Overcurrent protection design in power control chip

1 Introduction

The rapid development of home appliances, portable electronic devices and handheld appliances has made power adapter chips a major product category of integrated circuits. Since such chips have built-in integrated or externally connected power LDMOS tubes, the LDMOS tubes in applications need to be directly connected to high voltage and pass high current (current LDMOS tubes can withstand high voltages of hundreds or even nearly 1,000 volts). Therefore, how to ensure the safe operation of chips and LDMOS tubes is one of the key points of chip design.

Using the negative temperature characteristic of the on-chip diode forward voltage drop to monitor the thermal state of the chip and then control the switching of the power LDMOS tube is a feasible safety design method. However, due to the thermal inertia of the silicon chip, it is not possible to achieve instant control. This method is more suitable as the second line of defense for safety design.

From the perspective of chip design, to ensure the safety of the adapter chip, a better method should be to directly monitor the large current flowing through the LDMOS tube or the drain voltage of the LDMOS tube to monitor the working status of the chip in real time. Generally, two solutions are adopted: (i) a small resistor is connected in series with the source end of the power MOS tube to the ground to detect the source current, as shown in Figure 1 (a); (ii) the drain voltage of the LDMOS is monitored through a detection circuit, as shown in Figure 1 (b). The former solution has at least the following disadvantages: (1) due to the discreteness of the process, it is difficult to achieve an accurate resistance value (the error is about 20%); (2) after the source is connected in series with the resistor, the tube voltage drop of the LDMOS tube, which originally has a large on-resistance, is further increased, and the power handling capacity is weakened; (3) a large current flows through the resistor, consuming unnecessary energy and reducing the conversion efficiency of the switching power supply.

The latter solution uses the characteristics of integrated circuits (the resistance ratio accuracy of the voltage sampling circuit can easily reach 1%), and the circuit processing is not too complicated. The important thing is that the LDMOS tube has no source series resistance, which can reduce energy loss, does not affect the power handling capacity of the LDMOS tube, and improves the power conversion efficiency.

The design idea of ​​directly detecting the drain voltage to determine whether the LDMOS is overcurrent is to detect the LDMOS drain voltage through the sampling circuit when the LDMOS tube is turned on. After comparison, the overcurrent comparator outputs a low-level overcurrent signal to turn off the LDMOS tube; when the LDMOS tube is turned off, the sampling circuit does not work, and at the same time, in order to improve reliability, the comparator window level is appropriately raised.

Figure 9 Simulation of control logic circuit

Overall simulation of closed-loop control circuit

As shown in Figure 10, the circuit in Figure 3 and the external LDMOS form a closed-loop control system. The simulation results are shown in Figure 11: When there is no overcurrent, the duty cycle of the gate voltage is the largest; when overcurrent occurs, the overcurrent signal OverCurrent forces the gate voltage to a low level and turns off the LDMOS, thereby achieving an overcurrent protection effect.

Figure 10 Closed-loop overall simulation schematic

Figure 11 Closed-loop overall simulation waveform

3 Conclusion

This article describes several overcurrent detection methods and analyzes the advantages and disadvantages of each method. A closed-loop control overcurrent protection circuit is designed. It uses the method of directly detecting the drain voltage of the LDMOS tube to overcome the disadvantages of energy consumption and chip heating when using resistor detection, while improving the energy conversion efficiency of the switching power supply DC/DC. In addition, the ratio sampling circuit design is adopted to overcome the influence of process deviation and improve the sampling accuracy.

Based on the 3μm high-voltage BCD process, we used the circuit simulator Spectre in the Cadence design environment to simulate the control circuit in both sub-modules and overall modules. The results show that the circuit can well implement the real-time overcurrent protection function.