Does isolated full bridge converter (DC-DC) not need to use snubber?

You don't say whether you're using single switch or double switch half bridge. I recommend the latter for its various advantages, along with the 2-rectifier approach to steering off-cycle current back to the primary storage capacitor. You can find lots more info by searching on a phrase like "2-switch forward converter". Full bridges have their own set of problems. Transformer construction determines leakage inductance. Have you done everything possible to minimize it? It mainly depends on insulation volume between primary and secondary, but you can greatly reduce it by using a split primary, also. Choose a wide-window core if possible, too. You shouldn't find it necessary to snub the primary switches if you design the transformer well. However, snubbing the secondary rectifiers can be very beneficial, since ringing across them as they switch off is reflected back to the primary. If the optimal snubber has too high dissipation, check to see if a sub-optimal capacitance provides good enough reduction of ringing amplitude. Paul Mathews

Hi Paul, thanks for answering. Sorry for not having stated clearly. My original design was to use half bridge two MOSFET push pull converter, not forward. As for the transformer, I've heard that Lp can be reduced by doing a P-S-P strructure. However, there is only 6 turns in primary winding, so it is impractical to split it in to two sections. I think the small number of turns of Np is the main reason for poor coupling, which looks unavoidable (Another problem is too low the magnetizing inductance Lm = 200uH) due to design constraint, such as core dimension and wire thickness.

I bought a 300W 12V-230VDC pure sine inverter and inside it the circuit seems to use push pull for DC-DC step up, its transformer uses the same core as my design (ETD39, about 5cm x 5 cm). Is it possible that commercial designs use ZVS or ZCS resonant converter to reduce switching loss, or does this cure the leakage inductance problem? Sorry I'm pretty novice in power electronics and barely understand the resonant switching technique.

What are the main problems I'm supposed to see with full bridge converter? If RC snubbers are to be used, are they connected in parallel of all 4 switches? And you mentioned the snubber can be placed on secondary side ( I assume you mean placing it directly across output winding ). So if designed properly, would it have same effect as having two snubbers in parallel with the two MOSFETs on primary side? Once again thanks for you kind help.

You'd no doubt be better off with a forward than push-pull transformer configuration, but, sorry the pun, that's not your primary problem. With respect to the limitation of having only six turns on the primary, this is easily dealt with by using multiple paralleled turn sets made from much smaller wire and interleaving the secondary. Imagine you were using a wide copper strip for the primary, you could easily interleave the secondary. But instead of a copper strip you'll use multiple sets of primary turns, laid down side by side, and wired up in parallel at the end. I like to use litz wire in such a scene, because it can be easily smashed down and smeared into place. Note, this must be done separately for each primary half. If you meant six turns CT, then you'd need to redeploy with 4 or 8 turns CT, so you can interleave each set of 4 or 8 turns with 2 or 4 on each side of the secondary. Or, you can use a S-P-S winding setup instead. Be careful with your insulation, a layer of Kapton tape between each interleaved winding.

1. I agree completely with Winfield's comments about transformer construction. Also, ETD39 is an excellent core for minimizing leakage inductance. An additional advantage of increasing primary turns is that it increases magnetizing inductance, which has a stabilizing influence on feedback equivalent to some amount of slope compensation.
2. There should be no need to snub the switches themselves, whether you're using forward, full-bridge, or push-pull. The rectifiers, which are much faster switching, are generally what excites the tank circuits made up of parasitic and stray reactances. Minimize the strays by proper choice of components (low dynamic capacitance transistor switches) and layout (minimize leakage inductance, minimize loop areas and use wide traces in high di/dt loops, use low-profile components and maximize distances between high dv/dt nodes, etc). Then, the ringing frequencies will be quite high, so that snubbing capacitances will be low. When snubbing capacitance is low, power dissipation in their damping resistors is low. It all plays together. Books have been written on this subject. One good author: Sanjaya Maniktala.
3. Resonant and quasi resonant switching techniques can be applied to any topology, but this adds complexity which may not be worth the efficiency improvement. I'd start with the simplest control method and decide if it's good enough. For the same reason, I'd avoid full- bridge. If you do venture into that territory, be sure to learn the meaning of the term 'flux walking' and understand that the most important test conditions for any converter are power-up, power-down, and brownout. Paul Mathews

The energy stored in the primary is swished back and forth harmlessly for the most part. As long as primary current (due to inductance) doesn't exceed saturation current, you're fine. Leakage inductance is the most important figure.

Consider that the output rectifier and filter, which is choke input, maintains a roughly constant current due to the filter choke's flyback (at least one rectifier diode is always conducting). This clamps the secondary voltage down to roughly zero volts, which likewise keeps the primary waveform sequestered. The only issue on the primary is leakage inductance, which will cause a (much smaller) flyback pulse when the transistors switch off. This can be clamped with diodes in parallel with the transistors (or by using MOSFETs which have intrinsic reverse diodes, or co-pack BJTs or IGBTs). The remaining trash (that is, the ringing due to leakage inductance oscillating with the winding, transistor and diode capacitances) can be damped with an RC.

Tim

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Hi ! I have some problem in designing the dc-dc converter. Iam designing a converter with Push pull Boost Topology, 1500W from