UC3842 is a high-performance fixed frequency current mode controller, including error amplifier, PWM comparator, PWM latch, oscillator, internal reference power supply and undervoltage locking. Its structure diagram is shown in Figure 1.
The functions of each pin are introduced as follows:
Pin 1 comp is the output of the internal error amplifier. Usually, a feedback network is connected between this pin and pin 2 to determine the gain and frequency response of the error amplifier.
The 2-pin feed back is the feedback voltage input terminal. This pin is compared with the reference voltage (generally 2.5V) at the same direction input terminal of the internal error amplifier to generate the control voltage and control the pulse width.
3-pin iSense is the current sensing terminal. In the peripheral circuit, a small resistance sampling resistance is connected in series with the source of the power switch tube (such as VMOS tube) to convert the current of the pulse transformer into voltage, which is sent to pin 3 to control the pulse width. In addition, when the power supply voltage is abnormal, the current of the power switch increases. When the voltage on the sampling resistance exceeds 1V, UC3842 stops output, effectively protecting the power switch.
4-pin RT / CT is timing terminal. The sawtooth oscillator is externally connected with the common terminal of timing capacitor C and timing resistor R.
Pin 5 GND is grounded.
6 pin out is the output end, this pin is Tuten column output, and the driving capacity is Â± la. This totem pole structure is beneficial to the shutdown of the driven power tube, because when the triode VTL is cut off, vt2 is turned on, which provides a low impedance reverse extraction current circuit for the shutdown of the power tube and accelerates the shutdown of the power tube.
Pin 7 VCC is the power supply. When the power supply voltage is lower than 16V, uc3824 does not work, and the power consumption is less than 1mA. The input voltage can be obtained by reducing the voltage from the high voltage through a large resistance. After the chip works, the input voltage can fluctuate between 10 30V, and stop working below 10V. During operation, the power consumption is about 15mA, which can be provided through the feedback resistance.
8-pin VREF is the reference voltage output, which can output an accurate 5V reference voltage, and the current can reach 50mA.
The voltage regulation rate of uv3842 can reach 0.01%, the working frequency is 500KHz, the starting current is less than 1mA, the input voltage is 10 30V, the reference voltage is 4.9 5.1V, the working temperature is 0 70 â, and the output current is 1a.
UC3842 typical application circuit (I)
The following figure is a typical application circuit diagram with over-current and over-voltage protection composed of UC3842
Figure UC3842 application circuit diagram
Overcurrent protection principle: when the load current exceeds the rated value or the output short circuit causes the current of switch V4 to increase, and the voltage on R7 is fed back to pin 3. When the voltage on R7 is greater than 1V, the conduction width is narrowed through the internal current amplifier, the output voltage is reduced, and the working voltage of UC3842 is also reduced. When it drops below the set voltage, the overcurrent protection circuit works to protect the power tube. After the short circuit disappears, the power supply will automatically return to normal operation.
Overvoltage protection: when the output voltage is too high for some reason, the voltage stabilizing tube V6 is turned on, which triggers the thyristor V7 to turn on and short circuit the output end. It can be seen that the overvoltage protection appears in the form of overcurrent protection.
UC3842 typical application circuit (II)
UC3842 adopts a fixed working frequency pulse width controllable modulation mode, with a total of 8 pins. The functions of each pin are as follows: â the pin is the output of the error amplifier, and an external resistance capacitance element is used to improve the gain and frequency characteristics of the error amplifier; â¡ The pin is the feedback voltage input terminal. The pin voltage is compared with the 2.5V reference voltage at the in-phase terminal of the error amplifier to generate the error voltage, so as to control the pulse width; â¢ The pin is the current detection input. When the detection voltage exceeds 1V, reduce the pulse width to make the power supply work intermittently; â£ The pin is the timing terminal, and the working frequency of the internal oscillator is determined by the external resistance capacitance time constant, f = 1.8 / (RT) Ã CTï¼ï¼ â¤ The foot is the common ground end; â¥ The foot is a push-pull output terminal, the interior is a totem column, the rise and fall time is only 50ns, and the driving capacity is Â± 1A; â¦ The pin is the DC power supply end, with undervoltage and overvoltage locking function, and the chip power consumption is 15MW; â§ The pin is a 5V reference voltage output terminal with a load capacity of 50mA.
UC3842 typical application circuit (III)
AC-DC circuit turns AC into DC, which is a prerequisite for the design of flyback switching power supply. Figure 2 shows the AC-DC circuit, in which the input AC voltage is 60 220 V and the resistance 1r2 is a positive temperature coefficient thermistor to prevent overcurrent. 1r1 is a varistor used to bypass surge voltage. In PCB wiring requirements, serpentine wiring is adopted between the varistor and the zero line. If the varistor is damaged, the varistor can be disconnected in time without burning the circuit board due to the short circuit of the varistor. Common mode choke
And differential mode inductors (1l1 and 1L2) respectively filter out common mode and differential mode interference, and the DC current from the full bridge rectifier bridge is used to supply power to the subsequent circuit.
