4. Main power circuit The main power circuit is also called PWM switching power supply circuit. The circuit uses a two-switch forward mode, as shown in Figure 6. The so-called double pipe means that the switch tubes V805 and V806 are connected in series with the PFC and B+ power supplies, so that each switch tube is subjected to a voltage of only half of the supply voltage, which reduces the voltage resistance requirement of the MOs tube, and the circuit works more stably.
The main power circuit mainly outputs 24V and 28V (or 14V) voltage, 24V voltage is supplied to the inverter (Inverter), and the other channel is converted to 12V by DC-DC converter LM2576 (N811) to supply the main board; 28V (or 14V) The voltage is supplied to the sound amplifier circuit.
(1) Excitation control Since the N802 (NCP1217A) has only one drive pulse output, and the dual-tube forward architecture requires two pulses to drive two MOs, the transformer T804 and its peripheral circuits constitute an excitation circuit that drives the N802 output. The pulse becomes two paths. After the second power-on, 380V voltage is applied to the drain of the switch V806, and the 6-pin (VCC) of the N802 is powered. The internal oscillator circuit starts to work, and the excitation pulse is output from the 5-pin to the base of the V820 and V821. V820 and V821 form the OTL output circuit. The excitation pulse is output from the V820 emitter. It is coupled to the primary of T804 through C824. The T804 secondary outputs two excitation signals with the same phase and different DC potentials.
When the positive half of the signal arrives, V806 and V805 are simultaneously turned on, and 380V voltage → D, S pole of V806 → T801 primary coil → D, S pole of V805 → resistance R822 → ground form a loop. In this process, VD807 and VD808 are cut off, and the secondary coil of T801 is rectified and filtered by VD817, C854, C855 to output 24V voltage, which is supplied to the backlight circuit. The induced voltage generated by the 7-8 winding of T801 is regulated by VD815, C864 rectification filter and LM317 (N809), and outputs 28V voltage to supply the sound amplifier circuit.
When the negative half of the signal arrives, V803 and V804 are turned on to form a perfusion circuit, which quickly discharges the charge between the gate and source of V806 and V805, and V806 and V805 are quickly cut off. Since V806 and V805 turn off quickly, a high induced electromotive force is formed in the primary 1-3 winding of T801, and the direction is positive and negative, and VD808 and VD807 are turned on to form a current loop. In this period: in the T801 secondary circuit, Xin VD817 cut-off, L807 produces a left negative right positive induced electromotive force, when the freewheeling diode VD816 is turned on, constitutes a load freewheeling circuit, so that the electric energy can be fully utilized .
N802 is an oscillatory excitation integrated circuit. The pin function and measured voltage are shown in Table 4. The chip 3 pin is a current detecting end, and an external current detecting resistor 8822 is externally connected. When the current flowing through V806 and V805 is too large and the voltage of pin 3 of N802 exceeds the set threshold, N802 stops the pulse output.
(2) Voltage regulation control
The N pin of the N802 is the regulated feedback input terminal. When the voltage of 24V rises, the voltage of the control terminal (R) of the precision regulator T431 (N807) rises after being divided by R883 and R921, and the K-pole voltage drops. The phototransistor N806 has a deeper photoconductor connected to the 3rd and 4th pins of the N806. The voltage of the 2nd pin of the N802 drops, the frequency of the internal oscillating circuit decreases, and finally the output voltage drops, thereby achieving the purpose of voltage regulation. If the 24V voltage is reduced, the voltage regulation process is reversed from the above.
Tip: If the 24V voltage is too high or too low, then the dummy load can be connected, and the pins 1 and 2 of the optocoupler N806 are short-circuited instantaneously. If the 24V voltage can rise to nearly 30V, the fault point is in the secondary regulator circuit. Medium; otherwise, there is a problem with the primary circuit.
(3) Secondary output voltage overvoltage and load short circuit protection Secondary output voltage overvoltage and load short circuit (overcurrent) protection is realized by optocoupler N805, as shown in Figure 7. The N805's 3rd and 4th pins are connected to the PFC chip and the power supply control circuit of the main power control chip. During normal operation, the light-emitting diodes and phototransistors inside the N805 are in a conducting state, and the PFC chip and the main power control chip are electrically operated. V810 and V811 form an analog thyristor circuit. When the V811 base is at a high level, the circuit will be triggered to conduct, the voltage of pin 2 of N805 is pulled down, the internal circuit of N805 is cut off, and the PFC circuit and the main power circuit stop working.
When any of 24V, 28V, and 12V is overvoltage, one of the Zener diodes VZ811 to VZ813 is turned on. After the resistor divider and RC delay, the V811 base is extremely high, and the analog thyristor circuit triggers conduction.
When the main power supply is working normally, the base of the transistor V822 is very low level, V822 is saturated and turned on, its saturation voltage drop is about 0.2V, the collector voltage of V822 is 4.8V, and the transistor V823 is turned off due to the base being extremely high. If there is a short circuit in any of 24V, 28V, and 12V, V823 is turned on because the base is extremely low, then the V811 base is extremely high, and the analog thyristor circuit triggers conduction.
Tip: The pin 1 of the optocoupler N805 is the key test point. When it is normal, the pin voltage is about 3.8V. If the voltage of the pin is 0.7V, it indicates that the output voltage has overvoltage or overcurrent.
(4) When the open/standby control is in standby, the ON/OFF terminal of the CPU output is low level, V816 is cut off, and its collector is high level, which makes the internal circuit of N805 cut off, and the PFC chip and the main power control chip are powered off. And stop working; the other way to make V822 cut off, V823 has no working voltage, that is, the overcurrent protection circuit does not work, to prevent the circuit from being misprotected during the second boot.
After the second boot, the V816 is turned on due to the extremely high level. Under the action of the optocoupler N805, the PFC chip and the main power control chip are powered. At the same time, V822 is turned on, the V823 emitter is energized, and the load short circuit protection circuit enters the working state.
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