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The full-bridge converter is just a version of the push-pull converter with additional MOSFETs at the primary side and an untapped transformer from the primary side.

A full-bridge converter requires extra high-side MOSFETs to control and brings in more MOSFET losses so there must be advantages too.

What are the advantages and disadvantages of a full-bridge converter versus a push-pull converter?

I think it can be about the maximum voltages that the MOSFETs are exposed to, or core saturation, but I'm not sure.

Push-pull converter:

push pull converter

Full-bridge converter:

Full Bridge converter

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  • \$\begingroup\$ In your first diagram, are Nx1 and Nx2 two separate transformers? If so, then you run the risk of saturating the transformer core, as I think you alluded to. \$\endgroup\$
    – SteveSh
    Dec 16, 2022 at 19:19
  • \$\begingroup\$ And the full bridge (or H-bridge) is used in higher power applications than the push-pull configuration. \$\endgroup\$
    – SteveSh
    Dec 16, 2022 at 19:19
  • \$\begingroup\$ No, I guess there are 4 separate windings on a single core. Also I still don't understand why power rating of full bridge push-pull is greater. \$\endgroup\$ Dec 16, 2022 at 19:28
  • \$\begingroup\$ OK, that makes more sense. So core saturation is not an issue. \$\endgroup\$
    – SteveSh
    Dec 16, 2022 at 19:31

3 Answers 3

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For the same performance full bridge needs less copper, but more silicon.

As a result at low powers the simpler push-pull is preferred while at high power the material savings and efficiency gains by having a more compact trasformer win.

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  • \$\begingroup\$ Less copper means less winding and more silicon means more components right? \$\endgroup\$ Dec 17, 2022 at 11:40
  • \$\begingroup\$ yes, more copper in the transformer primary, versus more switching components on the circuit board \$\endgroup\$ Dec 20, 2022 at 0:07
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Eliminating the split winding is a large advantage. Driving it balanced is a further advantage. But the bridges have disadvantages, too.

First, about leakage inductance.

Transformers are made of wire. Wire has length, and length is proportional to inductance. Real transformers have leakage inductance proportional to the length of wire used to wind them (plus a geometry factor -- it's complicated, of course, but this is sufficient hand-waving for the average case).

When the push-pull converter switches (commutation), load and magnetizing currents must stop in one leg, and resume in the opposite leg. This cannot happen instantaneously, and physical delay manifests here as a lumped-equivalent inductance. Thus, there is a series inductor with each MOSFET drain, which causes overshoot and ringing.

Occasionally, the primary's leakage inductance can be made small enough that the overshoot/ringing is negligible, given the size of the transistors and the rate they are switched at. But it's a problem more often than not, leading to reduced efficiency, the need for snubbers, etc..

The easiest way to eliminate the leakage is to eliminate the split winding. Thus we might prefer a half or full bridge circuit.

The half bridge circuit has the advantage of needing only two transistors as well, but one is "flying" and so is more difficult to drive (typically a bootstrap gate driver IC is added).

Note that the stray inductance (between transistors and supply bypass capacitor) of the bridge is equivalent to the leakage of the transformer -- there is a direct map between push-pull and half-bridge topologies. The difference is, we can usually make this inductance lower, with more ease, than for the transformer case. Which means higher frequency and power for given efficiency / component ratings.

The three topologies are typically chosen based on supply voltage:

  1. At low voltages and high currents, push-pull is advantageous. Dividing the supply current between two legs, reduces the current each transistor has to switch. The leakage is less painful than the downsides of the other types.
  2. At high voltages and low to medium currents, half-bridge is advantageous. Dividing the supply voltage between two legs, reduces the voltage each transistor has to switch. Note that a capacitive divider, or bipolar supply, is required for the load-return path. At low voltages, the required capacitors might be quite large, making it especially inconvenient, on top of the higher switch current.
  3. At high voltages and currents, full-bridge is advantageous. The advantages of half-bridge are had (it's nothing more than two driven oppositely), and the requirement for capacitive divider or bipolar supply goes away. (A DC-blocking (coupling) capacitor may still be desirable for transformer loads, but it doesn't need to be rated for full supply voltage.) Of course, twice the drive and power circuitry is required.

EMI is also not to be underestimated. This is dominated by the capacitance between primary and secondary windings. If the primary is driven symmetrically (as in push-pull or full-bridge), and if it is wound in such a way that the capacitances from each end of the winding to the secondary are equal, then the switching waveform balances out across the isolation barrier and minimial (mostly high frequencies due to delay or timing errors between the two sides) common-mode emission results. In contrast, half-bridge necessarily puts an average 1/2 to 1/4 supply voltage (depending how you count it) full switching waveform across the isolation barrier, which may necessitate adding a shield to the transformer, or more filtering components.

There is a two-transistor configuration which has the same EMI advantage, without the burden of a full H-bridge: the two-switch forward converter benefits from the low EMI of a full bridge, but uses transistors for half the waveform (the other half is clamped by diodes). It is also popular for modest power levels (100-1000W?), a common sight in desktop computer PSUs.

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  • \$\begingroup\$ Most of our systems use fairly low output voltages, less than 15 V. The difference in when a full bridge and a half bridge is used depends more on the output current than the output voltage, with the break point being around 10 or 15 amps. For example, 3.3 V supply @ 4 A => half bridge. 2 V supply @ 30 A => full bridge. \$\endgroup\$
    – SteveSh
    Dec 17, 2022 at 2:08
  • \$\begingroup\$ Is the reason why two switch forward converter cannot utilize transistors on both half-cycles that there is no second winding on the secondary which has zero current flowing thus isn't ready to accept any magnetic flux? And this lowers the power rating of the converter I guess. Also, why does split winding increase leakage inductance? Magnetizing current must stop and change direction in both types of converters. \$\endgroup\$ Dec 17, 2022 at 12:38
  • \$\begingroup\$ @CaveScientist If it had full-wave rectification, it wouldn't be a half-wave converter!.. Well, tautology aside, it seems the savings over a primary full-bridge is worth the reduced output duty cycle (i.e., say, 40% vs. 80% at the rectifier output). Less flux is used (only half the B-H curve), but Fsw is typically high enough that you wouldn't be able to use anywhere near +/-Bsat anyway. So you don't lose capacity for that reason. Split winding: it has more leakage between ends, than stray between transistors in a half-bridge. \$\endgroup\$ Dec 18, 2022 at 6:27
  • \$\begingroup\$ Nice comparison! About the last paragraph: What is a two-switch-forward? Do you mean with a switch instead of diode in the demag winding path? If so, isn't this then in fact just a push-pull converter? \$\endgroup\$
    – tobalt
    Nov 25, 2023 at 10:13
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    \$\begingroup\$ @tobalt It's a half-wave forward with two switches: i.stack.imgur.com/KGjrv.jpg \$\endgroup\$ Nov 25, 2023 at 10:34
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This is simplified, but to operate at a given input current, each coil in the push-pull configuration gets full current half the time. Conversely, at the same input current the one coil in the bridge configuration gets full current all the time.

To design the transformer with the same overall efficiency and power dissipation, the push-pull transformer would need two windings in the primary, where the bridge transformer would just need one.

It's not that exact, but that's why the general trend is for more copper and less silicon in the push-pull configuration, and visa-versa for bridge.

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