Most popular boost regulator

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Playing with the boost regulator

when modeling the small signal of the power stage, the boost regulator has two disadvantages compared with the step-down regulator: first, it has a right half plane (RHP) zero point determined by the duty cycle and load, which increases the derivation complexity of the model; Second, boost regulators are not as common as step-down regulators, so they do not make much effort in deriving accurate small signal models. In this article, we will introduce a simplified model for current mode boost converter (used as voltage regulator), and give several suggestions for modifying the standard practice in order to predict the behavior of current regulator

peak current mode control (controlling the inductor/switch current in the boost regulator rather than the output current) can be seen everywhere in low-end controllers and monolithic ICs, whose control switch emitter/source is connected to the system ground. All common switch regulators implemented with low-end controllers, such as boost, flyback, single ended primary inductive converter (SEPIC) and Cuk Converter, have RHP zeros. By shifting an output LC pole to a higher frequency than the control loop bandwidth, current mode control simplifies the control to output conversion function. The performance of voltage regulator and current regulator can be predicted by the following power level conversion equation:

the difference between voltage regulator and current regulator in the formula can be referred to figure 1a and 1b below

dc gain

(voltage regulator on the left; current regulator on the right)

gi is the parameter of controller IC, ROP = VO/if

system pole

(voltage regulator on the top; current regulator on the bottom)

rhp zero point

(voltage regulator on the top; current regulator on the bottom)

for boost and up current regulators, The following values are the same:

duty cycle

(VD is the output diode voltage drop, typical value is 0.5V)

esr zero point

sampling bipolar quality factor

inherent inductance current slope

slope compensation

(VM is the parameter of controller IC; FSW is the switching frequency.)

sampling bipolar corner frequency:

up to now, the biggest change from voltage regulation to current regulation is DC gain, which comes from RD, which is very small compared with RO, and the resistance divider effect generated by the combined load and feedback path. Considering a voltage regulator with input 1 butter and output 36v/1a, the result of DC gain calculation is about 30dB. For comparison, drive the current regulator with 10 white LEDs ((VO ≈ 36V) measured with a dial indicator based on two force columns. The current is also 1a and the input is also 12V. Its DC gain is only 6dB

figure 1: (a) voltage regulation circuit; (b) Current regulation circuit

amplified current sensing

almost all regulators with adjustable output can be converted into an LED driver, but simply replacing the top feedback voltage dividing resistor with LED string and replacing the bottom feedback resistor with current sensing resistor will consume electric energy and generate heat. If the current induced voltage is not amplified to match the standard energy gap reference voltage of 1.25V, when 1A current passes through the LED, the power consumption of the current induced resistance is 1.25W. But it has little effect on the whole control loop. The increased gain is roughly offset by the reduced RSNs value. For the gain ASNs, the DC gain can be obtained by the following formula:

Figure 2 shows the practical application of the current sensing amplifier with low-cost operational amplifier. The 20 Ω injection res (HP's current partners include Evonik, BASF, Arkema and Lehmann Voss) will be placed between the op amp output and the FB pin of the regulator

Figure 2: current sensing amplifier


predict and measure the response of the LED control loop driven by the current raising transformer because the columns and beams constructed of steel structure are narrower and thinner than those of conventional concrete. It is necessary to modify the standard practice in the following aspects

led driver is regarded as a constant current source. It has no load transients that voltage regulators must overcome. It is attractive to ignore the "control to output" response and use an integrating circuit for simple compensation

hbled driver needs careful analysis to provide high DC gain (to ensure the accuracy of output current); In addition, due to the use of PWM dimming, the bandwidth should be as wide as possible. A fast control loop is needed to rapidly aim the output current responding to the dimming signal, which is as important as the load transient response of the voltage regulator

here, careful prediction, careful design and accurate measurement are as important to the boost LED driver as they are to the boost regulator

chris richardson

Application Engineer of power management product group

National Semiconductor Corporation (end)

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