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What is the reason that the duty cycle for Boost Converter should be around 50%?

Started by Myauk December 8, 2011
Dear All,

I am currently trying to design a solar inverter. I am looking at the
boost converter stage, At fist I was planning to boost from 48V to
400V using simple boost converter, but after calculation, the
theoretical duty cycle for it is more than 80%. People say it is not
good for such converters. May I know your opinions on why it is that
the duty cycle of Boost Converter should be around 50%?
Myauk wrote:
> Dear All, > > I am currently trying to design a solar inverter. I am looking at the > boost converter stage, At fist I was planning to boost from 48V to > 400V using simple boost converter, but after calculation, the > theoretical duty cycle for it is more than 80%. People say it is not > good for such converters. May I know your opinions on why it is that > the duty cycle of Boost Converter should be around 50%?
Don't know where you got that 50% opinion from. As a general statement it's wrong. The duty cycle depends on the ratio of input to output voltage. As a general rule of thumb 90% is usually a point where it pays to think about transformer-coupled alternatives such as a half-bridge. I have done 95% and more but due to the compromises in the switcher FET (high current plus high voltage doesn't pair up too well) it isn't so economical at higher power. -- Regards, Joerg http://www.analogconsultants.com/
But for my specific case, I need to make it to deliver at least 200
Watt of power, and I can't find many solutions for 48V to 400V DC to
DC Converter using simple boost converter. One solution from Microchip
uses Push Pull instead for 400V DC Boost Converter. So I have some
concerns.
:(
Rgds

On Dec 9, 2:32=A0am, Joerg <inva...@invalid.invalid> wrote:
> Myauk wrote: > > Dear All, > > > I am currently trying to design a solar inverter. I am looking at the > > boost converter stage, At fist I was planning to boost from 48V to > > 400V using simple boost converter, but after calculation, the > > theoretical duty cycle for it is more than 80%. People say it is not > > good for such converters. May I know your opinions on why it is that > > the duty cycle of Boost Converter should be around 50%? > > Don't know where you got that 50% opinion from. As a general statement > it's wrong. The duty cycle depends on the ratio of input to output > voltage. As a general rule of thumb 90% is usually a point where it pays > to think about transformer-coupled alternatives such as a half-bridge. I > have done 95% and more but due to the compromises in the switcher FET > (high current plus high voltage doesn't pair up too well) it isn't so > economical at higher power. > > -- > Regards, Joerg > > http://www.analogconsultants.com/
On Dec 8, 10:58=A0am, Myauk <aungkokot...@gmail.com> wrote:
> But for my specific case, I need to make it to deliver at least 200 > Watt of power, and I can't find many solutions for 48V to 400V DC to > DC Converter using simple boost converter. One solution from Microchip > uses Push Pull instead for 400V DC Boost Converter. So I have some > concerns. > :( > Rgds > > On Dec 9, 2:32=A0am, Joerg <inva...@invalid.invalid> wrote: > > > > > Myauk wrote: > > > Dear All, > > > > I am currently trying to design a solar inverter. I am looking at the > > > boost converter stage, At fist I was planning to boost from 48V to > > > 400V using simple boost converter, but after calculation, the > > > theoretical duty cycle for it is more than 80%. People say it is not > > > good for such converters. May I know your opinions on why it is that > > > the duty cycle of Boost Converter should be around 50%? > > > Don't know where you got that 50% opinion from. As a general statement > > it's wrong. The duty cycle depends on the ratio of input to output > > voltage. As a general rule of thumb 90% is usually a point where it pay=
s
> > to think about transformer-coupled alternatives such as a half-bridge. =
I
> > have done 95% and more but due to the compromises in the switcher FET > > (high current plus high voltage doesn't pair up too well) it isn't so > > economical at higher power. > > > -- > > Regards, Joerg > > >http://www.analogconsultants.com/- Hide quoted text - > > - Show quoted text -
Boost converters have a Right Hand Plane Zero in their control loop that only appears at D>0.5. To stablize at D>0.5 you must be smarter than the adverage bear. Cheers, Harry
Myauk wrote:
> Dear All, > > I am currently trying to design a solar inverter. I am looking at the > boost converter stage, At fist I was planning to boost from 48V to > 400V using simple boost converter, but after calculation, the > theoretical duty cycle for it is more than 80%. People say it is not > good for such converters. May I know your opinions on why it is that > the duty cycle of Boost Converter should be around 50%?
These kind of projects interest me. I'd like to hear what people are doing. Would be interesting to learn more about the whole project. Why did you pick 400V?? Making a lot of assumptions here in the USA...if I were doing a solar inverter, I'd pick 160V. Then I'd go buy a 12VDC to 120VAC modified sinewave inverter (possibly 240VAC in your jurisdiction, but the concept is the same) and fix the input to run off 48V. Maybe change the FET, inductors, caps and anything else in the input side that couldn't stand 48V. Shouldn't need to change the basic circuit at all. You can't purchase the high-voltage side components for the price of a cheapo off-the-shelf inverter. After I got familiar with that, and if it didn't meet my needs, I'd start on a custom design. Based on the wording of your question, I think you're gonna learn a LOT about problems you haven't yet considered. The answer to your question has already been answered, but I think it's the tip of the proverbial iceberg.
> Based on the wording of your question, I think you're gonna learn a LOT > about problems you haven't yet considered. > > The answer to your question has already been answered, but I think it's > the tip of the proverbial iceberg.
Thanks for the suggestion, mike, It is a school project and we need to design pure sine wave inverter. The reason 400 V is selected is because of the peak, due to line and load changes, it might affect the output AC voltage, so the Boost Converter output must be variable according to the battery voltage and load changes. Actually, my concern is not to blow capacitors and inductors by any chance. If possible my I know what are the important precautions? I am going to test out the configuration in lower voltage levels anyway, but, I still need to make it work at higher voltage levels too. Can anyone please suggest me on this? Regards
Myauk wrote:

