RESONANT PWM CIRCUITS ,ZVS and ZCS

[Vicky]

Hi,
Can anyone exlain in simple terms how a RESONANT PWM CIRCUITS differs
from an normal PWM circuit.I mean what is the basic concept of zero
voltage switching and zero current switching techniques.Can anyone
explain????
Vicky

 

[analog]

Can anyone exlain in simple terms how a RESONANT PWM CIRCUITS differs
from an normal PWM circuit. I mean what is the basic concept of zero
voltage switching and zero current switching techniques. Can anyone
explain????

Just explaining it would be a no brainer. No brainstrain, no gain.

So, then, let's pull out the brain teaser, shall we?

Q: Why use PWM at all? Why not a simple series pass element?

Q: In light of the answer to the prior question, how might ZVS and
ZCS further illuminate this advantage?

Good luck.

 

[The Al Bundy]

Hi,
Can anyone exlain in simple terms how a RESONANT PWM CIRCUITS differs
from an normal PWM circuit.I mean what is the basic concept of zero
voltage switching and zero current switching techniques.Can anyone
explain????
Vicky

I guess you meant a half or full bridge driven with PWM.
Some very basically half bridge:

--------
|
\ high
|
|---out
|
\ low
|
------

With the above circuit you can switch only 1 switch is on at a time,
otherwise you will short the supply When the switches are ideal you can
switch one off and directly the other one on, to get a square wave signal at
the mid point and there is no power loss.

When replacing these switches with MOSFET's you need to take care. Because
of parasitic capacitance between the drain - source (basically the switch)
you need some time to let those capacitors charge / discharge between
switching. When you don't do this the MOSFET's will dissipate energy.

Basically with ZVS you want to make sure the voltage across a MOSFET is
zero before you turn it on. When doing this there is already a current
flowing through the body diode of the MOSFET, which the Rds-on will take
over.
With ZCS is just the opposite, you want that no current will flow through
the MOSFET when you turn it on. (Voltage is also near zero across the
MOSFET)

With P = V * I you can see that in both cases the MOSFET(s) wont dissipate.
However due to the voltage drop of the body diode and Rds-on you always get
some dissipation, but less then with no ZVS/ZCS.

You need 2 things to have ZVS/ZCS, enough inductive load (never let switch a
half/full bridge unloaded or only with a capacitor...) and some so-called
deadtime between switching the MOSFETS.

The inductive load will charge/discharge the parasitic capacitance of the
MOSFETS and so creating ZVS/ZCS when running the half/full bridge with a
deadtime.
The term 'resonant' comes from that the inductive load charges / discharges
the capacitance of the MOSFET's.

PWM is just the modulation. However you need to take more care with PWM to
switch ZVS/ZCS then with a 50% duty-cycle.

Do some search with google and you will find more information.
Hope it helps a bit.

Al

 

[Malcolm Reeves]

Can anyone exlain in simple terms how a RESONANT PWM CIRCUITS differs
from an normal PWM circuit.I mean what is the basic concept of zero
voltage switching and zero current switching techniques.Can anyone
explain????

Have a search of the TI site (www.ti.com). AFAIR they have the
Unitrode lecture notes in pdfs. That will give you loads of
information. But to sum up, you add a tuned circuit to the mosfet.
Typically this uses parasitic elements like mosfet capacitance and
transformer leakage inductance although you can pad these with real
components too. When you switch off a tuned circuit it will ring.
That is it will rise up in sine wave fashion and then come back down
again. Turn on when it is at the bottom and you have losses switching
since you turn off at 0V and on at 0V and since P=I*V then P=0 (more
or less).

There are variations on this and I suggest you look at the unitrode/TI
notes.

Malcolm

--

....malcolm

Malcolm Reeves BSc CEng MIEE MIRSE, Full Circuit Ltd, Chippenham, UK
([email protected], [email protected] or [email protected]).
Design Service for Analogue/Digital H/W & S/W Railway Signalling and Power
electronics. More details plus freeware, Win95/98 DUN and Pspice tips, see:

http://www.fullcircuit.com or http://www.fullcircuit.co.uk

NEW - Desktop ToDo/Reminder program (free)

 

[Vicky]

Hi ,
I have typed down the following points as per the brasin teaser
asked and some answers i recieved in the groups.Please add to or
correct the points
if they are not perfect:

Q: Why use PWM at all? Why not a simple series pass element?

PWM's are used because of the higher effieciency they offer in
power supply design.The pass device will be operated in either cutoff
or saturation thus converting input DC to a high frequency
square/pulse waveform.But if we go for a series or linear regulators
there the pass device operates in linear region and efficiency is
lesser.


Q: In light of the answer to the prior question, how might ZVS and
ZCS further illuminate this advantage?

So main loss in a smps would occur when the device(MOSFET) is
actually switching.So if we have make the voltage across the switching
device or current throuh it actually zero before the actual switching
happens then the power losses are further minimised.
I hope this is the basic concept.
But how do we choose a topology when we start the actual design.Also
what exactly is the role of transformers in the SMPS.

Regards
Vicky

 

[analog]

I have typed down the following points as per the brasin teaser
asked and some answers i recieved in the groups. Please add to or
correct the points if they are not perfect:


PWM's are used because of the higher effieciency they offer in
power supply design. The pass device will be operated in either
cutoff or saturation thus converting input DC to a high frequency
square/pulse waveform. But if we go for a series or linear
regulators there the pass device operates in linear region and
efficiency is lesser.

