MOSFET IRF9610 design

[zhengchao201105]

hello everyone
this is a very simple design even I can not understand the influence of 9.1v zener,
you can find the simulation result,if D1 is de-populated,mosfet M4 source point and M1 drain point shoud be similar and equal to (15v-(-15v))÷(392+220)*392=4.22v.
the result is totally different if the zener is populated.can you help me to explain the reason?
The datasheet:
https://www.vishay.com/docs/91080/91080.pdf

 

[barry]

M4 is a constant-current source. You've got (9.1V-Vgs) across R15, regardless of the drain voltage.