In a synchronous buck converter, voltage step-down is achieved by using hysteresis control, also known as voltage mode control. The hysteresis control method is a popular technique for regulating the output voltage of a buck converter. It uses a simple and efficient feedback mechanism to maintain the output voltage within a specified range.
Here's how a synchronous buck converter with hysteresis control achieves voltage step-down:
Basic Buck Converter Operation:
A synchronous buck converter is a type of DC-DC converter that steps down the input voltage to a lower output voltage. It consists of two switches (typically MOSFETs), an inductor, a diode, and an output capacitor. When one of the switches is turned on, current flows through the inductor, storing energy. When the switch is turned off, the inductor releases this energy to the output load through the diode, effectively lowering the output voltage.
Hysteresis Control Overview:
Hysteresis control is a feedback control technique used to regulate the output voltage. It compares the output voltage with a reference voltage and decides whether to turn the switches on or off based on this comparison. Unlike traditional voltage regulation techniques (e.g., pulse-width modulation), hysteresis control does not use a fixed switching frequency. Instead, it regulates the output voltage by controlling the switching of the power switches based on predefined voltage thresholds.
Hysteresis Band:
In hysteresis control, a "hysteresis band" or "deadband" is defined around the desired output voltage. This band represents a voltage range in which the output voltage is considered acceptable, and no immediate action is taken. The width of the hysteresis band determines the regulation accuracy. A narrower band provides tighter regulation but might result in higher switching frequencies, while a wider band reduces switching frequency but might result in larger output voltage variations.
Feedback Loop:
The hysteresis control operates in a feedback loop that continuously compares the output voltage with the reference voltage. When the output voltage falls below the lower threshold of the hysteresis band, the control circuit turns on both power switches (synchronous rectification) to allow current flow through the inductor and charge the output capacitor. As the output voltage rises and crosses the upper threshold of the hysteresis band, the control circuit turns off both switches to stop the current flow and prevent overcharging the output capacitor.
Operation:
When the output voltage is within the hysteresis band, both switches remain off, and the inductor discharges into the output capacitor through the diode. This prevents unnecessary switching and reduces power losses.
Advantages of Hysteresis Control:
Simple implementation and fewer components compared to other control methods.
No need for an external compensation network, reducing component count and complexity.
Fast transient response due to immediate action when the output voltage crosses the thresholds.
However, hysteresis control may introduce some disadvantages like potential voltage ripple and higher output voltage variations when load changes occur.
In summary, a synchronous buck converter achieves voltage step-down using hysteresis control by comparing the output voltage with a reference voltage and turning the power switches on or off based on predefined voltage thresholds, effectively regulating the output voltage within a specified range.