T
I agree, but I would go further than you and say that not only is type 3 better, but it is essential for a voltage mode flyback in ccm, -type 2 just isn’t good enough here…so I am wondering why there app note “AN57” (in top post) says that a type 2 network is ok for their voltage mode flyback?…..they reckon they can get 1khz crossover frequency with the type 2 network...this surely isn't possible.?type 3 is best for volt mode as you need to counteract the inherent pole formed by the transformer and the o/p capacitors in volt mode...
it is current mode internally - so "type II" may be ok
This directly implies that the chip only operates in voltage mode control (where the duty cycle, not the peak current, is directly determined by control signal). Though throughout its load range, the actual method of modulating D changes from constant frequency to variable frequency and back to fixed frequency, and it's anybody's guess what the small signal transfer function is really doing for each mode.Although different modes operate differently to make transitions between modes smooth, the simple relationship between duty cycle and excess CONTROL pin current shown in Figure 6 is maintained through all three PWM modes
Feedforward to me refers to using the sensed input voltage to correct the duty cycle of a PWM modulator such that its average output is independent of the input voltage. See the UCC35705 datasheet for an example. This is very useful for voltage mode control loops, because otherwise the open loop gain would be proportional to input voltage. Also useful for making the loop response more independent of the source impedance.I was just wondering, the topswitch controllers have "feedforward", and I wondered if this means that a true , full "type 3" controller is not therefore needed for them?
..sorry but I don’t agree it’s a moot point whether its in CMC or VMC for DCM flyback….When operating in the variable frequency mode the controller sets the peak drain current, so it could be argued that it is effectively operating at a CMC, if it's DCM. But if it's in DCM, then the distinction between current mode and voltage mode control becomes moot, since they both have a single-pole response.
…again I see that like myself , you are trying to understand whether this controller is in CMC or VMC. I am becoming convinced that the topswitch-jx NEVER operates in CMC because for CMC, you MUST have a ramp signal going into your PWM comparator which comes from the primary current sense resistor. That never happens with topswitch-jx so from that viewpoint, topswitch-jx is NEVER in CMC.When operating in the variable frequency mode the controller sets the peak drain current, so it could be argued that it is effectively operating at a CMC, if it's DCM
No it doesn't. Both VMC and CMC can have the exact same transfer function at a given operating point in DCM. The devil is in the details, which are not given in those examples. There's no way to make sweeping generalizations based on the waveforms in those documents.https://www.edaboard.com/threads/351408/
..confirm that there is a big difference between DCM current mode flyback and DCM voltage mode flyback.
I don't see where it categorically states that a CCM flyback can be controlled with a type II compensator. Perhaps it assumes that most designs will be DCM over their entire operating range, which is typical for low power flybacks.However, having made that bold statement, I am then stumped by the fact that AN-57 (see top post for AN-57) by Power integrations states that a CMC Flyback with topswitch-jx, can be feedback compensated with a type 2 compensator to give crossover frequency around 1khz…….this just isn’t possible with a voltage mode controller, so I am wondering in what mode the topswitch-jx operates in?
I highly doubt the IC does anything to manipulate the dynamical response of the circuit (that is, the number or positions of the various poles and zeros). What it does do is vary the method of duty cycle control (which, in a small signal sense, should not have any effects on the dynamical behavior).I think Power integrations have hit on some superb new control method in topswitch-jx but are keeping it to themselves, as they are milking the huge sales of this excellent control chip, which indeed, is the most commonly used one in the world.
Yes, their charts are confusing, because the documentation is poorly written.If you look at fig 6 , page 4 of the topswitch-jx datasheet (below), then you see that as the control pin current (the error current), increases, the peak current level decreases, aswell as the duty cycle decreasing.
What possibly could be the reason to vary the peak current limit?, what advantage does it give?…it is something special, as the topswitch-jx is the world’s best seller.
thanks, so do you mean in the variable frequency region, the peak current is set to the level of either kps(upper) or kps(lower)?It can only control peak current to one of those levels.
Yes that’s true but its not my point, my point is , that a DCM current mode flyback is always intrinsically stable, whereas a DCM voltage mode flyback can be unstable.Both VMC and CMC can have the exact same transfer function at a given operating point in DCM.
Thanks, and of course, the next question is, why is the topswitch-jx flyback controller using voltage mode control?, .....when current mode is easier to stabilise for a flyback in DCMYes a DCM volt mode can be unstable, due to the inherent LC filter, formed by the Tx and the o/p caps (large), current mode operation removes the effect of the L, making closing the loop much easier, esp at light / no load.
I highly doubt the IC does anything to manipulate the dynamical response of the circuit (that is, the number or positions of the various poles and zeros).
From the diagram, in the "middle" variable frequency mode peak current is set to 55% of set maximum. In the multi-cycle modulation mode it will be 25%.thanks, so do you mean in the variable frequency region, the peak current is set to the level of either kps(upper) or kps(lower)?
The plot inherent to the converter itself, I don't think it can be changed. Depending on your design, you will make use of differing parts of the curves.What do the fig 6 graphs look like if you have set it up (NS/NP) for a max duty cycle of 0.2, say?
Either can be stable or unstable. It all depends on the compensation. The point is that in DCM there is no inherent advantage to CCM over DCM, as far as I can tell.Yes that’s true but its not my point, my point is , that a DCM current mode flyback is always intrinsically stable, whereas a DCM voltage mode flyback can be unstable.
Okay, that sounds believable.The following literature exerpt shows that a flyback that is stable in CCM current mode, will always be stable when it subsequently goes into DCM at lighter loading…..
I don't see this claim anywhere in the documentation.this is not the case for voltage mode flybacks.
this claim is shown in the documentation..AN-57...I don't see this claim anywhere in the documentation.
..thanks, but that's terrible news, as it means that the only way you can get full frequency PWM for the flyback at max load is to design the turns ratio so that the duty cycle is 0.74 at maximum load....and that is generally totally impracticalThe plot inherent to the converter itself, I don't think it can be changed. Depending on your design, you will make use of differing parts of the curves.
That's a terribly weak argument for your broad claims. A more thorough explanation would include bode plots at each operating point, not just a couple transient load responses. I'm assuming that the DCM oscillation is due to large signal nonlinearity, and is stable in the small signal sense.this claim is shown in the documentation..AN-57...
App Note AN57 on fig. 13, page 9 shows that a voltage mode flyback that is stable in CCM, can go unstable in DCM.
(the plot shows vout when load is stepped from high load in CCM, to lighter load where its in DCM)..
The only downside to high duty cycle is that it means your peak switch voltage will have to be higher to get flux balance. I think this is one of the reason that the topswitch controllers are rated to relatively high switch voltages...thanks, but that's terrible news, as it means that the only way you can get full frequency PWM for the flyback at max load is to design the turns ratio so that the duty cycle is 0.74 at maximum load....and that is generally totally impractical
We use cookies and similar technologies for the following purposes:
Do you accept cookies and these technologies?
We use cookies and similar technologies for the following purposes:
Do you accept cookies and these technologies?