Hi all, I have designed a DCDC converter 12V (battery) to 36V, with a output power max of about 250W (8A at 36V) and really it works as expected, but I have EMC problem!
I have followed the suggestions for better layout described in datasheet itself, but the results are not good, the EN55032 Class B limit is not satisfy in the area 30MHz-100MHz with peaks around 45dBuV (the limit is 30dBuV).
I am using ferrites and common mode choke (impedence about 1K ohm in 30-100MHz area) on the output , without these components the behaviour was worst. There are several ceramic capacitors on 12V input voltage, I put also the smd X2Y ceramic capacitors, With all these components the better board gives the result of 45dBuV! (Over EN55032 Class A!). Other boards (same layout and components) have dBuv more. Consider that we are not testing in the worst condition as output power, we are around 25%, 36V about 2A.
The frequency used is 125KHz, the inductor is 6.8uH (40A) with shunt 1.66mohm (3 shunt of 5mohm in parallel), the 2 external mosfet (60V 100A 2.6mohm) in D2PACK with heatsink on pcb.
I have stop the "high side" mosfet (cutting out its gate resistor and shorting the gate with source pins) using it as a diode (loosing the syncronous mode), and the EMC have improved a little bit, in this mode I have increased the gate resistor for "low side" mosfet to 22 ohm, and the EMC have improved a little bit more (Class A ok, but not Class B).
Coming back to syncronous mode (high side mosfet enabled again), I try to use 10 ohm as gate resistor for both mosfet, but in this way I have fired the mosfet itself, I believe that with this gate resistor the turn off time of the mosfet is too large while the other mosfet is already on.
So, the questions are:
1) which is the maximum gate resistor you suggest?
2) have you any idea to improve this kind of EMC problem?
Thanks in advance
Renato
