I have found this weird DC-DC converter topology in datasheet of MC34063 from ST.

Figure 24 MC34063 Datasheet

The primary charging is single ended, so at first it seems like flyback, but it has four diodes on the output instead of only two. Is this some forward or flyback hybrid? What is it called? Or is there a mistake in the datasheet?


It is a flyback.

Don't let the number of diodes confuse you here - that bridge rectifier is just necessary because the circuit strives to do full-wave rectification.

A flyback converter's transformer secondary is in principle isolated from the primary, so the output differential voltage can be referenced to any voltage.

  • \$\begingroup\$ The secondary is connected to the primary side via the feedback resistor in the circuit in the question, but the datasheet does show a way of isolating the ouput by using an additional secondary winding for the feedback. \$\endgroup\$ Mar 21 at 15:00
  • \$\begingroup\$ Oops, output, not ouput. \$\endgroup\$ Mar 21 at 16:18

This has some characteristics of a forward converter, (or it uses "forward" topology). See this wikipedia article or this guide.

Specifically, the current in the secondary of the transformer may occur at the same time as current in the primary.

However, it also has a characteristic of a fly-back.

Specifically, because the primary side of the circuit has no method to "reset" the flux induced by the DC component of the pulse train, (for example a diode and "reset" coil), the transformer must store (some) energy during part of the cycle, and release it to the secondary during another part of the cycle.

So, perhaps it is a hybrid between flyback and forward topologies? I originally thought it was a simple forward converter, but discussion with Jano952, gives me doubts.

  • \$\begingroup\$ But isnt reset winding needed in forward converter ? Or is it done using those two more diodes in output ? \$\endgroup\$
    – Jano952
    Mar 21 at 2:49
  • \$\begingroup\$ @Jano952 Look at the examples in "the guide" link in the answer. \$\endgroup\$ Mar 21 at 2:59
  • \$\begingroup\$ I have read it, but they mention the reset winding only in the two switch topology "two-switch topology does not require an auxiliary reset winding". Every other guide i saw mentioned using reset winding in single switch topology, so I am a bit confused :) \$\endgroup\$
    – Jano952
    Mar 21 at 3:12
  • \$\begingroup\$ Hmm. You make a good point. Maybe this is a hybrid topology. My reasoning that it is a forward topology is that, because the diodes form a bridge, secondary conduction can occur at the same time as primary conduction. However, the lack of a reset winding indicates that the transformer must store energy for part of the cycle and release it later, which is what characterizes a fly-back. \$\endgroup\$ Mar 21 at 3:22

Because the primary circuit goes open-circuit when Q1 is off - there is no snubbing or core-reset circuitry - I'm in the flyback camp.

IMHO - The reason for four diodes is that there are ((two)) outputs and it should improve efficiency.

In a standard, one-output flyback circuit, there is no transformer secondary current when primary current is moving because the output rectifier is reverse biased. That is why schematics show opposing transformer polarity dots. The "transformer" in a typical flyback circuit is not actually a transformer in the traditional sense; it is a pair of coupled inductors.

Anyway, in this circuit, the center-tapped secondary can create bipolar outputs with only two diodes. When Q1 is on and there is primary current, there also is secondary current because the other two diodes create a path for that direction of secondary current. that is one possible reason there are no transformer dots. Of course, the other is that the designer left them out. Still ... Cute.

  • 1
    \$\begingroup\$ Good points. But an aside not related to anything in particular. The transformer dots are unnecessary, because the polarity of the output is determined by the polarity of the diode bridge, not the polarity of the coils. The secondary transformer wires can be swapped, and it wouldn't affect anything. That's probably why the dots were omitted. \$\endgroup\$ Mar 21 at 3:44
  • \$\begingroup\$ Ummm ... , I think I said that. \$\endgroup\$
    – AnalogKid
    Mar 21 at 4:41
  • \$\begingroup\$ What exactly is the point of the multiple parentheses around the word two? Given my knowledge of other uses of multiple parentheses, it doesn't exactly leave me feeling comfortable to see.... \$\endgroup\$
    – Hearth
    Mar 21 at 4:47

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