Disadvantages of Using half-wave instead of full-wave in Pure Sinewave Inverter?

[KerimF]

Every pure sinewave inverter, I knew or heard of (12Vdc or 24Vdc to 220Vac), that uses laminated iron transformer (input: 1 coil, output: 1 coil) drives its transformer with a 4-MOSFET bridge.

Lately, I am designing a pure sinewave inverter but by using 2-MOSFET push-pull driver (lower side only) with a half-wave transformer (input: 2 coils, output: 1 coi).

I used to believe that simplifying a board likely creates some disadvantages.

So, I hope some experienced engineers here could help me know in advance certain expected disadvantages in using the push-pull configuration, in this case, instead of a bridge.

Thank you.
Kerim

[PCB.Wiz]

What do you mean by 2 mosfet push pull ?
If you mean 2 N MOSFETS in half wave drive, yes, that saves 2 MOSFETS but
* you now need 2 clamp circuits
* worse, is the current flows in half the transformer copper at a time.
 
It’s the same reason half wave rectifiers ‘waste’ transformer resource, and bridge rectifiers are preferred.

[coromonadalix]

and you think you can re-invent the wheel ?

the push pull is the best, if you dont use that,  it work in a flyback fashion, if the push pull is well made you have zero cross over defects

your single side mosfets seems  more like an class A amplifier, it will heat more for nothing

and pls  give some schematics to show your idea

[Ian.M]

Also, the end of the primary not currently driven must swing above the supply rail so you need MOSFETs with double the Vds rating.

[Zero999]

I've not seen any which use a large laminated iron transformer. The ones I've seen use an isolated DC:DC converter to a get DC of the peak voltage of the mains, followed by a MOSFET H-bridge and filter.

[KerimF]

What do you mean by 2 mosfet push pull ?
If you mean 2 N MOSFETS in half wave drive, yes, that saves 2 MOSFETS but
* you now need 2 clamp circuits
* worse, is the current flows in half the transformer copper at a time.
 
It’s the same reason half wave rectifiers ‘waste’ transformer resource, and bridge rectifiers are preferred.

Good remark,
If I understood you well, the price is more copper at low-voltage side in order to get the same resistance of the 1 coil transformer (driven by a bridge).
But let us also recall that the resistance added by a bridge is 2*Ron instead of 1*Ron in case of half wave drive.

[KerimF]

and you think you can re-invent the wheel ?

the push pull is the best, if you dont use that,  it work in a flyback fashion, if the push pull is well made you have zero cross over defects

your single side mosfets seems  more like an class A amplifier, it will heat more for nothing

and pls  give some schematics to show your idea

The driver is simple. The MCU (actually an ATmega8) generates two anti-phase SPWM (sinewave PWM) at relatively high frequency (actually 15,625Hz).
The N-MOSFET driver is a conventional one that uses 3 BJT transistors. The dead time between the two drivers is 1us.

[KerimF]

Also, the end of the primary not currently driven must swing above the supply rail so you need MOSFETs with double the Vds rating.

Another good remark.
Fortunately, the new power MOSFETs have ratings (Vds and Id) suitable for power inverters if supplied by 12Vdc or 24Vdc.

[coromonadalix]

why dont you use already know smps parts, pwm ic's  etc ...

[KerimF]

I've not seen any which use a large laminated iron transformer. The ones I've seen use an isolated DC:DC converter to a get DC of the peak voltage of the mains, followed by a MOSFET H-bridge and filter.

You don't surprise me because you refer to the most efficient topology which could be used for a pure sine wave inverter.

In where I live, I can't design and produce a product which needs ferrite transformers because I can't get the types I may need in quantities.
So, I had to use the available laminated iron cores if I like to add such inverters to my collection.
I used to believe that having/doing something (not very good) is much better than having/doing nothing

For instance, the small leakage inductance of an iron transformer does the filtering if a relatively small capacitor (about 1uF, high AC voltage) is added at its output.

[KerimF]

why dont you use already know smps parts, pwm ic's  etc ...

