Having some trouble with the above, My attempts are as follows:

Q1a:
VsDTs + (-VD)(1-D)Ts=0
Vo(1-D)Ts = -VsDTs
Vo = (-D/1-D)(Vs)

Q1b:
IL= 1/2 IL peak
IL = (1/2) (TsVs/L)(P)
IL = (1/2) (TsVo/L)(1-D)

Not to sure about C.

 

Hello,

Looks like an inverting 'boost' circuit which may or may not actually boost.

What dont you know about "C" ?

Ust a small note...
First energy is put into the inductor, then later it is dumped into the cap but some goes into the resistor.

 

Hello,

Looks like an inverting 'boost' circuit which may or may not actually boost.

What dont you know about "C" ?

Ust a small note...
First energy is put into the inductor, then later it is dumped into the cap but some goes into the resistor.

You mean an inverting buck-boost converter?

 

Assuming the switches alternate in action (only one closed at a time) the circuit is the prototypical flyback converter.

 

You mean an inverting buck-boost converter?

Hello,

You could call it that also yes. It all depends how you work the switches.

So what dont you understand about "C" ?

 

Assuming the switches alternate in action (only one closed at a time) the circuit is the prototypical flyback converter.

Hi,

Sorry but that's not really correct. A flyback is a boost circuit with a transformer with inductance, while a boost circuit just has an inductor and no transformer. So this circuit we usually call just a boost circuit. Because the switching action is not specified yet though, we could also call it a buck boost, inverting, but we cant really call it a flyback design because there is no transformer.

Once we know the switching action then we can narrow it down to either a inverting boost or an inverting buck boost.

 

I think the circuit has 2 modes of operation.

first : s1 closed and s2 open
second: s2 closed and s1 open

 

Had another attempt at A and came up with this...

 

No. It is a flyback converter. A flyback does not require a transformer and in fact the thing called a transformer in a flyback converter isn't a transformer at all in terms of delivery of power - it is an inductor with two (or more) windings. The defining feature of a flyback converter is that at no time is there current flow to the load while there is current flow from the input. Energy is stored in an inductor during part of the cycle and delivered to the load during another part of the cycle. In both buck and boost there is current flow from input to output during part of the switching cycle. Because of this action, the absolute value of the output voltage can be higher or lower than the input voltage, however the sign is changed unless multiple switches are used (2 is not enough, the second switch here is standing in for a diode - you can't have both switches open simultaneously except when there is no energy stored in the inductor; if you close both simultaneously the output capacitor is connected directly to the input supply and current is unlimited.)

See Switching Power Supply Topology Review by Lloyd H. Dixon, Jr.

[EDIT] I should acknowledge that the term "flyback" derives from beam retrace in magnetically-deflected CRTs. It would be more appropriate to call this an invertering converter, but it most definitely is neither a boost converter nor a buck converter in the accepted sense of either of those topologies. It is a very clearly distinct third fundamental topology.

 

No. It is a flyback converter. A flyback does not require a transformer and in fact the thing called a transformer in a flyback converter isn't a transformer at all in terms of delivery of power - it is an inductor with two (or more) windings. The defining feature of a flyback converter is that at no time is there current flow to the load while there is current flow from the input. Energy is stored in an inductor during part of the cycle and delivered to the load during another part of the cycle. In both buck and boost there is current flow from input to output during part of the switching cycle. Because of this action, the absolute value of the output voltage can be higher or lower than the input voltage, however the sign is changed unless multiple switches are used (2 is not enough, the second switch here is standing in for a diode - you can't have both switches open simultaneously except when there is no energy stored in the inductor; if you close both simultaneously the output capacitor is connected directly to the input supply and current is unlimited.)

See Switching Power Supply Topology Review by Lloyd H. Dixon, Jr.

[EDIT] I should acknowledge that the term "flyback" derives from beam retrace in magnetically-deflected CRTs. It would be more appropriate to call this an invertering converter, but it most definitely is neither a boost converter nor a buck converter in the accepted sense of either of those topologies. It is a very clearly distinct third fundamental topology.

Hello again,

What year was that book written?

That's not how the books i have read describe a flyback including a design guide from the old Magnetic, Inc. where i used to get my transformer core materials from.. In fact, when they say flyback they say that in order to distinguish it from a regular boost circuit that uses an inductor only.

