Split Phase Output From Full Bridge Inverter

[Wrydog]

How is a split phase output on a full-bridge inverter set-up?

I'm trying to accomplish having 240Vrms and 120Vrms outputs on an inverter

I have currently have a Psim simulation that has two capacitors connected in series after the LC filters of the upper and lower legs (see attached pic). For a load, I have two resistors. With both resistors, the simulated output is fine with Vout1 measuring 240Vrms and Vout & Vout0 both measuring 120Vrms. However, when I disable the lower resistor (simulating a resistive load plugged into the 120V receptacle), the voltage drops to nearly zero.

Thinking it might be simply a control issue, I added a DC offset to the amplitude of the modulating signal to try to boost the voltage of the upper leg across the resistor but didn't have any success. It this just the wrong circuit to do this with?


Ideally, I'd like to have 240V and 120V appliances running simultaneously. Obviously control is a major part of this but I wan to be sure I have a circuit that will support that.

Any suggestions?

Edit: Included basic overall topology.

Thank you.

[drussell]

The easiest way would probably be to do it just like the commercial inverter products do it.  Tie two inverters together with linked oscillator and controls.  Very simple with most 120 volt inverters, you just link the second one in with opposite phase so each handles any imbalanced 120 volt load and the pair produces the full 240 for any load across them.

I have experience in actual use of the Trace (now Xantrex) DR series at a friend's cabin, very, very simple and straightforward....  That kind of set-up should be a breeze to implement, even DIY from scratch, it just makes sense.

The operating manuals etc. for those commercial inverters are readily available and I think I even pulled service data and schematics at one point.

[jbb]

Are you building an inverter from scratch here, or looking to make some existing solution work?

If you're extending an existing solution, a simple 60Hz transformer (while not especially efficient) could be very effective.

If building from scratch, there are a few options, each with pros and cons.  You'll almost certainly need some more semiconductors to get good performance.  Also, I suggest you show an outline of the whole inverter concept including the DCDC stage and H bridge.

[David Hess]

I have no idea how real inverters do it.  Two floating outputs that are in phase could be stacked but I assume real split phase inverters do something more sophisticated.

My solution was to build a phase splitter from some big surplus 60 Hz power transformers that I had.  See the photograph below.

If you're extending an existing solution, a simple 60Hz transformer (while not especially efficient) could be very effective.

The efficiency of a transformer based phase splitter is just fine and better than an inverter.  It is just heavy.

[Zero999]

To get 240V you're using a full bridge with an output filter either side?

Can't you just make a neutral in the middle by adding some large capacitors to either side of the DC bus? Obviously the capacitors need to have a low enough impedance and high enough current rating to pass the desired current, without changing the voltage much. That way, you'll be effectively using a half bridge if you just connect a large load between one of the phases and neutral.

[retrolefty]

Why not just use a centered tapped output transformer, that's how my home is supplied from a street transformer. Center tap becomes the neutral, secondary ends become L1 and L2, ground the neutral at it's service panel?

 

[Zero999]

Why not just use a centered tapped output transformer, that's how my home is supplied from a street transformer. Center tap becomes the neutral, secondary ends become L1 and L2, ground the neutral at it's service panel?

Because there's no mains frequency transformer?

Battery -> DC:DC converter, giving 340VDC -> H-bridge, giving PWM, -> Low pass filter, giving 240VAC pure sine wave.

[T3sl4co1l]

If you have bipolar supplies, then an H bridge gives "+/-" 120V with 240V between the two.

Tim

[David Hess]

Why not just use a centered tapped output transformer, that's how my home is supplied from a street transformer. Center tap becomes the neutral, secondary ends become L1 and L2, ground the neutral at it's service panel?

Inverters with low frequency output transformers do this and easily support split phase operation.  Or you can add an external low frequency transformer to any inverter to do the same thing which is what my photograph shows.

I thought Wrydog was asking about how to do it in a design which lacks low frequency transformers.

