Venus TDA two-chip power supply

Relevant key device information applied in this article: d2294 BC558 CNX62A BC548 UA7812 This power supply is also called X53P power supply or The marked circuit numbers are different, but the circuit formula is exactly the same. Here we take Venus C498 as an example to briefly introduce the working principle. During the oscillation process, a DC voltage of about 300V on C708 is added to the B pole of the switch V720 through R729, R731, and L711. V720 begins to conduct, the C pole current increases, and the (1) and (12) pins of T705 generate an induced electromotive force. Its polarity is (1) positive (12) negative, and the positive feedback winding also produces (10) positive (9) negative induced voltage. This feedback voltage is added to the B pole of V720 through C713, V713, and R713, causing the current of V720 to With further increase, this cycle causes V720 to quickly saturate. After V720 is saturated, the current in the windings of pins T705 (1) and (12) increases linearly, and the magnetic field energy is stored in T705. When the C-pole current of V720 is greater than β×B-pole current, the C-pole current stops growing, and the polarity of the induced voltages in each winding in T705 is completely reversed. As a result of the positive feedback, V720 is quickly cut off. During the cut-off period of V720, V763, V765, and V770 are all turned on, establishing three sets of voltages of 115V, 26V, and 16.5V. After the magnetic field energy in T705 is released, the feedback voltage that maintains the cut-off of V720 gradually disappears, and V720 passes through R729 and R731. Obtain bias voltage and turn on again, starting to oscillate again. The moment when the switching tube V720 turns from on to off depends on the time when the pulse width modulation tubes V725 and V726 start to conduct. The voltage on C714 is obtained by the rectification of V716, and the polarity is positive below and negative above. During the period when V726 is on, C714 The lower end of the circuit is grounded through V726, and the negative voltage at the upper end is added to the B pole of the switch tube through V715 and V714. The switch tube is immediately turned off, and the conduction state of V725 and V726 is affected by the charge and discharge of C742. The moment when the switching tube V720 turns from off to on depends on the release of magnetic energy in T705 and the resonance of (1) and (12) windings with C710 and C711. Voltage stabilization process The voltage stabilization circuit consists of V745 and peripheral components. The (7) and (8) windings of T705 are sampling windings. They are rectified by V741 and filtered by C745. The voltage on C745 reflects the output voltage. If the output voltage increases , then the voltage on C745 also increases, the C-pole voltage of V745 increases, V725 and V726 are turned on in advance, the switch tube is turned off in advance, and the output voltage automatically drops to 110V. Protection circuit The protection circuit consists of V733, R734, V734 and other components. V734 is a 6.2V voltage regulator tube. Under normal circumstances, the pulse voltage at both ends of R742 is about 3.5V. V734 cannot conduct, and the protection circuit does not work. Once the output voltage occurs If it is too high, the pulse voltage at both ends of R742 will also increase. When it rises to greater than 6.2V, V734 will be turned on, V733 will also be turned on, and the positive terminal of C732 will be short-circuited to ground. All switching pulses will short-circuit C732, and the switching power supply will stop. Vibrate. Standby/power-on control When in standby, the POWER pin of the CPU outputs a high level, and the E pole of V760 is also at a high level. The optocoupler V741 is turned on, and V735 is also turned on. The positive terminal of C732 is short-circuited to ground, and all switching pulses are sent to C732. Short circuit occurs and the switching power supply stops oscillating. When starting up, the POWER pin of the CPU outputs a low level, and V760, optocoupler, and V735 are all cut off. This circuit has no impact on the switching power supply.