A coupled inductor boost circuit is not really the same as a flyback either because the flyback has the option of going to an ISOLATED output. Try that with an inductor

A transformer is a transformer, it does not matter how it is used, it's still a transformer. An inductor with a second winding becomes a transformer, like it or not. It has leakage inductance yes, but that's only part of the specification and operating properties unlike an inductor which only has inductance and no coupling factor for example, and certainly no turns ratio.

I think there might be some wiggle room here though so i wont try to force you to call it anything you dont want to call it. It's up to you what you want to call it.

 

Had another attempt at A and came up with this...

Here's my reply:




Too hard to read?
Sorry but yours was too
Maybe make bigger pictures or something, on white paper maybe?

 

The classic name for this topology is Buck-Boost converter (inverting converter). And flyback topology is really a Buck-Boost, with its usual single-winding inductor replaced by an inductor with multiple windings.

A coupled inductor boost circuit is not really the same as a flyback either because the flyback has the option of going to an ISOLATED output. Try that with an inductor

A transformer is a transformer, it does not matter how it is used, it's still a transformer. An inductor with a second winding becomes a transformer, like it or not. It has leakage inductance yes, but that's only part of the specification and operating properties unlike an inductor which only has inductance and no coupling factor for example, and certainly no turns ratio.

In power electronics, we distinguish between "coupled inductor" and "transformer". In the flyback converter, the "coupled inductor" behaves both as an inductor and as a transformer. It stores magnetic energy as any inductor would, but it also provides isolation just like any transformer would.
And by definition, the transformer does not store any energy in the core, only inductor can store the energy.
So when we are designing a Forward converter we can use the same equation as we would use to design "ordinary " mains transformer. Which is not true for a flyback converter (where "large" air gap is needed ).
And this is why in power electronics we distinguish between "coupled inductor" and "transformer".

 

The classic name for this topology is Buck-Boost converter (inverting converter). And flyback topology is really a Buck-Boost, with its usual single-winding inductor replaced by an inductor with multiple windings.

In power electronics, we distinguish between "coupled inductor" and "transformer". In the flyback converter, the "coupled inductor" behaves both as an inductor and as a transformer. It stores magnetic energy as any inductor would, but it also provides isolation just like any transformer would.
And by definition, the transformer does not store any energy in the core, only inductor can store the energy.
So when we are designing a Forward converter we can use the same equation as we would use to design "ordinary " mains transformer. Which is not true for a flyback converter (where "large" air gap is needed ).
And this is why in power electronics we distinguish between "coupled inductor" and "transformer".

Hi Jony,

Well here are a couple points to consider....

All real transformers have leakage inductance.
Coupled inductors really are transformers, because that's what a transformer is.
If we only have one winding we only call that an inductor.

So there are really only two types of devices:
1. Inductor.
2. Transformer.

The transformer has leakage inductance and that is what makes the flyback work.
The boost circuit does not require a transformer.

Also interesting is if you look up flyback converter, you always find a design with a transformer.

I did say however that there might be some wiggle room here. Sometimes people refer to a "boost' circuit as related to the input and output voltages not the topology itself. If we refer to that reference, then we have to call a flyback a boost circuit as well. The only catch here though is that the inverting boost (inductor only) may not actually boost the voltage it may just simply invert it. That would be used to generate a negative supply rail for example when there was only a positiver rail to begin with.

Check out these two links:
https://en.wikipedia.org/wiki/Flyback_converter
https://en.wikipedia.org/wiki/Boost_converter

and see what i mean. They always give the 'proper' form for each converter type.
Also interesting is that they state that the flyback has galvanic isolation and that can only come from a transformer based design.

If you look up design info on the old Magnetics Inc site they also make a distinction between the two circuits calling the one with the transformer the "flyback" and the one with the inductor only a "boost" circuit.
Of coruse the one we have here is buck boost, but it doesnt mean that it has to be used that way.

Back to the main point of the thread:
Yes there are two states:
1. s1 on, s2 off.
2. s1 off, s2 on.

The duty cycle governs what we get on the output. Efficiency is not quite as good as a regular boost circuit though.
The inductor charges up: v=L*di/dt
The cap charges up: i=C*dv/dt