DC/DC Converter Startup Frequency Ramping

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Solution Overview

Problem

Conventional boost controllers face issues during start-up due to high inductor current overshooting, leading to potential thermal failure of power MOSFETs, as existing solutions like foldback amplifiers are ineffective due to initial feedback voltage conditions.

Innovation Solution

A control circuitry that ramps up the frequency of the clock signal from a minimum level to a normal operation level during start-up, increasing the off-time of the power MOSFET, independent of the oscillator's operating frequency, using a voltage controlled oscillator and a switching circuit to manage the power switch, and a charging circuit to vary the voltage applied to the oscillator.

Engineering Contradictions & Design Principles

VSEngineering Contradiction Analysis

1Productivity

If the boost controller operates at a constant high frequency from startup, then the switching speed is fast and productivity is high, but the inductor current overshoots and causes thermal damage to power MOSFETs

Engineering Contradiction:

Improveswitching speedVSAvoidpower MOSFET safety

Core Design Contradiction:

ProductivityVSReliability

Solution Approach 1:

The oscillator frequency is made dynamic rather than fixed. During startup, the frequency is ramped up gradually from a low value to the normal operating frequency, allowing the inductor current to settle without overshooting. Once the system reaches steady state, the frequency transitions to the high constant value for normal operation. This dynamic frequency adjustment resolves the contradiction between fast switching and safe startup.

Inventive Principle:

Principle #15Dynamics

Solution Approach 2:

Before normal high-frequency operation begins, a preliminary low-frequency operation is performed during startup. This preliminary action allows the inductor current to build up gradually and prevents immediate high-current stress on the power MOSFETs. The soft-start mechanism prepares the system in advance, avoiding the harmful current overshoot that would occur with direct high-frequency startup.

Inventive Principle:

Principle #10Preliminary action

2Reliability

If the clock signal frequency is ramped up during startup, then the inductor current overshoot is prevented and reliability is improved, but the startup time increases and productivity decreases

Engineering Contradiction:

Improvepower MOSFET protectionVSAvoidstartup time

Core Design Contradiction:

ReliabilityVSLoss of time

Solution Approach 1:

The oscillator frequency parameter is changed dynamically during startup from a low initial value to the normal operating value. This parameter change allows the system to start up safely with gradual current buildup, preventing MOSFET damage. The frequency ramping is controlled to achieve the minimum necessary duration for safe startup, balancing reliability improvement with acceptable startup time.

Inventive Principle:

Principle #35Parameter changes

3Reliability

If foldback amplifier is used to control current during startup, then current limiting is attempted, but it is ineffective because the feedback voltage is initially zero or negative

Engineering Contradiction:

Improvecurrent controlVSAvoidstartup effectiveness

Core Design Contradiction:

ReliabilityVSEase of operation

Solution Approach 1:

The feedback-dependent current control mechanism (foldback amplifier) is replaced with a direct frequency control mechanism. Instead of relying on feedback voltage that is unavailable or negative during startup, the invention extracts the control function and implements it through oscillator frequency modulation, which is independent of feedback voltage conditions. This makes startup current control effective when traditional feedback-based methods fail.

Inventive Principle:

Principle #2Taking out (Extraction)

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 prevents inductor current from exceeding its steady state maximum level, thereby protecting the power MOSFET from damage by allowing the current to decay, ensuring safe and efficient start-up of the boost regulator.

Implementation Method 1

Methodology Applied

Scientific EffectVoltage controlled oscillator frequency modulation:

Data Source

PatentUS7843178B1DC/DC converter startup with frequency ramping

Publication Date: 2010.11.30 ANALOG DEVICES INT UNLTD CO

AI summary

Control circuitry controls a boost regulator during a start-up period. The control circuitry may comprise an oscillator for generating a clock signal. The oscillator may be configured for ramping up a frequency of the clock signal in accordance with an voltage to be applied to the oscillator and varied during the start-up period. The control circuit may further include a switching circuit configured for controlling a power switch of the boost regulator in response to the clock signal from the oscillator. The switching circuit can control the power switch to have an on-time which is largely independent of the operating frequency of the oscillator. The voltage to be applied to the oscillator may have the same initial voltage level upon startup of the control circuit, independent of an output voltage of the boost regulator.