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Old 05-11-2012, 02:30 AM
suzuki suzuki is offline
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Originally Posted by t_n_k View Post
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.
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Old 05-11-2012, 03:04 AM
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Ron H Ron H is offline
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Just remember this equation particularly applies to simulators:

Garbage In = Garbage Out
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Old 05-11-2012, 05:23 AM
t_n_k t_n_k is offline
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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.
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File Type: pdf Bridge Sweep.pdf (43.5 KB, 12 views)
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suzuki (09-12-2012)
Old 09-12-2012, 06:53 AM
suzuki suzuki is offline
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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.

Thanks for your help.
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Related Site Pages
Section Title
Worksheet Basic AC-DC power supplies
Worksheet Rectifier circuits
Textbook Rectifier circuits : Diodes And Rectifiers

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