新型有源箝位软开关PWM控制器UCC289X的原理及应用

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新型有源箝位软开关PWM控制器UCC289X的原理及应用 [Copy link]

摘要：介绍了电流控制型芯片UCC289X的基本原理，设计出了基于该芯片的同步整流有源箝位正激变换器的实用电路。实验结果证明，该芯片具有较好的控制特性和稳定性。

关键词：UCC289X；脉宽调制；有源箝位；峰值电流控制

　

0 Introduction

UCC289X系列是德州仪器公司（TI）继UCC3580之后，于2003年推出的新型PWM控制器[1]。该系列产品包括UCC2891、UCC2892、UCC2893和UCC2894等，其中，UCC2891和UCC2892的辅助输出与主输出同相，用于驱动PMOS；UCC2893和UCC2894的辅助输出与主输出反相，用于驱动NMOS。UCC289X系列适用于有源箝位正激或反激变换器，可提供辅助简单编程的延时，获得适当的有源箝位操作，从而为原边的MOSFET器件提供零电压开关(ZVS)功能，降低变换器的开关损耗。软开关功能可使这些器件在高频下获得更高的效率，并通过降低电磁干扰(EMI)和射频干扰(RFI)及电源组件的损耗提高系统的整体可靠性。

基于TI第一代UCC3580有源箝位控制器，这4种新的控制器在改善性能的同时又减少了许多外接元器件。UCC289X系列有内部可编程的斜率补偿电路，精确的最大占空比限制以及内置定时电容的1MHz同步振荡器，并具有可编程的软起动和线路监视功能。此外，UCC2891与UCC2893还具有内部110V启动功能，简化了电路设计。

1 Basic structure and main features of UCC289X

The UCC289X series chips have two package types: 16-pin SOIC and small outline TSSOP. The pins are shown in Figure 1 and the pin descriptions are shown in Table 1. Pin 16 of UCC2892 and UCC2894 is the input overvoltage detection terminal. UCC2891 and UCC2893 have an internal 110V high-voltage startup circuit, and their pin 16 (VDD) is directly connected to the input power terminal.

(a) Pin diagram of UCC2891 and UCC2893

(b) Pin diagram of UCC2892 and UCC2894

Figure 1 Chip pin diagram

Table 1 UCC289X pin description

Pinout	name	Function
1	RDEL	Delay setting
2	RTON	Timing capacitor charging time setting
3	RTOFF	Timing capacitor discharge time setting
4	VREF	Reference voltage
5	SYNC	External synchronous clock input
6	GND	Reference Place
7	CS	Current Sensing
8	RSLOPE	Slope compensation
9	FB	Feedback Input
10	SS	Soft Start
11	PGND	Gate drive ground
12	TO	Auxiliary output
13	OUT	Main Output
14	VDD	Working voltage (12V)
15	LINEUV	Undervoltage Lockout
16	COME	Chip input voltage

　

Figure 2 is the internal structure diagram of UCC289X, which mainly includes reference voltage circuit, undervoltage lockout and soft start circuit, synchronization circuit, error amplifier, slope compensation circuit and master-slave output, etc. Its main features are as follows.

Figure 2 UCC289X internal structure diagram

1) Peak current control mode is adopted, and the cycle-by-cycle current limiting function is provided. The threshold voltage of pin 7 (CS) of UCC2891 and UCC2893 is 0.75V, and that of UCC2892 and UCC2894 is 1.27V. Pin 7 is connected to the current detection resistor through the RC filter circuit. When pin 7 detects overcurrent, the hysteresis comparator inside the chip outputs a high level, resetting the SR trigger and turning off the main output, thereby protecting the circuit.

2) ±2A gate driver with integrated programmable dead time control Users can control the turn-on delay of the gate drive signal by connecting R _{DEL to chip pin 1,} R _DEL =( t _DEL－50×10 ^－9 )×0.87×10 ¹¹ .

The main and auxiliary output source and sink currents of UCC289X can reach 2A, so the main and auxiliary switch tubes can be turned on and off quickly.

3) UCC2891 and UCC2893 have internal 110V high-voltage startup circuit. Pin 16 (VDD) of UCC2891 and UCC2893 is directly connected to the input power supply terminal, and the undervoltage lockout circuit inside the chip monitors the VDD voltage. At startup, the JFET inside the chip is turned on to charge the energy storage capacitors C _BIAS and C _HF , and the charging current is nearly 15mA. When VDD rises to the undervoltage lockout start threshold of 13V, other circuits inside the chip start to work, a signal appears on the drive gate, the JFET is immediately turned off, and the bias voltage is provided by the bootstrap winding.