[Wrydog]

From scratch.

[Wrydog]

This is for a battery supplied inverter. Has to be comparatively lightweight.

[Wrydog]

Yes. My problem stems from the "what if only one 120V load is plugged in as a load?" My simulation shows nice waveforms with two balanced resistors, but that stops when only one is active.

[Ian.M]

You'd have to actively maintain the neutral point at the mean voltage of the two phases.  You cannot assume that the neutral voltage is half the DC bus voltage unless your inverter drive is fully symmetrical.. Use a potential divider across the two Line outputs to get the neutral reference voltage, and add another leg to the 'H' bridge PWMed in a servo loop to maintain the load neutral at the reference neutral voltage.

[Benta]

If I understand your setup correctly, you have a full-bridge PWM-driving a high-frequency tranformer to provide a sinusoidal output at 60 Hz.
Is that right?
If so, having two independent secondaries on said transformer, each delivering 120 Vrms solves the problem. After filtering, they can be series connected to 240 Vrms centre-tapped.

[Wrydog]

Thanks! This seems to be what I need. Can you explain further or point me to some more references?

[Zero999]

Yes. My problem stems from the "what if only one 120V load is plugged in as a load?" My simulation shows nice waveforms with two balanced resistors, but that stops when only one is active.

That's what you should expect to happen with your current schematic. The impedance of the neutral point is equal to that of C33 and C34 in parallel. C33 & C34 in parallel = 4.4µF, so Z = 1/(2pi×FC) = 1/(2pi×60×4.4×10^-6) 603R.

If I understand your setup correctly, you have a full-bridge PWM-driving a high-frequency tranformer to provide a sinusoidal output at 60 Hz.
Is that right?
If so, having two independent secondaries on said transformer, each delivering 120 Vrms solves the problem. After filtering, they can be series connected to 240 Vrms centre-tapped.

Thanks! This seems to be what I need. Can you explain further or point me to some more references?

All right, there's nothing wrong with that set up. It will work but then you say:

This is for a battery supplied inverter. Has to be comparatively lightweight.

How lightweight does it need to be?

What power rating are you after?

I hope you know that the transformer will need to be capable of passing 60Hz, which means it needs to be an ordinary mains transformer and those aren't lightweight.

The only way to make it more lightweight is to add a DC-DC converter, to boost the voltage to the peak mains voltage, before the h-bridge.

[Benta]

The best I could find at short notice is this:
https://www.onsemi.com/pub/Collateral/SMPSRM-D.PDF
page 12, Fig. 12.
Ignore the bridge rectifier etc., but imagine that the secondary is only filtered. Add an additional secondary with filtering, and you have your two 120 V independent supplies.

I hope you know that the transformer will need to be capable of passing 60Hz, which means it needs to be an ordinary mains transformer and those aren't lightweight.

 

Please explain. A full-bridge with a "12 V primary" and a "120 V secondary" is no problem (quotation sign, because with a switching supply the voltages are a bit different)

The only way to make it more lightweight is to add a DC-DC converter, to boost the voltage to the peak mains voltage, before the h-bridge.

 

Again, why? Running an H-bridge directly from a battery to generate a higher voltage is no magic.

You're confusing me.

[Zero999]

The best I could find at short notice is this:
https://www.onsemi.com/pub/Collateral/SMPSRM-D.PDF
page 12, Fig. 12.
Ignore the bridge rectifier etc., but imagine that the secondary is only filtered. Add an additional secondary with filtering, and you have your two 120 V independent supplies.

Good. That looks like an excellent suggestion for the DC:DC converter part of the inverter, to generate the high voltage DC bus.

I hope you know that the transformer will need to be capable of passing 60Hz, which means it needs to be an ordinary mains transformer and those aren't lightweight.

 

Please explain. A full-bridge with a "12 V primary" and a "120 V secondary" is no problem (quotation sign, because with a switching supply the voltages are a bit different)

That's no problem at all.

