Absolutely usable switching power supply: Hitachi NP8C power supply

Models using Hitachi NP8C power supply include: Hitachi CTP236, CEP320D, CRP350D, 450D, Furi HFC-236, 450, Venus C37-401, C46-1, C563, etc. This power supply was widely used in my country's early color TVs.

During the oscillation process,
the 300V DC voltage is added to the B pole of the switching tube Q901 through R911, R907, and R908. Q901 begins to conduct, and the primary winding of T901 begins to have current flow through it. At the same time, an induced voltage is generated, and its polarity is positive up and negative down, so this The secondary winding also generates an induced voltage, with the polarity of positive at the bottom and negative at the top. This voltage is sent to the B pole of Q901 through R902, R909, and C908, causing Q901 to further conduct. The strong positive feedback causes Q901 to quickly saturate.
During the saturation period of Q901, D906 and D907 cut off, T901 stores energy, and at the same time, the positive feedback voltage continuously charges C908. As C908 is continuously charged, the B electrode voltage of Q901 continues to decrease until Q901 saturation cannot be maintained, and Q901 exits the saturation state.
Once Q901 exits the saturation state, the polarity of the induced voltages in each winding of T901 will all flip, and another strong positive feedback will cause Q901 to quickly cut off.
During the cut-off period of Q901, D906 and D907 are turned on, establishing 108V and 54V DC voltages on C909 and C910. At the same time, D905 is turned on, and C908 forms a discharge circuit through the feedback windings of R902, R909, and T901. At the same time, the 300V voltage passes through R911, R907, R908 reversely charges C908, causing the B-pole voltage of Q901 to continue to rise until Q901 is turned on again and enters the next oscillation process.

Voltage stabilizing circuit
The voltage stabilizing circuit consists of CP901, Q902, and Q903. Pin (4) of CP901 provides the reference voltage for the E pole of Q902. The size of the output voltage can affect the conduction degree of Q902, thereby affecting the conduction degree of Q903. Therefore, it can control the oscillation frequency of Q901 and stabilize the output voltage. size.
In addition, the oscillation frequency of Q901 is also controlled by the horizontal frequency. The horizontal reverse pulse right C912 is introduced, and the oscillation frequency of Q901 is controlled by the horizontal frequency, so a stable voltage output can be obtained.