Optimization and improvement of color display switch power supply based on UC3842

Nowadays, UC3842 integrated circuits are widely used to form high-performance current-type switching power supply control circuits, which are suitable for offline switching regulated power supplies or DC-DC converter circuits. They have low starting current (I<1mA), automatic compensation, enhanced load response, undervoltage lockout output, and a maximum operating frequency of up to 500KHZ.

Analysis of the principle of the color display switching power supply circuit composed of UC3842

UC3842 is a new type of PWM pulse width modulation integrated circuit, which contains a pulse signal generator, a voltage stabilization circuit, a pulse width adjustment circuit, a voltage and current detection circuit, etc. It is a single-ended isolated current type pulse width modulation integrated circuit. The typical application circuit of the switching power supply composed of UC3842 is shown in Figure 1. The circuit mainly consists of a demagnetization circuit, an input power grid anti-interference circuit, an AC to DC circuit, a voltage conversion circuit, a voltage stabilization control circuit, and a switching power supply frequency locking circuit.

(1) Start the oscillation circuit

In Figure 1, after the power switch SW901 is turned on, the 220V AC voltage is input through the fuse F901, the conjugate filter L901 and L902L, and the DC voltage of about 300V is obtained by the bridge rectification of VD901~VD904 and the filtering of C908. This voltage is added to the drain of the switch tube VT901 through the primary winding between the ⑤~⑦ pins of the switch transformer T901 through the resistor R913; the other way is through the active voltage limiting circuit composed of R907, R909, R908, ZD901, VT902, etc., so that the emitter of the VT902 tube obtains a DC voltage of about 45V, and then charges C913 through the resistor R911. When the voltage across C913 reaches 16V, the reference voltage generator in N901 (UC3842) starts, generates a 5V reference voltage, and outputs it from the ⑧th pin, and is sent to the oscillator in the ④th pin through R926 to start the oscillator.

(2) Voltage stabilization process

When the grid voltage increases or the load decreases, causing the output voltage of the switching power supply to increase, the 15V voltage generated by VD909 rectification and C914 filtering increases at the same time, causing the sampling voltage formed by R915, RP901, R916, R917, etc. (i.e., the voltage on the second foot of N901) to exceed 2.5V. This voltage is compared with the 2.5V reference voltage at the same-phase input terminal of the internal amplifier to obtain the error control voltage. After amplification, it controls the duty cycle of the oscillator output pulse to make it smaller, and the on-time of the switch tube VT901 is relatively shortened, thereby reducing the output voltage; conversely, when the grid voltage becomes lower or the load becomes smaller, causing the output voltage of the switching power supply to become lower, the on-time of VT901 becomes longer, thereby increasing the output voltage.

(3) Protection circuit

Reverse peak voltage absorption circuit

R912, VD907, and C912 form a reverse peak voltage absorption circuit for protecting the switch tube VT901, which is used to absorb the reverse peak voltage generated when VT901 enters cutoff from saturation.

Overcurrent protection

Pin ③ of N901 is the overcurrent protection control terminal, and resistor R924 is the overcurrent sampling resistor. When the load is too heavy, the source current of the switch tube VT901 becomes larger, and the voltage on resistor R924 will increase proportionally. In this way, the potential of pin ③ of N901 will increase through the isolation resistor R925. When the potential of this pin rises to about 1V, the locking circuit inside N901 will be activated, causing the trigger to flip and shut off the drive pulse of pin ⑥. VT901 is always in the cut-off state, thereby preventing the fault from expanding.

Undervoltage protection

When the starting voltage on the N9017 pin is less than 16V, the circuit will not start, thus prohibiting the display from working under too low a grid voltage. After the circuit is started and enters normal working state, if the voltage on the N9017 pin is less than 13V due to excessive load or abnormal rectification and filtering circuit composed of components such as VD908, C913, and R914, the locking circuit inside the 7th pin will be activated to judge the driving pulse and realize undervoltage protection.

Overvoltage protection

This circuit is also set in the ⑦ pin of N901. The voltage of this pin is stable at about 14V during normal operation. When the AC voltage of the mains increases significantly, the voltage after rectification and filtering will also increase. This voltage is coupled through L901, and the voltage induced on the ②~③ windings after rectification and filtering by VD908 and C913 will also increase accordingly and be added to the overvoltage protection circuit in the ⑦ pin. When the voltage reaches the 20V start-up voltage of the overvoltage protection circuit, the overvoltage protection circuit will be activated to control the internal oscillator to stop oscillating, and there will be no pulse signal output on the ⑥ pin, so that VT901 is cut off, thereby achieving the purpose of overvoltage protection. Circuit optimization and improvement

Through the analysis of the principle of the color display switching power supply circuit composed of UC3842 , it is found that the overcurrent protection function of UC3842 is realized through pin ③. When the voltage detected on pin ③ is higher than 1V, the comparator inside UC3842 will be flipped, the PWM latch will be set to zero, and the pulse modulator will be in the off state, thereby realizing the overcurrent protection of the circuit. Since the detection resistor can sense the peak inductor current, a pulse-by-pulse current limiting circuit is naturally formed. As long as the level on the detection resistor reaches 1V, the pulse width modulator will be immediately turned off. Therefore, this peak inductor current detection technology can accurately limit the maximum output current, so that the magnetic components and power devices in the starting power supply do not need to be designed with a large margin, and the operation of the voltage-stabilized power supply can be guaranteed to be reliable. However, the sampling resistors commonly used are metal film or oxide film resistors. This type of resistor is inductive. When the current flowing through the sampling resistor may not be fully discharged before the next oscillation cycle arrives, the current borne by the sampling resistor will become larger and larger, which will cause UC3842 to operate incorrectly, and even cause the circuit to not work properly and components to be burned out. Therefore, the overcurrent protection function of UC3842 sometimes cannot play a good protective role and has certain defects. In view of this situation, two improved methods are proposed.

(1) Connecting to an emitter follower

An emitter follower is connected to the sampling voltage of UC3842, thereby adding an artificial slope synchronized with the pulse width modulation clock to the control voltage, which can reduce the △I disturbance to zero in the subsequent cycle. As shown in Figure 2, the signal of the emitter follower is input from the base and output from the emitter. It has the characteristics of high input impedance, low output impedance, and the input signal and output signal have the same phase. Therefore, even if the system operates under the condition of duty cycle greater than 50% or continuous inductor current, the system will not be unstable.

(2) Use non-inductive resistor instead of sampling resistor

The non-inductive resistor is a two-wire winding resistor with high precision and easy to achieve high power. After using the non-inductive resistor, its impedance will not increase with the increase of frequency. In this way, even in high-frequency conditions, the power consumed by the sampling resistor will not exceed its nominal power, so there will be no circuit failure and component burnout.

Summarize

By optimizing and improving the UC3842 integrated circuit, the circuit performance and voltage regulation rate will be further improved, and the circuit's load-carrying capacity will be greatly enhanced, thereby achieving a more stable circuit operation.