Q: When is Full-Wave not Full-Wave?

Posted by Triode_Kingdom (A) on October 15, 2016 at 15:34:08

A: When it's a voltage doubler.

Explanation...

Below is a schematic of a full-wave voltage doubler:

Note that the output ripple frequency of this supply is 120 Hz. By definition, it's a legitimate full-wave rectifier. However, something is happening internally, a hidden downside to the design. In addition to the 2X increase in current demanded from the power transformer (compared to a standard full-wave or full-wave-wave bridge), there's a degradation in effective filter capacitance that's easy to miss.

Looking at the top of C2, there's 75uf of total capacitance (two series 150uF). If each cap has an ESR of 2 ohms, total ESR is 4 ohms. Combining the ESR with XC, the total Z across the circuit should be 18.14 Ohms. Unfortunately, this is an illusion.

Below is a SPICE plot showing output ripple voltage (Blue) and currents through C1 (Green) and C2 (Red). The ripple is indeed 120 hz, but currents through the filter capacitors aren't. Each cap only sees a 60 Hz charging current. This greatly reduces the filtering ability of these components compared to a standard rectifier/filter. Halving the frequency doubles XC. It also doubles ESR. This means the two series caps actually exhibit 8 Ohms ESR and 35.4 Ohms XC. These combine to create a total shunt Z of 36.28 Ohms, exactly twice the previous number.

None of this was apparent to me until just recently. Harman Kardon used this topology frequently in their amplifiers and receivers, and I'm rebuilding several of those. As a result of this analysis, I'll be making substantial changes to add more energy storage and to reduce ripple in all of them.

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Topic - Q: When is Full-Wave not Full-Wave? - Triode_Kingdom 15:34:08 10/15/16 (16)

RE: Q: When is Full-Wave not Full-Wave? - used-hifi 17:34:02 10/19/16 (0)

RE: Q: When is Full-Wave not Full-Wave? - Tom Bavis 07:46:50 10/17/16 (0)

RE: Q: When is Full-Wave not Full-Wave? - Eli Duttman 19:56:24 10/15/16 (9)

RE: Q: When is Full-Wave not Full-Wave? - Triode_Kingdom 22:11:16 10/15/16 (8)

RE: Q: When is Full-Wave not Full-Wave? - Eli Duttman 23:46:49 10/15/16 (7)

RE: Q: When is Full-Wave not Full-Wave? - Michael Samra 05:19:01 10/16/16 (0)

RE: Q: When is Full-Wave not Full-Wave? - FenderLover 00:34:03 10/16/16 (5)

High power rectifiers - Triode_Kingdom 08:33:15 10/16/16 (3)

What filter capacitors did they use? - FenderLover 17:05:11 10/17/16 (2)

RE: What filter capacitors did they use? - Triode_Kingdom 19:22:19 10/17/16 (1)

RE: What filter capacitors did they use? - FenderLover 20:21:38 10/17/16 (0)

RE: Q: When is Full-Wave not Full-Wave? - Michael Samra 05:16:46 10/16/16 (0)

"Each cap only sees a 60 Hz charging current." - FenderLover 18:21:55 10/15/16 (3)

RE: "Each cap only sees a 60 Hz charging current." - Triode_Kingdom 21:46:09 10/15/16 (2)

RE: "Each cap only sees a 60 Hz charging current." - FenderLover 22:23:48 10/15/16 (1)

RE: "Each cap only sees a 60 Hz charging current." - Triode_Kingdom 08:12:38 10/16/16 (0)

Q: When is Full-Wave not Full-Wave?

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