A synchronous buck converter is a type of DC-DC converter used to step down voltage levels efficiently. Hysteresis control with digital compensation is a control technique employed in synchronous buck converters to regulate the output voltage accurately. Let's break down how this control scheme works:
Basic Synchronous Buck Converter:
A synchronous buck converter consists of two power switches, typically MOSFETs, and an inductor connected in series with the load. The operation is based on switching the MOSFETs on and off at a fixed frequency to control the output voltage. When the high-side switch (HS) is ON, current flows through the inductor, storing energy. When the low-side switch (LS) is ON, the inductor discharges its stored energy to the output capacitor and load.
Voltage Feedback Loop:
To regulate the output voltage, a feedback loop is employed. The output voltage is compared to a reference voltage, and the error voltage is used to adjust the duty cycle of the high-side and low-side switches. If the output voltage is too low, the duty cycle is increased to deliver more energy to the output. If the output voltage is too high, the duty cycle is reduced to limit the energy transfer.
Hysteresis Control:
Hysteresis control introduces a small voltage band around the reference voltage. Instead of regulating the output voltage precisely to the reference voltage, the converter allows some voltage variation within the hysteresis band before taking corrective action. This introduces a simple form of voltage regulation and reduces the number of control operations, resulting in faster transient response and reduced switching frequency.
Digital Compensation:
To improve the accuracy and stability of the hysteresis control, digital compensation techniques are used. A microcontroller or digital signal processor (DSP) is employed to monitor the output voltage and control the switching operation. The digital controller measures the output voltage and compares it with the reference voltage. If the output voltage falls below the lower hysteresis threshold, the controller triggers the high-side MOSFET ON; if the output voltage rises above the upper hysteresis threshold, it turns the high-side MOSFET OFF.
Zero Current Detection:
To enhance the efficiency of the converter, synchronous buck converters use synchronous rectification. This means that the low-side MOSFET is turned on only when the current through the inductor becomes zero. This zero current detection is typically achieved using a current sensing mechanism, such as a shunt resistor or a current sense amplifier.
By combining hysteresis control with digital compensation, the synchronous buck converter can achieve accurate and efficient regulation of the output voltage. The digital compensation ensures stable control, while the hysteresis control provides faster response and reduces switching frequency, leading to improved efficiency and reduced switching losses.