Intelligent Power Switch Power Supply IC Design

1 Introduction

Switching power supply is one of the focuses of the power supply market in recent years. Its biggest advantage is that it greatly reduces the size and weight of the transformer, thus reducing the size and weight of the entire system. Generally speaking, the weight of a switching power supply is 1/4 of that of a linear power supply, and the corresponding volume is about 1/3 of that of a linear power supply. Therefore, the switching power supply poses a threat to low-end linear power supplies, especially those below 20W, and is likely to replace them. However, in addition to PWM and power MOSFET, traditional switching power supplies also include about 50 discrete components, which not only increases the cost and volume, but also affects reliability. This is mainly due to the production process. Switching power supplies have not made any breakthroughs in integration.

In recent years, with the maturity of production technology, low-voltage control units and high-voltage high- power tubes have been integrated into the same chip. TI, ON Semiconductor, Power, Integrations and other companies have similar products, while there is almost no domestic market. Due to the advantages of switching power supply in volume, weight, efficiency and reliability, its research and development speed is amazing. Its main application areas are: ① Post and telecommunications: as a power supply for program-controlled switches and mobile communication base stations; ② Computers : as a switching power supply for various PCs, servers, and industrial control machines; ③ Home electronic products: Currently, home electronic products that use switching power supplies include televisions, DVD players, etc.; ④ Other industries: such as electricity, aerospace, military and other fields.

According to the development of technology and market needs, the core power MOSFET and low-voltage PWM controller are integrated into a chip. At the same time, it also has functions such as overheat protection, overvoltage protection, undervoltage lockout, automatic restart, and overcurrent protection. This new type of switching power supply integrated circuit brings many advantages to the power supply system. The AC input of the chip can be directly connected to the power grid, with low power consumption, low cost, and small size. At the same time, it also improves the stability of the system, reduces costs, and makes the design of electronic engineers simpler. The chip can be used to drive a single-ended grounded power supply system, such as connecting an oscillating flyback secondary coil transformer to output a DC voltage.

2 Working Principle

This switching power supply is a medium frequency integrated module with a design frequency of 100kHz and a maximum duty cycle of 70%. It includes a constant frequency pulse width modulator and a highly integrated power switch circuit, and its structure is shown in Figure 1. The high-voltage side of this combined switch can continuously control the AC voltage from 85 to 265V and can be applied to most conventional power supply systems.

Through a photocoupler, the load change is fed back to the chip. The feedback signal generates a voltage drop on the 2.7k resistor, and the high-frequency switching noise is filtered out through a 7kHz low-pass filter. It is input into the PWM module in the form of a DC voltage for adjustment, generating a pulse wave whose duty cycle changes with the feedback signal, and driving the power MOSFET through the drive circuit, thereby realizing PWM adjustment. In addition, a resistor is connected to the source of the power MOSFET to achieve current limiting protection per cycle.

Under normal circumstances, the 1/8 frequency divider output signal turns on the power MOSFET. If a fault occurs, its output signal turns off the power MOSFET, and it starts counting. In the first cycle, the power MOSFET turns on. If the fault is not eliminated, it will continue to cycle in this way; if the fault is eliminated, it will enter the normal working state. After the IC is connected to an external transformer to realize the AC-DC function, transformers of different specifications can obtain different DC voltages.

3 Introduction to internal functional modules

3.1 Oscillator Circuit

As shown in Figure 2, the oscillator uses two comparators to conduct in turn to charge and discharge the capacitor, and obtains a sawtooth wave with a voltage of 2.7~4.1V. Its design frequency is 100kHz and the duty cycle is 70%. When charging and discharging the capacitor, the constant current charging and discharging is achieved by using the principle that the working current of the MOS tube is constant in the saturation region. Its equivalent simplified circuit model is shown in Figure 3. When charging, the switch S is closed to the 3 terminal, and it can be obtained

DQ=DU×C (1)

And DU=4.1-2.7=1.4v ​​(3)

In the formula, C = 40pF, IP = 18.6mA, and TP = 3ms can be calculated. When discharging, the switch S is turned to the 8 terminal, and we can get

nbsp; In the formula, IN=8mA, we can calculate TN =7ms.

T=Tp+TN=10ms (5)

The design of the duty cycle also needs to be considered. When the duty cycle is increased, the power efficiency of the entire IC and external circuit will be improved.

But it can't be infinitely improved to 100%, mainly because the establishment and recovery of the transformer flux is time-limited. At the same time, the power MOSFET is easy to burn out if it is turned on for a long time.

3.2 Bias Circuit

The circuit uses a three-transistor bandgap reference power supply, as shown in Figure 4. The emitter voltage of T2 is shown in formula (6). It can be seen from the formula that by using the positive temperature coefficient of the equivalent thermal voltage Vt and the negative temperature coefficient of Vbe to compensate each other, the temperature coefficient of the output reference voltage can be close to zero (since the Vbe of T6 and T2 is the same, the output voltage Vref is the same as the emitter voltage of T2).

3.3 PWM modulation circuit

The signal of load change coupled by the optocoupler first passes through a 7kHz low-pass filter, and then is sent to the PWM comparator for comparison with the sawtooth wave generated by the oscillator, thereby achieving pulse width modulation. The frequency response of the low-pass filter is

Can be used as a design parameter.

