Voltage Detector# Technical Documentation: KA75250Z Switching Power Supply Controller
Manufacturer : FAIRCHILD Semiconductor (Note: Document references "FAILCHIL" which appears to be a typographical error; the correct manufacturer is FAIRCHILD, now part of ON Semiconductor)
Component Type : Current-Mode PWM Controller IC for Offline Switching Power Supplies
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## 1. Application Scenarios
### 1.1 Typical Use Cases
The KA75250Z is primarily employed as a fixed-frequency, current-mode pulse-width modulation (PWM) controller in offline flyback and forward converter topologies. Its core function is to regulate output voltage by controlling the duty cycle of a power switch (typically a MOSFET or bipolar transistor).
* Primary Configuration : Most commonly used in flyback converters for low-to-medium power applications (up to approximately 250W). It provides the necessary drive, feedback control, and protection functions.
* Standby Power Supplies : Ideal for auxiliary or standby power rails in larger systems (e.g., TVs, monitors, appliances) due to its low startup current and ability to operate from a high-voltage DC bus.
* Battery Chargers : Used in constant-voltage/constant-current (CV/CC) charger designs for consumer electronics, where its current-mode control ensures good line and load regulation.
### 1.2 Industry Applications
* Consumer Electronics : Power supplies for LCD/LED televisions, set-top boxes, audio equipment, and desktop computer standby power.
* Office Equipment : Low-power internal supplies for printers, scanners, and monitors.
* Industrial Controls : Auxiliary power for control boards, sensors, and relays in industrial environments, benefiting from its built-in protection features.
* Lighting : Drivers for LED lighting systems requiring isolated, regulated DC power.
### 1.3 Practical Advantages and Limitations
Advantages:
* Integrated Functionality : Combines an oscillator, error amplifier, current sensing comparator, PWM latch, and high-current totem-pole output driver in one package, reducing external component count.
* Current-Mode Control : Provides inherent cycle-by-cycle current limiting, simplified feedback loop compensation, and improved line transient response compared to voltage-mode controllers.
* Low Startup Current : Typically < 1 mA, allowing the use of a high-value startup resistor, which improves efficiency at light loads.
* Comprehensive Protection : Features under-voltage lockout (UVLO), programmable soft-start, and a pulse-by-pulse current limit. The PWM latch ensures single-pulse operation per cycle for reliable over-current protection.
* Wide Operating Range : Can be designed for universal AC input (85VAC to 265VAC) with appropriate external component selection.
Limitations:
* Fixed Frequency : Operating frequency is set by external RC components and is not synchronized to an external clock, which can be a drawback in noise-sensitive multi-converter systems.
* Maximum Duty Cycle Clamp : Internally limited to approximately 50% (typical), making it less suitable for wide-input-range forward converters without additional slope compensation considerations.
* Driver Capability : While the totem-pole output can source/sink significant current, it may require a gate driver transistor for very high-power or high-frequency MOSFETs to minimize switching losses.
* Aging Technology : As a legacy FAIRCHILD part, it may face eventual obsolescence, and direct second-source alternatives should be verified for parametric equivalence.
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## 2. Design Considerations
### 2.1 Common Design Pitfalls and Solutions
* Pitfall 1: Inadequate Slope Compensation. In continuous conduction mode (CCM) at duty cycles > 50%, sub-harmonic oscillations can occur.
* Solution : Add an external ramp (via a resistor and capacitor from the oscillator
