HW13 (d)&(e)

Anyone having any luck with part (d) and (e) of HW13?  I'm trying to figure out if we're supposed to build our own model to do plotting, etc?

Problem 18.14 a. choosing value of C

For 18.14 part a. I assume that we must first determine a value for C to satisfy the Gvd transfer function for the loop gain of Ti = (va/ig) * (Gc) * (1/Vm) * (Gvd). Is this the same approach other folks are using?

18.10d&c RMS Ripple Assumptions

In these problems it asks us to find the worst-case rms currents of the inductor, MOSFET and diode.  When it refers to rms currents is that over a line cycle such as in Table 18.3?

Furthermore, the expressions in Table 18.3 neglect the effect of switching ripple.  Does this mean that we cannot use the expressions given in Table 18.3 and that we need to derive new expressions that take into account the switching ripple with the inductor that we have chosen?

Question for 18.1

Hello,

I am confusing about the define of the LOAD POWER in 18.1 (d). Does this contain only the power on the resistor or also contain the power both on resistor and capacitor?

Thanks.
NY

Re for 18.2

So I am starting to confuse myself on what Re should be for 18.2.  Do we use the DCM boundary to determine the Re for DCM or is it the standard 2L/d^2Ts?  What decides Re?
It seems that because the vg/ig=Re in both the DCM of Chpt 11and in our current model, Re is the same but if this were a Cuk or Sepic then Re would be different.  Is this correct?

Suggestions for OpAmp model

I have suggestions for the OpAmp model to use for HW10 if you have problems.

Try using "UniversalOpamp2" instead of LT1368.
The default parameters for this model is sufficient for our simulation.
The ideal opamp seems to not work well for many people.

If the design is done correctly, there should be no convergence problems, and no need for initial conditions.

Beom

PWM comparator questions

I have not been able to find a comparator that will work in this application. I'm sure there is one, but I'm not having much luck with getting the output for d correct. I have the sawtooth, and Vm , but I'm struggling with the comparator.
Suggestions?

Extra Credit

Hi All,

I am a little confused as to what the difference is between the last part of problem 1d and the extra credit is... they both ask to verify problem 1 using spice...

Can anyone elaborate on what the difference is between the two?

Thanks!

HW10 P16.1

In part (d), I am a little confused about the question statement. Are we supposed to find two power factors, one for input (surface S1), and one for output (surface S2)? Thanks.
- KT

HW 10-1 Creating PWM Spice block.

Hi,

I've read in the text that the PWM block is modeled in Spice as simply vx/VM where vx is the input and VM is the sawtooth peak value. The text also says to limit the output to between 0.1 and 0.9. A couple of questions:
1) I used the LTSpice voltage dependent voltage source, put the output of the compensator into the component and set the scale factor to 0.5. Does this work?

2) Where do the limit values 0.1 and 0.9 come from and how can they be implemented?

Thanks.

-Drew

failure to find a DC operating point

* C:\Program Files\LTC\LTspiceIV\homework 9.asc
Vg 1 0 12 AC 1
L 2 4 4µ
R 4 0 1
C 4 0 100µ
XU3 Vc N003 N005 N006 5 CPM Rf=.1 Va=0.3125 L=4u fs=100k
E2 N005 0 1 4 1
E3 N006 0 4 0 1
H1 N003 0 value={i(L)}
XU1 1 2 2 0 5 CCM-DCM1 L=4uH fs=100k
XU2 N001 N004 Vc opamp Aol=100K GBW=10Meg
R1 4 N001 10k
R2 N001 N002 9.1k
C2 Vc N002 11n
C1 Vc N001 700p
Vref N004 0 5
.lib switch.lib
.ac dec 101 10 100k
.lib opamp.sub
.backanno
.end

--------------------------

With a higher value for R (around 10 ohms) spice can resolve to a DC operating point. At R = 1 I get "Failed to find DC finding operating points for AC analysis".

Any suggestions?

I have tried (most) of the suggestions listed. 

compensation circuit questions

On homework 7 we were given a type 2 compensation network for problem 2 as part of a Buck voltage regulator operating in discontinuous conduction mode. As we are using this same compensation circuit for Homework 9 I have a few questions regarding the solution given on homework 7.

I have a calculated transfer function of vc/v is equal to: (sC2R2 + 1)/[sC1R1*(1+sR2C1C2/(C1+C2))].  If the pole at C1R1 is designed to be equivalent to C2R2 then (sC2R2 +1)/sC1R1 becomes 1 + w/s, an inverted pole.

In the solution, it states that "at mid-frequencies around fc, Gc(s) = R2/R1". Can anyone give me any help on where this comes from and how we determine it?

Additionally, the equation used to determine the crossover frequency where T = unity gain is given as:
1/Vm * R2/R1 * Gd0 * fp/fc = 1
where does the fp/fc come from?


UDPATE: I actually found a good paper on the Intersil site that discusses this compensation network.
www.intersil.com/data/tb/tb417.pdf

I'm still confused about the unit gain equation, and anyone who can give help on this would be appreciated.

Gallium Nitride (GaN) presentation tonight in DLC

In last Friday's lecture Professor Maksimovic mentioned that the big trend at APEC was the increasing use of wide-bandgap and compound semiconductor devices, like those made from GaN and SiC.

I would like to invite you to my presentation tonight in the DLC called "Putting GaN To Work In Your Power Supply".  More information is here: http://www.denverpels.org/Meetings.html.

The meeting starts at 6 pm with pizza and networking.  This is a great way to meet local people working in power electronics.  The presentation will start about 7 pm.  The meeting is open to all interested persons.  You do not have to be a member of the IEEE or the IEEE Power Electronics Society to attend so please join us in you are interested.

Problem 9-2 factoring denominator

Hi,

I've started problem 9-2, and I've run into an issue. I began with the same analysis as the text and the lecture notes. I used equation 12.89 (for Buck) and substituted in Fm = Fm/(1+s/wx). The denominator becomes 3rd order which means you can't simply rearrange to find new expressions for Qc and wc.

Do we have to do some factoring of the denominator, or some approximation to isolate the effective high frequency pole?

Thanks.

-Drew