20 Vdc to 230 Vac Inverter Design Approach

[skyflyer54]

Hi,

I'm currently tasked to build a 20Vdc to 230Vac(rms) inverter but I'm not sure which approach will be better to build. Can someone please comment based on your valuable experience and expertise. The following are the two approaches that I have in mind.

Approach 1:
First, design a DC DC boost converter to boost the 20Vdc to 325Vdc (Peak) than use this 325Vdc to inverter to 325Vac(Peak) where I can get 230Vac(rms)

The DC AC Inverter i'm having in mind is using a full bridge SPWM topology where 4 IGBT switches will be used.

Approach 2:
First, I will design the inverter so that it can convert 20Vdc to 20Vac(Peak) by using a full bridge SPWM topology where 4 IGBT switches will be used.
Than use a step up transfomer to step up the 20Vac(Peak) to 325Vac(Peak) so that I can obtain 230Vac(RMS).

 

[skyflyer54]

Another thing that come across my mind will be the short circuit protection.

As we know that the top and bottom IGBT switches cannot turn on together at any instance, else, short circuit occurs. A deadtime circuit will help to minimize the problem.

But is there any way to protect the whole circuit when this type of short circuit occurs and which approach will be good for designing a deadtime circuit.

Million Thanks in advance to those reply.

 

[audioguru]

Where is 20VDC at many Amps going to come from? Car batteries are not 20VDC.

A cheap Chinese Pure-Sine-Wave inverter uses a TL494 PWM IC driving Mosfets at a high frequency into a small ferrite stepup transformer to make a 327VAC square-wave that is rectified and filtered into 325VDC.
Then it uses another TL494 to make a sine-wave at 50Hz or 60Hz with PWM that has many steps. The PWM drives a high voltage push-pull pair made with Mosfets and their stepped output is smoothed with a little high frequency filter.

 

[Ubergeek63]

NEVER use IGBTs with low supply voltages! They waste power.

You neglect to mention how much power you need. I can give you 230VAC with an LM324 opamp and an audio transformer, but I know that is not what you want.

 

[skyflyer54]

Where is 20VDC at many Amps going to come from? Car batteries are not 20VDC.

A cheap Chinese Pure-Sine-Wave inverter uses a TL494 PWM IC driving Mosfets at a high frequency into a small ferrite stepup transformer to make a 327VAC square-wave that is rectified and filtered into 325VDC.
Then it uses another TL494 to make a sine-wave at 50Hz or 60Hz with PWM that has many steps. The PWM drives a high voltage push-pull pair made with Mosfets and their stepped output is smoothed with a little high frequency filter.

Hi,
Sorry to mention my source... I'm taking my 20Vdc from solar panel with about 20Amps.

If i will to use TL494, do I still need a driver to drive the IGBT?

Lastly, from your reply, you will recommend boost up the DC Voltage first than Invert to AC right?

Thanks for your reply.

 

[skyflyer54]

NEVER use IGBTs with low supply voltages! They waste power.

You neglect to mention how much power you need. I can give you 230VAC with an LM324 opamp and an audio transformer, but I know that is not what you want.

Oh... this mean u will recommend me to boost the low supply voltages to high voltage than invert DC to Ac right.?

Thanks for your reply.

 

[Ubergeek63]

Actually the points was at low voltages the 1-2V drop per IGBT takes 10-20% of your power before you even start.

Low voltage FETs are inexpensive to get a 0.6V drop at 20A when an IGBT will drop 1-2V. Would you rather install a heat sink for 12W or 80W?

 

[Hero999]

Where is 20VDC at many Amps going to come from? Car batteries are not 20VDC.

Stuff that's designed to work from a 24V lead acid battery also needs to run at 20V to compensate for the batteries running down.

A cheap Chinese Pure-Sine-Wave inverter uses a TL494 PWM IC driving Mosfets at a high frequency into a small ferrite stepup transformer to make a 327VAC square-wave that is rectified and filtered into 325VDC.
Then it uses another TL494 to make a sine-wave at 50Hz or 60Hz with PWM that has many steps. The PWM drives a high voltage push-pull pair made with Mosfets and their stepped output is smoothed with a little high frequency filter.

