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LM3481-FLYBACKEVM: LM3481 oscillates under load

Part Number: LM3481-FLYBACKEVM
Other Parts Discussed in Thread: LM3481

Hello all,

I've got the LM3481-FLYBACKEVM to use it as a testplatform for a LM3481-based Fyback-powersupply.

I changed the following parts: R13 from 162k to 105k (new switching frequency: 200 kHz)

   R16 from 3.83k to 1k (new output voltage: 5 V)

   T1 : Np:Ns (primary to secondary) = 11:14

   Lp = 19 µH, ER11/5-Core, N87

The powersupply should deliver 1.5 W at the output. However at 700 mW the LM3481 starts to oscillate at the DR-Pin, the core

of the transformer saturates than too (see the screenshots from measurement). The duty-cycle of the PWM-Signal for the range from 0 to 700 mW  is shown below.

Below there are some screenshots from measurements of the following signals:

CH1: LM3481:DR (PWM switching signal)

CH2: Rsense (sensed Primary current)

CH3: LM3481:COMP (output voltage feedback)

CH4: uds (drain source voltage of switching Transistor) 

regards

Adam