In this video prof. Sam Ben-Yaakov discusses regulated switched capacitors in converters, its efficiency, stress and voltage regulation
Switched capacitor converters (SCCs) are integral in modern electronics, offering compact solutions for voltage conversion without relying on inductors. This article delves into their basic configurations, efficiency challenges, stress factors, and mechanisms for voltage regulation.
Basic Configurations of Switched Capacitor Converters
A fundamental SCC, often illustrated with schematics from Texas Instruments and Analog Devices, operates using MOSFET switches and flying capacitors:
- Inverting Converter (Texas Instruments Model):
- Design: Four MOSFET switches connect the input through to a flying capacitor, then to the output.
- Operation: In one phase, the capacitor connects to the input, storing charge. In the next phase, it connects to the output, releasing charge. The output voltage is negative relative to the grounded positive terminal, effectively inverting the input.
- Voltage Doublers/Dividers (Analog Devices Model):
- Design: Configures capacitors in either series or parallel arrangements.
- Operation: Depending on the input/output definition, the output can be double or half the input voltage.
Efficiency Challenges in SCCs
Efficiency in SCCs is influenced by internal losses due to:
- Resistive Elements: MOSFET on-resistance and Equivalent Series Resistance (ESR) of capacitors contribute to power dissipation.
- Load Conditions: At low currents, voltage drops are minimal, but as load increases, internal resistance causes significant drops.
- Switching Frequency and Capacitor Size: Higher frequencies and larger capacitors reduce equivalent internal resistance, enhancing efficiency.
Voltage Stress and Regulation Techniques
Stress Factors:
- Switch Stress: MOSFETs endure voltage swings that depend on the switching cycle.
- Capacitor Stress: Capacitors face charge/discharge cycles leading to thermal and electrical stress.
Regulation Methods:
- Adjusting Switching Frequency: Changing the frequency alters the internal resistance, thereby regulating the output voltage.
- Control of MOSFET Resistance: Varying gate drive levels modifies MOSFET conduction resistance, affecting output voltage.
- Hybrid Solutions: Incorporating Low Dropout Regulators (LDOs) in series with SCCs for fine-tuning output voltages.
Simulation Insights and Practical Considerations
Simulations confirm theoretical models:
- Load Impact: Higher load resistances reduce output current, minimizing voltage drops and improving efficiency.
- Parasitic Inductances: Influence current waveforms, necessitating careful PCB layout to reduce oscillatory behavior.
Conclusion
Switched capacitor converters offer compact, efficient solutions for voltage conversion. While their efficiency can rival that of LDOs under certain conditions, understanding and mitigating internal losses and stress factors are crucial for optimal performance. By leveraging advanced control techniques and meticulous design, engineers can harness SCCs for a wide range of power management applications.