UC3842 typical application circuit (IV)
The typical circuit of switching power supply made of UC3842 for the protection of 3842 circuit is shown in Figure 1. Overload and short circuit protection is generally realized by connecting a resistor (R4) in series with the source of the switch tube to send the current signal to the third pin of 3842.
When the power supply is overloaded, the 3842 protection acts to reduce the duty cycle, the output voltage and the power supply voltage Vaux of the 3842. When it is too low to work, the whole circuit is closed, and then the next startup process is started by R1 and R2. This is called hiccup protection. In this protection state, the power supply only works for a few switching cycles, and then enters the start-up process for a long time (hundreds of MS to a few s). The average power is very low. Even if the output short circuit for a long time will not cause damage to the power supply.
Due to leakage inductance and other reasons, some switching power supplies have large switching spikes in each switching cycle. Even when the duty cycle is very small, the auxiliary voltage Vaux cannot be reduced sufficiently. Therefore, a resistor (R3) is generally connected in series on the rectifier diode of the auxiliary power supply, which forms RC filtering with C1 to filter out the spikes at the moment of opening. By carefully adjusting the value of this resistance, satisfactory protection can generally be achieved. When using this circuit, attention must be paid to selecting a relatively low auxiliary voltage Vaux, which is generally 13 15V for 3842, so that the circuit can be easily protected.
Figure 1 is the most widely used circuit, but its protection circuit still has several problems:
1. During mass production, due to the difference of components, some power supplies cannot be well protected. At this time, the value of R3 needs to be adjusted individually, causing trouble to production;
2. When the output voltage is low, such as 3.3V and 5V, due to the large output current, the output voltage drops little during overload, and it is difficult to adjust R3 to an ideal value;
3. In forward application, although the auxiliary voltage Vaux also changes with the output, it has a greater relationship with the input voltage HV, and it is difficult to adjust R3 to an ideal value. At this time, if the auxiliary circuit is used to realize the protection shutdown, a better effect will be achieved. Implementation principle of auxiliary shutdown circuit: in case of overload or short circuit, the output voltage decreases and the optocoupler with voltage feedback is no longer on. When the auxiliary shutdown circuit detects that the optocoupler is no longer on, it acts after a delay to turn off the power supply.
Figures 2, 3 and 4 are common circuits. Figure 2 turn off the power by pulling down the first pin. Fig. 3 adopts the method of disconnecting the oscillation circuit. Fig. 4 adopts the method of raising the second leg and then lowering the first leg. In these three circuits, R3 resistance can still be well protected even if it is not used. Pay attention to the function of C4 in the circuit. The power supply starts normally and the optocoupler is not connected. Therefore, C4 is used to delay the action of the protection circuit for a period of time. In case of overload or short circuit protection, it also plays the role of time-delay protection. When the starting current of bulbs and motors is large, the value of C4 should also be larger.
UC3842 typical application circuit (V)
The power factor correction circuit composed of UC3842 is shown in the figure below:
The input voltage is rectified by the EMI filter composed of L1, L2, L3, etc. brc1, one is sent to the PFC inductor, and the other is sent to the PFC controller after R1 and R2 voltage division as the sampling of the input voltage, so as to adjust the duty cycle of the control signal, that is, change the on and off time of Q1 and stabilize the PFC output voltage. L4 is a PFC inductor, which stores energy when Q1 is on and releases energy when Q1 is off. D1 is the starting diode. D2 is PFC rectifier diode, C6 and C7 filter. One way of PFC voltage is sent to the rear stage circuit, and the other way is sent to the PFC controller after being divided by R3 and R4 as a sample of PFC output voltage to adjust the duty cycle of control signal and stabilize PFC output voltage.
UC3842 typical application circuit (VI)
The input overvoltage and undervoltage protection circuit based on UC3842 is shown in the figure below. The input overvoltage and undervoltage protection principle of AC input and DC input switching power supply is roughly the same. The sampling voltage of the protection circuit comes from the input filtered voltage. The sampling voltage is divided into two channels. One channel is divided by R1, R2, R3 and R4 and then input to the â§ pin of the comparator. If the sampling voltage is higher than the reference voltage of the â¡ pin, the output of the 1 pin of the comparator is off level to control the main controller to turn it off, and the power supply has no output. The other circuit is divided by R7, R8, R9 and R10 and then input to pin â¥ of the comparator. If the sampling voltage is lower than the reference voltage of pin â¤, pin â¦ of the comparator outputs a high level to control the main controller to turn it off, and the power supply has no output.
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