That's by far the best method, except I'd go for a slightly higher DC bus than the mains peak like 350VDC.

The DC-DC converter also needs to be isolated for safety reasons so the feedback needs to be done through an opto-isolator.

 

[skyflyer54]

Stuff that's designed to work from a 24V lead acid battery also needs to run at 20V to compensate for the batteries running down.

That's by far the best method, except I'd go for a slightly higher DC bus than the mains peak like 350VDC.

The DC-DC converter also needs to be isolated for safety reasons so the feedback needs to be done through an opto-isolator.

Oh.. What you mean by "need to be isolated for safety reasons"
Can you explain and how i go about doing the feedback? Sorry for asking so much as i'm very new to converter and inverter.

 

[skyflyer54]

Actually the points was at low voltages the 1-2V drop per IGBT takes 10-20% of your power before you even start.

Low voltage FETs are inexpensive to get a 0.6V drop at 20A when an IGBT will drop 1-2V. Would you rather install a heat sink for 12W or 80W?

... Thanks for your advice.. Probably I will change my design to MosFet instead of IGBT.

By the way, do you have a good MosFet to recommend? Sorry for asking so much as I'm still very new to designing converter and inverter.

 

[Ubergeek63]

By the way, do you have a good MosFet to recommend? Sorry for asking so much as I'm still very new to designing converter and inverter.

Depends on what current you need:
The SPW52N50C3 looks reasonable at $12 from mouser. once you have HV you can use the IGBTs if you are going that route.

if you have a 20V 500VA transformer you can do the bridge for much less at LV. I am not sure why hero is saying you need isolation, since you are running off a solar panel. A 55V 80A STP80NF55-06 fro $1.50 from mouser would do for each of the bridge switches if you are driving a transformer.

 

[skyflyer54]

Depends on what current you need:
The SPW52N50C3 looks reasonable at $12 from mouser. once you have HV you can use the IGBTs if you are going that route.

if you have a 20V 500VA transformer you can do the bridge for much less at LV. I am not sure why hero is saying you need isolation, since you are running off a solar panel. A 55V 80A STP80NF55-06 fro $1.50 from mouser would do for each of the bridge switches if you are driving a transformer.

Oh.. Ok.. Thanks for your advise. Previous I had design the circuit using STGW19NC60WD. Wonder is this ok or not.

 

[Hero999]

Oh.. What you mean by "need to be isolated for safety reasons"
Can you explain and how i go about doing the feedback? Sorry for asking so much as i'm very new to converter and inverter.

If you choose to use a high voltage DC buss then it needs to be isolated because the output from an h-bridge isn't isolated from the input. Both the lines on the mains output will have 20VDC superimposted alternately on each other. This means that if an earth fault occurs on either of the mains lines, the 20VDC system will float at 230V and the insulation on the solar pannels isn't designed for this.

If you go for the transformer option, then unless you want to use a plain squarewave then use a 12V transformer, not a 20V transformer. If you use a 20V transformer, then the peak output voltage when fully loaded will only be about 200V so the RMS voltage will be about 140V which is totally useless for powering most loads.

The secondary coil on a 12V transformer will be wound so it gives about 14VAC, this means that it'll be fine connected to a 20V modified sinewave source which will have an RMS voltage of 14V.

 

[mneary]

STGW19NC60WD is an igbt. It's wasteful at 20V (the 2V or more drop x 2) but OK at line voltage (40A rating to handle 2A).

Since you're only handling 400VA, a 50 Hz transformer is sort of practical (if a little heavy), and it only needs one stage (20V PWMed into a 14:230 winding) as suggested by Hero999.

But even as low as 40VA, I have cheap (US$20) inverters that take two steps; create 180V first (500 kHz switching supply) then switch it into 120VAC rms "modified sine wave". (I prefer to call it modified square wave ).