3.4 Overvoltage protection, undervoltage lockout circuit

The designed internal circuit working voltage environment is 7.5~8.6V. The circuit is shown in Figure 5 and consists of comparators C1, C2 and resistors R1, R2, R3, and R4. Due to the effect of the hysteresis comparator, when Vcc is at 7.5~8.6V, the IC operates normally. When Vcc>8.6V, C1 outputs a high level, directly turning on the discharge NMOS tube for discharge. The NMOS tube is designed to be relatively large, so that it can be discharged quickly and the IC can return to a safe state in time. If the Vcc fault still exists, an eight-frequency counter will be used to count. This eight-frequency counter turns off the power MOSFET, and the capacitor will be repeatedly charged and discharged in 8 consecutive cycles. After 8 cycles, if the fault is eliminated, the entire IC enters a normal working state and the power MOSFET is turned on. This design can greatly reduce the power dissipation of the power MOSFET. When the internal working voltage Vcc is 7.5V, C1 outputs a low level, turns off the drive, and drives the high-voltage startup circuit at the same time to charge the external 10μF capacitor. At the same time, the low level is also sent to the counter to count, thus realizing the self-starting function. Generally speaking, Vcc 7.5V is caused by the output voltage loss of the additional coil of the power transformer due to load short circuit or overload, and there is not enough voltage to power the chip.

The thermal shutdown circuit is shown in Figure 6. Under normal conditions, T = 25°C, Vz = 6.3V, V BE1 = 0.75V, VBEH = 0.65V, at which point VH = R3 ( Vz -VBE1) / (R2+R3) = 0.43V VBEH

Therefore, Q1 is not conducting, and Vout is at a high level.

In the fault state, the temperature coefficient of the voltage regulator is positive, while the VBE of the transistor is negative. The designed temperature protection capability (when T=150℃) is

The same calculation can get VH (150℃) = 0.46V, so Q2 is turned on and Vout is low level. This signal directly turns off the power MOSFET. At the same time, this pulse signal is also input to the 1/8 frequency divider for counting.

3.6 High Voltage Starting Circuit

The high voltage startup circuit is shown in Figure 7. When the IC is powered on, the entire IC is in the state of establishing a working environment. When the gate of VDMOS is high, the tube is turned on and there is a charging current at the Out end. When Vcc reaches 8.6V, the overvoltage protection circuit sends a signal Vstart of low level, which turns on P2, so that VDMOS is turned off. In addition, the role of R1 is to turn on P1 and Q1 when the charging current is too large, so that

VDMOS is cut off to protect the system. The design value of this charging current capability is 3mA. If the value exceeds this, VDMOS will be cut off. According to calculations, the time required for the entire IC to establish a working environment is 40ms, which is consistent with the actual simulation results.

3.7 Driving Circuit

The purpose of designing the driving circuit is to remove the burrs of the driving signal and protect the gate of the power MOSFET (Figure 8). Normally, N1, N2, and N3 are all in the cut-off state. When the power supply voltage Vcc inside the circuit suddenly changes from a low level to a high level, the voltage across the capacitor C cannot change suddenly, so N1 is turned on and the output is 0. In addition, when the IC is suddenly powered on, due to the existence of the gate-drain capacitance of the power MOSFET, the gate voltage is high, but due to the existence of resistors R and N3 added in the design, the gate is bypassed and plays a protective role. Finally, if the IC is suddenly powered off, there is no large current supplied to the drain of the power tube. If the drive is high at this time, the current can be unloaded from R, and finally the low level becomes low. In short, N1, N2, and N3 protect the gate of the power MOSFET.

3.8 Leading-Edge Blanking Circuit

The leading edge blanking circuit is shown in Figure 9. Under normal conditions, the voltage at point A is low, and the 2nd tube is turned on, so the output of C2 is high level; when a fault occurs, that is, when the current of the power MOSFET is too large, the potential at point A rises, causing the 2nd tube to be turned off, so that the output of C2 is low level, and a fault pulse appears. It is worth mentioning that the gate input signal of the 2nd tube is not synchronized with its source input signal. The advantage of this design is that it can avoid the situation where the current is too large in a short period of time. If the current is always large, the leading edge blanking effect can be played. The delay size of these two signals is composed of several levels of inverters and capacitors, among which the contribution of the capacitor is the largest, and its designed delay time is 200ns.

4 Simulation Results

During the simulation, the analysis focused on normal operation, overvoltage, undervoltage, overcurrent, overload and other conditions. Figure 10 simulates the change in the output of the power MOSFET when the load changes. The bottom waveform is the voltage after the load condition passes through the optical coupling and low-pass filter, the middle waveform is the IC internal voltage Vcc signal, and the top waveform is the drive voltage signal on the gate of the power MOSFET. It can be seen that due to charging, Vcc continues to increase and stops increasing when it reaches 8.6V (the overvoltage protection circuit works), and the IC starts to work. When the load gradually becomes smaller, the feedback voltage increases, which increases the signal fed back to the IC, and the duty cycle of the drive voltage of the power MOSFET gate decreases, eventually reaching 0.

Figure 11 simulates the situation when the internal voltage of the IC is abnormal. The bottom waveform is the gate drive voltage of the power MOSFET, the middle waveform is the working signal (Vstart) of the automatic restart circuit, and the top waveform is the internal voltage Vcc signal of the IC. It can be seen that when Vcc rises to 8.5V, the automatic restart circuit is turned off, and the counter starts counting at the same time. At this time, the power MOSFET is still in the working state. When Vcc drops to 7.5V, the automatic restart circuit starts to work and charges the external 10μF capacitor. This is repeated 8 times. In the ninth cycle, the power MOSFET works again, meeting the original design requirements.

5 Conclusion

This paper designs a power switch IC suitable for portable devices. Through the analysis of its functions and characteristics, the circuits of each submodule are designed and simulated. The results show that the load regulation is sensitive and accurate, and the various protection circuits act timely and reliably.