evans, yes it can be used, provided the supply never exceeds 28 V, and make sure to add external diodes across D/S of the MOSFET. You can try with 3 mosfets initially, check if they get too hot with a heatsink, in that case you can add a couple of more, until the dissipation is manageable with a reasonably large heatsink

Thank you so much for help

sir Swagatam thanks allot

please I want you to help on this if they ok.Sir supposing I’m using a transformer from 5kva stabilizer for 4kva inverter under 24v battery.

total current will be 4000/25 = 167A sir

if 10 fet (3205) per side then current handle by each fet will be 167/20 = 8.35A because they connected parallels sir. also power handle by each fet will be 24v *8.36a = 200w is this ok

Sir can I use this transformer for 5kva inverter sir? thanks

Osei, each channel will be subjected to 4000watts, therefore each FET will share 167/10 = 16.7 amps according to me, and power will be 24 x 16.7 = 400 watts

Thank you sir or should I call you teacher

It’s my pleasure Osei!

Sir Swagatam I have followed your link for a number of weeks now and I find it interesting. sir I want you help me on this

let me use you 3kva circuit with 3 fet transistors per channel sir

if I want to use 10 on each side , what will be the current consume by each transistor (irf 3205) under 24v battery and also what will be the maximum power delivered by the inverter

thank you sir

Osei, Please check the ID and VDS ratings of the mosfet through its datasheet, and multiply them to get the maximum tolerable wattage of the device. Make sure your expected power output is less than this, and the MOSFETs are mounted on heatsink.

Output power is decided by the transformer and the battery, the MOSFETs only work like switches and must be rated appropriately to handle and execute the specified amount of power.

thanks for your quick response sir

OK, thanks for your response. Am working on a 3kva inverter, how many mosfets do I use in total. Thanks

Good day sir,I have been following your post all this while and i find them motivating especially to people like me.

pls sir I need your help cocerning protecting my inverter .

and i dont really know some causes ,how can i go about it ;

some of them are ; mosfet got burnt ,unable to control the charging ,making noise that is fan make some umming noise when inverter is in use ,how do i protect my oscillator,…

Hi Ag, I have covered all these topics in this website, it will be difficult for me to explain all these separately through comments, you can search the topics through the search box, you might find some help

Hello sir Swagatam,

Thanks a lot for the article. The article didn’t cover how to accurately calculate the number of MOSFETs that will match a specific transformer and battery ratings in an inverter system. Kindly shed more light on that.

Anticipating your usual prompt response. Thanks.

Hello Godson,

mosfets are nothing but electronic switches, therefore they simply need to be rated as per the transformer consumption. if the transformer primary is rated to consume say 20 amps max, the mosfets must be rated at 25 amps, that’s a little higher than the maximum consumption value.

Hello sir Swagatam,

Thanks for the prompt reply. What I meant was that supposing I want to build a 2KVA inveter that will run on 24V battery, how do I know how many MOSFETs to use?

Hi Godson,

divide 2000 with 24, this will give you the mosfet current, make sure to select 10 amps above this value for the mosfet on each channel.

voltage rating will be 10V higher than 24V that is 34V .

higher values will work, but not the lower values.

Thanks for the reply sir.

Using the example of the 2KVA, 24V above, here is what I understood from your explanation:

2000/24 = 83.3A

83.3 + 10 = 93.3A

Using IRFP150N MOSFET which has a continuous drain current of 42A and voltage rating of 100V,

Number of MOSFETs on each channel = 93.3/42 = 2

Is this calculation correct sir?

Where does the voltage rating that you mentioned above come in to this calculation?

According to me that’s correct!!

the voltage rating has to be simply 10V higher than the battery voltage…

make sure to add good heatsinks, which is always compulsory for inverter devices.

please sir how can i know the number of mosfet for a 2000w inverter

multiply the V and I of the mosfet and divide 2000 with this result

https://www.homemade-circuits.com/2013/04/how-to-modify-square-wave-inverter-into.html?m=1

The above link is the link mentioned in my previous post. Please reply soon.

Regards,

Santanu

yes, 1N4148 can be used instead of 1N4007, that would make no difference in the performance of the system.

1.bp.blogspot.com/-NgqlieM8TX8/U7zL0tGH6qI/AAAAAAAAHgE/WCKLVG9pVVs/s1600/mosfet+transistor+driver+circuit.png

In the above given link there are 2 nos. of 1n4007 diodes for pwm injection. I need to know that if i use 1n4148 diodes in place of the 1n4007 diodes as shown in one of your post, does it make any difference? I think no. Please rectify me if i am wrong. Link given in next post.

Regards,

Santanu

1N4148 can be used instead of 1N4007, that would make no difference in the performance of the system.