TEA1504 switching power supply low power control IC

1 Introduction

Switching power supply is used by more and more electronic and electrical equipment due to its high power supply efficiency, large voltage stabilization range and small size. It will be used in the standby or stand-by state of large-screen TVs, monitors, computers and other electrical appliances. To this end, Philips developed a high-voltage switching power supply control chip called Green Chip TM using the BiCOMS process. The voltage regulation range of this type of integrated chip (IC) is 90~276V (AC), which can reduce the standby power consumption of the switching power supply to less than 2W. Its own standby loss is less than 100mW, and it has a fast and efficient on-chip starting current source. ; When the load power is low, it can also automatically switch to low-frequency operating mode, thereby reducing the loss of the switching power supply. The high level of integration technology greatly reduces the peripheral components of the IC to achieve miniaturization, high efficiency and high reliability of the switching power supply. The TEA1504 introduced in this article is one of the important members of the Green Chip TM series of ICs.



2 Working principle of TEA1504

TEA1504 adopts a 14-pin dual in-line (DIP14) plastic package. Its pin functions are listed in Table 1, and the internal schematic block diagram is shown in Figure 1. The IC integrates analog circuits and digital circuits inside. In addition to the error amplifier, oscillator, pulse width modulator (PWM), sawtooth wave generator and other general switching power supply control IC unit circuits, it also integrates a high-voltage starting current source, a unique on and off function circuit and a burst standby (burst mode stand-by) circuit. TEA1504 has three operating modes, namely: normal on/off operating mode, burst standby mode and light load power low frequency mode. The working efficiency of the switching power supply can be greatly improved by flexibly setting the working mode.

Table 1 TEA1504 pin functions

2.1 Internal startup current source and power Vaux management

TEA1504 is designed with an advanced starting current source inside, so there is no need for an external high-energy-consuming continuous charging circuit. The startup current source is input from the external main voltage from the Vi terminal (pin1), which can provide charging current for the IC's power capacitor Caux, and also provide operating current for the IC's internal control circuit. When the power capacitor at the Vaux end is charged to 11V, the oscillator starts to oscillate, and the IC outputs a pulse width modulation signal (PWM) to drive the power MOSFET, so that the secondary of the switching transformer outputs the DC voltage Vo. The voltage on Caux undergoes a charge and discharge process during startup, and Caux is charged by the starting current source during startup. When the voltage on Caux rises to 11V, the circuit will oscillate and output a PWM wave. At the same time, the voltage on Caux begins to drop. When the voltage drops to the lower threshold value 8.05V (UVLO), the switching transformer outputs the voltage, so that Caux is recharged to 11V by the auxiliary winding. The normal startup waveform of TEA1504 is shown in Figure 2(a).

In addition, the starting current source can also help achieve safe restart or "hiccup" operation mode under system fault conditions. Generally in a fault state, the IC will stop normal operating mode. Because when the IC detects an output fault state, it will immediately block the drive pulse output, so that Caux cannot be recharged, causing its voltage to drop. Once the voltage on Caux drops to the voltage lower limit lock value, the starting current source will be recharged. Activate and charge Caux to 11V, the system begins to enter the safe restart mode again, and so on. In the "hiccup" operating mode (its operating waveform is shown in Figure 2(b)), in order to achieve a safe "hiccup" operating mode, in the safe restart mode, the charging current Irestart of Caux should be 0.53mA, while the normal The charging current Istart in working mode is 1mA, thus ensuring that system components will not be damaged in the event of an output short circuit. The 2.5V reference voltage with temperature compensation in the IC can generate a bias current IREF that is not affected by temperature by connecting the external reference resistor RREF to the REF pin (pin8). However, it should be noted that the value of RREF will affect the oscillation frequency.

Figure 2 TEA1504 typical waveform

2.2 Pulse width modulator (PWM) and oscillator

TEA1504 uses a unique voltage feedback structure. Its primary voltage feedback signal is input from the DEM terminal (pin13) through RDEM. The sampling and holding circuit works by the sampling current flowing into the DEM terminal. The size of the sampling current is related to the voltage on RDEM. The magnitude of the secondary sampling current is stored in the external capacitor CCTRL of the CTRL pin, and it sets the duty cycle of the driving pulse to the PWM modulator. In secondary feedback circuits, the feedback voltage is typically provided through an optocoupler.

The PWM unit consists of an inverting error amplifier and a comparator. The duty cycle of the PWM wave it outputs is inversely proportional to the control voltage at the CTR terminal (pin9). The signal from the oscillator is sent to the drive stage of the power MOSFET through the trigger to turn the MOSFET on, while the signal from the pulse width modulator or the duty cycle limiting circuit signal can turn the MOSFET off. When the PWM output waveform is unstable, the flip-flop will stop outputting the PWM waveform. The maximum duty cycle of the PWM waveform is 80%.

