I do not ever remember running into a "A to Z" article for a multiple output mains converter with all the important aspects covered, but i can tell you that many datasheets and application notes from the various IC giants (Texas Instruments, Analog Devices, Onsemi, Maxim, etc, etc) contain various information in varying quantities. The mains AC poses high risks during the design and debugging process and there are many practical pitfalls both in schematic design and in the layout of the design, such as parasitics.īut regarding the question the design resources are quite scattered. I would like to warn you that full blown mains converter design, at any reasonable power level is not for beginners, far from it. Typical values of 47-100 pF are often seen in commercial adapters and they also offer some snubbing advantage too. Adding a bit of capacitance across the drain-source of the MOSFET clearly helps on the \$RCD\$ power dissipation as long as the saved power is not lost in switching losses because of too big a capacitor. By doing so, there is less current circulating in the \$RCD\$ network during the reset time. This is because part of the energy stored in the leakage inductance is used to charge the parasitic capacitance lumped at the drain until the diode conducts. It is the turn-off (recovery) time that is much longer however but this lazy diode is often used in \$RCD\$ clamps of cheap adapters below 30 W because it nicely damps the oscillations at turn-off and reduces radiated EMI.Īlso, surprisingly, the peak current going into the \$RCD\$ is often less than the power switch peak at the opening event. People usually believe that the diode should be ultra-fast but it is little known that the turn-on time of a 1N4007 nicely competes with that of a MUR160 for instance. The capacitor value is more in the vicinity of 1 to 47 nF perhaps, as a very rough figure while the resistance cannot be too low considering the dissipated power. Depending on the drive scheme, the MOSFET size, the input voltage and the primary inductance, there can be some significant overshoots destroying the MOSFET quite quickly. That duration is the time needed by the controller to effectively turn the MOSFET off when the current sense pin has detected a maximum. Now, I have simulated my circuit and tweaking values on C and R I came out with C=680uF and R=2.2kOhm the problem is they are simulated values and I've really not applied any formula. I already know the diode to use I just need to design the R and C value. I have a DC/DC flyback converter and I need to design a snubber.
0 Comments
Leave a Reply. |
AuthorWrite something about yourself. No need to be fancy, just an overview. ArchivesCategories |