Q: In light of the answer to the prior question, how might ZVS and
ZCS further illuminate this advantage?

So main loss in a smps would occur when the device(MOSFET) is
actually switching. So if we have make the voltage across the
switching device or current throuh it actually zero before the
actual switching happens then the power losses are further minimised.
I hope this is the basic concept.

Very good. You seem to understand the motivation behind ZVS and ZVC
designs. By the way, the main loss in a smps (even a hard switching
type) is not necessarily switching (although it may well be).

But how do we choose a topology when we start the actual design.

I know of no simple answer to this question. It all depends on your
design constraints. For example, some of the simplest and best
performing lossless switching designs operate with a line and load
dependent variable frequency. This may be incompatible with your
design constraints.

Also what exactly is the role of transformers in the SMPS.

How were you able to answer the first two questions and not know
the answer to this much more basic question? Perhaps you are a
student and have no practical experience? Also, if you don't mind
my asking, what country are you from?

A transformer is most often used to provide two properties. One is
galvanic isolation. What the other is, is given away by its name.
Care to guess what it may be?

 

[Vicky]

Also what exactly is the role of transformers in the SMPS.

How were you able to answer the first two questions and not know
the answer to this much more basic question? Perhaps you are a
student and have no practical experience? Also, if you don't mind
my asking, what country are you from?

A transformer is most often used to provide two properties. One is
galvanic isolation. What the other is, is given away by its name.
Care to guess what it may be?

Hi,
I do know that two main functions of a transformer are :
1>Electrical isolation
2>Stepup/stepdown voltage.

But if we go through the smps reference manual(page22) from ONSEMI it
is mentioned as below:

"In forward mode topologies, the transformer is only used for stepping
up or down the AC voltage generated by the power switches. The output
filter (the output inductor and capacitor) in forward mode topologies
is used for
energy storage. In boost mode topologies, the transformer is used both
for energy storage and to provide a stepup or stepdown function."
I did not get the point here.that was the reason behind asking the
role of transformer.i mean how does it differ in the various modes.
Regards
vicky

 

[analog]

I do know that two main functions of a transformer are :
1>Electrical isolation
2>Stepup/stepdown voltage.

This is a good start, but always bear in mind that current is
transformed (stepped down and up) inverse to that of voltage. An
ideal transformer stores no energy, only transforming it in such a
way that the product of instantaneous voltage and current on the
secondary winding is always equal to that of the primary winding
(at least for a transformer with only two windings). A more
general observation would be that net ampere-turns must always
equal zero (do you know what this means?).

But if we go through the smps reference manual(page22) from ONSEMI it
is mentioned as below:

"In forward mode topologies, the transformer is only used for stepping
up or down the AC voltage generated by the power switches. The output
filter (the output inductor and capacitor) in forward mode topologies
is used for energy storage. In boost mode topologies, the transformer
is used both for energy storage and to provide a stepup or stepdown
function."

I did not get the point here.that was the reason behind asking the
role of transformer.i mean how does it differ in the various modes.

This manual is confusing coupled inductors with transformers. At least
for the sake of understanding, it is probably best to think of coupled
inductors (such as used in flyback converters) as a combination of an
idea transformer with an ideal inductor connected in parallel with its
primary winding.

 

[ROBIN MARSTRAND]

Vicky.
If you go to Google and search RESONANT PWM CONVERTERS, IONVERTERS AND
COMBINATIONS OF THESE WORDS you'll get mountains fo information in the form,
in many cases of MSc and PhD thesis. IEEE is closed if your'e not a member
and os are some fo the universities aroung the world but, in the main you'll
find some very interesting but VERY technical materil with schematincs and
the rest.
Happy searching!
ROBIN

 

[ROBIN MARSTRAND]

To All on this thread.

I saw from other posting below that there could be some confusion.
I am learnign but would like to clarify on some points:-

PWM = Pulse Width Modulation (everybody knows this)
ZVS = Zero Voltage Switching (as applies to PWM power converters and
Inverters)
ZCS = Zero Current Switching (as applies to PWM power converters and
Inverters)

These techiniques allow for less stress in the switching devices and rely
upon a whole variety of circuit-additional features which add complexity but
improve performance in may respects.
The subject is very deep and a rapdily growing and developing technolgy,
especially as semiconductors are now capable of massive power handling at
reasonably high frequencies - megawatts of power, indeed.

Best. ROBIN.

 

[ROBIN MARSTRAND]

Vicky,

You have grasped the concept.
I suggest you follow my suggestions an earlier posting on this thread, re
searching for info.

Best, ROBIN

 

[ddwyer]

I did not get the point here.that was the reason behind asking the

role of transformer.i mean how does it differ in the various modes.
Regards
vicky

At the risk of being obvious the various characteristics of switched
mode escaped me initially, this may also help:
Energy can be stored in an inductor in a similar but different manner to
a capacitor.
Line (mains) PSU generally rectify the mains to DC and then chop it up
by charging an inductor for a time that determines how much energy is in
the inductor the energy is released in one of a number of ways including
the switching element going into the off state providing the flyback
condition to boost the voltage.
Transformers (with switching circuits) with mutual inductance less than
1 can also be used simultaneously to store energy transform the voltage
and limit ripple.
If you get into the practical realms I found that unlike signal and
signal analogue circuits switched mode prototypes can generate heaps of
blown IC s and Power transistors.
LT spice is most helpful but does not include breakdown in its models
perhaps this helps sell ICs!



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