TMS320F28069: Bidirectional isolated DC-DC

Part Number:TMS320F28069

Hi TI Experts,

Following to the previous discussion posted in the below link, i would like to post my query once again.

https://e2e.ti.com/support/power_management/isolated_controllers/f/188/t/619494

I am working with the same converter configuration where i am facing some issues with active clamp circuit.
Under No load condition,i could acheive the DC output of 400V with VDCin : 48V.
But when I loaded output with very small power ~ 120W,Clamp device is experiencing voltage spikes with high frequency ringing as shown in the Figure.Actually,it should not happen.

The loaded output power is very low ~ 120W with VDCin: 20V and VDCout:150V. Pls refer the below circuit & waveforms for main devices & active clamp device attached.

I need to find out the answers for the following questions.

1,What is the reason for the voltage spikes induced across the Active clamp device? 

2,Why the voltage spikes across the Main Devices also not been suppressed ?

I also found the some information from the following TI links.

e2e.ti.com/.../for-efficiencies-sake-how-to-integrate-bidirectional-power-flow-into-your-ups-design-part-1

e2e.ti.com/.../for-efficiencies-sake-how-to-integrate-bidirectional-power-flow-into-your-ups-design-part-2

Pls help me to solve the above issue.

regards,

Ramesh P