In the pulse width modulation circuit, the duty cycle of the PWM waveform can be adjusted by comparing the sawtooth wave voltage output by the oscillator with the output of the error amplifier. The oscillator is fully integrated in the IC and generates a sawtooth wave through the charging and discharging of the internal capacitor. The ramp section of the sawtooth wave accounts for 80% of the entire oscillation cycle, so the maximum duty cycle of the IC output waveform is 80%. Changing the resistance value of the external reference resistor RREF (RREF can be selected between 16.9k Ω and 33.2k Ω ) can change the oscillation frequency between 50 and 100kHz. There is a frequency control unit inside the IC, which can automatically make the oscillator work in a low-frequency or high-frequency state according to the weight of the output load. When the output power of the switching power supply is less than 1/9 of the maximum output power, TEA1504 will switch to low-frequency operating mode, and the frequency ratio of low-frequency to high-frequency operating modes is 1:2.5. Low-frequency operation can reduce the switching loss of the switching power supply and will not affect the output voltage regulation during conversion.

The driving pulse forward current output by TEA1504 can reach 120mA, and the reverse pulse current can reach 550mA. It allows fast switching on and off of power MOSFETs. The purpose of choosing a lower forward pulse is to limit the dV/dt (voltage rise rate) when the MOSFET is turned on, so as to reduce the electromagnetic interference (EMI) of the circuit and at the same time reduce the current peak through the resistor Rsense.

2.3 Protection function of TEA1504

The protection functions of TEA1504 mainly include over-current protection (OCP), over-voltage protection (OVP), 140°C over-temperature protection and magnetic saturation protection. The magnetic saturation protection is to ensure intermittent power output, simplify the design of the feedback control circuit and provide a faster transient response, thereby preventing magnetic saturation of the transformer and inductor components when starting or when the energy storage component releases energy. The stress endured is too great. In addition, when the output of the switching power supply is in a short-circuit state, the magnetic saturation protection can also provide cycle-by-cycle current protection for the switching power supply. Figure 3 Application circuit of

secondary feedback switching power supply 3 TEA1504 in on/off mode

The main components of the switching power supply composed of TEA1504 include EMI filter, full-bridge rectifier, filter capacitor, switching transformer, power MOSFET tube and buffer circuit. After the sampling resistor converts the primary current into a voltage and adds it to the ISENSE terminal (pin5), the IC will set the peak current of the switching current based on this voltage. The auxiliary coil is used to provide energy to Caux, thereby providing the internal power supply of the IC. This coil is also part of the primary output voltage regulation circuit. Resistor RREF determines the reference current entering REF (pin8). The value of the capacitor CCTRL is very small, generally 0.2~2nF, and is usually connected to the CTRL terminal (pin9). Therefore, the primary feedback can be adjusted through the internal sample and hold circuit. At the same time, this terminal is also the signal input terminal of the secondary photocoupler. The input terminal OOB (pin14) can select on/off mode or burst standby mode. The main input power supply is connected to Vi (pin1) and can be used as a current source within the IC during startup, while charging the capacitor Caux during startup and safe restart modes.

Figure 3 is a feedback switching power supply using on/off mode. In the figure, one end of switch S1 is connected to the OOB terminal (pin14), and the other end is connected to ground or 2.5V voltage. If VOOB is low level, the IC enters shutdown mode, and the typical current consumption of the VI pin is 350 μA ; if VOOB is 2.5V, the IC will start the startup sequence and start normal operation, at this time Ivi=60 μA .

Figure 4 is another switching mode switching power supply using three resistors. Assume that the resistance of R3 is very high. Then, when the IC starts, if VOOB=2.5V and R1>>R2, then, VOOB=VmainsR2/( R1+R2) can be concluded: Vmains=VOOBR1/R2, which ensures that only when the main voltage is higher than a certain value (for example, Vmains=80V), the switching power supply can enter the working mode, thereby reducing the current flowing through R1. The IC's OOB pin (pin14) can also be used in burst standby mode. In the IC standby state, the switching power supply enters a special low-power consumption state, and its power consumption is less than 2W. In fact, Figure 4 is also a feedback switching power supply that utilizes burst standby and on/off modes. In the figure, when the microprocessor ( μP ) closes the secondary switches S2 and S3, the system enters the burst standby state. The switch S2 connects the secondary winding to the microprocessor capacitor (C μc ) to bypass the output capacitor C0. . When the voltage on C μc is higher than the breakdown voltage of the voltage regulator tube (Vz), the optocoupler is triggered and the feedback signal is sent to the OOB terminal, so that the IC stops working and enters "hiccup" mode. The "hiccup" mode in the system fault state is different from the "hiccup mode" during the burst mode operation. When the system fails, the output power in the safe restart state is very small, while the burst mode needs to output enough power to provide the micro Processor. To prevent the transformer from emitting noise, the peak current of the transformer should be reduced by 3.3 times. That is to say, the burst standby mode continues until μP opens switches S2 and S3. Once S2 and S3 are opened, the system enters the startup sequence And start normal switching.

Figure 4 Secondary feedback switching power supply in burst standby mode

4 Main electrical performance

The maximum repetitive voltage of the Vin pin of TEA1504 is 600V, and the operating current is 20~100 μA . The maximum voltage of the OOB pin is 14V, the maximum current of the DEM pin is ±1mA, the maximum voltage range of the Vaux pin is 0.3~+18V, the maximum voltage range of the VCTRL and Isense pins is -0.3~5V, and the maximum current of the REF pin is 1mA, operating temperature range is -10~+140℃.

The starting voltage of pin 6 of TEA1504 is 11±0.6V, and the voltage drop threshold is 8.05V. The maximum starting current of pin 6 is -1mA, and the normal operating current is 3.85mA.