Bipolar Power Supply Load Current Imbalance And Transformer Core Saturation

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mediatechnology
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Re: Split Power Supply Load Balancing

Post by mediatechnology » Sun Nov 15, 2020 2:19 pm

It has occurred to me that the "full wave" portion of the term "full wave center tapped" may be somewhat of a misnomer.

The FWCT voltage output appears to be full-wave - the same as a four diode "full bridge" - but the current waveform and flux are half-wave in the sense that they occur only on one polarity of peak.
The current and flux peaks are at 2*Fin (resembling FW) but they are still unipolar. (Resembling HW.)
The output is full-wave but the winding and core respond similarly to a half-wave rectifier due to secondary Idc.

Though FWCT and FW-Bridge are both called "full wave" they are entirely different in terms of secondary Idc and how the transformer core responds.
The true FW-Bridge has no Idc other than insignificant levels due to diode mis-match.

Speaking of secondary Idc...

In the FWCT configuration, from an AC perspective, the windings are series-stacked and the end taps are in anti-polarity with respect to the CT.
At DC, the windings appear to be in parallel but only for one polarity. (In the opposite non-conducting polarity they are unconnected.)
It looks like the magnetization from each secondary adds in the core rather than subtracts or "bucks."
This appears to be the case for a transformer with two independent secondaries (four wire) connected to form a common CT.
I'm not sure if magnetization adds or bucks for a three wire secondary with an actual CT.
Not many low-VA universal input transformers are made that way anymore but there may be some CT "filament" transformers with a classic CT used to make a split supply that would behave differently.
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terkio
Posts: 242
Joined: Thu Feb 07, 2019 4:17 am

Re: Split Power Supply Load Balancing

Post by terkio » Mon Nov 16, 2020 12:04 am

From my simulation: Average currents over an exact period.
WindingA -463.49mA
WindingB 463.49mA
Center Tap -926.98mA

Loads 10 Ohm and 50 Ohm.
1.2301A and 303.13mA
Difference -926.98mA
_________________________
Edit. Now, with balanced loads

WindingA: 0A
WindingB: 0A
Center Tap: 0A
Loads 10 Ohm and 10 Ohm.
1.2301A and 1.2301A
____________________________
Now, with total imbalance.

WindingA: -614.45mA
WindingB: 614.45mA
Center Tap: -1.2289A
Loads 10 Ohm and None.
1.2301A and 0A

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