I sometimes long for those far off days when my circuits were simple enough that I could just plug in my power supply and not worry about noise in my power delivery network. Ignorance truly is bliss. It wasn’t until I built my first board for an electro-optical system that I realized the importance of power and signal integrity in my PCBs.
With integrated circuit processes reaching Gbit/GHz levels, power integrity in PCBs has become more important than ever. Gone are the days when a simple decoupling capacitor between power and ground was all that was needed to suppress noise in your power delivery network. A thorough understanding of noise in your power system can help you identify potential noise problems that can cripple your board.
Signal Integrity Problems from DC Power Supply Noise
Many tricky signal problems actually originate from power supplies as a result of excess ripple in the power output. DC power supplies can produce large fluctuations in the output power signal. And excessive IR Drop in the layout hinders IC components from working normally. A stable and clean power delivery network (PDN) is very important for signal integrity, as the PDN provides return paths for the signals. These systems can also produce a ripple signal that is strong enough to cause involuntary switching in the logic gates they power because of ground bounce for example.
When a circuit board is mounted near a DC power supply, the power supply itself can actually induce noise in nearby signal traces. Even though signal traces may be routed directly over their ground plane and present a small loop area, the switching action and voltage variation in the DC power supply’s full wave rectifier are strong enough to induce significant noise signals in these AC traces. This increases BER values and can even cause logic circuits to switch involuntarily.
Variation in the output from a DC power supply with a full wave rectifier arises from two charging/discharging cycles during a single 60 Hz AC oscillation from the wall outlet. This variation can be removed from the power signal with filtration or by simply placing a huge capacitor across the power supply output. Unfortunately, this does nothing to block the field emitted from the power supply itself from reaching sensitive traces or components.
In a switching power supply, there is another noise source that arises from the internal switching action. Typical switching power supplies use MOSFETs in the output stage to provide fast switching, but an architecture with faster switching actually intensifies the induced noise, thanks to Faraday’s Law. Similarly, the strength of the induced noise signal from switching will increase when the required output current increases. Removing switching noise from the power supply output requires passing the output through a filter (usually an LC filter will do the trick).
For example, a power supply that supplies 5 A of DC current with 30 ns rise time in the output stage can induce a voltage spike reaching 3-4 V in a nearby trace, assuming a typical situation where the parasitic inductance in a trace is on the order of 10 nH. This is large enough to easily cause involuntary switching in a nearby TTL logic circuit.
Suppressing Noise with the Right Layout
Noise suppression in a PCB takes two forms: reducing or blocking the electromagnetic field that reaches traces and components (radiated EMI), and reducing the power supply noise in the signal that is used to power those components (conducted EMI). Minimizing noise in the output signal only suppresses noise induced from the power plane or power rails, but it does nothing to decrease the field from the power supply itself.
Tackling this first point requires making the right layout choices. Power supply manufacturers do not recommend placing sensitive components or traces on a PCB near power sources unless the power source has less than 5% variation in the output power. Components should be judiciously arranged so that more sensitive components are farther from the power supply.
Sensitive signal traces can also be routed between a power and ground plane on the inner layers to protect them from power supply noise. You might also consider using small shielding cans or meshes around sensitive components to block EMI from your power supply.
Only so much noise can be removed from the power signal. Once the power output hits your power plane and the remainder of your power delivery network, any noise in your power signal will propagate to your components. In order to prevent noise in your power delivery network from inducing noise elsewhere in the board, you need to suppress resonance for noise signals.
When your power delivery network is not impedance matched, the impedance spectrum for your components and your power delivery network will contain resonance and anti-resonance peaks. Signals in your power delivery network with these frequencies can create large voltage spikes elsewhere in your board.
Using a decoupling capacitor between your power and ground planes, as well as where recommended by component manufacturers, will smooth out the impedance spectrum, which reduces the strength of any noise resonance. In addition, you should reduce the impedance of the actual power delivery network, which is typically at micro or milli-Ohm levels. For your power integrity and signal integrity needs, ensure you have PCB design software you trust.
Using a strong power aware signal integrity simulation tool during your design phase can help you diagnose potential noise issues in traces and your power delivery network before finalizing your design and sending your board off for production. You’ll be able to test different layout and stackup options and determine the best design for your board.
If you’re looking for layout software that has an easily integrated process for simulation and analysis, Cadence has the PCB design tools for you. With leading signal integrity and power integrity analysis backing boundary-pushing electronic innovation, nothing is out of reach.
If you’re looking to learn more about how Cadence has the solution for you, talk to us and our team of experts.