Power circuit of STR5412

Color TVs using STR5412 thick film blocks have been used a lot in the past few years. It is a thermal base plate movement.

Oscillation process

The 300V DC voltage on C810 is added to the B pole of the internal switch tube of pin (2) of STR5412 through R811, and at the same time, it is added to the C pole of the internal switch tube of pin (1) of STR5412 through the windings (1) and (4) of T802. The switch tube begins to conduct, and an induced voltage is generated at the (1) and (4) pins of T802. After coupling by T802, an induced voltage is also generated in the windings (7) and (8). The polarity is (7) positive (8) negative. , this positive feedback voltage is added to pin (2) of STR5412 through C811 and R812, which further increases the current of the internal switch tube, and the strong positive feedback causes the switch tube to quickly saturate.

After the switch tube is saturated, the current in pins (1) and (4) of T802 increases linearly. The current flows out from pin (4) of STR5412, which is the E pole of the switch tube, and a DC voltage of 112V is established on C812. At the same time, T802 stores Magnetic field energy. At this time, the positive feedback winding continuously charges C811, causing the voltage of STR5412(2) pin to continuously decrease, and finally forces the internal switch tube of STR5412 to exit the saturation state.

After the switch tube exits the saturated state, the current in the T802 (1) and (4) pins decreases, causing all the polarities of the induced voltages in each winding of T802 to reverse. At this time, the polarity of the induced voltages in the windings of T802 (7) and (8) is (8) positive (7) negative, the voltage of T802 (7) pin is added to the (2) pin of STR5412 through C811 and R812, which further reduces the current of the internal switch tube. This cycle makes the switch tube quickly cut off.

During the cut-off period of the switching tube, the polarity of the induced voltage on the windings of T802 (6) and (8) is (8) positive (6) negative, the freewheeling diode D807 is turned on, and the magnetic field energy in T802 is released to C812 through D807, causing C812 to The 112V voltage is smoother. At the same time, the voltage on C811 is discharged through the windings of R812, T802 (7), and (8). The 300V voltage reversely charges C811 through R811, making the voltage at the positive terminal of C811 higher and higher. On the other hand, the reverse direction The process pulse is sent to pin (2) of STR5412 through D808 and R813, causing the cut-off period of the switching tube to end early and enter the next oscillation cycle.

Regulator circuit

The voltage stabilization circuit of STR5412 is automatically completed internally. The output voltage of STR5412 is 112V when designed. If you want to change the output voltage, you can connect resistors in parallel at pins (4) and (5) or (3) and (5). When you connect resistors in parallel at pins (4) and (5), , the output voltage drops (the resistance of the parallel resistor is above 150K, depending on the need); when the resistors are connected in parallel at pins (3) and (5), the output voltage rises (the resistance should be gradually reduced from 47K, and the resistor cannot be If it is too small, the output voltage will rise significantly).

protect the circuit

D806 is an output overvoltage protection voltage regulator tube. If the output voltage is too high (greater than 130V), D806 will break down and the switching power supply will stop oscillating.