Research on PWM-based current-source DC/DC converter

　　1 Introduction

　　The power supply is the core of all equipment, and its importance is like the heart of the human body, but the power supply has many forms. The parameters of the power supply generally include voltage, power, load current, noise, and the changes of various parameters under load dynamic conditions, etc. Under the same parameter, there are also indicators such as weight, volume, efficiency, and reliability. Therefore, the forms of power supply are extremely numerous. Ordinary electricity (mains electricity) needs to be converted before it can become what our various devices need, for example: AC becomes DC, high voltage becomes low voltage, low frequency becomes high frequency, etc. In fact, the process of conversion is that we obtain the power form we need through certain means.

　　According to the industry, AC-DC (AC converted to DC, AC is alternating current, DC is direct current) is called rectification, including rectification and offline conversion; DC-AC is called inversion, AC-AC is called AC-AC frequency conversion (accompanied by voltage conversion), and DC-DC is called DC-DC conversion. We can use a variety of methods to achieve the purpose of conversion, and currently the main method is to use semiconductor devices to achieve conversion. Generally speaking, any circuit that uses semiconductor power devices as switches to convert one form of power into another is called a switching converter circuit. As long as the automatic closed-loop stable output and protection function are used during the conversion, it is called a switching power supply. The main component of the switching power supply is the DC-DC converter, which is the core of the conversion and is related to the frequency, and the frequency achieved by the DC-DC conversion is the highest. We often hear about offline switching converters, which are actually AC-DC conversions, and some people call them switching rectifiers, but they are not rectification in the pure sense, but DC-DC conversion after rectification.

　　2 High frequency current type pulse width controller UC3825B

　　UC3825B is a high performance pulse width controller . The controller contains a precise voltage reference, micro power startup circuit, soft start, high frequency oscillator, wideband error amplifier , fast current limit comparator, dual pulse suppression logic and dual totem pole output driver. The signal passes through the current limit and comparator, logic and output driver with very short transmission delay.

　　UC3825B has the following features: suitable for voltage-type or current-type switching power supply circuits; the actual switching frequency can reach above 1MHz; the maximum transmission delay time of the output pulse is 50ns; it has two large-current push-pull outputs with a peak current of 2A; it has soft-start control; it has a pulse-by-pulse current limiting comparator; it has a blocking overcurrent comparator with full-cycle restart; the starting current is very small - the typical value is 100mA; during undervoltage lockout, the output low level and no-load current can be 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 Schematic Diagram

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

　　3.3 Current Limitation and Duty Cycle Control of UC3825B

　　After the transformer primary current 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 the 9th pin of UC3825B through RC filtering, thereby achieving cycle-by-cycle current limiting. Under normal working conditions, the 9th pin input voltage of UC3825 must be lower than the 1V threshold voltage. When the 9th pin input voltage exceeds 1V, the pulse width will become narrower. When the 9th pin input voltage exceeds 1.4V, the output current is interrupted, and UC3825B starts the soft start procedure.

　　UC3825B can realize current control or conventional duty cycle control by using the ramp RAMP pin (pin 7) input signal. When this pin is connected to a timing capacitor, UC3825B can realize duty cycle control. When the RAMP pin is connected to a current sampling resistor, UC3825B can realize current control. In this application circuit, the primary current waveform passes through a very small RC filter network to generate a ramp waveform. The role of the RC network is slope compensation. The dynamic range of this input signal is 1.3V, which is usually 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, whose forward voltage drop was about 0.4 ~ 0.65V. The on-state power consumption was very large at low voltage and high current. Because the forward voltage drop of power MOSFET is very small, power MOSFET is used as the output rectifier. Compared with Schottky diodes, the advantages of using power MOSFET are not only small forward voltage drop, but also high blocking voltage and small reverse current. Figure 2 shows the output full-wave synchronous rectifier circuit. Power MOSFET tubes VT1 and VT2 are two rectifier tubes (VD1 and VD2 are the internal anti-parallel diodes of VT1 and VT2 respectively). When the same-name terminal of the transformer secondary winding is positive, VT2 and VD2 are turned on at the same time, VT1 and VD1 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; conversely, when the same-name terminal of the transformer secondary winding is negative, VT1 and VD1 are turned on at the same time, 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.

　　Adopting this power MOSFET tube rectifier circuit can greatly improve the rectification efficiency. Output +5V/20A, adopting a power MOSFET tube with an on-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 tube loss alone is reduced by 8W, and the efficiency can be increased by about 6%.

　　3.5 Transformer Manufacturing

　　The primary winding N2 and the secondary winding N4 are tightly coupled, while the coupling from the primary winding N1 to the primary winding N2 is not very strict.

　　3.6 High Frequency Design

　　Special attention should be paid to the layout of external conductors and components to reduce unnecessary inductance and capacitance. All wire lengths must be as short as possible. The printed circuit board should be carefully arranged with components and connections. The resistor of the power MOSFET gate should be a carbon component resistor to reduce the series inductance.

　　4 Conclusion

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