Understand these six key issues of voltage regulator and become an expert in seconds
A voltage regulator produces a fixed output voltage with a constant preset amplitude regardless of changes in its input voltage or load conditions.
Voltage regulators are divided into two categories: linear and switching -
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Linear regulators use active (BJT or MOSFET) pass devices (in series or parallel) controlled by a high-gain differential amplifier. It compares the output voltage to a precision reference voltage and adjusts the pass device to maintain a constant output voltage.
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A switching regulator converts a DC input voltage into a switching voltage that is applied to a power MOSFET or BJT switch. The filtered power switch output voltage is fed back to a circuit that controls the on and off times of the power switch so that the output voltage remains constant regardless of changes in the input voltage or load current.
What are the topologies of switching regulators?
There are three common topologies: buck, boost, and buck/boost. Other topologies include flyback, SEPIC, Cuk, push-pull, forward, full-bridge, and half-bridge.
How does switching frequency affect regulator design?
Higher switching frequencies mean that the regulator can use smaller inductors and capacitors. This also means higher switching losses and more circuit noise.
What are the losses in a switching regulator?
The power required to turn the MOSFET on and off causes losses and is related to the MOSFET gate driver. Similarly, the switching time from the conducting state to the non-conducting state takes a certain amount of time, which causes power dissipation in the MOSFET. In addition, the energy required to charge and discharge the MOSFET gate capacitance between the threshold voltage and the gate voltage also causes losses.
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What are some common applications for linear and switching regulators?
Given an input and output voltage, the power dissipation of a linear regulator is proportional to the output current, so typical efficiencies can be 50% or less. By optimizing the device, switching regulators can achieve efficiencies of 90%. However, the noise output of a linear regulator is much lower than that of a switching regulator with the same output voltage and current requirements. In general, a switching regulator can drive higher current loads than a linear regulator.
How does a switching regulator control its output?
A switching regulator needs some way to change its output voltage in response to input and output voltage changes. One method is to use PWM to control the input of the associated power switch, thereby controlling its on and off time (duty cycle). In operation, the filtered output voltage of the regulator is fed back to the PWM controller to control the duty cycle. If the filtered output changes, the feedback applied to the PWM controller changes the duty cycle to maintain a constant output voltage.
Which design specifications are important for voltage regulator ICs?
Basic parameters include input voltage, output voltage, and output current. Depending on the application, other parameters may also be important, such as output ripple voltage, load transient response, output noise, and efficiency. Important parameters for linear regulators include dropout voltage, PSRR (power supply rejection ratio), and output noise.
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