DC/DC converters generally have two operating modes: current-mode control and voltage-mode control. The majority of systems that require a DC/DC converter will require a voltage source meaning a power source that regulates voltage to some specified value. This is the case with logic circuits and most analog circuits. When a constant voltage is required from a regulator, how is this implemented in a DC/DC converter?
This article will run over the standard mechanisms to implement voltage-mode operation in a DC/DC converter. The ability to spot voltage-mode regulation in these systems requires looking at the feedback loop, and specifically which components are used in the feedback loop to provide regulation. The strategies outlined here can generally be used in multiple topologies, where the goal is to provide some voltage measurement into a feedback loop.
DC/DC Converter Feedback for Voltage-Mode Control
DC/DC converters generally operate in two possible feedback regimes to supply required power to components:
- Voltage-mode control - The output voltage is adjusted to a fixed value while current demands can change up or down
- Current-mode control - The output voltage is adjusted to a fixed value, but a dynamic current measurement is used for feedback
Voltage-mode control comprises the vast majority of power designs. As such, most DC/DC converters are designed with specific types of feedback loops, depending on whether the design needs galvanic isolation.
Voltage-mode control generally involves comparing the output voltage from a DC/DC converter with a precision reference voltage, such as those used in ADCs. Most DC/DC controller ICs will contain this reference voltage internally, typically as a silicon bandgap reference. The block diagram below shows one example of a voltage-mode controller with an internal reference. This reference voltage is used with an error amplifier/comparator to set a control signal to a PWM controller circuit.
Reference voltage comparison (V1) in a switching regulator IC for voltage-mode control.
Voltage-mode control can be spotted by looking at the block diagram of the controller, or it can be spotted by looking at the feedback mechanism.
Resistor Divider Feedback
The simplest and most common method of voltage-mode feedback is through a resistor divider network. Resistor dividers are placed on the output side of a power regulator, and they step down the voltage to the target Vref threshold level. An example is shown below.
Voltage-mode feedback with a resistor divider in an LDO. The same type of feedback is used in switching regulators.
These divider networks do not draw appreciable current into the feedback pin because those pins are connected to the input of an error amplifier (see above). However, they do divert some current to ground. Therefore, it is advisable to use kOhm or larger resistors in the resistor divider network. In general, the current entering the resistor divider should be much smaller than the target output current, otherwise the resistor divider will load down the power regulator.
Precision Shunt Regulator Feedback
Precision shunt regulators are used in DC/DC converter feedback loops to implement either voltage-mode or current-mode control. They are specifically used to bias an optocoupler so that the primary-side current is set to a certain value; voltage-mode control is then implemented by dropping that voltage across a resistor to produce the required threshold voltage. The image below shows a simple schematic for a flyback controller implementing voltage-mode control with a shunt regulator.
Filtering and feedback implemented with a precision shunt regulator (U1).
The challenge here is to ensure the current reaching the feedback resistor is the correct value to translate to the target Vref voltage. This means a designer needs to carefully read the optocoupler bias specification to ensure the correct voltage is brought to the primary side of the system.
Voltage Regulation With Current-Mode Feedback
While it is not recommended, it is possible to use a current-mode SMPS controller to implement voltage-controlled feedback. This involves measuring the output voltage, converting it to a current value, and supplying that current to the feedback pin. If the output voltage droops, the system will interpret this as a current droop and will adjust the PWM duty cycle to compensate and restore the output voltage to a target value.
One reason this is not recommended is the speed of the current compensation loop, which can respond so quickly that the device enters an oscillating state. The other possibility is incorrect translation between output voltage changes and the rate of change in the duty cycle. Because current mode converters are so fast, they can greatly change the duty cycle for a small change in output voltage. This translation needs to be carefully simulated, or an application note should be followed.
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