Designing a High-Performance Flyback Converter Using the onsemi UC3845DR2G Current Mode PWM Controller
The flyback converter remains a dominant topology in low-to-medium power AC/DC and isolated DC/DC applications, prized for its simplicity, cost-effectiveness, and ability to provide multiple isolated outputs. Achieving high performance in such designs—characterized by efficiency, stability, and reliability—demands a robust control strategy. Current mode control is the preferred method for its inherent line regulation, simplified loop compensation, and superior transient response. The onsemi UC3845DR2G current mode PWM controller is an industry-standard IC that provides the essential building blocks for creating such high-performance power supplies.
This article outlines the key design considerations when implementing a flyback converter with the UC3845DR2G.
Core Advantages of the UC3845DR2G
The UC3845DR2G is a fixed-frequency PWM controller specifically designed for current mode control. Its key features include:
Precise Oscillator Frequency: A single external resistor (Rt) and capacitor (Ct) set the switching frequency, which can be programmed up to 500 kHz, allowing optimization for size and efficiency.
Current Sensing and Control: The IC uses a sense resistor in the source of the power MOSFET to accurately measure switch current. This signal is fed into the current sense comparator, which terminates the output pulse when a threshold set by the error amplifier's output is reached. This cycle-by-cycle current limiting is critical for protecting the power switch and simplifying control loop dynamics.
Built-in Undervoltage Lockout (UVLO): The UC3845DR2G features an optimized UVLO threshold with 6V turn-on and 10V turn-off hysteresis. This ensures reliable startup and prevents erratic operation during brownout conditions.
High-Current Totem Pole Output: The output stage is capable of sourcing and sinking over 1A of peak current, enabling direct, efficient driving of large power MOSFETs with minimal switching losses.
Critical Design Steps for a High-Performance Converter
1. Defining Specifications: Begin by establishing key parameters: input voltage range (e.g., 85-265 VAC), output voltage(s) and current(s), target efficiency, ripple requirements, and isolation needs.
2. Transformer Design: The flyback transformer is the heart of the converter. Its design dictates performance, safety, and EMI. Key calculations include:
Determining the primary inductance (Lp) to ensure the converter operates in Continuous Conduction Mode (CCM) or Discontinuous Conduction Mode (DCM) as desired, often a trade-off between peak current stresses and loop stability.
Setting the turns ratio (Np:Ns) based on input/output voltages and duty cycle limits.
Ensuring the core size can handle the required power without saturation.
3. Power Stage Component Selection:
MOSFET Selection: Choose a switch with a voltage rating significantly higher than the maximum input voltage plus the reflected output voltage. Low gate charge (Qg) and low on-resistance (Rds(on)) are crucial for minimizing switching and conduction losses.
Output Rectifier: For higher efficiency, a Schottky diode is preferred for outputs below 60V due to its low forward voltage. For higher voltages, a fast-recovery diode is necessary.
Snubber Network: A well-designed RCD snubber across the primary winding is essential to clamp voltage spikes caused by transformer leakage inductance, protecting the MOSFET.
4. Control Loop Compensation with the UC3845DR2G: Stability is paramount. The compensation network, typically connected between the COMP pin and ground, shapes the feedback loop's response. The UC3845’s internal error amplifier allows for Type II or Type III compensation. Proper compensation ensures stability across load and line variations and provides excellent transient performance. Analyzing the right-half-plane zero (RHPZ), inherent in CCM flyback converters, is critical during this phase.
5. Protection Circuits: Leverage the UC3845DR2G’s features to build robust protection. The current sense pin can be used for cycle-by-cycle overcurrent protection. Additional circuitry can be added to implement overvoltage protection (OVP) and overtemperature protection (OTP).
The onsemi UC3845DR2G provides a robust and versatile foundation for designing high-performance, cost-effective flyback converters. Its integrated current mode control architecture simplifies the feedback loop design while offering essential protection features. A successful design hinges on meticulous transformer design, careful power component selection, and precise control loop compensation to achieve optimal efficiency, stability, and reliability in the target application.
Keywords:
1. Current Mode Control
2. PWM Controller
3. Flyback Converter
4. Loop Compensation
5. Power Supply Design