The answer is simple. They are not available, or their prices are relatively high, in my local market. (Unfortunately, after year 2011, ordering electronic components from abroad by ordinary people like me became out of question... my golden years are things of the past after the world... saved the people among whom I was born and live).

On the other side, the price of the available ATmega8 is not high (perhaps because it is old and replaced by new ones). Also, the price of BJTs is good.
And to gain my daily bread, my designed products have to compete similar ones that are imported from abroad (lately by the privileged rich families only) or made locally.

 

[PCB.Wiz]

Good remark,
If I understood you well, the price is more copper at low-voltage side in order to get the same resistance of the 1 coil transformer (driven by a bridge).

Basically, yes. Each winding has the same peak current as a bridge, but half the time.
So you have wasted space in the transformer and can get less loss, by using a larger copper gauge, single winding in the same physical space.

But let us also recall that the resistance added by a bridge is 2*Ron instead of 1*Ron in case of half wave drive.

In 2024 that's immaterial, the days when low milliohms FET were hard to come by, are long gone.

[KerimF]

Good remark,
If I understood you well, the price is more copper at low-voltage side in order to get the same resistance of the 1 coil transformer (driven by a bridge).

Basically, yes. Each winding has the same peak current as a bridge, but half the time.
So you have wasted space in the transformer and can get less loss, by using a larger copper gauge, single winding in the same physical space.

I agree with you. And this is indeed a disadvantage which is the price for using a much simpler circuit.
 

But let us also recall that the resistance added by a bridge is 2*Ron instead of 1*Ron in case of half wave drive.

In 2024 that's immaterial, the days when low milliohms FET were hard to come by, are long gone.

Knowing that the primary resistance (in a power inverter) is around a few mR, the low values of Ron (also around a few mR) are not negligible even if every set is formed by 4 N-MOSFET in parallel.

[coromonadalix]

and you do know, i hope 

using any mcu for generating a sinal will not be pure as you need,  it will be called pseudo sinusoidal, you'll need good filtering, if you want to acheive a somewhat good sinus wave out of it

you could have some equipment compatibility problems

IE  i have  some laptop chargers who wont refuses to start even with pseudo signal inverters,  with pure sinewave they start  etc ....  some tv's  dont work too (they use ac-dc adapters)

while i salute your idea,   you may face other challenges you dont even know
i can understand  some material could not be available ...  etc ...  but its not a reason to play with safety, protection  etc .. 

[Kim Christensen]

Every pure sinewave inverter, I knew or heard of (12Vdc or 24Vdc to 220Vac), that uses laminated iron transformer (input: 1 coil, output: 1 coil) drives its transformer with a 4-MOSFET bridge.

The driver is simple. The MCU (actually an ATmega8) generates two anti-phase SPWM (sinewave PWM) at relatively high frequency (actually 15,625Hz).
The N-MOSFET driver is a conventional one that uses 3 BJT transistors. The dead time between the two drivers is 1us.

Are you attempting to drive a standard laminated iron power transformer at 15Khz instead of 50Hz or 60Hz?
How much power (Watts @ 220Vac) will this inverter be outputting?

[KerimF]

using any mcu for generating a sinal will not be pure as you need,  it will be called pseudo sinusoidal, you'll need good filtering, if you want to acheive a somewhat good sinus wave out of it

You are right in case the code is not well written. For instance, I had to write my codes, since early 80's, in assembly language only because I couldn't get, for free or else, high language compilers as of C for example.

I managed to output the SPWM signals at pins OC1A and OC1B of ATmega8 (using fast PWM of timer 1). The rate is 15,625 Hz. The number of samples per cycle (50 Hz) is 312.
The amazing thing is that the small leakage inductance of the iron power transformer does the filtering with a relatively small high voltage capacitor (around 1 uF). I am not sure how good (speaking THD) the sinewave, I got, is. But it looks on the scope screen it looks a sine wave without ripple.

