Application of TOP224P in switching power supply

Crystal oscillator

Quartz Oscillator 3225 20M OSC

1 Introduction

The first generation of TOP switch chips produced by TOPSWitch in the United States has been widely used in the design of switching power supplies. At present, the company has recently introduced the second generation switch chip TOPSWitch-II series, which has greatly improved performance compared with the first generation TOP switch, AD/DC efficiency is up to 90%, and has a large output power range and low cost. , high degree of integration, simple circuit design and so on. Many of the circuits in the chip have been improved to make the design of the switching power supply easier. The TOP224P used in this paper is the chip in the TOPSWitch-II series.

2 TOP224P chip introduction

2.1 Pin Introduction

The chip consists of three terminals: the drain terminal, the control terminal and the source terminal.

The drain terminal (DRAIN pin) is connected to the drain of the output MOSFET. At startup, an internal bias current is provided, and the control terminal (CONTROL pin) controls the output duty cycle, which is the input of the error amplifier and feedback current. During normal operation, the internal shunt regulator provides the internal bias current. It can also be used as the contact between the current bypass and the auto-start/compensation circuit capacitor. The source terminal (SOURCE pin) is connected to the source of the output MOSFET. Common points and reference points of the circuit.

2.2 Introduction to the internal working principle of the chip

The internal working principle block diagram of the chip is shown in Figure 1. The chip consists of MOSFET.PWM controller, high voltage start circuit loop compensation and fault protection circuit. The specific working process is as follows: under the control of the loop oscillating circuit of the control terminal, the current input chip at the drain terminal provides an open-loop input. When the input is connected to the error amplifier through the parallel regulated voltage regulator, the control terminal performs closed-loop adjustment to change the IFB. The PWM controls the output duty cycle of the MOSFET and finally achieves dynamic balance.

Figure 1 chip internal working principle block diagram

2.3 chip features introduced

The TOP224P is a self-programmable, self-protecting current-duty cycle linear control converter. Due to the CMOS process, the conversion efficiency is greatly reduced compared to the use of dual integrated circuits and discrete components, eliminating the need for external resistors for current conduction and providing a startup bias current. During normal operation, the duty cycle of the internal MOSFET output pulse decreases linearly as the CONTROL pin current increases.

The TOP224P maintains the control voltage VC between 4.7 and 5.7V by turning the high voltage current source on and off. The internal voltage of the chip is taken from the temperature-compensated bandgap reference voltage, which produces a fine-tunable temperature-compensated current source for accurately adjusting the frequency of the oscillation and the gate drive current of the MOSFET. The rated frequency of the oscillator is chosen to be 100KHz, which reduces EMI and improves the efficiency of the power supply. The gate drive current can be fine-tuned by cycle current limit to improve accuracy.

In addition, the chip also has its own protection functions such as break/auto restart and overheat protection.

3 TOP224P in the design of dual output switching power supply

3.1 Switching power supply circuit and working principle

Take the +5V, +12V, 20W dual output switching power supply as an example. The circuit diagram is shown in Figure 2.

Figure 2 20W dual output switching power supply circuit diagram