The majority of systems that use DC/DC converters will implement voltage-mode control to hit a constant voltage target. Current-mode control is a less common control method in DC/DC converters, but it is important in certain applications requiring constant DC current output to an arbitrary load. This is an important area of analog electronics that arises in sensors, medical and industrial systems, and precision measurements.
This article will show the standard mechanisms to implement current-mode control in a DC/DC converter. Implementation of current-mode control relies on sensing the output current and adjusting the output by varying the duty cycle, frequency, or both. By sensing the output current, the circuit can have a faster response and will avoid certain sub-harmonic oscillations that can arise in PCB layouts for DC/DC converters.
What is Current-Mode Control?
Current-mode control is an alternative method for regulating output voltage in a DC/DC converter. The goal is to achieve a constant output voltage from a switching DC/DC converter by measuring the current passing through the inductor. This should not be confused with current regulation, or constant-current regulation; these terms refer to regulation of the output current of a target value.
The topology of current mode control can use two possible measurements to determine the error in the target output voltage:
- Current feedback loop: The current passing to the output is measured in series with the inductor (output from the switching node)
- Voltage feedback loop: The output voltage is measured as part of a feedback loop and is used as an input to the current feedback
Within current-mode control, there are two possible measured values that can be used for control: average current and peak (or valley) current. The feedback stage must be set up to operate on these values; an integrator circuit will be used for the current measurement when average current control is desired.
Example Buck Converter
The example below shows the circuitry required to implement peak current-mode control in a buck converter. In this example, OA1 provides a voltage measurement across a precision current-sense resistor (RS). The measurement is then adjusted and output to one of the terminals on OA3, which is wired as a comparator to compare the reference voltage measurement and the current-sense measurement.
Buck converter with current-mode control.
Here we have a standard slow feedback loop along the voltage divider, which is normally seen in voltage-mode control. This would be the standard setup used to connect to the feedback pin in LDOs and non-isolated switching converter controllers. The other loop is the fast current loop, which comprises OA1, OA3, and the flip-flop.
Why Use Current-Mode Control?
While non-isolated voltage-mode control is just a simple step of connecting a feedback line, current-mode control does offer some advantages. Current-mode control maintains regulation by adjusting the output voltage based on current response, so it can respond faster to changes in load impedance. This is because current mode control inherently provides a more accurate measurement of the actual power delivered to the load, compared to voltage mode control.
Another advantage of current-mode control is its ability to prevent sub-harmonic oscillations that can occur in voltage mode control when the converter is operating in continuous conduction mode. The problem with sub-harmonic oscillations is that they can cause the output voltage to fluctuate at a frequency that is lower than the converter's switching frequency. This can result in high ripple voltage and lower power conversion efficiency. Sub-harmonic oscillations can also generate strong electromagnetic interference (EMI).
The reason sub-harmonic oscillations can be eliminated in current-mode controlled DC/DC converters is because parasitics in the output filter are not altering the measured switching current. With fewer parasitics on the measurement path, the switching current can be measured more accurately.
Is There Current-Mode Control in VRMs?
Because the feedback mechanism in current-mode control is much faster than in voltage-mode control, we might expect current-mode control to be used with devices like VRMs for large processors. However, this is not the case; commercially designed VRMs implement voltage-mode control as a standard approach. This means that board designers must focus on reducing voltage fluctuations by providing lowest possible impedance in the PDN.
While not used in VRMs commercially, there is recent research into using current-mode control in VRMs to provide faster responses to output voltage fluctuations. Current-mode control has its fast response time advantage as well as its ability to measure the current being drawn directly from the switching node. As power regulators for advanced digital components continue to see greater demand on their feedback and response capabilities, VRMs may end up switching to current-mode control for stable power delivery.
When you’re ready to design and simulate your DC/DC converter designs with current-mode control, use the complete set of simulation tools in PSpice from Cadence. PSpice users can access a powerful SPICE simulator as well as specialty design capabilities like model creation, graphing and analysis tools, and much more.