Naturally, to maintain 220V at the output, the MCU reads the output voltage in every cycle (50 Hz) to adjust the gain of the PWM for the next cycle. This voltage regulation allows a soft start at boot (from 0V to 220V, in 800ms). The MCU has other functions related to safety in case of faults.

you could have some equipment compatibility problems
IE  i have  some laptop chargers who wont refuses to start even with pseudo signal inverters,  with pure sinewave they start  etc ....  some tv's  dont work too (they use ac-dc adapters)

In the last 12 years, I produced and sold about 5 thousand 'adaptive square wave' inverters (regulated by varying the duty cycle). Lately, it became clear that every device (made for 50Hz sinewave) whose power supply uses a series capacitor to limit the internal supply current, is damaged, sooner or later, by a square wave inverter. Summer is at the door and electric fans will be used soon. Most of these fans are controlled by electronic boards supplied internally by a series capacitor (no more by a small transformer). So, I decided to also produce low-cost sinewave inverters, at least for these fans.

I admit that with the limited available components around me I can't do miracles by making such inverters also with high efficiency. Losses in the iron transformer whose available laminated core is also of the lowest possible grade! (Bs=1.05 Tesla, Br=0.7 Tesla and Hc=100 A/m) is much higher than of ferrite's.

while i salute your idea,   you may face other challenges you dont even know

In the initial tests, it became clear that the transformer acts as a current source when overloaded. In this case, the output cannot reach the sine flat top and starts to look like a triangular wave instead. Fortunately this wave doesn't damage the series capacitor supplies, as the fast edges of a square wave do.

[KerimF]

Are you attempting to drive a standard laminated iron power transformer at 15Khz instead of 50Hz or 60Hz?

I am somehow surprised that most members here didn't hear yet how pure sine wave inverters can also use a laminated iron power transformer. I heard of such inverters in many other EE forums, since more than a decade ago.

Unfortunately, I also didn't believe first that this can be done, mainly without external filtering. But since my private business has slowed down lately, I had to re-think carefully about it. And I realized that the small leakage inductance of the iron transformer is good enough to acts as LPF for the sinewave PWM high frequency (modulated to produce 50Hz sinewave if filtered) by adding a small high voltage capacitor.     

How much power (Watts @ 220Vac) will this inverter be outputting?

The planned powers are 500W and 1000W on 12Vdc, 1000W and 2000W on 24Vdc.

[coromonadalix]

well you do sound like a specialist  with your written text tone     who frankly did not need to come here i think .......

[KerimF]

well you do sound like a specialist  with your written text tone     who frankly did not need to come here i think .......

Good remark.
But I never believed that I could reach perfection in knowledge (unless it is about the spiritual realm) no matter how long I may live.
So, in the material world, I see myself always in the learning stage. After all, I am not that smart. It took me about 10 years to believe that a conventional iron power transformer can be used practically in a pure sinewave inverter (though with lower efficiency).

By the way, if I didn't come asking for possible new knowledge, I may have missed, and I simply presented what I did lately, I would look also as a bad guy This happened to me when I started a thread (in the RF forum) about my simple reliable topology to demodulate DSB-SC signal which combines the two known ones, Costas Loop and Squaring Loop. It has no I-Q signals (as in Costas Loop) and has no selective filter at 2*fc (as in the Squaring Loop). Lately I called it 'Harmonic Loop' since its PLL locks to the second harmonic of the suppressed carrier frequency. I did it as an MS thesis at the American University of Beirut (AUB) in spring 1979. But I didn't submit it (as a document) because I had to return home early for financial reasons. But I took advantage of it in my short-range RF links (between home and workplace, 3 Km) for many years in the 80’s.

[Kim Christensen]

I am somehow surprised that most members here didn't hear yet how pure sine wave inverters can also use a laminated iron power transformer. I heard of such inverters in many other EE forums, since more than a decade ago.

Because there are much better ways of doing it. (I know you can't do it the standard way due to lack of parts). You're going to have large eddy current losses in the transformer which will cause it to heat up. This will reduce efficiency and require extra cooling of the transformer and/or require it's power output to be derated. After Googling about others who have done the same as you're proposing, they run into exactly this problem. The transformer overheats and requires extra cooling.

[Zero999]

Why not put the filter before the transformer?

Only half of the power in the PWM waveform is delivered to the load. Half of it is blocked by the filter. There's no point in the transformer passing the unnecessary harmonics and generating the associated losses. It's better to filter them out first.

