All About Circuits Forum AC sweep (using PSIM)
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#11
05-11-2012, 02:30 AM
 suzuki Senior Member Join Date: Aug 2011 Posts: 119

Quote:
 Originally Posted by t_n_k If you are actually looking at the frequency response of the bridge rectifier plus RC load then that is a more uncertain problem. Keep in mind that the diodes operate in discontinuous mode with the presence of a load filter capacitor. A simplistic view is to consider the main AC supply cycle period to be infinitely long in comparison with the sweep frequency cycle period range. One would then consider one opposing diode pair of the diode bridge to be in quasi-static forward bias. In that case one would have the equivalent dynamic diode resistances in series with the load and the sweep frequency source. So one might expect to see a typical single pole -20dB per decade roll off. It eludes me as to why one would want to know this ....???
I actually really like this idea. So in each half cycle, you essentially just have 2 diode resistances, the load R and C. I think this should give a pretty good idea of the frequency response.

I guess to clarify, i had the dc source there as a test for the frequency response. It should be ac going into the rectifier, and the capacitor is mainly for filtering.
#12
05-11-2012, 03:04 AM
 Ron H E-book Developer Join Date: Apr 2005 Location: Idaho, USA (GMT-7) Posts: 7,050

Just remember this equation particularly applies to simulators:

Garbage In = Garbage Out
#13
05-11-2012, 05:23 AM
 t_n_k Senior Member Join Date: Mar 2009 Posts: 4,949

I tried using PSIM for this 'concept' but obtained a better result using Simetrix.

The attachment shows the simulation model and two frequency response results for two different bridge AC input voltages - namely 2V p-p and 5V p-p. The result of differing diode dynamic resistances is readily observed with the lower cut-off frequency resulting from a lower AC source voltage.The dB ratio is obtained from the load capacitor back to the AC small signal source sensor / isolator.
Attached Files
 Bridge Sweep.pdf (43.5 KB, 12 views)
 The Following User Says Thank You to t_n_k For This Useful Post: suzuki (09-12-2012)
#14
09-12-2012, 06:53 AM
 suzuki Senior Member Join Date: Aug 2011 Posts: 119

Thought I would bump this thread and continue rather than start a new one.

That is a pretty interesting result, t_n_k. Sorry, I did not see it until now as I've been away. The fact that the response changes so much based on the input voltage seems to make it even more difficult to model or at least approximate the bridge rectifier as a single frequency response transfer function.

Since this time, what I have tried to do is the following, although I'm not sure if it is correct, so I'd love to have some input regarding my method.

If we take that circuit you provided, I called the input port $V_i$ and then output of the diode bridge (before R1 and C1) $V_d$, and lastly the port across C1 is $V_o$

As you suggested in a previous post, consider the path through the diode bridge as a series resistor $R_d$. Therefore, the total output voltage is given by $V_o =V_d*Z_o/(Z_o+R_d)$ where $Z_o=R1/(1+sC1R1)$.

Would this voltage divider then give the correct transfer function? If you solve for $V_o/V_in$, you get a single pole, but it doesn't appear that I would get the same DC gain values as you did in your SIMetrix model.

 Tags psim, sweep

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