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I've been experimenting with a half-bridge converter and there is something that I don't understand: what is the purpose of the capacitance and extra winding in the gate base driver?

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The extra winding is magnetically coupled with the other winding of the driver transformer.

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    \$\begingroup\$ The capacitor you have circled is here to prevent the transformer primary-side winding to be dc-biased during switching operations. Usually, there are two caps (runonielsen.dk/Half_bridge_control.pdf). Now, regarding the link to the gate-drive transformer, it looks like a kind of speed-up circuit further biasing \$Q_1\$ and \$Q_2\$ as soon as they start conducting. It could be the case, especially if these two bipolar transistors need a large base current to turn on. I would personally use MOSFETs instead and remove this winding. \$\endgroup\$ Jul 13, 2017 at 13:26
  • \$\begingroup\$ thanks for your help. but if the extra winding is used as speed-up circuit, then after turning the bjt on it will never turn off. Also I have a half bridge converter and I shorted this winding and it did not work at all. \$\endgroup\$
    – Masood
    Jul 13, 2017 at 16:49
  • \$\begingroup\$ Your remark regarding the speed-up action makes sense. If you want to get rid of this winding, do not short it as you short the drive also! Leave it unconnected instead (or load it with a resistor) but connect the left-side of the current sense transformer to the half-bridge node so that the controller still senses the current. \$\endgroup\$ Jul 13, 2017 at 17:09
  • \$\begingroup\$ You are right, shorting the winding is not a good idea. \$\endgroup\$
    – Masood
    Jul 13, 2017 at 18:15
  • \$\begingroup\$ I've read the article (runonielsen.dk/Half_bridge_control.pdf). But there is nothing in it about the mentioned capacitor. It describes about two capacitance (C5 & C6 in picture above). I think C10 (mentioned capacitor) is used to prevent high voltage transformer to saturate, is it right? \$\endgroup\$
    – Masood
    Jul 13, 2017 at 18:26

2 Answers 2

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The below circuit shows a quick simulation file concentrating on the power section only. The thing is to crank the power supply by forcing a self-relaxing activity through the extra winding you mention. An initial start-up current is given by \$R_2\$ which biases \$Q_1\$ at power-up. Given the presence of the speed-up winding and its polarity, this strengthens the biasing of \$Q_1\$ until its collector current no longer rises given the available base current. As \$\frac{d\phi}{dt}=0\$ in the transformer, the transistor blocks because the extra bias disappears. The polarity reverses on the speed-up winding, effectively blocking \$Q_1\$ and now biasing \$Q_2\$ until a new cycle repeats. Then I believe the TL494 will soon drive operations once the auxiliary supply is alive. The start-up sequence seems to be at low frequency and nicely oscillating considering the arbitrary values I put there. It should be enough to understand the role of this extra winding which blindly (there is no over current protection or over voltage protection in the sec. side either) ensures a self-relaxing start-up until the TL494 takes over. I have looked at the Czech site in which the owner gathered a lot of power supply schematics (nice collection!) and it was the way cheap ATX power supplies were designed a while ago. I however do not see the need for a self-relaxing circuit like this one when a 5-V USB auxiliary power supply is present in the silver box as this power supply starts first and then provides a \$V_{cc}\$to the control circuit. Nowadays, most ATX boxes uses a 2-SW forward converter with MOSFETs and a dedicated control section powered from a standby power supply.

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  • \$\begingroup\$ Good answer! By the way, I've asked a similar question nearly a year ago but the extra winding in my question serves a different purpose. Interesting to see such a different approach. So I fav'd the question and answer. I also want to append some extra info: ... \$\endgroup\$ Nov 15, 2017 at 5:15
  • \$\begingroup\$ ... I however do not see the need for a self-relaxing circuit like this one when a 5-V USB auxiliary power supply is present. Actually, there's always the extra winding in the gate-drive transformer even if the power supply has an independent SB supply circuit (example). But, as I said, those windings serve a different purpose: Since the bridge switches are BJTs, the switching losses will be much higher than MOSFETs. So those extra windings provide a soft transition of primary current (think of a resonant pump) and decrease the the switching losses. ... \$\endgroup\$ Nov 15, 2017 at 5:25
  • \$\begingroup\$ ... And yes, those extra windings are not needed when MOSFETs are used and provided an independent SB supply circuit. By the way, the most big disadvantage of the OP's circuit is the absence of the standby power supply circuit because the circuit "may" be unstable on light loads even if the FAN is used as a dummy load. Nowadays, most ATX boxes uses a 2-SW forward converter with MOSFETs. True. But, half-bridge and full-bridge are the most common topologies for new designs due to the need of 80+ GOLD, PLATINUM or TITANIUM certificates for high power (>500W) supplies. \$\endgroup\$ Nov 15, 2017 at 5:35
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No one answered the specific question "what is the purpose of the capacitor". The answer is: to prevent transformer dc bias in 110-V grid operation.

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    \$\begingroup\$ Your answer would be true without "... in 110-V grid operation" phrase. \$\endgroup\$ Nov 15, 2017 at 5:52
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    \$\begingroup\$ @Cubrilo I agree with H&H, but your addition to the end of "in 110-V grid operation" is not in H&H. \$\endgroup\$
    – W5VO
    Nov 20, 2017 at 13:44
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    \$\begingroup\$ @Cubrilo You're talking about the supplies having 110V/220V selection switches. But blocking capacitor usage is not related to this. Yes, the two series capacitors provide the midpoint and an inequality caused by them leads to flux imbalance (remember the Half- and Full- Bridge converters' cores operate on 1st and 3rd quadrants of their BH loop) and the BH loop to "walk" which we never want. And the series blocking capacitor is placed to avoid this. But come on! This is not related to the voltage doubler! \$\endgroup\$ Nov 22, 2017 at 13:31
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    \$\begingroup\$ @Cubrilo Did you really think about why the series blocking capacitor is not needed in LM5039 eval board? You said because -SURPRISE- they don't need it, but it is not related to 110V operation. The series blocking capacitor is not needed because LM5039 measures the average current (via T2 current transformer and BR1 bridge rectifier), thus the potential flux imbalance is automatically avoided. You can check it from p.12 of the datasheet. \$\endgroup\$ Nov 22, 2017 at 13:47
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    \$\begingroup\$ @Cubrilo Result: If current mode control is used then the series blocking capacitor is not needed. You can check this from any academic or non-academic source you want. Two good examples: Switching Power Supply Design (from Abraham Pressman) and Switchmode Power Supply Handbook (from Keith Billings). \$\endgroup\$ Nov 22, 2017 at 13:52

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