Connecting DC Boost Converters in Series

[tim9000]

Hi,
I have a graduate level understanding of power electronics. The other day I decided I wanted to series the output of two DC Boost converters. I remembered the simple conceptual circuit of how they worked, and I jumped onto google to see if this could be done safely.

What I read was that it could if the boost converters were the isolated kind, from which I infer means uses a high frequency transformer?

It is at this point that I should specify, that I am intending on using two 12V batteries, to supply the boost converters, to generate two 60V outputs, which I seek to put in series (creating 120V). Now, I haven't taken delivery of the cheap non-isolated chinese boost converters (from ebay) yet, so I can't google the driving chip datasheet or examine the circuitry. But I want to know, from a Boost converter topology point of view, if you were supplying each boost converter from it's own separate battery: why would this be a bad idea to series the two 60V outputs into one 120VDC supply to a load?

Thanks.

 

[.Scott]

There is definitely some caution required.
I assume the 12Vdc batteries are similar to automobile batteries.

What are you going to choose for Earth ground? Or do you plan on floating everything - including the load.
Whatever you choose, there will potential differences among different components that exceed 60Vdc.
This could create an unexpected surprise for anyone else working with the setup.

I would be careful to keep all parts of the circuit - including the battery terminals well protected from accidental contact with people or tools.
Their potential relative to other parts of the circuit or surrounding conductors could be well more than the expected 12Vdc.

 

[Rive]

from a Boost converter topology point of view

From that point of view it is OK to connect them in series if they are on a separate battery. The mentioned requirement that they should be the isolated type is not about frequency: it means that the input and the output side is not connected, so the ground on the input side is not the same as the ground on the output side.
Most common boost converters are not this type, but if they are running from isolated sources, then that is still OK..

120V DC can be quite dangerous. I hope you are aware of the safety requirements and you are adequately scared.

 

[anorlunda]

We are treading on thin ice here, vis a vis the PF policy on dangerous topics. Use great discretion.

The first question I have is why? What are you trying to accomplish?

 

[gneill]

Moderator Note: Changed the thread title to reflect the thread topic.

 

[essenmein]

Hi,
I have a graduate level understanding of power electronics. The other day I decided I wanted to series the output of two DC Boost converters. I remembered the simple conceptual circuit of how they worked, and I jumped onto google to see if this could be done safely.

What I read was that it could if the boost converters were the isolated kind, from which I infer means uses a high frequency transformer?

It is at this point that I should specify, that I am intending on using two 12V batteries, to supply the boost converters, to generate two 60V outputs, which I seek to put in series (creating 120V). Now, I haven't taken delivery of the cheap non-isolated chinese boost converters (from ebay) yet, so I can't google the driving chip datasheet or examine the circuitry. But I want to know, from a Boost converter topology point of view, if you were supplying each boost converter from it's own separate battery: why would this be a bad idea to series the two 60V outputs into one 120VDC supply to a load?

Thanks.

In principle its fine, where you will run into problems is if there are any signals to each, for example an enable line, that is referenced to the converter local ground, for the one on top, that is its ground is now at +60V compared to the lower converter, you cannot connect any signal connections or grounds together.

Then I assume you know that 120V DC comes with significant danger, its not like AC where at least you could let go every 8ms or so at a zero crossing...

 

[berkeman]

The other day I decided I wanted to series the output of two DC Boost converters.

What does that mean? Could you post a block diagram of how you would try to make that connection?

A few other thoughts:

• I could see how to connect two converters in parallel (a boost stage on top and an inverting boost stage on the bottom, with the two common rails tied together in the middle. That would generate +/-60Vdc, which would give you 120Vdc overall between the outside rails.
• As pointed out already, 60V is SELV, but above that is not considered SELV (since you have the power electronics background, I'll assume that you are familiar with SELV). What you do with the output to try to keep users safe changes significantly when you are outputting voltages above SELV.
• If there were a way to series-connect two boost stages, you would need to do some analysis and testing to be sure that the stages were stable. The problem is that the first stage is not going to like the chopped current draw of the second stage, and the second stage is not going to like the ripple at its input side from the first DC-DC converter. I've had to do some pretty detailed analysis of input voltage noise and output resonant current draw on some of my DC-DC circuits, and it can become *very* difficult to keep a switcher stable when it's trying to deal with other switching (either in its input power or its output load) anywhere near its own switching frequency or harmonics.