 

[skyflyer54]

If you choose to use a high voltage DC buss then it needs to be isolated because the output from an h-bridge isn't isolated from the input. Both the lines on the mains output will have 20VDC superimposted alternately on each other. This means that if an earth fault occurs on either of the mains lines, the 20VDC system will float at 230V and the insulation on the solar pannels isn't designed for this.

If you go for the transformer option, then unless you want to use a plain squarewave then use a 12V transformer, not a 20V transformer. If you use a 20V transformer, then the peak output voltage when fully loaded will only be about 200V so the RMS voltage will be about 140V which is totally useless for powering most loads.

The secondary coil on a 12V transformer will be wound so it gives about 14VAC, this means that it'll be fine connected to a 20V modified sinewave source which will have an RMS voltage of 14V.

Oh.. It getting confusing to me now. So actually with your recommendation, which approach will be more recommended and how should I come abt designing an isolation circuit for fault occurrence?

 

[skyflyer54]

STGW19NC60WD is an igbt. It's wasteful at 20V (the 2V or more drop x 2) but OK at line voltage (40A rating to handle 2A).

Since you're only handling 400VA, a 50 Hz transformer is sort of practical (if a little heavy), and it only needs one stage (20V PWMed into a 14:230 winding) as suggested by Hero999.

But even as low as 40VA, I have cheap (US$20) inverters that take two steps; create 180V first (500 kHz switching supply) then switch it into 120VAC rms "modified sine wave". (I prefer to call it modified square wave ).

Oh... US$20 Inverters? Can I have the model number so that I can take a look at the specification? I seem to be good as it do two things for me. Haha..

 

[Hero999]

Oh.. It getting confusing to me now. So actually with your recommendation, which approach will be more recommended and how should I come abt designing an isolation circuit for fault occurrence?

First you have to establish your requirements.


The power output.

Whether you need a squarewave, modified sinewave or a pure sinewave.

Does it need to be lightweight?

If you need a simple solution then a transformer is the easiest but it's heavy and the voltage regulation will be poor.

As far as the isolation thing is concerned.
Imagine a 20V to 350V DC-DC converter which has the 0V input connected to the 0V output connected to an h-bridge. The mains side and extra low voltage DC side will be electrically connected to each other. This is a very bad idea, to avoid this you use some sort of isolation transformer to separate the two circuits.

 

[skyflyer54]

First you have to establish your requirements.


The power output.

Whether you need a squarewave, modified sinewave or a pure sinewave.

Does it need to be lightweight?

If you need a simple solution then a transformer is the easiest but it's heavy and the voltage regulation will be poor.

As far as the isolation thing is concerned.
Imagine a 20V to 350V DC-DC converter which has the 0V input connected to the 0V output connected to an h-bridge. The mains side and extra low voltage DC side will be electrically connected to each other. This is a very bad idea, to avoid this you use some sort of isolation transformer to separate the two circuits.

Oh.. I will need a pure sinewave for my design. If I need lightweight than I can't use isolation transformer right as from what i know is that isolation transformer is very heavy. Am I right?

Can an opto_isolator be used to do the isolation between the two circuit?

By the way.. I have already done my DC DC Boost Converter (20VDC to 325VDC with 94% duty cycle) and DC AC Inverter but the problem i'm facing now is the isolation part and also the short circuit protection.

Another problem i'm facing is the deadtime problem for my DC AC Inverter. when i'm input the supply voltage for the inverter to below 20Vdc, it works well. But when i increase the supply voltage, its keep giving me short circuit problem. Luckily my supply current is not very high to the extend of burning the MOS switches. Is there a good way to implement deadtime delay for rising edge.

Another possibility i'm suspecting is that it might be noise that create the short circuit.

Any recommendation or suggestion?

 

[audioguru]

A TL494 and many other PWM controller ICs have a variable dead time adjustment. A second one can make the PWM stepped sine-wave.

 

[skyflyer54]

A TL494 and many other PWM controller ICs have a variable dead time adjustment. A second one can make the PWM stepped sine-wave.

Hi,

As of what u have recommended. TL494 is providing PWM with fixed duty cycle right? I'm using SPWM which is not the normal PWM where duty cycle are the same.