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I've been experimenting on a lot of half-bridge-based PC PSU schematics. There's one thing I cannot understand:

enter image description here

As can be seen from above, there's an extra winding in series with the high side driving winding (marked with red ?) and one end of it goes directly to the primary of power transformer.

What is its purpose?

Thanks.

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  • \$\begingroup\$ Please clarify the dot for each winding. \$\endgroup\$
    – winny
    Sep 21, 2016 at 18:26

3 Answers 3

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The extra winding is proportional drive circuit. Its current adds (through magnetic coupling) to corresponding primary drive transformer winding. It is connected in series with main (power) circuit, so its effect to drive transformer is proportional to transistor main (collector) current. As a result the drive power is partially provided from the collector circuit, the drive requirements from the Q5-Q6 drive circuit are smaller.

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  • \$\begingroup\$ After a few months I revisited this. If I understood correctly, the purpose of that extra winding is to provide kinda "current support" for driving the transistors. So it's not necessary if the main switches are MOSFETs since they are driven with voltage instead of current. Correct? \$\endgroup\$ Jun 25, 2020 at 12:31
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Can you tell if the winding is actually magnetically coupled to the other windings? If not it is likely a shim inductance in order to facilitate resonant soft switching. Which controller is this design using? (Would have left this as a comment, but for some reason the comment link isn't working.)

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  • \$\begingroup\$ Actually I cannot. Also, neither of the schematics show the dots on the winding. Anyway, regardless of the controller IC shown in the schematics (which can be accessed from this site: danyk.cz/s_atx_en.html - categorized by the topologies), all the gate driving transformers have that extra winding on the high side. \$\endgroup\$ Sep 22, 2016 at 5:31
  • \$\begingroup\$ After some extra research, I realized that it's a shim inductance: Assisting/providing soft switching --thus decreasing switching losses. Some designers put this inductance externally (a few uH). Also, LLC half bridge converters do take advantage of that shim inductance (primary's leakage inductance, actually). Without further ado, accepting the answer. Thanks anyway. \$\endgroup\$ Dec 21, 2016 at 17:09
  • \$\begingroup\$ I had same question. To check and be sure whether this winding is used as shim inductance, I short this winding in my half bridge converter that has this extra winding and my power supply did not work. actually the winding is magnetically coupled to the other windings. So it is not used as shim inductance. \$\endgroup\$
    – Masood
    Jul 13, 2017 at 12:38
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this AUX winding is used to ensure CCM at light loads when using higher switching frequencies (over approx. 100 kHz)

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