 

[CWatters]

Tim9000... Why do you want to do this?

 

[tim9000]

Hi everyone,
Thanks for the replies, I'll re-read them all in an hour or so, I'm just getting ready for work, so this is just a quick reply to clarify a couple things.

Here is the block diagram:

As for the safety specifics: I am using an electrical box (IP55) all with nylon bolts, all the banana plugs are the safety kind, so there is no exposed conductor on either the male or female plug, and I think everything is CAT IV. When I'm finished I'll make sure everything has heatshrink etc. so it's hopefully impossible to come in contact with either the LV: 180V AC/DC.

From that point of view it is OK to connect them in series if they are on a separate battery. The mentioned requirement that they should be the isolated type is not about frequency: it means that the input and the output side is not connected, so the ground on the input side is not the same as the ground on the output side.
Most common boost converters are not this type, but if they are running from isolated sources, then that is still OK..

120V DC can be quite dangerous. I hope you are aware of the safety requirements and you are adequately scared.

I had a feeling the isolation was more to do with the supply side being from the same source, so I was hoping it would be different for batteries. (Yes, I should have put emphasis on the "transformer" word in the sentence. Its just that I am assuming it is done via transformer, because I've never seen a circuit of an isolated boost converter (that wasn't a fly-back for e.g.).

If there were a way to series-connect two boost stages, you would need to do some analysis and testing to be sure that the stages were stable. The problem is that the first stage is not going to like the chopped current draw of the second stage, and the second stage is not going to like the ripple at its input side from the first DC-DC converter. I've had to do some pretty detailed analysis of input voltage noise and output resonant current draw on some of my DC-DC circuits, and it can become *very* difficult to keep a switcher stable when it's trying to deal with other switching (either in its input power or its output load) anywhere near its own switching frequency or harmonics.

This is a great point, if there was some issue, could I possibly add a large-ish DC capacitor on the output for stability? (what software would I use for the DC analysis?)

Thanks so much everyone!

 

[CWatters]

The battery terminals will also be at high voltage.

How are you charging them?

Have you considered building a 180V battery? The cells could be smaller. Not just because the current draw would be perhaps 5 times lower but also because there would be no losses in step up converters. Harder to charge perhaps.

 

[tim9000]

The battery terminals will also be at high voltage.

How are you charging them?

Have you considered building a 180V battery? The cells could be smaller. Not just because the current draw would be perhaps 5 times lower but also because there would be no losses in step up converters. Harder to charge perhaps.

The connection between the batteries series connection is disconnected via a switch for charging, and there are three separate chargers. The batteries are 9.8Ah, I would have bought larger capacity but I didn't want to spend the extra $.

 

[tim9000]

There is definitely some caution required.
I assume the 12Vdc batteries are similar to automobile batteries.

What are you going to choose for Earth ground? Or do you plan on floating everything - including the load.
Whatever you choose, there will potential differences among different components that exceed 60Vdc.
This could create an unexpected surprise for anyone else working with the setup.

I would be careful to keep all parts of the circuit - including the battery terminals well protected from accidental contact with people or tools.
Their potential relative to other parts of the circuit or surrounding conductors could be well more than the expected 12Vdc.

Everything will be floating (including the load) and well insulated from the Earth.
What components did you have in mind that would exceed the (previously stated) 60Vdc?

 

[essenmein]

Another way you could get your voltage is one of those 12v inverters more often than not they have a push pull DC-DC front end, not sure the exact operating voltage but its enough to convert to 120Vac.

 

[.Scott]

Everything will be floating (including the load) and well insulated from the Earth.
What components did you have in mind that would exceed the (previously stated) 60Vdc?

