A critical element of electronic circuit design is providing every component with needed power. Although a simple concept on paper, designing power distribution networks (PDNs) that reliably meet power delivery requirements at acceptable voltage and current levels can be challenging. Doing so often requires a regulated power supply design, which includes filtering elements that must adhere to specific guidelines.
Important PCB Power Supply Filter Design Considerations
Incorrect power supply filter designs lead to unreliable hardware, which is distressingly common. A proper design can help obviate a host of circuit problems and improve power supply bypassing. To create the latter, follow these steps:
Power Supply Filter Design Considerations
- Understand the power supply filter requirements.
- Use simple rules of thumb to find the component values.
- Iterate the design using a circuit simulator.
A high-frequency ripple passes right through a linear regulator. The ripple comes from switching power supplies, digital circuits, and radio interference. At frequencies above 10kHz, most linear regulators begin to lose effectiveness. The small bypass capacitors distributed among ICs become effective at about 1MHz. A simple power supply decoupling filter made from an inductor and capacitor can cover the gap between 10kHz and 1MHz. An adequate decoupling filter design ensures that it won’t cause more problems than it solves.
PCB power supply filter design frequency ranges
The chart above shows typical power supply filtering frequency ranges. Careful design with high-performance components can extend these ranges, and not all designs have the same requirements for ripple rejection.
Power Supply Filter Design Model
A single inductor and damped capacitor, as shown below, serve as a model for a satisfactory PCB power supply design.
PCB Power supply filter design example
This is referred to as an LC filter. Other designs with more or fewer components are possible. The design process includes generating the requirements for the inductor LB, choosing a candidate for the inductor, and then designing the filter around it. If an acceptable filter can’t be designed, figure out what was wrong with the inductor, choose a better one, and try again.
In the figure above, the example power supply regulator is off-board and a regulated voltage is received through a connector. When there is a local regulator, the design is simpler, resulting in a reduced number of components. The power supply filter comes after the regulator, so it needs to have a low DC voltage drop. The inductor datasheet contains a value for the DC resistance, and the voltage drop is about 20% more than this resistance times the current. The extra 20% accounts for the increase in the inductor’s copper wire resistance at higher temperatures.
The Power Supply Filter Design Process
The inductance value needed for the filter isn’t too difficult to calculate. It should be around ten times larger than every other inductance in series with the power supply. If there are no other inductors or ferrite beads in the supply, this inductance is due to cables and printed circuit board traces. To approximately calculate this inductance, take the maximum length for the power to travel and multiply it by 1nH per millimeter. The inductance of power planes is much lower, and for this calculation, the length of power plane paths can be ignored.
In this example, the board needs to work on an extender cable that is around 300mm long. The board itself is about 100mm X 100mm. A generous overall length is 500mm, which means the power distribution inductance is around 500nH. To make the power supply filter inductor about 10 times larger than this, a 10uH +/- 30% inductor was utilized. The extra inductance accounts for the -30% tolerance. In addition to the initial tolerance, the inductor value drops with increasing current. For this part, at 2.4A, the inductance drops by another 35%.
For this example, the Bourns SRU1028 series inductor was chosen due to its low height, self-shielding capabilities, and availableness. The Bourns datasheet is preferred as it lists the specifications needed to construct a reliable simulation model of the inductor.
Calculating filter components from a datasheet
This inductor model uses four components. The inductance L is the same as the datasheet L. The series resistance RESR is the same as RDC from the datasheet. The values of RQ and CSRF are calculated from the datasheet values for fSRF, Q, and Q test frequency.
Filter frequency response
These extra components cause the inductor to behave in a particular way as shown in the impedance plot above. The solid curve is the dB magnitude of the impedance and the dashed curve is the phase angle of the impedance. Below about 1kHz, the inductor acts like a small resistor RDC. Above 1kHz, it acts like an inductor—up to near the Self Resonant Frequency (SRF). For a narrow range of frequencies near the SRF, the inductor acts as a large value resistor with the value RQ. Above the SRF, the inductor acts as a capacitor CSRF.
From this point on, a circuit simulator saves time. The above graph was created using the simulation schematic below.
Simulation circuit for the example filter
The voltage source, V1, is a 1V AC source. The impedance can be graphed with the expression -1/(i(V1)). To learn LTspice, see my Simulation Series tutorials on YouTube. LTspice AC analysis can be found in part one and part two, and transient analysis is in part three.
It’s easy to transform the PCB power supply filter’s inductor model schematic into a low-pass filter by adding a capacitor to it. For the example below, the Kemet capacitor T491A106010A was chosen. It’s a 10uF polarized tantalum capacitor with a maximum ESR of 3.8Ω and a voltage rating of 10V.
Low-pass version of example power supply filter design
The frequency response of this filter is V(VOUT)/V(VIN), but since V(VIN) = 1 in this simulation, the same answer is produced from a graph of V(VOUT).
