Reduce Flyback Reflected Voltage Ringing?
So basically when the current goes to zero on the secondary side, there is a oscillation between the main inductance and the various capacitances (such as Coss of Mosfet and Tranformer).  

ds() =  +  ∙ 00( − 0)

So you have a n of (1/7) .
Vin = 24V
Vout = 180
So  around Vo is 180 V  . 
The Equations leads to a minimum of  :
Vds = 24 - 25.7 = -1.7 V 

As This happens the body diode of the fet starts to conduct and the resistance limiting the current through it, is only the sense resistors. 

There is not a lot you could do about the reflected voltage despite changing N, but what you could try is adding some resistance ( a few ohms) in the drain of the Mosfet to limit the current through the body diode.

Other option is to work in Continous conduction mode, as in CCM there is no oscillation in that operation mode. But that would mean that you probably require a different transformer.