TO USE OR NT USE SNUBBER IN SMPS

[sekhar]

i have made the given smps and it works fine ......... but in the original schematics the snubber was 470E & 102pf for a frequency of 50khz.

but i have increased the frequency to 90khz and the given snubber burns up . so how do i calculate the the new snubber. And the smps works even with out the snubber..........

help me with some formula with which i can calculate the snubber.......



regards
sekhar

[metalsculptor]

Going from 50 to 90 khz will almost double the snubber resistor loss, assuming constant rise time and no ringing. The snubber is chosen for rise time and and stray inductance. Also the snubber only needs to draw current until the body diode in the mosfets Q2 and Q3 have chance to conduct during the converter dead time.

Without a detailed simulation it is difficult to calculate a snubber, get out the Oscilloscope and look for overshoot spikes, decrease the capacitor until they just start to appear if the resistor is still smoking then use a higher wattage device or consider using external fast recovery diodes instead of the mosfet body diode.

If there are no large overshoot spikes then the snubber is probably unnecessary.

It would help if you can measure the dv/dt at point 5 during switching and the current in the DC link.