Buck Converter Transient Response to Load Step Down
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Solution Overview
Problem
Existing synchronous buck converters experience voltage overshoot during load step-down due to inductor current dissipation, which is not efficiently corrected by existing body braking techniques, particularly when the inductor current reaches zero.
Innovation Solution
A second sensing circuit is introduced to detect when the inductor dissipation current reaches zero, allowing the shunt MOSFET to be turned on until the output voltage reaches the regulated value, enabling reverse current flow through the MOSFET channel to quickly correct the overshoot, thereby disabling the shunt MOSFET entirely during step-down to utilize the larger voltage drop across the body diode and Shottky diode.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If the shunt MOSFET is kept on during load step-down to maintain current flow, then the inductor current can continue to flow through the MOSFET channel, but the output voltage overshoot cannot be quickly corrected and large output capacitors are needed
Solution Approach 1:
The patent applies preliminary action by detecting when the inductor current reaches zero before the output voltage overshoot is fully corrected, and immediately turning on the shunt MOSFET to enable reverse current flow. This proactive timing allows the MOSFET channel to conduct the reverse current needed to quickly reduce the output voltage, rather than waiting for the overshoot to naturally decay.
Solution Approach 2:
The patent implements dynamics by dynamically switching the shunt MOSFET state based on real-time inductor current conditions. The MOSFET is turned off during normal operation, then turned on when inductor current reaches zero to enable reverse current flow, and turned off again when output voltage is corrected. This dynamic control adapts the circuit behavior to the transient condition.
2Power
If the shunt MOSFET is turned off entirely during step-down to utilize body diode voltage drop, then the voltage drop across the body diode and Shottky diode is larger which helps correct overshoot, but the inductor current dissipation path is lost
Solution Approach 1:
The patent uses preliminary action by turning on the shunt MOSFET precisely when the inductor current reaches zero, which is the optimal moment to switch from body diode conduction to MOSFET channel conduction. This timing ensures that the MOSFET channel is ready to immediately conduct reverse current when needed, while the body diode has already provided the necessary voltage drop for overshoot correction.
Solution Approach 2:
The patent applies parameter changes by transitioning the shunt MOSFET from off-state (using body diode with larger voltage drop) to on-state (using MOSFET channel with lower resistance). This parameter change in the MOSFET's conduction state allows the circuit to switch between two different current paths with different electrical characteristics, optimizing performance for different phases of the transient response.
3Reliability
If large output capacitors are used to reduce voltage overshoot during load step-down, then the output voltage stability is improved, but the cost and device size increase
Solution Approach 1:
The patent implements feedback by using a sensing circuit to detect when the inductor current reaches zero during load step-down. This feedback signal triggers the shunt MOSFET to turn on, enabling reverse current flow that actively corrects the output voltage overshoot. This closed-loop control allows the system to maintain output voltage stability without relying on large output capacitors.
Solution Approach 2:
The patent substitutes the passive mechanical approach of using large output capacitors to absorb voltage overshoot with an active electronic control mechanism. Instead of relying on the physical size and energy storage capacity of capacitors, the system uses controlled MOSFET switching to actively manage current flow and correct voltage deviations, replacing a passive component-based solution with an active control-based solution.
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
This approach rapidly reduces the output voltage to the desired level by enabling reverse current flow through the MOSFET channel, improving transient response and reducing the need for large and costly output capacitors.
Implementation Method 1
A second sensing circuit is introduced to detect when the inductor dissipation current reaches zero
Implementation Method 2
enabling reverse current flow through the MOSFET channel to quickly correct the overshoot
Implementation Method 3
disabling the shunt MOSFET entirely during step-down to utilize the larger voltage drop across the body diode and Shottky diode
Data Source
AI summary
A synchronous buck converter having an improved transient response during output current includes a first switching transistor connected between an input node and a first node, a second switching transistor connected between the first node and a second node, a series inductor connected between the first node and an output node, an output capacitor connected between the output node and the second node, a first driver circuit operative to turn the first switching transistor one and off according to a variable duty cycle determined by an error signal representing the difference between the voltage output of the converter and a reference voltage, a sensing circuit operative to provide a control signal output when the duty cycle for the first switching transistor is zero, a second drive circuit responsive to the control signal output of the sensing circuit to turn off the second switching transistor and a second sensing circuit operative to provide a second control signal when a dissipating current of the inductor is zero, wherein the second drive circuit is responsive to the second control signal to turn the second switching transistor on when the dissipating current of the inductor is zero.