HW5 Prob 2 Value Check

Hey everyone, just wanted to do a value check on HW5 prob2:
b) when I compute the steady state V expression @ fso= 110k, I get Vout = 23.9-j48.2
I'll skip small signal since it can't be evaluated without 's' as well as 2c's expression
d) I get Genv(0) = 27.5 this seems wrong to me.

For part d I did the following:
Genv(s) = v_hat / d_hat (Kpwm term multiplies into this but doesn't change the value)
v_hat = ((V* x v_hat) + (V x v_hat*)) / (2 x ||V||)

Last question, the 's' in the small signal voltage AM expression 's+jwso' (part b of this problem) is the update rate of the duty-cycle modulation correct?

Ugly Algebra

Is anyone getting a manageable version of ||H(jwso)||? I am at the point where I can barely fit the equation on a single sheet of paper and it's not looking good about reducing the thing much more...

Here's what I got for ||H(jwso)||:

RCwso/(sqrt((RCwso)^2 + (1-LCwso^2)^2))

Just wondering if anyone got anything would would get rid of the radical and help to reduce the massive ugliness that ensues with Genv(s).

Thanks!

The principle of modulation frequency

Hey everyone, just want to make sure I am thinking of modulation frequency, fmod, correctly. fs is the switching frequency and basically fmod is how often I am adjusting the switching frequency, is that correct? So in a real system, fmod would be fixed value, a lower value for slower output regulation, and a higher value for faster output regulation, correct?

Also, the 's' in Genv(s) is the modulation freq (s = j*2*pi*fmod), correct?

Thanks for the clarification.

HW 3 19.8c

I need some help finding fm from 19.8 c. Here is how I'm starting
Zi0(jw)=1/jwC+jwLs Ziinf(jw)=1/jwC+jw(Ls+Lp)
Zio(jw)=j*(wLs-1/wC) Ziinf(jw)=j*(w(Ls+Lp)-1/wC)

||Zio(jw)||=wLs-1/wC ||Ziinf(jw)||=w(Ls+Lp)-1/wC

When I set these two equal to each other I end up with wLs=w(Ls+Lp).
Let me know how I should approach this problem.

Question about example problem and solution

Hi,

I was working through the example problem and solution provided and came up with an inconsistency that relates to homework problem 1.

In the solution, on page 4, the denominator polynomial is shown as having two roots at

s = (-wo/2Q) +/- j(wo/2Q)..............

My problem is with the (wo/2Q) terms. After working through the problem on my own, everything seems fine, except those terms should instead be (1/(2Qwo)).

Even if you work out the problem using R's, C's, and L's, my solution would still be simplified to (1/(2Qwo)). Because the tank transfer function is the of the same form as homework problem 1, this makes a difference for the homework assignment. Can someone please confirm if the example solution is correct or if there is an error in the solution? Thank you.

Problem 19.9

Is anyone getting a Rcrit < 500 Ohms? I was going good until I hit part e and I think I may have done it all wrong..

HW3 Problem 19.6

Hi All,

I am getting started on HW 3 and have run into some confusion on the first problem. Can anyone steer me in the right direction... Here's what I was thinking: the 200 kHz that is mentioned in the problem is fs. In part a it says to derive expressions for Voc and Isc... I am assuming that Voc will be (4/pi)*Vg * H_inf(jws) and Isc = I/sqrt(1-(V/Voc)^2), where I=Vnom/Rnom and V = Vnom. The issue that I am finding is that I am not seeing how to get an F term in the expressions, any hints?

HW 2 Prob 19.4 below vs above resonance

I believe in the converter operates such that. Is this correct?

below resonance, vc(t) leads ig(t)
above resonance, vc(t) lags ig(t)

Thanks

HW 2 Prob 19.3b - sketch bode?

I'm a bit confused on the wording of the problem. 19.3b: "Sketch a Bode diagram of the parallel LC parallel tank resonant."

Isn't this the same as H(s)?

But H(s) = (sL||1/sC||Re)/(sL||1/sC||Re) = 1 given that Zo(s)=Zin(s).

If that's the case, what's the point of plotting H(s)? Or is the problem asking to plot the bode for sL||1/sC? Thanks.

