Designing Power Supply Loops: Simplified in Three Key Steps - Part 3: Crafting Loops through Basic Sequences
In modern electronic systems, switch-mode power supplies (SMPS) are commonly used for their high efficiency and power density. One popular current-mode control step-down buck converter features an inner current sensing loop and an outer output voltage regulation loop. The Type 2 compensation network plays a critical role in stabilizing the feedback loop of these power supplies, ensuring stable regulation and good transient response.
Role of the Type 2 Compensation Network
The Type 2 compensation network provides a pole-zero pair to cancel out the power stage’s dominant pole and introduce a zero to increase phase margin, improving loop stability. It controls the error amplifier’s frequency response, allowing the controller to maintain a constant output voltage despite variations in input voltage and load current. In current mode control, this network helps manage the inner current loop dynamics and the outer voltage loop, ensuring fast transient response and avoiding subharmonic oscillations especially when duty cycles exceed 50%.
Design Process
The design process for the Type 2 compensation network involves several steps:
- Identify Power Stage Characteristics: Determine the power converter’s power stage dominant pole frequency (typically set by the output LC filter) and any ESR zero from output capacitors.
- Select Compensation Type: The Type 2 compensator includes one zero and one pole and is often implemented as a series RC network within the error amplifier feedback path.
- Place Zero to Cancel Dominant Pole: The zero is chosen to cancel the power stage’s dominant pole (usually the LC output filter’s resonant low-frequency pole).
- Place Pole at High Frequency: A high-frequency pole is placed to roll off the loop gain and reduce high-frequency noise and jitter.
- Adjust Gain and Crossover Frequency: The compensator’s gain is tuned to achieve a target bandwidth (crossover frequency) with sufficient phase margin (~45°-60°).
- Implement and Validate: The compensation network is often integrated or externally configured. Testing with load transients and varying input voltage ensures stability and performance.
Focus on ADI Power Products
Analog Devices' power products that use current mode control, such as the µModule regulators (e.g., LTM4739), incorporate internal current mode controllers with internal or external compensation options.
- The LTM4739 provides internal compensation, simplifying the design by integrating the compensation network tuned for the control loop parameters.
- For designs needing custom compensation tuning, users configure an external Type 2 compensation network by selecting resistors and capacitors that set the zero and pole frequencies according to the power stage characteristics.
- ADI documentation typically guides users on how to calculate these frequencies based on the inductor, capacitor, and ESR values, along with recommended crossover frequency ranges for stable and efficient operation.
Summary
The Type 2 compensation network stabilizes the voltage regulation loop in current mode controlled switch-mode power supplies by introducing one zero and one pole. Its design process involves canceling dominant power stage poles, placing poles at high frequency for noise roll-off, and tuning the loop bandwidth to optimize transient response and stability. ADI’s power products like the LTM4739 µModule often have internal compensation optimized for their internal current mode controllers, easing design complexity but still allowing external compensation adjustments if needed.
The total power supply loop gain is given by Equation 1 in the article. Figure 3 shows the Type 2 compensation network. The Type 2 compensation network is used to design and optimize the power supply loop. Figure 2 shows the conceptual power supply loop gain diagram. A simple Type 2 compensation network is adequate to optimize supply loop stabilities and transient performances.
For a current-mode control switching supply, it is important to attenuate switching noises in the feedback loop to minimize jittering of the switching node waveform, with greater than 8 dB attenuation at being preferred. A phase margin greater than 45° is usually required, with a margin greater than 60° phase margin being recommended. Gain margin, defined as the gain attenuation where the loop phase is -180°, should be at least 8 dB to 10 dB. The example Type 2 compensation network includes , , and components on the error op amp output pin.
- Technology plays a crucial role in the design of the Type 2 compensation network, as it introduces a pole-zero pair to optimize the feedback loop's stability and transient response in modern switch-mode power supplies.
- In the design process of the Type 2 compensation network, technology such as analog devices helps engineers integrate internal compensation networks or configure external Type 2 networks for specific power supply requirements, providing a simple and efficient solution for current-mode controlled switch-mode power supplies.