Two of the battery terminals will be have a 72Vdc potential between them.

 

[tim9000]

Two of the battery terminals will be have a 72Vdc potential between them.

So from positive to positive terminals it is 72Vdc and from positive to negative it is 84Vdc?

Thanks

 

[CWatters]

Quite curious to know what the application is? Why did the original circuit use a 180V motor in the first place if its battery powered?

 

[Rive]

120V DC motors are often used for high power/torque applications => most likely there will be some awful big currents on the battery side.
The project is still doable (safely, if you do know what you are doing) but more and more feels like this would not be the most optimal way to do this.

I'm too curious about this project of yours, could you please give us more details?

 

[tim9000]

120V DC motors are often used for high power/torque applications => most likely there will be some awful big currents on the battery side.
The project is still doable (safely, if you do know what you are doing) but more and more feels like this would not be the most optimal way to do this.

I'm too curious about this project of yours, could you please give us more details?

Quite curious to know what the application is? Why did the original circuit use a 180V motor in the first place if its battery powered?

It's actually a 230VAC inducton motor, but I don't need the rated speed or torque. I measured the current on the secondary side of an isolation transformer (using a VARIAC) and it only drew about 1.2-1.5A (at 50hz).

In a few weeks I'll be able to start a whole thread on the project, although it won't be using boost converters actually, I'll be using a 4.5 ratio step-up transformer. I'm just waiting on an M16 tap to arrive so I can tap the thread of a ten tooth sprocket for the motor shaft...I'm using a raspberry pi and arduino to electrify my pushbike, but I've only just started.

 

[Rive]

I see.

Well. As I looked up your previous threads for clues it seems you are working on this project for some time already, what is a good sign. But I would prefer you work on it a bit longer, because it is just not any good this way (neither with those boost converters, neither with a step-up transformer).
Just keep it at low voltage. Get a 48V DC motor for this. The price of the motor will be lower than the price of the extra electronics needed if you want to do this at high voltage.

 

[tim9000]

I see.

Well. As I looked up your previous threads for clues it seems you are working on this project for some time already, what is a good sign. But I would prefer you work on it a bit longer, because it is just not any good this way (neither with those boost converters, neither with a step-up transformer).
Just keep it at low voltage. Get a 48V DC motor for this. The price of the motor will be lower than the price of the extra electronics needed if you want to do this at high voltage.

I would agree with you if my motives were different. In fact, if my intent was to electrify a bike I'd fork out for one of those Bafang systems.

This project is actually just a good segue for me to learn how to practically implement PWM and dip my toe in the water of Analogue to digital conversion. After I can successfully use v/f control I'm going to try multi-level PWM. Then after I master that I'm going to build my own inverter from IGBTs or MOSFETs. This is all ground work for my larger project: Building a Single Phase Universal Power Conditioner, where I can inject reactive VArs and filter [real power] harmonics (and power factor correction). There are relevant papers about single phase applications of space vectors.

Also, the induction motor was a salvage. I was thinking that if I am successful I might upgrade my [very] old electric scooter, and for that project I might the boost converters, and I'd use a DC motor because the gear ratio would be lower, so I'd get maximum torque on start-up via the DC motor.

 

[Windadct]

A lot of things are "possible" but it largely depends in that devices control system, and will they interfere with one another, does it make sense, is there abetter way, -- many factors...I am not really seeing why you are not putting the batteries in series, and then a single Boost to ~325VDC, and then H bridge inverter drive for 230 VAC- if you are just starting out building these - why make it more complicated.

But - to build from scratch, yes, you can series as many basic boost converters as you can manage - also there are a few "variations on a theme" for this. For example - you can have boost inductors in the positive or negative - or BOTH ( one Mosfet and 2 FWD +/-) ... keeping in mind the ground reference for all for the signals and what potential they will see.

You do realize that you can also series connect H bridges - right? If you phase shift the carriers, the resulting output has effectively 3x the Fsw of each one.

 

[anorlunda]

The original question has been adequately answered.

Thread closed.