HW2 19.1d

Hello everyone, I have some questions on HW2 19.1 (traveling at the moment and running behind on homework):
19.1) The effect of the transformer on the inductor seems to be kicking my fo to be in the 800kHz range which seems awfully high. Did I do something horribly wrong? For Zo I have L/400 || C || Re. I get an answer that is very close to 19.31 except with a scaling factor (for the transformer and due to the input square wave having a DC offset).

Also, when I get to part D, my fs is less than fo when I assume n=1. For n=3 I get imaginary values for F. I would have expected an fs higher than fo since that is assumed above. I also would have expected fs and fo to be very close...

thanks for your input,
charlie

Etymology of "resonant tank"?

Brief change of pace: anyone know the origins of the word "tank" as it's used in this class, and electrical engineering in general?

Wikipedia claims it's got to do with the image of water sloshing back and forth in a tank, but doesn't cite any sources, and I can't find any other clues in my brief search.

HW #2 19.1 E and G

I am having a little trouble figuring out the peak current through the transistor for 19.1 Part E.  So far my thoughts are that the peak transistor current will match the peak ig, input current, which is also the same as the peak current value through Cb. So I get something like Ig = Is1cos(phi_s)/pi and want to solve for Is1. I feel like I should relate the equation back to power, i.e. Pg = VgIg = <ps(t)>, but I don't know how to solve for the input power either without Ig. I've been circling around this for a little while, and feel like I missing some minor(?) detail. Any help is appreciated. Thanks!

in SRC (lecture 4), is phi_s = phi_r ? [i_s(t) = i_r(t)]?

SRC of lecture 4, we said: v_r(t) ~ Vr1 * sin(ws*t - phi_r) and i_r(t) = Ir1 * sin(ws*t - phi_r).
Also, we said that i_s(t) = Is1 * sin(ws*t - phi_s).
Since the resonant circuit is in series, one would think that i_s(t) = i_r(t). Then, Is1 = Ir1 and phi_s = phi_r. Is that true?
Regards,
Nitish

HW 2 Problem 19.3

I find that my equivalent circuit model for part A is identical to Figure 19.22, with a different tank network, and hence a different H(s). Because L and C are in parallel, Vc(t) (or Vs(t) depending on which figure you are looking at) equals Vr(t), making H(s) equal to one. This would then make M, which was equal to Eq. 19.27, solely equal to 8/(pi^2), which would not depend on Qe and F. Any ideas on how I am doing this so drastically wrong? Thanks

Lecture 8, 9, 10

Hello Class

Are we having the correct lecture videos for 8,9 and 10? Thanks.

Spice sim of pll res DC-DC converter lecture 5

I was trying to simulate the pll resonant DC -DC converter of lecture 5, to see if I got a square wave shape i_R(t). I noticed that i_R(t) appeared square wave like only before the steady state was reached, after which it appeared like DC. Moreover, when i_R(t) did look like a square wave, its amplitude kept slowly changing over several cycles.
I used a square wave signal gen +/5V, 100kHz
Lr = 1.591uH, Cr = 1.591uF
Lf = 20mH, Cf = 1500uF

Any comments would be appreciated.

Zero Current Switching in practice

Hi,

In the book (and also in the lectures) it is explained that ZCS occurs when (for a full bridge) Q1 and Q4 are on and the current becomes negative (below resonance, current leads voltage, see section 19.3.1). The diodes D1/D4 then automatically take over the currents of Q1/Q4 and there is ample time to turn of the switches.

The part I don't completely understand is why the diodes would take over the current. That may be the case with ideal diodes that have forward voltage Vf=0V. In practice, a threshold voltage is required to forward bias the diodes, so Vf~0.7V. Since MOSFETs are bidirectional devices, they would conduct the negative current and their Ron*Isd would have to be larger than Vf before the diode turns on and takes over (some of) the current. Now suppose Ron*Isd is smaller than Vf, the diode would not conduct and the transistor would carry all the current

For kW type converters, ZCS may be easy to achieve, since probably Ron*I>Vf. But is it also practical to use ZCS in smaller converters?

Any comments would be much appreciated.

Regards,

Toby

H(s) for 19.1

I am getting something really ugly for H(s) for the resonant tank...

I am starting with:

H(s) = Zo/((Ls/n)+ n/Cbs)

where Zo is (Ls/n + n/Cbs) || 1/Cs || Re

Did anyone approach this differently?