Please post below text.


> But for my specific case, I need to make it to deliver at least 200 > Watt of power, and I can't find many solutions for 48V to 400V DC to > DC Converter using simple boost converter. One solution from Microchip > uses Push Pull instead for 400V DC Boost Converter. So I have some > concerns.
200W is pretty much half-bridge turf, or push-pull. You can do a boost though if you really want to for some reason. The FET will get kind of big and expensive because it must withstand >5A but also over 400V. In a half-bridge topology the current is the same but the FETs do not have to withstand a very high voltage. [...] -- Regards, Joerg http://www.analogconsultants.com/
Myauk wrote:
>> Based on the wording of your question, I think you're gonna learn a LOT >> about problems you haven't yet considered. >> >> The answer to your question has already been answered, but I think it's >> the tip of the proverbial iceberg. > > Thanks for the suggestion, mike, It is a school project and we need to > design pure sine wave inverter. The reason 400 V is selected is > because of the peak, due to line and load changes, it might affect the > output AC voltage, so the Boost Converter output must be variable > according to the battery voltage and load changes. > Actually, my concern is not to blow capacitors and inductors by any > chance. If possible my I know what are the important precautions? I am > going to test out the configuration in lower voltage levels anyway, > but, I still need to make it work at higher voltage levels too. Can > anyone please suggest me on this? > > > Regards
I've given almost zero thought to this, but I can tell you some things to think about. First is SAFETY. It's obvious that there's enough energy stored to KILL you. What ain't so obvious is that you can seriously injure yourself when you touch a live circuit and fling yourself across the room. Always, ALWAYS wear safety glasses. 50 years ago, I was testing tubes in a TV. I grazed the crt HV wire and went flying out the back of the set. In the process, my arm caught the antenna wire and flung it into my eye. Scratched the cornea and was lucky I wasn't blinded. Same can happen when you drop a screwdriver into the circuit and bits of vaporized metal head your way. There's an old Tektronix building that still has pieces of a FET embedded in the ceiling, put there when I was torture testing a power supply. Always, ALWAYS wear safety glasses. Your friends may laugh at you, but at least you'll be able to see them when you visit them in the hospital. At these power levels and voltage ratios, you're probably gonna need a push-pull forward converter. I'd consider a full-wave voltage doubler on the output to get 400V. BIG caps that are derated to operate reliably at 400V are hard to find. The doubler lets you use 200V caps. There are other tradeoffs, so weigh all your options carefully. For a boost converter, you can tap the inductor and get more voltage out than the transistor switches. That can help with your 10X voltage ratio, but it's not free. You end up with big negative voltages to deal with. Read up on "safe operating area" for bipolar transistors. Even if you use FETS, the lessons learned from SOA will serve you well. Just cause the data sheet says 200V 20A 200W, doesn't mean that you can get ALL at the same time. As the voltage goes up, the amount of current you can switch goes down DRAMATICALLY. For every point during the switching cycle, you need to know the voltage and current at that time. Make sure it's inside the repetitive transient ratings of your switching devices. Choose your switching devices CAREFULLY. Read up on "snubbers". For a forward converter, the transformer design is critical. If there's any imbalance, the thing will saturate in a heartbeat and you'll need new FETs. Transformer design is an art. You want low leakage inductance and perfect balance. Core material can depend on switching frequency. You also need a perfectly symmetrical drive waveform. Stick a current probe on the primary wire and change the drive duty factor from 50%. You can watch the current walk right up the B-H curve and saturate just before the FETs explode. Use a digital storage scope, cause it happens quickly...you don't wanna miss it. Be particularly careful with the physical circuit layout and where the currents go. It takes very little coupling from the load or source to transient the duty factor off 50%. That was the problem with the commercial supply I mentioned before. When a load transient coupled through an improperly