Would you agree that the Two Transistor Forward Converter with synchronous rectifiers (2TFC+SRs) is potentially a death trap….if not dealt with properly?
In a 2TFC+SRs, if the output is suddenly no-loaded, then the output inductor current will try and reverse and flow “the wrong way” through the synchronous rectifiers. This is because the “catch” Sync FET has an opposite drive to the main primary fets, so if the main primary fets go continuously off, then the ‘catch’ sync fet goes continuously ON…..disaster!
The attached LTspice sim and schematic is the best way to counter this. Would you agree?
....The sync FETs are only permitted to ever turn ON if the load current monitor indicates that the load current is above 33%. However, there is more to it than just that. Supposing the 2TFC+SRs had been no-loaded for some time, and then suddenly went into full-load…..the output capacitor would then supply high current to the load before the PWM controller had chance to start switching, and so the ‘catch’ FET would be continuously ON if then re-enabled…resulting in massive current reversal through the sync fet……..so the ‘catch’ sync fet must be disabled …until the COMP pin voltage on the PWM controller has built up high enough to mean that the 2TFC+SRs has started switching again. Also, it must be switching with a long enough on-time…because otherwise the “Forward” sync FET wont really turn ON, and so the “catch” sync fet will be continuously ON……which leads to disaster.
Some 2TFC+SRs actually have expensive controllers which can manage it so that the inductor current can indeed reverse in times of no-load…..the output inductor current is simply made to oscillate positive and negative such that its average is zero….however, controllers capable of managing this are far too expensive for most Power supply budgets.
One other problem of reversing output inductor current in a 2TFC+SRs is that the current in the primary side sense resistor also reverses, causing a slight negative voltage into the PWM controller current sense pin…which will kill the controller immediately….a Schottky cant be used to counter this as its leakage current at high temperatures is too great, and it will form a divider with the current sense filter resistor.
…A current sense transformer can however be used to counter this.
One other problem of ever allowing the output inductor current to reverse in a 2TFC+SRs, is that of the problem if the sync fets somehow both get turned off, and the converter also stops switching….because in this case the reversed output inductor current has nowhere to flow and so will overvoltage the fets and diodes severely.
The above also applies to a Flyback with sync rects, would you agree?
Really, to play safe, a microcontroller is needed to manage the situation. ….When the load current goes below 33%....then the micro should really disable the sync rects until the load current has been above 33% for 10 seconds or so…the exact amount of time depends on how long it would take for the output capacitor to discharge, and for the converter to resume switching following re-connection of the load.
The attached sim shows the 2TFC+SRs starting up, then getting no loaded, then getting fully loaded again…….the combo of output current monitor and COMP pin monitor both prevent disaster….take either of these out of the sim and you soon see disaster unfold.