The only way to make it more lightweight is to add a DC-DC converter, to boost the voltage to the peak mains voltage, before the h-bridge.

 

Again, why? Running an H-bridge directly from a battery to generate a higher voltage is no magic.

You're confusing me.

I was under the impression that the idea was to use an h-bridge to convert the DC voltage from that battery to PWM AC and step it up to the mains voltage with a transformer. That will work the transformer still needs to be able to pass 60Hz.



Perhaps I misunderstood what you meant? No schematic has been posted of the actual inverter.

[Wrydog]

Yes. My problem stems from the "what if only one 120V load is plugged in as a load?" My simulation shows nice waveforms with two balanced resistors, but that stops when only one is active.

That's what you should expect to happen with your current schematic. The impedance of the neutral point is equal to that of C33 and C34 in parallel. C33 & C34 in parallel = 4.4µF, so Z = 1/(2pi×FC) = 1/(2pi×60×4.4×10^-6) 603R.

If I understand your setup correctly, you have a full-bridge PWM-driving a high-frequency tranformer to provide a sinusoidal output at 60 Hz.
Is that right?
If so, having two independent secondaries on said transformer, each delivering 120 Vrms solves the problem. After filtering, they can be series connected to 240 Vrms centre-tapped.

Thanks! This seems to be what I need. Can you explain further or point me to some more references?

All right, there's nothing wrong with that set up. It will work but then you say:

This is for a battery supplied inverter. Has to be comparatively lightweight.

How lightweight does it need to be?

What power rating are you after?

I hope you know that the transformer will need to be capable of passing 60Hz, which means it needs to be an ordinary mains transformer and those aren't lightweight.

The only way to make it more lightweight is to add a DC-DC converter, to boost the voltage to the peak mains voltage, before the h-bridge.

Sorry, I had meant to reply to IanM saying that was what I was looking for. There is a high frequency full-bridge DC-DC stage on the front end of the inverter to boost the inverter's input voltage in order to negate the need for a  60Hz transformer. I was looking for more references to control strategies for the neutral point.

[Benta]

Hero999, first a big Thank You for providing much better graphics than what I found.

I was under the impression that the idea was to use an h-bridge to convert the DC voltage from that battery to PWM AC and step it up to the mains voltage with a transformer. That will work the transformer still needs to be able to pass 60Hz.

Yes, but why include the extra 60 Hz transformer? Just step it up directly to 2 x 120 VAC with two separate secondaries and filters. Then the outputs can be used freely.

[Benta]

Sorry, I had meant to reply to IanM saying that was what I was looking for. There is a high frequency DC-DC stage on the front end of the inverter in order to negate the need for a  60Hz transformer. I was looking for more references to control strategies for the neutral point.

This is still not clear, but you get back what you offer: a fractional schematic, unclear questions and, quite frankly, no real idea what you want.
The quality of the answers correspond to the quality of the input.

[Wrydog]

Yes. My problem stems from the "what if only one 120V load is plugged in as a load?" My simulation shows nice waveforms with two balanced resistors, but that stops when only one is active.

That's what you should expect to happen with your current schematic. The impedance of the neutral point is equal to that of C33 and C34 in parallel. C33 & C34 in parallel = 4.4µF, so Z = 1/(2pi×FC) = 1/(2pi×60×4.4×10^-6) 603R.

If I understand your setup correctly, you have a full-bridge PWM-driving a high-frequency tranformer to provide a sinusoidal output at 60 Hz.
Is that right?
If so, having two independent secondaries on said transformer, each delivering 120 Vrms solves the problem. After filtering, they can be series connected to 240 Vrms centre-tapped.

Thanks! This seems to be what I need. Can you explain further or point me to some more references?

All right, there's nothing wrong with that set up. It will work but then you say:

This is for a battery supplied inverter. Has to be comparatively lightweight.

How lightweight does it need to be?

What power rating are you after?

