Lecture 28 Slide 9

My question is regarding the CPM Buck converter example on slide 9 of lecture 28. If Ma = M2 = M1 = 1A/us, how do we arrive at .11/A for Fm?

line commutated rectifier applications

In lecture 30, it was mentioned that the chapter on line commutated rectifiers may be eliminated as they don’t provide power factor correction (PFC) and thus not much used. I talked to Prof. Maksimovic about that statement and he clarified that there are indeed applications where such rectifiers are well suited for relative simplicity, low cost, or power levels. I would like to take this opportunity to mention one such application.

The application I am familiar with is where the ac voltage is not from the power line, but from an alternator (for instance, the generator in a small wind turbine system). The alternator source has no PFC requirement, per se, as it is isolated from the power grid. In such systems, the simple (line commutated type) rectifier is used to convert the variable frequency ac output of the wind turbine generator, i.e., alternator, to (variable) DC, which is then boosted and then inverted to power line AC (for connection to the power grid). The line commutated rectified DC is generally boosted using a simple conventional boost circuit. That boost circuit does provide some power factor correction action. As a result, the alternator current no longer looks like current spikes (when the rectifier diodes conduct), but actually with a nice current waveform (although, not quite sinusoidal). In my experience, the generator current lags the generator voltage by no more than, say, 15degrees, in the worst case (which is generally under light loads and also depends on the generator winding inductance and resistance). With such a design, one does has to be careful about the alternator current waveform at various operating powers. Even though there is no requirement for PFC, it is important that the alternator current waveform be not too much distorted or too much out of phase with the alternator voltage, as it results in torque ripple and acoustic effects. High current harmonic content may also lead to excessive loss in the windings.

Component differences in peak vs. avg. current mode control?

I am trying to figure out the difference in components between a peak and an average current mode control (ACMC) setup. There doesn't appear any difference between the block diagram of fig. 12.12 (which is in the peak current mode control section) and the block diagram presented for ACMC. Both have the voltage loop and the current loop as well as the comparator to compare iL and ic. I am guessing, then, the differences are: 1) The current sensor in peak control will be sensing the instantaneous inductor current, while the one in average control will sense the average inductor current. 2) The comparator comparing iL and ic in peak control operates in the open loop condition (no feedback to reduce its gain) and hence puts out either "1" or "0", whereas the one in ACMC operates with a finite gain (closed loop form). 3) Finally, the current command (variously called i^c*Rs and i^c*Rc in class) would need to reflect the peak and average inductor currents respectively. Would these be the only differences between a peak control and ACMC setup? Nitish

HW8 Prob 2

At the risk of coming across as a simpleton, how do you go about calculating the steady state duty cycle for differing loads in part a? Every time I try a different approach I come up with the same duty cycle for all loads which cannot be the case.

Midterm Exam Solution?

Will there be a midterm exam solution posted?

I do not see one, nor do I see anyway to get a PW for it on the CU Learn.

thanks
Mark

HW 7 Spice solution , part D link ?

the link for the HW 7 Spice sims netlist for part D,. points to the same as part C. Is this a mistake?

thanks
Mark

HW7 Prob2(b)

In deriving the loop gain T for this converter, I'm trying to understand what the impact R1 has. Can anyone please explain what it's supposed to do? Does it affect the sensor gain H(s)? Thanks.

Post replies

Has anyone else had an issue making replies to posts? I can make a new post - like this one - but I can not reply (or add to) any post. When I click 'post comment' or 'preview' nothing happens. I am definitely signed in. I have tried with both safari and explorer. Unfortunately if someone else is having the same issue, you will not be able to reply to this post! :-)

Also I have never been able to stream a continuous un-interrupted feed of any lecture. It always eventually hangs and I have to restart it and forward to where it hung up. Is anyone else having this issue? Has anyone found a fix to either issue?

Thank-you! Tim

HW7 Problem 1 Zout

How do you go about getting plots for Zout on problem 1? For Gvd I sweep Vg with Vd at the duty cycle voltage, for Gvg I reverse that by sweeping Vd and setting Vg to 48V but I am unable to get plots for Zout. I've tried placing another source across the output and sweeping it while plotting V/I but that doesn't work. Someone drop me a note if you get a chance. Thanks.

