The TEA1733P is a low-cost 75W switching power supply controller (SMPS) from NXP for flyback topology power supplies. The chip belongs to the GreenChipm series, and the same category is TEA1733T, TEA1733LT and TEA1733LP.
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The TEA1733P uses fixed frequency mode and frequency jitter technology to reduce electromagnetic interference (EMI). Its operating voltage is 12V ~ 30V, operating frequency is 65kHz, internal high voltage regulation, undervoltage lockout detection, oscillator, RS trigger, Gate drive, soft start, undervoltage identification and undervoltage protection (UVP), overvoltage protection (OVP), overcurrent protection, overload protection (OPP), over temperature protection (OTP) and other circuits, as shown in Figure 1. The TEA1733P is available in an 8-pin package. The pin functions and measured data are shown in Table 1. Since the control power of TEA1733P can reach 75W, it is now used in 2637-inch LED liquid crystal color TV. The following is an example of the operation principle of Skyworth 168-P32ETU-04 power board.
First, the circuit analysis Because the total power of the power supply is small, no power factor correction (PFC) circuit is set. At the same time, the power supply is not provided with a separate standby power supply (also called sub-switching power supply) circuit, but the on/off control is performed by means of on/off control of the output voltage, and its structural block diagram is shown in FIG. 2 .
This structure not only keeps the power supply at a lower power consumption level, but also makes the circuit very compact, greatly reducing the board size and reducing its production cost. Therefore, this structure has been widely used in medium and large screen LED liquid crystal color TVs.
After 220V AC power is filtered by EMI and bridged, it is sent to TEA1733P (IC100) oscillation circuit to make it work normally; the voltage of secondary rectification and filtering output is converted by DC-DC and output 5V voltage, which is sent to the main board as standby voltage. After being regulated by the voltage regulator circuit on the motherboard, it is sent to the CPU. After the second boot, the CPU output power-on signal is sent to the power board, the open/standby control tube is turned on, and the main voltage is output, which is respectively supplied to the main board and the LED backlight driving circuit.
1. EMI rectification and filtering circuit The EMI rectification and filtering circuit of the power supply is shown in Figure 3. F101 is a fuse for overvoltage or overcurrent protection; TH101 is a thermistor, and TNR101 is a varistor for preventing surge shock. (including lightning protection); CY101, CY102 are common mode filter capacitors for absorbing common mode high frequency noise; LF101 and LF102 are common mode inductors for suppressing common mode interference (there should be no relationship between the reference body and the reference ground) The potential difference is); CX101 and CX104 are differential mode capacitors for suppressing differential mode interference; R101 to R106 are bleeder resistors for discharging the accumulated charge on CX101 and CX104 at appropriate time to avoid charge accumulation and affecting filter characteristics. In addition, after unplugging the power plug, make sure there is no potential difference between the live line and the neutral line; D1011~D104 form a rectifier circuit to prevent AC interference.
2. Oscillation Circuit The oscillation circuit of the power supply board is composed of the switching regulator control block IC 100, as shown in FIG. IC101 (PC817C) is a photocoupler, and IC300 (AS431 H) is a reference voltage regulator integrated circuit. C210, R210, D205, F201 form a spike absorbing circuit, and Q201 is a protection switch tube. R313 and R316 are sampling resistors, R314 is the bias resistor of IC300, and C320 and R315 are used to eliminate parasitic oscillations and prevent circuit malfunction.
After the 220V AC power is passed through the EMI and rectification and filtering circuits, a DC power VDD of about +300V is obtained, and the primary 5-7 winding of the pulse switching transformer T101 is applied to the D pole of the switching transistor Q201. At the same time, the AC220V voltage is divided by the bleeder resistor and half-wave rectified by one diode in the whole bridge, and then stepped down by R201 and stabilized by ZD201 to obtain a voltage of about 27V, which is added to the 1 pin of IC100 as the starting power supply.
At this time, IC100's 1-pin internal high-voltage voltage regulator circuit works to supply internal related circuits, and the internal oscillation circuit starts to oscillate; the output pulse signal is driven by soft start (delay) and control gate, and passes through IC pin 3 And the resistors R207 and R208 are applied to the G pole of the switching transistor Q201. After the Q201 enters the switch state, the changed current is passed through the 5-7 winding of T101. Due to electromagnetic induction, the secondary winding of T101 senses the corresponding electromotive force. Among them, the induced voltage on the 1-2 winding is continuously regulated by R215 current limit, D207 rectification, C209 filtering, and ZD201 voltage regulation to provide continuous power supply for IC 100's 1 pin. At this time, the voltage regulator circuit inside the 1 pin is automatically turned off.
The electromotive force induced by the secondary winding of T101 is sent out in two ways. One channel is filtered by D301 rectification, C301, R301, L301 and other components to obtain 24V voltage; the other channel is filtered by D302 rectification, C305, R302, 002 and other components to obtain 12V voltage. A 12V voltage is then converted to 5V by DC-DC conversion to supply power to the motherboard related circuit.
3. Voltage regulation control When the voltage of the output terminal is high for some reason, the rising voltage is sampled by R312 and R314, and the error of the IC300 is amplified, and the voltage of the A pole is lowered, so that the light in the pins 1 and 2 of IC101 is passed. The current of the diode increases, and its illumination is enhanced. The current passing through the phototransistor in pins 3 and 4 of IC101 will increase accordingly, that is, the equivalent resistance between pins 3 and 4 of IC101 decreases, and the voltage of pin 7 of IC100 decreases. The internal circuit of IC100 adjusts the working state of the corresponding circuit according to the voltage of pin 7, so that the on-time of the switch tube becomes shorter in one cycle, the induced voltage on the winding of T1015-7 decreases, and the induced voltage on each secondary winding It will also be reduced accordingly, so that the output voltage will drop to achieve the purpose of voltage regulation. When the voltage at the V output is low for some reason, the voltage regulation process is opposite to the above.