What is Power Supply Rejection Ratio? And Why is it Important?
November 13, 2023
Power Supply Rejection Ratio and Lighthouses
Light houses started out as oil lamp based lighting. Then evolved to technologies like carbon arc, or sodium vapor lighting, with the event of LED lighting and it’s advantages of redundancy, reliability, life and power efficiency, and potentially illumination power, the importance of power supply rejection ratio became apparent.
Power supply rejection ratio in this example implies the ability for the LED control power electronics to reject changes and noise in the power input from the generator and battery backup. This means as the generator varies in speed the output of the Lighthouse lamp remains constant. It also means that if the generator stops and the batteries switch in, the switchover glitch is rejected.
Other electronics like operational amplifiers, current buffers, and even digital logic and FPGA’s can have important power supply rejection ratio considerations. PSRR is fairly intuitive for op amps and current buffers. However for digital logic and FPGA’s, PSRR is part of a set of complex power integrity design considerations. For SRAM FPGA’s, if the power dips below a certain level the device may reset or sometimes, in older devices, may corrupt the SRAM fuse map data.
There are many patents for methods of improving PSRR. One can search databases like patents.google.com for ideas in solving PSRR issues. Other sources includ articles in periodicals and books on analog and transistor level design. One simple idea is a post regulator on the power source such that a Low Drop Out Regulator (LDO) is placed between the op amp power rail and the power supply. Others are brute force like a large array of low effective series resistance capacitors in a variety of configurations. When rejecting RF and microwave power supply noise sources often some high frequency porcelain capacitors are required close to the output. To reject low frequency, ripple super capacitors can be effective. Ferrite chokes can help with UHF and Microwave noise rejection. Sometimes to reject something, a 60HZ very large choke like a 30 Henry choke, can help (very heavy and large and expensive) but can also be invaluable and very rugged. In a high Interference environment tubular EMI and Microwave filters with metal walls and EMI gaskets may prove incredibly useful.
William is an Applied Scientist with over 45 years experience in the workforce. He graduated with a BSEE from SD MINES, and had graduate work at UTD, UTA, The Borden Institute, USAF Flight Test Engineering, and others.