Suitable For Automotive Cold Start Applications, Two
Generally speaking, we use buck-boost topology topology to address automotive applications in a wide input voltage range and cold-start demand. This article will explain the requirements of cold start, and describes two different solutions. One is the traditional SPE IC Topology, while the other is relatively new and more Switch Buck / Boost topology.
Described below, the advantages and disadvantages of each program, and will focus on that two-switch buck / boost topology compared to the advantages of traditional SEPIC topology. In addition, this article will be combined with the National Semiconductor Latest LM5118 current-mode buck / boost controller for application instructions.
Cold start conditions Start car is actually driven by internal combustion engine electric starter motor. Starter motor power consumption of power from the car Battery Provide. Start the motor needs a large load will cause the battery voltage has dropped. For the car start, the worst is the “cold start.” This happens when the low temperature environment, the low temperature makes it more difficult for automotive cold start. When the car is in low temperature environment, the internal combustion engine rotational resistance will rise to the highest, requiring greater mechanical strength can be mobilized. Therefore, the power consumed by the motor starting peak current than when in a warm environment, a higher wage. Another in the “cold start” under the influence factor is the car battery voltage declines as the temperature dropped, and dropped the old battery the more the greater the magnitude.
Temperature effect of the two car batteries will make the minimum supply voltage has dropped significantly. ISO7637 standard setting the car on cold start conditions, the basic voltage waveform. Figure 1 indicated their cold start conditions, the voltage characteristics, which generally defined as two voltage levels of voltage. First of all, when the starter motor began to turn the power to overcome the initial mechanical resistance, the supply voltage will be at a minimum. Then mechanical systems up and running, the required voltage also increased. Finally, when the power is turned off starter motor, the system voltage will return to normal levels.
Generally speaking, we use buck-boost topology topology to address automotive applications in a wide input voltage range and the demand for cold start. This article will explain the requirements of the cold start, and describes two different solutions. One is the traditional SPEIC topology, while the other is relatively new multi-switch buck / boost topology.
Described below, the advantages and disadvantages of each program, and will focus on that two-switch buck / boost topology compared to the advantages of traditional SEPIC topology. In addition, this article will be combined with National’s new LM5118 current-mode buck / boost controller for application instructions.
Cold start conditions Start car is actually driven by internal combustion engine electric starter motor. Electric starter motor from the car battery power consumption. Start the motor needs a large load will cause the battery voltage has dropped. For the car start, the worst is the “cold start.” This happens when the low temperature environment, the low temperature makes it more difficult for automotive cold start. When the car is in low temperature environment, the internal combustion engine rotational resistance will rise to the highest, requiring greater mechanical strength can be mobilized. Therefore, the power consumed by the motor starting peak current than when in a warm environment, a higher wage. Another in the “cold start” under the influence factor is the car battery voltage declines as the temperature dropped, and dropped the old battery the more the greater the magnitude.
Temperature effect of the two car batteries will make the minimum supply voltage has dropped significantly. ISO7637 standard setting the car on cold start conditions, the basic voltage waveform. Figure 1 expressed under cold start voltage characteristics, which generally is defined as the voltage of two voltage levels. First of all, when the starter motor starts turning the power to overcome the initial mechanical resistance, the supply voltage will be at a minimum. Then the mechanical system up and running, the required voltage also increased. Finally, when the power is turned off starter motor, the system voltage would be back to normal levels.
Order under wide input voltage range to provide high precision output voltage regulation, we must use the appropriate control method to drive two switch MOSFET, in order for the buck and buck / boost modes to provide a smooth over. The controller input and output of the conditions under three different mode:
1. Antihypertensive action Vin> Vout: Vout if Vin is greater than a sufficient amount, the regulator would be a form of traditional buck regulator to run. In this mode, the step-down conversion function Vout / Vin = D, where D is the duty cycle of Q1, while the simple step-down operation mode can be obtained to ensure optimal efficiency and moderating effects.
Vout when Vin fell relative to the duty cycle close to 70%, the boost switch to a minimum duty cycle will be activated, so that regulators into a soft buck / boost mode (Figure 3a).
2. Buck / Boost operation Vin Vout: With Vin further reduced to close to Vout, step-down switching duty cycle will decrease, while the boost switch duty cycle increased . This step-down operation mode can also smoothly transition to boost operating modes.
3. Buck / Boost operation Vin With this operating mode, the output voltage Vout will continue to close at Vin stable, because no voltage during the mutation, only a buck and boost mode from a gradual transition between.
Imitation peak current mode control scheme
In order to ensure a wide output voltage can be adjusted input voltage range, must use current-mode PWM control scheme. The reason is the current mode control provides inherent line feed-forward, by periodic current limit and simple loop compensation and so on.
Traditional current-mode application program only limitation is its sensing path of the noise is extremely sensitive and difficult to meet the required high input voltage applications with low duty cycle. Therefore, National Semiconductor has developed a special new current-mode control scheme “current-mode” and will limit the application of the past swept away.
Current-mode can be rebuilt Inductors Slope current. The specific method is: first freewheeling diode measured at the end of the switching cycle, current, and then combined with inductor current slope is proportional to the slope. In order to imitate the inductor current slope part of an external Capacitor By a constant current charge, while the fixed current and the input and output voltage proportional to the difference between. Thus, the final
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