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

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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%?

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

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

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

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

Please post below text.


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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 /

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

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

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



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


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Which means pretty much everything.


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You have to. The PWM chip that is often supplied from the 12V input
cannot be supplied directly with a 48V input.


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Not even the low voltage side parts:

(Amazon.com product link shortened)


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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.


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That is indeed so. It can also become the proverbial can of worms :-)

--
Regards, Joerg

http://www.analogconsultants.com /

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

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  Headroom.

Re: What is the reason that the duty cycle for Boost Converter should be around 50%?
Dear All,

Thank you so much for your kind suggestions on the design.

Now I have decided to boost 48V to 180V Maximum only. I have selected
150kHz operating frequency for Boost Converter, with inductor 25uA,
3A. And the rated power is supposed to be 250W. My question are:

1. Is the operating frequency and inductor value selected practical?
2. What are the important things to consider for sizing components in
this design?

regards

Re: What is the reason that the duty cycle for Boost Converter should be around 50%?
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1, Depends on core losses, switch losses and diode trr
2, Heat

 Most PFC's boost 0 to 141Vdc to 375Vdc with PW>80% for part of the
input AC cycle. The trick is; BW<12Hz to control the dreaded RHPZ.

 Regards,
 Harry

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