In my mid-30s, I was still a fierce opponent in the art of Taekwon-do. Until a roundhouse kick floored and almost knocked me out cold. I recovered, but to the realization that age has dull my senses as I should have seen that kick coming.
In PCB design, you could suffer a knockout blow by the least expected component. An oversight in switching power supply design will bring the entire circuitry down by ripples in the power network. Power ripple can cause major signal integrity issues when left undetected.
What Causes Power Ripple
Power ripple is a phenomenon caused by the switching characteristic of a power supply. It usually occurs in switch-mode power supplies where the inductor and capacitor introduce noises that result from the switching frequency. Power ripples that are introduced by the switching frequency often resonated in low frequencies.
However, low-frequency power ripples aren’t the sole concern for PCB designers. High-frequency power ripples or ringing may also be introduced into the output of a switching power supply through the parasitic capacitance of the inductor. Ringing happens during the on-off transition of a switching power supply and are identified by spikes along the ripple.
How Power Ripple Affects Electronics Circuit
Boats would have no problem cruising through gentle ripples of waves. However, the same could not be said on the electronics circuit that is dependent on a clean power supply. Power ripples, regardless of both low and high frequency, can be detrimental to the function of a circuitry.
Ripples that are present in the voltage trace may result in crosstalk to adjacent circuitry. Higher frequency ringing on the power line will be coupled over to signal pins on an IC. While it’s true that most ICs can tolerate a certain amount of noise, higher frequency noises are beyond the power supply rejection ratio.
When switch mode noises and ringing starts coupling into signals, the accuracy of input readings will be compromised. Communication signals, coupled with power ripples, suffer from signal integrity issues. Power ripple is more than a mere annoyance. It’s a serious issue that can bring down an electronic circuit.
Power ripple is inevitable in a SMPS design.
How To Minimize Power Ripple In PCB Design
Unless you’re designing with a linear power supply, you will have to deal with power ripples. To reduce the implication of power ripples, you’ll have to reduce the amplitude of the initial peak and the subsequent ripple that follows.
Here are some handy tips for tackling power ripples in your design.
1. Snubber Circuit
A snubber circuit, which consists of a resistor and capacitor can be placed across the switching node of the low-side MOSFET in a switching power supply to reduce ringing. Both the RC combination functions as a dampener that absorb the energy released when the MOSFET changes state. Using a snubber circuit helps to reduce EMI emitted from ringing.
2. Boot Resistor
A boot resistor limits the initial offshoot of the ringing noise. It is placed across the gate of the high-side MOSFET in series with the bootstrap capacitor. When a boot resistor is added to the design, the charging time of the MOSFET is increased to dampen the initial peak value. You’ll want to be cautious with the value of the boot resistor as too little current passing through to can affect the performance of the switching power supply.
Ripple can be minimized by limiting charging current to the MOSFET.
3. Feedthrough Capacitors
It may occur to you that using a capacitor to smoothen the ripple is a good idea. But typical capacitors only help in filtering out low-frequency noise. To reduce high-frequency ripples, using a feedthrough capacitor is a better option. A feedthrough capacitor has three terminals that result in better insertion loss at high frequency.
And of course, above all, trusting a reliable simulation tool to accurately model power supply and power ripple through a vast library of up-to-date model parameters will improve your odds massively of designing a circuit that isn’t overcome by power ripple.
Often, all the above methods are used simultaneously to deliver optimal performance for the switching power supply. As a PCB designer, you’ll want to minimize power ripple without compromising the efficiency of the power supply.
Using a PCB design software with the advanced analysis tools helps accurately detect and mitigate power ripple issues. Whether its considering voltage necessities, mitigating parasitics, or determining thermal analysis for your board design, PSpice simulation can account throughout your design process. OrCAD PSpice allows you to simulate the power supply schematic to catch potential problems early on.
If you’re looking to learn more about how Cadence has the solution for you, talk to us and our team of experts.