Are resonant converters bi-directional? - Page 1
Forum section: electronics / beginners

author: state_of_flux
author_rank: Contributor
timestamp: February 13, 2020, 02:38:02 pm
content: A bit of a general question here! In resonant converters, the power is typically transferred from the source to the load. However, is it possible to transfer power bi-directionally such as within a dual-active-bridge?In some applications where the load current can transition from full load to zero load, and no energy is transferred to the secondary side through the transformer - where does the resonant energy held within the inductors and capacitors go? Does it just gradually dissipate, or is it held in them until the load is connected again? Is there a way to transfer this resonant tank energy back to the source, or is this just defeating the purpose of a resonant converter?At the moment, I have a buck regulated resonant half bridge converter. However when the load current transitions from full load to zero load, the buck converter turns off. Obviously this is expected because no current is transferring through the transformer. But what I don't understand is what happens to the energy stored within the tank at this instant? Could the buck be replaced with something else which is bi-directional to recover the energy back to the source?

author: radiolistener
author_rank: Super Contributor
timestamp: February 15, 2020, 02:12:52 am
content: If you're meaning network of inductors and capacitors, tuned to resonate at a specific frequency, then yes - it is bidirectional.Energy is stored as oscillations in the network of inductors and capacitors. Technicaly it is stored in reactive fields (electric field for capacitor and magnetic field of inductor). Such LC network has heat loss, so part of energy will be dissipated into heat.

author: radiolistener
author_rank: Super Contributor
timestamp: February 15, 2020, 02:15:15 am
content: Quote from: state_of_flux on February 13, 2020, 02:38:02 pmaccumulated energy in the tank is gradually dissipated into heat and electromagnetic radiation, because tank has some loss for heat and electromagnetic radiation.

author: radiolistener
author_rank: Super Contributor
timestamp: February 15, 2020, 02:21:16 am
content: Quote from: state_of_flux on February 13, 2020, 02:38:02 pmWhen there is no load, energy is consumed to charge resonant LC network. With increasing energy in LC network amplitude will also increasing. As result there is higher energy loss for heat and EMI radiation. In addition it may lead to electrical breakdown due to high voltage. This is why converter turns off.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: February 15, 2020, 03:31:19 am
content: What is it, LLC type? The first L's energy decays in less than a cycle, returning to the supply. The LC tank rings down, if still having more amplitude than the supply then it also discharges into the supply partially. The leftover rings down gradually. It may ring down incompletely before the next pulse starts (if pulse skipping / burst mode control is used), in which case some energy may be returned; or more energy may be consumed if the pulse is out of phase with the ringdown.I think you will find, for typical, recommended values, the amount of energy lost is quite small, so it's quite easy to tolerate such losses on an infrequent basis. The efficiency may not be fantastic, but the absolute power loss needn't be.Consider a tank delivering 100W output power, with 100VA reactive power, made with components giving a total Q of 100. Every cycle, 1W of that reactive power is dissipated as heat and when it's turned off, it rings for, roughly 100 cycles (give or take what time constant you consider it "done" ringing). If the control restarts within say 10 cycles, not much energy will have been lost. If more than 100, then ~all the energy in the tank is gone. If Fo is 100kHz, the resonant energy is VAR / (2*pi*Fo) = 160uJ, and if the pulse rate is 1kHz then the dissipation is all of 0.16W, a fraction of the continuous-duty losses.If you're only delivering 1W at that condition, your efficiency is less than 86% (compared to < 99% at nominal load), pretty pitiful, but that's a lot to expect of dynamic range to expect from a power supply and I would suggest be less picky.Tim