Current-mode controlled DC-DC switching voltage converters ("switching regulators") are popular because they provide the high efficiency of switching power supplies while overcoming the shortcomings of conventional voltage-mode control devices.
However, when the duty cycle of the pulse width modulated signal (the PWM used to set the output voltage) rises above 50%, the current mode design can suffer from instability. To overcome this instability, design engineers use a technique called slope compensation to restore reliable operation over the entire PWM duty cycle range.
This article describes the important, but largely obscure, technique of slope compensation, how it is implemented, and how it affects the performance of current-mode controlled switching regulators. The article then goes on to incorporate examples of commercial voltage controllers from major power module manufacturers in this technology.
Benefits of Current Mode Control
In conventional voltage-mode controlled switching regulators, a pulse-width modulated (PWM) signal is generated by applying a control voltage to a comparator input and a fixed-frequency sawtooth voltage (PWM ramp) generated by a clock. The output voltage of the regulator is proportional to the duty cycle of the resulting PWM square wave.
Another design, the current-mode controlled regulator, has become popular because it offers many advantages. For example, a current-mode control regulator responds more quickly to line or load voltage changes than voltage-mode, it eliminates loop-gain changes caused by input-voltage imperfections in voltage-mode, compensation is easier to implement, and the circuit has Also has a higher gain bandwidth. (See Hi-Tech's article "Voltage and Current Mode Controlled PWM Signal Generation in DC Switching Regulators".)
A current-mode controlled regulator differs from the second-loop equivalent voltage-mode by increasing the feedback current, which then contributes to the derivation of the PWM ramp. The feedback signal includes the AC ripple current and the DC or average value of the inductor current. The amplified version of the signal is routed to one input of the PWM comparator, while the error voltage (VE, the difference between the reference voltage and the output voltage) forms the other input. As with the voltage-mode control method, the system clock determines the PWM signal frequency (Figure 1).
Image of Texas Instruments Current Mode Control Switching Regulators
Figure 1: Current-mode control switching regulator. Here, the PWM ramp is generated by the signal from the output inductor current.
While current mode control has many advantages, it is not without its complexities. The most notable of these is the instability of the "inner" control loop (carrying the inductor current signal) with duty cycles above 50%. Pioneering design engineers quickly found a solution to this problem by "injecting" a small amount of slope compensation into the inner loop. This technique guarantees stable operation for all values of PWM duty cycle.
Many power supply designers, while aware of the technology, understand how to work and how to implement the performance impact of slope compensation circuits. Let's take a closer look.
Stable circuit
For example unstable sources, the current-mode controlled buck ("Barker") regulator operates in continuous mode (ie, during switching cycles when the inductor current is not zero) because the controller drives the inductor by regulating the peak inductor current output, the load current is equal to the average inductor current to set the output voltage. Figure 2 shows that the average currents (I1 and I2) remain constant (regulated) with the peak current of the duty cycle.
Texas Instruments Inductor Average Current Image
Figure 2: In a current-mode control regulator, the average inductor current is proportional to the duty cycle.
The difference between the peak and average inductor current forms is wrong (ΔI), which is large at the maximum input voltage. At lower duty cycles, the error is smaller than the problem because it remains constant, but at duty cycles above 50%, the error increases for each successive inductor charge-discharge cycle, causing instability ( image 3).
Texas Instruments Error Between Inductor Peak and Average Current
Figure 3: At larger duty cycles (D2), the error (ΔI) between the peak and average current of the inductor multiplied by successive charge/discharge cycles induces instability.
By summing the negative sawtooth ramp voltage with VE amplified at the control input of the comparator (as shown in Figure 1), engineers can introduce slope compensation into the circuit. For perfect compensation, the compensation ramp must have a slope at the input of the other comparator that is equal to half the downslope of the voltage waveform, which is the voltage analog of the inductor current downslope over the current-sense resistor.
When implemented correctly, slope compensation addresses both the problem of current errors that vary over the duty cycle range and the instability of duty cycles beyond 50%. Figure 4 shows the slope compensation, regardless of changes in VIN and duty cycle, the average current remains the same. Figure 5 shows how the circuit remains stable because slope compensation reduces the error for any value of the duty cycle.
Texas Instruments slope compensation image
Figure 4: Slope compensation, the average current of the circuit remains constant and the duty cycle varies.
德克萨斯仪器斜率补偿图像减小电流误差
图5：斜坡补偿减少占空比的任何值在连续循环上的电流误差。
简化设计
对于一个工程师从离散组件构建电流模式控制调节器，产生斜坡补偿所需的负斜坡可能是棘手的。一种解决方案是通过将PWM通过电压分压器施加到相反的比较器输入(然后与上述电感电流的电压模拟求和)来使用PWM IC的正斜坡。
一种日益流行的替代方案是将电路围绕一个开关控制器模块集成，将PWM控制电子器件集成到单个设备中。这些模块中的许多现在包括由制造商纳入的斜坡补偿。
功率模块包括电流模式控制与斜坡补偿的例子包括德克萨斯仪器(TI)tps43060。该装置是一个1 MHz开关控制器，从4.5-38 V输入高达95%的占空比操作。该芯片具有内部斜坡补偿，以避免占空比高于50%的次谐波振荡。
美心max15004开关稳压器控制器––电流模式控制装置的设计与输入电压范围从4.5-40 V––采取更进一步通过使用外部电容器提供可编程斜率补偿的汽车应用。该公司声称这简化了设计。
它的一部分，Intersil还提供一个开关稳压器，可编程斜率补偿的isl6726(图6)。制造商说，该设备是一个电流模式PWM控制器的许多特点，旨在简化其使用。与马克西姆装置一样，通过使用外部电容器再次设置斜率补偿。为isl6726数据表提供了一个方便的指导如何计算坡度的电容值。
Intersil的isl6726图像
图6：Intersil的isl6726使可编程斜率补偿。