USB PD VBUS Discharge Using SR Control to Limit Thermal Dissipation
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Solution Overview
Problem
Integrated circuits (ICs) controlling USB power delivery face thermal reliability risks due to high power dissipation and temperature increases during voltage transitions, requiring external power transistors and increased area and cost to manage heat.
Innovation Solution
The implementation of secondary-side-controlled AC-DC and DC-DC converters with control logic that monitors VBUS voltage and triggers partial discharge of output capacitors, reducing peak current and thermal dissipation through efficient SR transistor control, eliminating the need for external power transistors and GPIO pins.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Loss of energy
If on-the-die VBUS discharge using resistive-type method is used, then the VBUS voltage can be discharged, but the power dissipation increases significantly due to higher current at higher VBUS voltage
Solution Approach 1:
The patent changes the discharge method from resistive-type to transistor-controlled discharge. By using a transistor switch instead of a simple resistor, the discharge current can be precisely controlled to limit peak current while maintaining effective voltage discharge. This parameter change in the discharge mechanism resolves the contradiction by reducing power dissipation without compromising thermal reliability.
Solution Approach 2:
The patent introduces an intermediary transistor component between the VBUS capacitor and ground to control the discharge process. This intermediary element (transistor) acts as a controlled switch that regulates the discharge current, preventing the excessive power dissipation that would occur with direct resistive discharge while ensuring reliable voltage reduction.
2Reliability
If external power transistors are employed to control VBUS discharge, then thermal reliability is improved, but the area and cost of the USB device increase
Solution Approach 1:
The patent merges the VBUS discharge control function with the existing USB controller and voltage converter circuitry. Instead of using separate external power transistors and GPIO pins, the discharge control is integrated into the existing control logic of the voltage converter. This consolidation maintains thermal reliability while reducing device area and eliminating additional external components.
Solution Approach 2:
The patent makes the existing voltage converter and control logic multi-functional by enabling it to perform both voltage conversion and VBUS discharge control. The same control circuitry that manages power conversion is also used to control the discharge transistor, allowing one component to serve multiple functions and thereby reducing overall device complexity and component count.
3Ease of manufacture
If higher thermal resistive packages like SOIC or TSSOP are used, then assembly cost is reduced, but die temperature increases significantly causing reliability risk
Solution Approach 1:
The patent implements preliminary action by proactively controlling VBUS discharge before excessive power dissipation can occur. The control logic monitors VBUS voltage and triggers discharge when voltage exceeds the sink voltage threshold, preventing the buildup of excessive heat that would otherwise occur during uncontrolled discharge or voltage transitions. This preventive approach allows the use of cost-effective packages without thermal reliability issues.
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 solution effectively manages thermal dissipation within ICs, reducing die temperature, costs, and design area by avoiding external components and additional pins, while maintaining flexibility and programmability based on capacitance and inductance values.
Implementation Method 1
a higher thermal resistivity (or theta-Ja) of 50-90° C./W results in a sudden increase in die temperature, causing a reliability risk due to a temperature reaching higher than 150° C.
Data Source
AI summary
A secondary side controller for a flyback converter can include a synchronous rectifier (SR) gate driver pin coupled to a gate of an SR transistor on a secondary side of the flyback converter. An error amplifier is coupled to an output of a voltage bus of the flyback converter, the error amplifier to generate an error signal indicative of a voltage of the output of the voltage bus. Control logic is coupled to the error amplifier and to the SR transistor, the control logic to: detect when the voltage is at least a threshold percentage higher than a sink voltage required by a sink device coupled to the output of the voltage bus; detect assertion of a skip mode signal; and cause the SR transistor to be driven during a skip mode such as to partially discharge an output capacitor coupled to the output of the voltage bus.