I hope you know that the transformer will need to be capable of passing 60Hz, which means it needs to be an ordinary mains transformer and those aren't lightweight.

The only way to make it more lightweight is to add a DC-DC converter, to boost the voltage to the peak mains voltage, before the h-bridge.

Thank you, the low combined impedance was a big factor in the simulated voltage drop. I think I'll have to redesign with higher value caps.

[Wrydog]

You'd have to actively maintain the neutral point at the mean voltage of the two phases.  You cannot assume that the neutral voltage is half the DC bus voltage unless your inverter drive is fully symmetrical.. Use a potential divider across the two Line outputs to get the neutral reference voltage, and add another leg to the 'H' bridge PWMed in a servo loop to maintain the load neutral at the reference neutral voltage.

Thanks Ian, this is helpful.

Can you explain further about adding a third leg or point me to other resources/search terms?

I've added a diagram of the basic overall scheme to the original post.

[Ian.M]

Another leg: another pair of upper and lower MOSFETs like a three phase H bridge, only instead of driving the phases to approximate sine waves  at 120 deg to each other after filtering, you drive two phases as a conventional H bridge output inverter and drive the third phase to maintain the neutral.

[T3sl4co1l]

Some variations on the output network.

Note that the supplies are already shown as bipolar, so I added a ground reference, implying the supplies are not just plus and minus of a single supply, but a dual, symmetrical supply.


Original


Two notes: 1. The transformer must carry the full low frequency content regardless.  2. LF transformers have high core losses at high frequencies.  Might as well move the filter in front.


The supplies and phases are complementary and symmetrical, so we can divide the LPF in half against ground (or, since the ground terminal is just capacitors, it doesn't care what DC is -- either supply would work as well, if you prefer).  This is identical if the phases are perfectly symmetric, so seems like a lot of component duplication, however in practice they will never perfectly match, and this provides equal common mode and differential mode filtering.  Huge EMI win!


Continuing the symmetry theme, the primary can be CT-grounded without problems*.

*Given that the flux walking constraint now applies to both outputs individually, not just their difference.  That is: DC output current must be very close to zero, for both outputs.

Also, as long as we're putting taps on things, the secondary can be tapped anywhere you like (not just half), getting the split phase that was asked about, or any other voltages for that matter.


Since it's symmetrical, we can even eliminate the transformer altogether -- assuming isolation is not required.

Now it is especially important for the LPFs to be independent, rather than balanced, because there's no potential filtering benefit from the transformer.

Tim

[jbb]

So, now that we've seen a concept schematic we can start to say really confusing things 

The advantage of the 2 stage approach is that the DC DC transformer only handles high frequency and can be quite small & light. We can probably do better than the proposed PWM full bridge converter by moving to a phase shifted full bridge if size is critical ('filter' inductance is integrated into the transformer).

If we modify the DCDC stage to output +-170V, the ground point can be used to split the two output phases.
This corresponds to the last suggestion from T3sl4co1l. The downside is that each output phase will need some separate control elements (e.g. current sensors and current control loops), but that's hard to avoid.

[NiHaoMike]

How many kW are we dealing with? And what kind of loads?

I think it would be better to split the electronic loads from the motor and resistive loads. In particular, you can really reduce the power usage of motor loads with V/Hz scaling.

[jbb]

How many kW are we dealing with? And what kind of loads?

I think it would be better to split the electronic loads from the motor and resistive loads. In particular, you can really reduce the power usage of motor loads with V/Hz scaling.

This is an interesting point. It can be decoded to 'do you have motor loads that could benefit from running on a Variable Frequency Drive (VFD) aka Variable Speed Drive (VSD)?' It's often possible to supply 340V DC power straight into a VFD. This means that the DC to AC output stages can be smaller.

[Zero999]

Hero999, first a big Thank You for providing much better graphics than what I found.

I was under the impression that the idea was to use an h-bridge to convert the DC voltage from that battery to PWM AC and step it up to the mains voltage with a transformer. That will work the transformer still needs to be able to pass 60Hz.