4) 1MHz synchronous oscillator with programmable maximum duty cycle control The UCC289X series has a synchronization input (pin 5) to synchronize the oscillator with the system clock, thereby limiting the maximum duty cycle of the converter, as shown in Figure 3. Normally, the maximum pulse width of the chip's main output is limited by the charging time of the timing capacitor. When the synchronization function is used, the rising edge of the synchronization signal stops charging the timing capacitor and starts discharging. Once the voltage of the timing capacitor drops to a preset threshold, a new charging cycle automatically begins. This synchronization method makes the charging and discharging process of the timing capacitor no longer affected by the converter's operating mode, thereby maintaining the converter's maximum duty cycle.

Figure 3 Synchronous waveform

5) Programmable internal slope compensation function Users can perform slope compensation on the circuit by connecting R _F at pin 7 and R _SLOPE at pin 8 .

6) Programmable soft-start function UCC289X has an internal precise DC current source. Users can control the soft-start current through the external resistor R _{ON of pin 2, that is,} I _SS = ×0.43A.

7）准确的输入欠压与过压传感阈值 UCC289X通过脚15即可实现输入欠压滞环控制。输入欠压时，箝位电容放电（见图4）。当输入低于欠压锁定关断阈值8V时，主输出驱动脉冲闭锁，辅助输出端仍有脉冲输出，软启动电容C_SS缓慢放电。随着辅助输出端脉宽的增大，箝位电压逐渐减小，从而实现了欠压保护功能。UCC2892和UCC2894没有高压启动装置，其脚16用于提供过压保护功能。同样，用户通过脚15即可实现输入过压滞环控制。这是由于脚15的电压是对输入的分压，其输入过压关断阈值是1.27V。当脚15检测到其输入高于1.27V时，主输出停止工作，同时软启动电容CSS缓慢放电。当CSS为0.5V，过压消失后，电路通过软启动恢复正常工作。

(a) Undervoltage lockout shutdown waveform

(b) Input overvoltage protection

Figure 4 Input undervoltage lockout shutdown waveform and input overvoltage protection

2 Applications of UCC289X

UCC289X is suitable for low-voltage and high-current communication power supplies with small and medium power, such as servers, data communication adapters, remote control equipment, DSP, ASIC, etc.

图5所示为采用控制芯片UCC2891设计的100W同步整流有源箝位正激变换电路。电路的试验参数如下：输入电压48V，输出电压3.3V，输出电流30A，开关频率300kHz，最大占空比设为0.65。其中，主开关采用N沟道MOSFET(Q₁)，箝位开关采用P沟道MOSFET(Q₂)，主副开关间的死区时间由脚1的外接电阻R_DEL控制。T₁是主变压器，C_CL是箝位电容。为使电流检测端的功耗最小，采用电流检测变压器T₂。副边采用同步整流技术，Q₃和Q₄是相应的同步整流管。与以往通过主变压器的辅助绕组获得偏置电压的方式不同，本文利用输出滤波电感L_O的耦合电感作为原边控制芯片的偏压绕组，从而既可以为芯片提供稳定的偏压，又避免了采用常规线性调节器时产生的功耗。线性光耦SFH690BT与可调式并联稳压器TLV431将输出反馈至芯片的FB端。

图5 采用UXX2891实现的同步整流有源箝位正激变换器电路

Setting the appropriate delay can make both the master and slave switches have ZVS turn-on conditions. Pin 1 (DELAY) of UCC2891 is specifically used to control the delay between OUT and AUX. Figure 6 shows the ratio between the two delays. In the process of transitioning from the main switch to the auxiliary switch, the delay is not very critical for the ZVS condition. In the first half of the OUT turn-off process, the body diode of the auxiliary switch is turned on, so AUX can achieve ZVS turn-on at any time thereafter. The transition from the auxiliary switch to the main switch is more critical. The energy in the parasitic inductance at the end of the AUX pulse width can be used to discharge the parasitic capacitance of the main switch tube during the delay period. The delay (Delay1) should be 1/4 of the resonant period determined by the parasitic inductance and parasitic capacitance. However, due to the influence of other parasitic parameters of the circuit, the resonance may change, making it impossible to achieve ZVS turn-on in some cases. It can be seen that in a specific circuit, the most appropriate delay depends on the operating conditions. Therefore, specific problems should be treated specifically.

Figure 6 Relationship curve between Delay1 and Delay2

The test results show that the output voltage fluctuation range is within 4mV (0.1%) within the entire input and load variation range. This circuit can achieve full load to no load regulation, and the converter efficiency is above 90% within a fairly wide operating voltage range, as shown in Figure 7.

Figure 7 Converter efficiency curve

3 Conclusion

将UCC2891应用于100W同步整流有源箝位正激变换电路中，试验结果证明了该芯片具有较好的控制特性和稳定性。

　

About the Author

Yu Li (1978-), female, master's student, research direction is high-frequency switching power supply technology.

Wang Zhiqiang (1951-), male, is an associate professor and master's supervisor at the Power Electronics Teaching and Research Section of the School of Electrical Engineering of South China University of Technology. He is mainly engaged in the research of high-frequency switching power supplies and power conversion technology.