laid-out circuit board into the control circuit, the edge got shifted resulting in core saturation and FET pieces embedded in the ceiling. The take-away here is to make sure you have lots of FETS. If you have to wait for delivery of replacements, your project will be late. Where are you getting your 48V? If it's 4 lead acid batteries, the input can go from 40-60V. If you're trying to run right off the solar panel, it's much worse. Plot power vs output voltage for various levels of insolation. There's a voltage that corresponds to maximum power. There are regions that look like negative resistance, so you can easily get limit-cycle oscillations in the system. Battery maintenance is a critical item. You want the batteries to charge at the maximum power point of the solar panel when sun is available, independent of the load... up until the battery is fully charged. I dealt with this problem on a solar-powered ham radio repeater by just shunting the excess solar power into a resistor. Wasteful, but the power was free and that was the simplest solution. Have no idea if it will work, but I've toyed with the idea of skipping the DC altogether. Compare the output waveform to a sinewave and use a microcontroller to adjust the switcher duty factor on the fly to track the sinewave. This might have serious issues with non-resistive loads. The literature on power factor correction might be instructive. Are we having fun yet?
mike wrote:
> Myauk wrote: >> Dear All, >> >> I am currently trying to design a solar inverter. I am looking at the >> boost converter stage, At fist I was planning to boost from 48V to >> 400V using simple boost converter, but after calculation, the >> theoretical duty cycle for it is more than 80%. People say it is not >> good for such converters. May I know your opinions on why it is that >> the duty cycle of Boost Converter should be around 50%? > > These kind of projects interest me. I'd like to hear what people > are doing. > Would be interesting to learn more about the whole project. > Why did you pick 400V?? > > Making a lot of assumptions here in the USA...if I were doing a solar > inverter, I'd pick 160V. Then I'd go buy a 12VDC to 120VAC modified > sinewave inverter > (possibly 240VAC in your jurisdiction, but the concept is the same) > and fix the input to run off 48V. Maybe change the FET, inductors, ...
... and this is where frustration can set in. It's usually a forward converter so you'll sit there drilling out rivets to get the pot core apart. Once the rivet are out and the metal splinters removed for your fingers you might find that the core halves are held together by some oriental super-glue that is also used to hold spacecraft parts together. Assume you get that apart without cracking the ferrite you might just find that the whole winding package has been dipped in some sort of lacquer that is designed to survive world wars three, four and five.
> caps and anything else in the input side that couldn't stand 48V.
Which means pretty much everything.
> Shouldn't need to > change the basic circuit at all. ...
You have to. The PWM chip that is often supplied from the 12V input cannot be supplied directly with a 48V input.
> ... You can't purchase the high-voltage > side components for the price of a cheapo off-the-shelf inverter. >
Not even the low voltage side parts: http://www.amazon.com/Cobra-400-Watt-12-Volt-120-Volt-Inverter/dp/B001RNOHBC/ref=sr_1_1?ie=UTF8&qid=1323382502&sr=8-1
> After I got familiar with that, and if it didn't meet my needs, I'd start > on a custom design. >
If you manage to rewind the transformer the rest is not so difficult anymore. At least for me, I've rewound many but can't say that I've ever enjoyed it.
> Based on the wording of your question, I think you're gonna learn a LOT > about problems you haven't yet considered. > > The answer to your question has already been answered, but I think it's > the tip of the proverbial iceberg.
That is indeed so. It can also become the proverbial can of worms :-) -- Regards, Joerg http://www.analogconsultants.com/
On Thu, 8 Dec 2011 10:13:48 -0800 (PST), Myauk <aungkokothet@gmail.com>
wrote:

>Dear All, > >I am currently trying to design a solar inverter. I am looking at the >boost converter stage, At fist I was planning to boost from 48V to >400V using simple boost converter, but after calculation, the >theoretical duty cycle for it is more than 80%. People say it is not >good for such converters. May I know your opinions on why it is that >the duty cycle of Boost Converter should be around 50%?
Headroom.