AN1686 is an application note about L5991 chip, introduces how the chip realizes overload protection function
The document describes a design example of a switch-mode power supply (SMPS) using the STMicroelectronics L5991 PWM controller. It addresses applications where load peaks are too long for output capacitors but short enough that thermal time constants can be managed, such as in motors and audio systems.
The note details modifications to an original 45W design to allow a peak output power of 75W (lasting less than 0.5 seconds) while maintaining regulation. Key changes include increasing the input bulk capacitor from 100 µF to 150 µF, adjusting sense resistors and transformer specifications to handle higher currents without saturation, and implementing a circuit for delayed shutdown upon overpower or short-circuit conditions.
| Maximum Continuous Output Power | 45 W |
|---|---|
| Peak Output Power (t< 0.5 s) | 75 W |
| Regulated Output Voltage | 18V + 2% |
| Normal Operation Switching Frequency | 70 kHz |
| Light Load Switching Frequency | 18 kHz |
The converter can handle a peak output power of 75 W for a duration of less than 0.5 seconds.
The converter shuts down in latch mode within 10 to 100 ms if the load demand exceeds the peak power.
The input bulk capacitor size needs to be increased from 100 µF to 150 µF (or 120 µF worst case) to reduce peak primary current.