PWM Controller Feedback Pin Dual-Function for HV Startup
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Solution Overview
Problem
Switching mode power supplies with flyback topology face challenges in startup, as the PWM controller is not fully operational to drive the control transistor, requiring a high voltage startup circuit that increases pin count in integrated circuit chips, either internally or externally, complicating design and functionality.
Innovation Solution
A PWM controller IC chip with an external HV startup transistor controlled through the feedback pin, which turns ON during startup to generate supply voltage from the input voltage and turns OFF once the supply voltage reaches a sufficient level, allowing the control transistor to receive operating current from the auxiliary winding, thus reducing the pin count by utilizing the same pin for both feedback and HV startup control.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Ease of operation
If an internal HV startup circuit is included in the PWM controller IC chip, then the PWM controller can receive current from the input voltage during startup, but the pin count of the PWM controller IC chip increases by at least two pins (HV pin and NC pin)
Solution Approach 1:
The feedback pin is assigned dual functionality: it receives the feedback signal during normal operation and controls the HV startup transistor during startup. This multi-functionality eliminates the need for separate dedicated pins for HV startup control, thereby reducing the overall pin count of the PWM controller IC chip while maintaining both feedback and startup capabilities.
2Device complexity
If the feedback pin is used to control the HV startup transistor, then the pin count is reduced, but the feedback pin must perform dual functions
Solution Approach 1:
The feedback pin dynamically changes its function based on the operational state of the power supply. During startup, the feedback pin controls the HV startup transistor. Once the supply voltage reaches the startup voltage level and normal operation begins, the feedback pin switches to receiving the feedback signal. This dynamic functional switching allows the pin to adapt to different operational requirements, resolving the conflict between pin count reduction and functionality versatility.
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 simplifies the PWM controller IC chip design by reducing the pin count and eliminating the need for additional external circuitry, ensuring efficient startup and operation of the SMPS while maintaining regulated output voltage.
Implementation Method 1
The HV startup transistor turns ON during startup to generate a supply voltage from the input voltage of the power supply. The HV startup transistor turns OFF when the supply voltage reaches a startup voltage level that is sufficient to start the switching operation of the control transistor and thereby receive operating current from an auxiliary winding of the transformer.
Implementation Method 2
A switching mode power supply (SMPS) with flyback topology typically includes a transformer with a primary winding that is controlled by a control transistor. The control transistor is driven by a pulse-width modulation (PMW) controller to induce current on a secondary winding of the transformer. A supply voltage of the PWM controller may be generated from operating current induced on an auxiliary winding of the transformer.
Data Source
AI summary
A power supply includes a control transistor that controls a primary winding of a transformer to induce current on a secondary winding of the transformer to generate an output voltage. A pulse width modulation (PWM) controller integrated circuit (IC) chip drives the control transistor through a gate pin. The PWM controller IC chip has a feedback pin that receives a feedback signal indicative of the output voltage. A high voltage (HV) startup transistor is controlled through the feedback pin. The HV startup transistor turns ON during startup to generate a supply voltage from current received from the input voltage of the power supply. The HV startup transistor turns OFF when the supply voltage reaches a startup voltage level that is sufficient to start the switching operation of the control transistor and thereby receive operating current from an auxiliary winding of the transformer.