Development of a current-mode DC/DC converter

1 Introduction

　　Switching converters usually adopt two control methods: voltage type and current type [1]. Voltage-type controllers only have voltage feedback control, while current-type controllers add current feedback control. Current-type control has many advantages over voltage-type PWM control. It can automatically correct symmetry, realize week-by-week current limiting, and output parallel operation is convenient and faster. Excellent load dynamic response and simple loop compensation and other characteristics.

2 High frequency current type pulse width controller UC3825B

　　UC3825B is a high-performance pulse width controller [2]. The controller contains an accurate voltage reference, micropower start-up circuitry, soft-start, high-frequency oscillator, wideband misband amplifier, fast current-limit comparator, dual-pulse suppression logic, and dual totem-pole output drivers. The signal passes through current limits and comparators, logic and output drivers with very short propagation delays.

　　UC3825B has the following characteristics: suitable for voltage or current switching power supply circuits; the actual switching frequency can reach more than 1MHz; the maximum output pulse transmission delay time is 50ns; it has two high-current push-pull outputs with a peak current of 2A; it has soft Start-up control; has a pulse-by-pulse current limit comparator; a blocked over-current comparator with full-cycle restart; the starting current is very small - the typical value is 100mA; during the under-voltage lockout period, the output low level and no-load current can reduced to the starting current value.

3 1MHz current mode PWM DC/DC converter

3.1 Main technical parameters

　　Output voltage: 36V±3V

　　Switching frequency: 1MHz

　　Output voltage: 5V

　　Output current: 20A

　　Rated output power: 100W

　　Efficiency: 86%

3.2 Circuit diagram

Figure 1 is a schematic block diagram of a 1MHz current-mode PWM DC/DC converter; the main circuit principle of the converter is shown in Figure 2. The current control circuit uses UC3825B as the core, with a switching frequency of 1MHz; the converter adopts a push-pull type [3] main circuit; the synchronous rectification adopts a power MOSFET controllable rectification circuit; the auxiliary current is composed of a resistor and a 12V voltage regulator tube (can also be used Bootstrap circuit), providing +12V power supply for UC3825B; current sampling is to take the voltage on the primary series resistor of the transformer (see resistor R in Figure 2).

3.3 Current limiting and duty cycle control of UC3825B

　　After the primary current of the transformer flows through the sampling resistor R, a voltage proportional to the primary current is generated at both ends of R. This voltage is added to pin 9 of UC3825B through RC filtering, thereby achieving cycle-by-cycle current limiting. Under normal operating conditions, the input voltage of pin 9 of UC3825 must be lower than the 1V threshold voltage. When the input voltage of pin 9 exceeds 1V, the pulse width will become narrower. When the input voltage of pin 9 exceeds 1.4V, the output current is interrupted and the UC3825B starts the soft-start procedure.

　　Using the ramp RAMP pin (pin 7) input signal, UC3825B can implement current mode control or conventional duty cycle control. When this pin is connected to a timing capacitor, UC3825B can achieve duty cycle control. When the RAMP pin is connected to the current sampling resistor, UC3825B can achieve current mode control. In this application circuit, the primary current waveform generates a ramp waveform after passing through a small RC filter network. The function of the RC network is slope compensation. This input signal has a dynamic range of 1.3V and is typically used to generate PWM slope compensation.

3.4 Synchronous rectification circuit

　　In the past, low-voltage output DC/DC switching converters used Schottky diodes as synchronous rectifiers, with a forward voltage drop of about 0.4 to 0.65V. The on-state power consumption was very large at low voltage and high current. Because the forward voltage drop of the power MOSFET is very small, the power MOSFET is used as the output rectifier. Compared with Schottky diodes, the advantages of using power MOSFET tubes are not only small forward voltage drop, but also high blocking voltage and small reverse current. Figure 2 shows the output full-wave synchronous rectification circuit. Power MOSFET tubes VT1 and VT2 are two rectifier tubes (VD1 and VD2 are internal anti-parallel diodes of VT1 and VT2 respectively). When the same terminal of the secondary winding of the transformer is positive, VT2 and VD2 are turned on at the same time, and VT1 and VD1 are blocked. During the freewheeling period of L1, VT1 and VT2 are turned off, and VD1 and VD2 are turned on at the same time for freewheeling; conversely, when the secondary winding of the transformer When the same terminal of the winding is negative, VT1 and VD1 are turned on at the same time, and VT2 and VD2 are blocked. During the freewheeling period of L1, VT1 and VT2 are cut off, and VD1 and VD2 are turned on at the same time for freewheeling.

　　Using this power MOSFET tube rectifier circuit can greatly improve the rectification efficiency. Output +5V/20A, using a power MOSFET with a conduction resistance of 10mΩ, the conduction loss is:

　　PON=10mΩ×(20A)2=4×103mW=4w

　　If a Schottky diode rectifier circuit is used and the conduction voltage drop of the Schottky diode is 0.6V, the conduction loss is:

　　PON=0.6V×20A=12w

　　It can be seen that the rectifier loss alone is reduced by 8W, and the efficiency can be increased by about 6%.

3.5 Manufacturing of transformers

　　There is a tight coupling between the primary winding N2 and the secondary winding N4; while the coupling between the primary winding N1 and the primary winding N2 is not very strict.

3.6 High frequency design

　　Special attention needs to be paid to the placement of external conductors and components to reduce unnecessary inductance and capacitance effects. All wire lengths must be kept as short as possible. The printed circuit board should be carefully laid out with components and their connections. The resistor of the power MOSFET tube gate should be made of carbon to reduce the series inductance.

4 Conclusion

　　The 100W, 1MHz current-mode DC/DC converter developed using high-frequency current-mode PWM controller UC3825B fully meets the index requirements in design; and due to the use of power MOSFET tube full-wave synchronous rectification circuit, the efficiency is as high as 86%; this is also It is shown that current mode control has many advantages.

References

1 Ye Huizhen et al. Novel switching regulated power supply. Beijing: National Defense Industry Press, 1999

2 Wang Honglin et al. Modern communication power supply. Beijing: People's Posts and Telecommunications Press, 1998

3 Zhang Zhansong et al. Principle and Design of Switching Power Supply. Beijing: Electronic Industry Press, 1999