[Xena E]

The rate is 15,625 Hz. The number of samples per cycle (50 Hz) is 312.

Interesting.
(To be pedantic the number of samples per second is 312.5 BTW).

This post really jumps out at one who designs this sort of thing for a living.

Now if I were to use standard power line frequency iron, which even if it were a low frequency design, I wouldn't  I'd say that you could perhaps expect the most efficient frequency to transfer power at would be in the range of 500Hz to 2kHz.

Any fancy synthesis of a 50Hz wave form would need to be filtered before hitting the primary of your power transformer.

Also has anyone noted that 15,625Hz is the horizontal scan frequency of the 625 line analog television standard?

Very Faringdon-esque O/P

Still, I'll continue watching, I may learn something from the other  honorable posters.

Regards Xena.

 

[KerimF]

Why not put the filter before the transformer?

Only half of the power in the PWM waveform is delivered to the load. Half of it is blocked by the filter. There's no point in the transformer passing the unnecessary harmonics and generating the associated losses. It's better to filter them out first.

For instance, Zero999 (reply #4) reminded us of the most efficient topology to build a pure sinewave:
"The ones I've seen use an isolated DC:DC converter to a get DC of the peak voltage of the mains, followed by a MOSFET H-bridge and filter."
I guess you know that in this case the DC:DC converter and the output filter have to be made on ferrite cores which I can't get in quantities.
Should I forget producing pure sine wave inverters just because the new world allows me to use the lowest grade of laminated iron only (besides many other limitations)? On the other side, almost all engineers around the world (I hope) can get, in one way or another, whatever they may need in their designs.

I even chose the half wave drive instead of the full wave one to reduce even more the cost and labor.

Now, returning to your suggestion, how the filter could be done without using a ferrite choke? I hope you get the idea.

[KerimF]

The rate is 15,625 Hz. The number of samples per cycle (50 Hz) is 312.

Interesting.
(To be pedantic the number of samples per second is 312.5 BTW).

Your calculation is right. But, on my side, the calculation is:
64 us (15,625 Hz) * 312 = 19968 us, that is 50.08 Hz

Now if I were to use standard power line frequency iron, which even if it were a low frequency design, I wouldn't  I'd say that you could perhaps expect the most efficient frequency to transfer power at would be in the range of 500Hz to 2kHz.

The main reason of generating a pure sign wave 'at any efficiency' is that, where I live, consumers are more concerned for the price and reliability (a product that doesn't need to be repaired one a while). And as long the efficiency is not too bad, they don't complain.

Any fancy synthesis of a 50Hz wave form would need to be filtered before hitting the primary of your power transformer.

I think I have to repeat what I replied to some others.
If I can get ferrite cores suitable for my designs, I simply build my new inverters as the one which Zero999 (reply #4) was referring to.
 

Also has anyone noted that 15,625Hz is the horizontal scan frequency of the 625 line analog television standard?

It is just a mere coincidence. The MCU crystal is 8 Mhz. The timer 1 prescaler is 1 (its clock = 0.125 us). The PWM register is 9 bit (512). Therefore, the PWM period is 64us (512*0.125) and its frequency is 15,625 Hz.

[KerimF]

I am somehow surprised that most members here didn't hear yet how pure sine wave inverters can also use a laminated iron power transformer. I heard of such inverters in many other EE forums, since more than a decade ago.

Because there are much better ways of doing it. (I know you can't do it the standard way due to lack of parts). You're going to have large eddy current losses in the transformer which will cause it to heat up. This will reduce efficiency and require extra cooling of the transformer and/or require it's power output to be derated. After Googling about others who have done the same as you're proposing, they run into exactly this problem. The transformer overheats and requires extra cooling.

I agree with all you said.
About the extra dissipation of the transformer, this may be a problem if the inverter is loaded at its maximum power for a long time. Actually, it is not the case for most local consumers, if not all, who have to preserve their battery charge as long as possible.
Soon after year 2011, we lost the mains network completely. With time, mains electricity has been restored gradually. And now (after 12 years), we get the mains voltage (220V, 50Hz) 6 hours per day at best.