Peak Current Servo for Buck Converter Accuracy
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Solution Overview
Problem
Existing DC-DC switching power converters face challenges in peak current limit accuracy due to compensation ramps in peak-mode control and output current offset in valley-mode control, especially at higher switching frequencies, leading to sub-optimal performance and potential negative control currents.
Innovation Solution
A peak current servo circuit is implemented, featuring a sampling circuit and control loop that samples the peak high side device current just before the high side device switches off, using a comparator to adjust a stored value and offset signal to match the peak coil current to a reference, thereby improving accuracy and removing output current offset in both peak-mode and valley-mode switching converters.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Stability of the object's composition
If a fixed compensation ramp is subtracted from the peak current limit in peak-mode control, then sub-harmonic oscillation is compensated, but the peak current limit accuracy deteriorates and varies with duty cycle
Solution Approach 1:
The patent transforms the static fixed compensation ramp into a dynamic variable compensation ramp that adapts to the operating conditions. The compensation amount is no longer fixed but varies based on the duty cycle and load conditions, allowing the system to maintain both stability compensation and current limit accuracy across different operating points.
Solution Approach 2:
The patent changes the parameter of the compensation ramp from a fixed value to a variable value that depends on operating conditions. By making the compensation ramp parameter dynamic rather than static, the system can adjust the compensation level to maintain accuracy while still preventing sub-harmonic oscillation.
2Reliability
If a high clamp level is used for over-current protection, then over-current protection is ensured, but the peak current limit cannot rise above this level reducing control flexibility
Solution Approach 1:
The patent applies preliminary action by setting a baseline clamp level that provides over-current protection, then using dynamic compensation to adjust the effective current limit within safe boundaries. The system prepares a protective clamp level in advance but allows controlled variation around this level through the dynamic compensation mechanism.
Solution Approach 2:
The patent introduces dynamics to the current limiting system by making the compensation ramp variable. This allows the peak current limit to dynamically adjust within the bounds set by the clamp level, providing both protection and flexibility. The system transitions from a static hard limit to a dynamic controlled limit.
3Stability of the object's composition
If a fixed compensation ramp is added to the control current in valley-mode control, then sub-harmonic oscillation is compensated, but output current offset is introduced and control currents may become negative
Solution Approach 1:
The patent applies inversion by reversing the approach: instead of adding a fixed compensation ramp that causes offset, it subtracts or adjusts the compensation dynamically to eliminate offset. The system inverts the conventional valley-mode compensation approach to achieve both stability and accuracy.
Solution Approach 2:
The patent changes the compensation ramp parameter from fixed to variable in valley-mode control, similar to peak-mode. This dynamic adjustment allows the system to compensate for sub-harmonic oscillation while maintaining current accuracy and avoiding negative control currents by adapting the compensation level to operating conditions.
4Productivity
If higher switching frequencies are used, then productivity is improved, but the existing control loops become less effective requiring higher compensation
Solution Approach 1:
The patent uses dynamics to adapt the compensation mechanism to higher switching frequencies. By making the compensation ramp variable rather than fixed, the system can automatically adjust its compensation behavior to remain effective at higher frequencies without requiring excessively high compensation levels that would introduce other problems.
Data Source
AI summary
The proposed disclosure combines peak-mode monitoring with valley-mode control, in a Buck switching converter, by means of a peak-current sampling circuit, not to turn the high side device off, but to control a slow loop, which in turn controls a variable offset incorporated into the loop control current. This helps the loop control current define the exact peak current, regardless of what other offsets, compensation ramp or peak-to-peak current ripple, are applied to the loop control current. The peak current is determined by an operational transconductance amplifier (OTA), whose maximum current is clamped to a programmed value. The loop control current is most likely implemented using a digital successive approximation register (SAR) system, but may also be implemented using a slow analog control loop.