One way to provide good negative-voltage regulation is with a low-drop-out positive-voltage regulator operating from a well-isolated secondary winding of a switch-mode circuit transformer. The technique works with low-dropout types.
Since a secondary winding is galvanically isolated , a regulator 12V output can be referenced to ground. Therefore , in the case of a negative-voltage output , the positive-voltage terminal of the regulator connects to ground , and the 12V output comes off the anode of D1. The Vin terminal floats at 1.5V ore above ground.