Yes, but why include the extra 60 Hz transformer? Just step it up directly to 2 x 120 VAC with two separate secondaries and filters. Then the outputs can be used freely.

I agree. I didn't mean there needed to be two transformers. You don't even need two secondary windings with separate filters. A single centre tapped secondary winding will do.

My point was, that I thought the original poster needed a light and compact design, but a mains frequency transformer is big and heavy.

Some variations on the output network.

Note that the supplies are already shown as bipolar, so I added a ground reference, implying the supplies are not just plus and minus of a single supply, but a dual, symmetrical supply.

I don't think that was intentional. I can't remember where I got the original schematic from. I think it was for a modified sine inverter. I modified it and posted it on another forum about 10 years ago. If I remember rightly, it was meant for the h-bridge to be run off a 12V battery, hence the transformer. It should have been +12V and 0V, rather than +Vdd and -Vss.

Original


Two notes: 1. The transformer must carry the full low frequency content regardless.  2. LF transformers have high core losses at high frequencies.  Might as well move the filter in front.


The supplies and phases are complementary and symmetrical, so we can divide the LPF in half against ground (or, since the ground terminal is just capacitors, it doesn't care what DC is -- either supply would work as well, if you prefer).  This is identical if the phases are perfectly symmetric, so seems like a lot of component duplication, however in practice they will never perfectly match, and this provides equal common mode and differential mode filtering.  Huge EMI win!


Continuing the symmetry theme, the primary can be CT-grounded without problems*.

*Given that the flux walking constraint now applies to both outputs individually, not just their difference.  That is: DC output current must be very close to zero, for both outputs.

Also, as long as we're putting taps on things, the secondary can be tapped anywhere you like (not just half), getting the split phase that was asked about, or any other voltages for that matter.

Those are very good ideas. Thanks for posting them.

Since it's symmetrical, we can even eliminate the transformer altogether -- assuming isolation is not required.

Now it is especially important for the LPFs to be independent, rather than balanced, because there's no potential filtering benefit from the transformer.

Tim

It also assumes that +Vdd and -Vss are equal to the mains peak voltage, i.e. they're generated by DC:DC converter, otherwise the big and bulky transformer is still needed for voltage conversion.

[Zero999]

Here's what I was thinking of originally. C1 & C2 need to be low impedance enough to ensure the neutral point doesn't shift too much and be capable of carrying the required current. They could be rated to a lower voltage than 340V (I'd go for 250V), as they only see half the DC bus voltage, but balancing resistors would be required.



Note that the DC converter is isolated. The gate drivers would also need to be isolated if the PWM circuit is powered off 12V.

[jbb]

EDIT: removed incorrect screenshot.

That's the right sort of idea, but some refinements are needed to get it to work.

First, we need to make the split DC supply behave nicely when subject to unequal loading.  For this (and because Wrydog mentioned size is a concern) I would suggest something like a phase shifted full bridge converter. (First Last image).

The two legs of the H bridge (M1+M2 and M3+M4) switch at constant frequency and 50% duty cycle.  Control is achieved by varying the phase lag between the two legs of the bridge. This approach gives soft (or at least less hard) switching, it can operate at fairly high frequencies (> 50kHz) with acceptable efficiency.  A split winding output transformer generates a split DC output rail.  Large capacitors C1 and C2 are required to buffer energy over the 60Hz line cycle.

Secondly, we need an output stage.  In general, there will be two phases, each with their own power switches (MOSFET / IGBT), LC output filter and control system (second first image).  Current sensing for each phase will be needed somewhere - on the inductor output is traditional but not the only option.

The exact nature of the output stage is open to debate.  The simplest is a pair of half-bridges (second, third, fourth images) using MOSFETs or IGBTs.  MOSFETs will have lower conduction losses, but their internal body diodes can have large reverse recovery losses and therefore lead to high switching losses.  IGBTs have higher conduction losses, but much lower reverse recovery losses (because the diode is a separate piece of silicon, integrated into the same package).  These power stage both supply +180V or -180V to the output filters (i.e. 2 output voltage levels).

