High-Side Active Clamp Charging to Limit Forward Converter Stress
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Solution Overview
Problem
Existing power supplies using forward converter topology experience transformer saturation and high voltage stress on components due to residual energy in the primary winding, leading to component failure and inefficiencies.
Innovation Solution
Incorporation of a high-side active clamp configuration with a bootstrap charge circuit that includes a resistor network and charge voltage storage device to generate and store charge voltage, which is then supplied to the bootstrap voltage storage device during specific cycles to reduce voltage stress on switches and transformers.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If a forward converter topology is used with a transformer, then galvanic isolation between input power source and load is achieved, but voltage stress on primary side components causes transformer saturation or component failure
Solution Approach 1:
An active clamp circuit is introduced as an intermediary component between the primary winding and the power supply voltage input. The clamp circuit includes a clamp switch and a clamp capacitor that work together to capture and dissipate residual energy from the primary winding, thereby reducing voltage stress on the primary side components including the transformer and power switch.
Solution Approach 2:
The invention changes the voltage parameters across the primary winding by introducing the active clamp circuit. The clamp capacitor voltage is maintained at a specific level (e.g., 0.7 times the power supply voltage) to limit the maximum voltage stress on primary components, transforming the voltage waveform and reducing peak voltage excursions that would otherwise cause saturation or failure.
2Reliability
If an active clamp circuit is added to reduce voltage stress, then component reliability is improved, but device complexity increases due to additional switches and control circuits
Solution Approach 1:
The clamp switch control circuit is designed to perform multiple functions: it controls the clamp switch timing, generates the gate drive signal for the clamp switch, and incorporates a bootstrap circuit that also serves to charge the clamp capacitor. This multi-functionality reduces the need for separate dedicated circuits for each function, thereby mitigating the increase in overall device complexity.
Solution Approach 2:
The bootstrap circuit automatically charges the clamp capacitor during specific phases of the switching cycle without requiring external intervention or complex control logic. The charging circuit uses the transformer winding and existing switch nodes to generate the charge voltage, allowing the clamp capacitor to self-charge and maintain its voltage, thus reducing the complexity of external power supply circuits.
3Ease of operation
If a bootstrap circuit is used to charge the clamp capacitor, then the active clamp can operate, but the bootstrap voltage storage device may not be charged timely under certain duty cycles
Solution Approach 1:
The charging circuit is designed to charge the clamp capacitor during the on-time of the primary switch, which occurs before the clamp switch needs to be activated. By performing the charging action in advance during the first portion of the switching cycle, the bootstrap voltage storage device is fully charged and ready before the second switch needs to operate, eliminating timing issues and ensuring reliable clamp operation.
Solution Approach 2:
The charging circuit dynamically adapts to different duty cycles by utilizing the available on-time of the primary switch. The charging process is synchronized with the switching cycle, and the circuit automatically adjusts the charging duration based on the duty cycle, ensuring that the clamp capacitor is charged sufficiently before the clamp switch needs to operate regardless of the specific duty cycle 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
Reduces transformer saturation and high voltage stress on components, extending the life of switches and improving operational efficiency by ensuring timely charging of the bootstrap voltage storage device.
Implementation Method 1
a resistor network configured to generate a charge voltage in response to an input voltage supplied to the first voltage input by the primary winding during a first portion of a switching cycle
Implementation Method 2
a charge voltage storage device coupled to the resistor network and configured to store at least a portion of the charge voltage during the first portion of the switching cycle
Implementation Method 3
to supply the at least a portion of the charge voltage to the bootstrap voltage storage device via the voltage output during a second portion of the switching cycle
Data Source
AI summary
A power supply comprises a transformer having a primary winding, a first switch coupled to the primary winding and to a voltage input, an active clamp circuit coupled in parallel with the primary winding, and a clamp switch control coupled to the active clamp circuit. The power supply further comprises a bootstrap circuit coupled to the clamp switch control and having a bootstrap voltage storage device coupled to the clamp switch control. A charging circuit has a resistor network configured to generate a charge voltage in response to an input voltage supplied by the primary winding. A charge voltage storage device is configured to store at least a portion of the charge voltage during the first portion of the switching cycle and to supply the at least a portion of the charge voltage to the bootstrap voltage storage device during a second portion of the switching cycle.