HW 7 problem 2 steady state

HW 7 problem 2a seems pretty straightforward in nature, but I am having a difficult time with the number of unknowns in the problem. It seems like the problem does not give us R, R1, or V. Because of this, I am having trouble finding equations for D1 and D2 that are not dependent on these unknown quantities. I have solved for I(D1,D2) as the average value of the inductor current, and I have the buck equation for M(Re, R) and Re(D) in DCM from the book, but I am still having trouble piecing it together with these unknowns. Could anyone give me a hint on what I am missing?

solving DCM buck, and boost for M

on page 420, we see the steady state model for the buck, and boost converters in DCM.

I do not understand how to solve these for M (M= output Volage/Vg in steady state). The buck-boost on page417 was easy.

Can anyone help me to understand how to solve for these?

thanks
Mark

HW5, prob.1, determination of s^2 numerator zero contributed by L2C1

Hello,
In HW5, problem 1, I cannot seem to determine the s^2 numerator zero contributed by L2,C1. For determining the first order zero contributed by C1, I assumed a samll resistance 'r' in series with L2 and the result was: v^test/i^test = r. Now, to determine the zero contributed by L2C1 together, I have set C1 as short (abnormal) and put the test source across L2 (with shorted L1 represented by a small resistance 'r') . I was expecting the resistance see would be 1/r....but I cannot get that.
Any hints would be useful.
Regards,
nitish

Null Loop Gain Question

For question 1 of HW6 I have expressions for G0, Ginf, and T. I'm now chasing Tn. It's tempting to just use reciprocity to find it but the next step is to check the reciprocity relationship so that would be somewhat redundant. Starting with iy/ix with vout = 0, it appears that ix would be zero since vout = 0 (This is with the test source between gm and R0 and ix is the current between gm and R0). That being the case, Tn will either blow up or be zero depending on the value of iy. Obviously I'm missing something here. Any hints on how to find Tn?

Problem 11.1

For Problem 11.1,

are we supposed to derive a model of the switches for DCM?

The problem does not actually say to do that, however part 1 is a step in that direction.

thanks
Mark

Lecture 21 Quesion?

In the slides for Lecture 21, the professor showed a signal "c" and "c^", are these supposed to represent the Vc(t) input from the compensator to the PWM ?

thanks
Mark

prob. 11.1(e); Lp =0; does Re become 0? which means j1 and j2 = infinity?

Hello everyone,
In prob. 11.1(e); Lp =0; Does that allow us to make Re = 0 as it has Lp in its Numerator ? Making Re=0 would make j1 and j2 = infinity as they have Re in their denominator.
Any comments would be useful.
Thanks,
Nitish

DCM waveform levels for the buck-boost converter (fig. 11.3)

Hello everyone,
In fig. 11.3, the buck-boost converter DCM waveform levels are shown in terms of instantaneous values of vg and v....but shouldn't they be written in terms of their average values ,vg> and ?
For example, v2(t) waveform level during interval d1Ts is shown to be vg-v. But shouldn't it be - ? Similarly, the vL(t) voltage level during d1Ts is shown as vg. It appears to me that it should be shown as .....and so on, for the rest of the levels. If representing only in terms of vg and v, the implicit assumption is that they are constants.
Also, eqn. 11.1 should be written as ipk = d1.Ts/L instead of vg.d1.Ts/L.
I would appreciate any comments.
Regards,
Nitish Agrawal

General Feedback Theorem

I came across this paper on General Feedback Theorem



I have not read it, but it looks well written.

Mark

lecture 18, slide 34 (when expressed in terms of R, L, C....)

Hi everyone,
I did not understand the lecture 18, slide 34 summary statement, "when expressed in terms of R, L, C, and M (not D), the small signal transfer functions are he same in DCM and in CCM".
Can anyone shed light on this?
Regards,
Nitish