If size is critical, tricks like moving to a three level inverter are possible. (Final Fourth image).  This requires some more switches, but allows for the power stage to output +180V, 0V and -180V.  Because the applied voltages can be controlled to be closer to what you actually want, the output filter inductor can be approximately halved.  Additionally, because the switches are only switching 180V at a go (instead of 360V), it may be possible to increase the switching frequency (and shrink the inductor) without decreasing the efficiency.

[Zero999]

That's the right sort of idea, but some refinements are needed to get it to work.

First, we need to make the split DC supply behave nicely when subject to unequal loading.  For this (and because Wrydog mentioned size is a concern) I would suggest something like a phase shifted full bridge converter. (First image).

I don't understand the variac and isolating part. I presume the low frequency transformers are for test purposes and will be replaced with a switched mode DC:DC converter later?

I don't see why a split DC supply is essential. A half bridge converter will work, if AC coupled to the neutral, as I showed above. Unequal loading won't be a problem, as long as the voltage drop in the filter and switches is minimal.

The only problem I can see with my idea, is that the load needs to draw the same charge on positive and negative cycles, otherwise one of the capacitors will charge up more, causing imbalance. If it's likely such a load will be connected, i.e. load powered by a half wave rectifier, then a split DC power supply is the only way to go.

Note that the above can also be a problem with a transformer: if net flux becomes DC, then the core can saturate, resulting in excessive current draw and overheating.

[jbb]

I don't understand the variac and isolating part. I presume the low frequency transformers are for test purposes and will be replaced with a switched mode DC:DC converter.

Oh dear, that would be the wrong image... I'll replace it later.
Sorry

[T3sl4co1l]

IGBTs are chosen when they have lower conduction losses.  The switching losses are higher, so the switching frequencies go down, too.

For a 340V supply, IGBTs are meh.  For industrial (400VAC+) equipment, they're pretty good, and in large modules (100A+) they're quite good (as you can't really get MOSFETs that large, and they'd be too fast to switch that much current safely anyway, so why not get something cheaper).

In the <400V range (like here), it's better to throw enough MOSFETs at the problem to reduce conduction losses below IGBT levels.  You can even reduce it below diode levels: this prevents body diode forward-bias and subsequent recovery losses.

If you're in a situation where cost is more critical than efficiency and size, you might go more towards IGBTs.

At lower voltages (Vds < 300V), it's no contest.  They don't even make sub-300V IGBTs, no point.   (The ones they do make -- 300 to 400V, 300A+ pulsed, are used for driving plasma displays; or, they were, as PDPs pass out of vogue, now.)

Body diode recovery is kind of hard to avoid, and it is a big problem in a converter that has to handle four quadrant operation (that is, positive or negative, current or voltage, at the same time).  Correct snubber design helps a lot to mitigate recovery.

Tim

[jbb]

What kind of snubbers were you thinking?

A typical commercial product would use hard switching with as little snubbing as possible, but there are a lot of options out there.

[T3sl4co1l]

dI/dt snubbing is for diode recovery.

Tim

[Zero999]

I've just done a calculation on the vales of C1 & C2 in the schematic I posted previously and they'd need to be quite big: 1500µF for a neutral impedance of <1R @ 60Hz and at 250V is quite large. It would be smaller than a transformer, but I think a bipolar DC supply would be more compact, than massive capacitors, even if it does make it more complex.

[jbb]

C1 and C2 will be quite big anyway to handle 60Hz power swings - in a quality scheme, their voltages are allowed to wobble around a bit (i.e. store & release energy) which prevents the 120 Hz power ripple from getting back to the battery bank.

Of course, the size of the cap banks depends on the power rating, so once again:

How many kW are we dealing with? And what kind of loads?