Switch Mode Power Supply Input Capacitor Discharge Control
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Solution Overview
Problem
Existing methods for discharging input capacitors in switch mode power supplies, particularly under no-load or brownout conditions, are inefficient as they dissipate significant power through resistive components, and require complex and costly control solutions to prevent local oscillations and ensure safe operation.
Innovation Solution
A method and controller for switch mode power supplies that discharge input capacitors through a power switch by repeatedly charging and discharging its control terminal, using a comparator to manage the capacitor discharge current, allowing energy dissipation within the power switch rather than resistive components, and avoiding linear mode operation to prevent power loss and oscillations.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If resistive discharge networks are used to discharge input capacitors, then the capacitors can be discharged to safe voltage levels, but significant power is dissipated especially under no-load conditions
Solution Approach 1:
The patent changes the discharge mechanism from resistive to inductive by utilizing the power switch inductor. The inductor value is dynamically adjusted through PWM control to achieve the desired discharge rate without the continuous power loss associated with resistive networks. The effective inductance can be varied by changing the duty cycle, allowing optimization between discharge speed and power consumption.
Solution Approach 2:
The power supply's own power switch and inductor are utilized for the discharge function, eliminating the need for separate resistive discharge networks. The existing components serve dual purposes: normal power conversion during operation and capacitor discharge during shutdown, reducing overall component count and power loss.
2Measurement precision
If linear mode operation is used to control the power switch during discharge, then precise control of discharge current is achieved, but power loss and local oscillations occur
Solution Approach 1:
The patent employs periodic PWM pulses to control the power switch during discharge instead of continuous linear mode operation. The switch operates in brief on-off cycles, allowing the inductor to naturally limit current while the control circuit maintains precision. This periodic switching achieves accurate discharge current control without the continuous power dissipation of linear mode.
Solution Approach 2:
The patent replaces the linear analog control mechanism with a digital PWM control system. Instead of using linear mode operation to control current, the system uses frequency and duty cycle modulation of switching pulses, substituting a more efficient switching mechanism for the lossy linear approach while maintaining control precision.
3Reliability
If the power supply is disabled under brownout conditions, then operation safety is improved, but the input capacitors cannot be discharged to acceptable levels within the required time
Solution Approach 1:
The patent initiates the discharge process immediately upon detection of brownout or shutdown conditions, before the capacitors fully discharge naturally. The control circuit activates the discharge mode proactively, using the power switch and inductor to force a controlled discharge that meets the time requirements without compromising safety.
4Reliability
If discharge resistors are used in parallel with the mains input, then capacitors can be discharged under all conditions, but the circuit complexity and cost increase
Solution Approach 1:
The patent makes the power switch serve multiple functions: normal power conversion during operation and capacitor discharge during shutdown or brownout conditions. This multi-functionality eliminates the need for separate discharge resistors and their associated control circuits, reducing overall system complexity while maintaining discharge reliability across all operating conditions.
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 efficiently discharges input capacitors within specified times, reducing power dissipation and avoiding the need for expensive control solutions, while ensuring safe and reliable operation by dissipating energy in the power switch and minimizing power consumption under no-load conditions.
Implementation Method 1
discharging the input capacitor through the power switch... dissipating energy in the power switch
Data Source
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AI summary
A method is disclosed of discharging an input capacitor of a switch mode power supply comprising a power switch and the input capacitor, through the power switch and in response to disconnection of the switch mode power supply from a mains supply, the power switch having a control terminal and main terminals; the method comprising a repeated sequence, the sequence comprising: charging the control terminal to partially close the power switch until a comparator indicates that a capacitor discharge current from the capacitor through the main terminals is equal to a reference signal; and thereafter discharging the control terminal, thereby stopping the capacitor discharge current. A corresponding control and power supply is also disclosed.