Plot of low-pass filter design
High-Q, low-ESR ceramic capacitors have replaced tantalum capacitors in many applications. Next, a simulation with a low-ESR ceramic capacitor instead of a tantalum was conducted:
Frequency plot with low-ESR capacitor
The peak at 15.9kHz is the resonance of LB and CB calculated from the equation below:
Resonant frequency equation
Supply ripple at this frequency will increase instead of decrease. Because of the narrow frequency range of this resonance, the effects of the resonance are easy to miss in testing. The values of LB and CB have a loose tolerance and drift over time and temperature. Adding a series resistor is recommended to solve this resonance problem. A good initial guess for the value of a damping resistor is as follows:
Adding a damping resistor to filter design
Use a circuit simulator to find the first resonance and adjust the resistor value to find the best value for good damping. The ceramic capacitor and resistor have a more repeatable design than a tantalum capacitor, largely owing to the wide range of possible values for the ESR of a tantalum capacitor.
Modeling the Load Network
So far, this example doesn’t have a load impedance or any load current. To see how this filter will perform on a circuit board, the simulation needs to include printed circuit board trace inductance and bypass capacitors. At frequencies higher than 100MHz, transmission line effects further complicate the model. The next circuit example has a simplified model that represents typical loads found in PCB power supplies. You can look at your own circuits to estimate trace inductance using the rough inductance approximation of 1nH per millimeter. More accurate models are possible with a Power Integrity (PI) CAD tool.
Filter design with load circuit model.
These typical trace inductances demonstrate additional resonances in the power distribution network.
Frequency response with load model
The extra simulated resonances are caused by the output load of the inductors and capacitors. The performance of this filter is still solid, even with these resonances. The overall shape of the filter is preserved because the inductor is much larger than the sum of the small load inductors and the damped capacitor is much larger than the sum of the bypass capacitors.
There are still differences between this board and a real-world circuit version. The latter would have a different response above 100MHz due to high-speed transmission line effects. Also, other small capacitors and inductors become important, especially at frequencies above 500MHz.
Excluding the power supply filter or using a large, undamped capacitor without an inductor leads to resonances such as these:
Unfiltered filter frequency response
Local bypass capacitors provide local charge storage that supplies transient current to high-frequency pulsating loads. To hold the supply voltage steady, larger pulsing load currents require larger bypass capacitors. An example of a pulsed load is a processor going in and out of low-power sleep mode. Analyze each high-current pulsed load for voltage ripple in the supply.
Bypass capacitors can also resonate with inductance in the power distribution network.
Damping the resonances at the power supply input filter does not guarantee that all resonances due to load current will also be damped, but it often helps. To demonstrate a potential problem, here is the undamped (R3 = 0.01 Ohm) version of the filter with an AC current source at one of the load points:
Pulsed load example simulation circuit
The impedance at VLOAD is v(VLOAD)/i(I1). Since the AC current in I1 is set to 1, the impedance is just v(VLOAD):
Pulsed load plot example
The undamped resonance circled above is 1.87 MHz. This is one frequency where a pulsed load will cause a problem.
Code example for simulation of pulsed load
The pulsed load with a pulsed current source was simulated as shown in the schematic above. This example shows pulses with an amplitude of 20mA and a period of 535ns. The largest voltage swings occur when the period of the pulsed current source is the reciprocal of the frequency of the resonance.
The sine wave shape of the ripple voltage in this example is typical for undamped, high-Q resonances in the power distribution. The undamped resonance acts as a filter that converts the current pulses into a sinusoidal voltage waveform:
Results of undamped resonance
If the voltage is still growing at the end of the simulation, increase the simulation time to find the maximum level. Sharper (higher Q factor) resonances take longer to settle. In the example of sleep-mode current pulses, a software change can cause the frequency of the pulses to change. The large voltage swings due to resonance only occur when the sleep cycle period aligns with the resonant frequency. In development, this can cause mysterious bugs that appear to be software-related but are actually caused by hardware.
In production, component variation will shift the resonant frequencies and cause yield problems. In use, temperature changes and component drift will shift the resonant frequencies, leading to product failure. The next simulation shows the damped version, with resistor R3 set to 3.8 Ohms. The AC analysis shows that the largest two high-Q resonances have been damped:
Plot of damped pulsed load example
This changes the shape and decreases the voltage waveform caused by the pulsed load current.
Pulsed current waveform for damped case
DFM for HDI Printed Circuit Boards
The triangular waveform shape is typical for a pulsed load. It is from the charge and discharge cycle of the local bypass capacitors. The amplitude of this triangle wave will increase with larger bypass capacitors. If the ripple waveform shape looks more like a square wave, it is due to resistance of the bypass network, and is reducible with lower ESR bypass capacitors or wider traces. The long, slow pulse at turn-on is caused by the damped low-frequency resonance at 100KHz. The short spikes are feedthrough of the 10ns current source edges, and are reducible with lower inductance through the bypass capacitor path. The remaining resonance at about 4MHz requires further simulation.
Filter Optimization for Power Supply Design
Optimal PCB power supply filter designs require strict adherence to clear and effective guidelines, as discussed above. For example, you can avoid power distribution resonances by using a correctly designed and damped low-pass filter with:
- An inductor value that is much larger than the stray inductances
- A capacitor value much larger than the sum of the bypass capacitors
- Damping resistance to eliminate high-Q resonances
- Bypass capacitors sufficient to supply pulsed loads
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