#### Summary of Topologies and Characteristics | Topology | Min Voltage (Typical) | Components Required | Efficiency | Applications | | ----------------------- | --------------------- | -------------------------- | ---------- | ----------------------------- | | Joule Thief Variants | 0.1-0.4V | Transformer, diode, BJT | Moderate | LED drivers, sensors | | Charge Pumps | 0.02-0.3V | Capacitors, diodes | Low-Medium | Thermoelectric, solar | | Boost Converters | 0.02-0.3V | IC, inductor, capacitor | High | Energy harvesting | | Inductor-Based Doublers | <0.1V | Inductors, MOSFETs | High | TEGs, low-power IoT | | SEPIC | ~0.3V | Inductor, capacitor, diode | High | Solar, wearable electronics | | Transformer Oscillators | ~50mV | Transformer, oscillator | Moderate | RF, piezoelectric energy | | Synchronous Rectifiers | <0.1V | MOSFETs, control circuit | High | Low-voltage energy harvesting | Each topology has specific strengths depending on the application, input voltage range, and energy source characteristics. For ultra-low-voltage applications, **boost converters with sub-threshold startup** and **charge pumps** are among the most effective solutions. #### Circuit Topologies for Low-Voltage Energy Extraction Low-voltage energy extraction often involves harvesting energy from weak sources such as thermoelectric generators (TEGs), piezoelectric devices, solar cells, or RF energy. Specialized circuit topologies are required to operate effectively at these low input voltages. Below is an overview of key circuit topologies and their characteristics. --- ##### 1. **Joule Thief Variants** ###### Description - A basic **Joule Thief** circuit can be modified to improve efficiency and work at even lower voltages. These modifications often involve: - Using high-turn-ratio transformers. - Employing low-Vf Schottky diodes. - Replacing the transistor with an ultralow-threshold MOSFET. ###### Applications - Powering low-power LEDs or small sensors from weak sources such as dead batteries. - Can achieve oscillation from ~0.1V with the right design. --- ##### 2. **Charge Pump (Voltage Doublers and Multipliers)** ###### Description - Charge pump circuits use capacitors and diodes to boost the voltage through charge transfer. Low-voltage Schottky diodes and low-leakage capacitors are critical for efficient operation. - Variants like the **Dickson charge pump** or **Cockcroft-Walton multiplier** can achieve significant voltage amplification. ###### Advantages - Can start with extremely low voltages (~20-30mV in well-optimized designs). - No inductors required, simplifying design. ###### Applications - Thermoelectric and photovoltaic energy harvesting where the source voltage is very low. --- ##### 3. **Boost Converters with Sub-Threshold Startup** ###### Description - Modern **boost converter ICs** are specifically designed for ultra-low-voltage startup. These include energy-harvesting ICs that can start from input voltages as low as **20mV** and then boost to usable levels (e.g., 3.3V or 5V). ###### Examples - **LTC3108 (Linear Technology/Analog Devices)**: Starts at 20mV. - **BQ25570 (Texas Instruments)**: Starts at ~330mV but with high efficiency. - **MAX17710 (Maxim Integrated)**: Optimized for energy harvesting. ###### Applications - Thermoelectric generators, RF harvesting, and small solar cells. --- ##### 4. **Inductor-Based Voltage Doublers** ###### Description - These circuits use two inductors and synchronous rectification to perform voltage doubling with minimal losses. A common topology involves a coupled inductor and synchronous MOSFET switching to achieve high efficiency. ###### Advantages - Operates at very low input voltages (<100mV). - High efficiency due to reduced diode losses. ###### Applications - Thermoelectric energy harvesting. - Low-power IoT devices. --- ##### 5. **SEPIC (Single-Ended Primary Inductor Converter)** ###### Description - The SEPIC topology allows for a single inductor to boost or buck the voltage depending on the input conditions. It is particularly suitable for energy sources with fluctuating voltage levels (e.g., solar cells or TEGs). ###### Advantages - Can handle a wide range of input voltages, including very low levels. - Provides regulated output voltage. ###### Applications - Solar energy harvesting. - Wearable devices with variable input sources. --- ##### 6. **Synchronous Rectification Circuits** ###### Description - These circuits replace diodes with actively controlled MOSFETs, drastically reducing losses from forward voltage drops. - Synchronous rectifiers are particularly suited for low-voltage applications as they reduce the forward voltage requirement to near-zero. ###### Advantages - Highly efficient at low voltages. - Often integrated into boost converter designs. ###### Applications - Energy harvesting systems where diode losses are prohibitive. --- ##### 7. **Transformer-Based Oscillators** ###### Description - Similar to Joule Thief circuits but optimized with better transformers and oscillators. These circuits leverage magnetic coupling to step up very low voltages to usable levels. ###### Advantages - Simple implementation. - Effective at very low voltages (~50mV). ###### Applications - RF energy harvesting. - Piezoelectric vibration energy extraction. --- ##### 8. **Capacitor Pre-Charge Circuits** ###### Description - These circuits use a capacitor to accumulate and pre-charge a small amount of energy before releasing it into a boost converter or other topology. The circuit waits until enough charge is accumulated to start operation. ###### Advantages - Useful for very weak intermittent energy sources. - Compatible with energy storage (e.g., supercapacitors). ###### Applications - RF harvesting. - Piezoelectric harvesting from infrequent vibrations. --- ##### 9. **H-Bridge Oscillators** ###### Description - H-bridge circuits can be used to create oscillations even at low input voltages. Combined with step-up transformers, they can operate effectively in low-voltage environments. ###### Advantages - Oscillations are self-sustaining once initiated. - Transformer provides step-up capability. ###### Applications - TEGs and small-scale renewable energy. --- ##### 10. **Resonant Converters** ###### Description - Resonant converters operate by using the resonance of inductors and capacitors to achieve efficient energy transfer. **Series-resonant** or **parallel-resonant** converters are common in ultra-low-power designs. ###### Advantages - High efficiency with weak input sources. - Can operate at low voltages if designed carefully. ###### Applications - Wireless energy harvesting. - Weak solar or RF energy extraction. ---