Zero-Current POR Circuit for Fast Switching Converter Startup
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Solution Overview
Problem
Conventional switching power converters experience prolonged startup times due to power-on-reset (POR) circuit current consumption, which increases the delay before the power supply voltage reaches the POR threshold, and existing solutions either reduce efficiency or increase complexity and cost.
Innovation Solution
A POR circuit design with two current paths, including a diode-connected transistor and a current mirror transistor, that consumes virtually no current until the POR threshold is reached, allowing a robust startup resistor to minimize delay without the need for depletion-mode FETs.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Loss of time
If the resistance of the startup resistor is reduced to speed up the POR process, then the startup time is reduced, but power dissipation increases which lowers operating efficiency and system reliability
Solution Approach 1:
The POR circuit operates periodically rather than continuously. The first current path with the zener diode is enabled only when V_CC exceeds the POR threshold voltage, allowing the circuit to achieve fast startup when needed while consuming minimal power during normal operation. This periodic activation resolves the contradiction between fast startup (requiring low resistance) and low power dissipation (requiring high resistance).
Solution Approach 2:
The circuit changes its effective resistance parameter dynamically. When V_CC is below the threshold, the high zener diode breakdown voltage creates a high-impedance state that minimizes power dissipation. When V_CC exceeds the threshold, the zener diode conducts and the effective resistance decreases, enabling fast POR operation. This dynamic parameter change allows the circuit to satisfy both contradictory requirements at different operational stages.
2Loss of time
If the capacitance of the power supply capacitor is reduced to speed up startup, then the POR delay is reduced, but the capacitance must be sufficient to power internal control circuitry during startup
Solution Approach 1:
The POR circuit is segmented into two separate current paths with distinct functions. The first current path (through the zener diode) is responsible for generating the POR output signal with zero current consumption during charging. The second current path (through the current mirror transistor) provides the necessary current to power the control circuitry. This segmentation allows the power supply capacitor to be sized appropriately for both functions without compromise.
3Reliability
If conventional POR circuit current consumption is used, then the POR circuit can power control functions, but the current discharge prolongs the startup delay
Solution Approach 1:
The zener diode acts as an intermediary element that enables voltage threshold detection without continuous current consumption. The high breakdown voltage of the zener diode allows it to block current during the charging phase, creating a zero-current state that minimizes startup delay. When the threshold is reached, the zener diode conducts to enable the POR output signal. This intermediary component resolves the contradiction between powering control functions and minimizing startup delay.
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 design significantly reduces startup time with minimal power consumption and lower complexity, maintaining efficiency and reducing design costs by eliminating the need for additional isolation components.
Implementation Method 1
a first current path including a diode-connected transistor coupled to a zener diode through a first resistor
Implementation Method 2
the diode-connected transistor coupled to a zener diode through a first resistor. The first current path thus cannot begin conducting until the voltage across the zener diode reaches its breakdown voltage
Implementation Method 3
A second current path includes a current mirror transistor in series with a third resistor. The current mirror transistor will thus mirror a current conducted by the diode-connected transistor
Data Source
AI summary
A switching power converter is provided with a power-on-reset (POR) circuit that discharges essentially no current until a power supply voltage exceeds a POR threshold voltage.