Cut emissions with integrated data, power isolation: Steps 1-3
Article By : Anand Reghunathan, Koteshwar Rao, Anant Kamath
Designs often require an isolated power supply along with data isolation. Here are the first of the steps you need to know to effectively use products that integrate data and power isolation on one chip.
Isolation prevents DC and unwanted AC between two parts of a system, while still allowing data and power transfer between those two parts. Isolation is used in a wide variety of applications, including factory automation and grid infrastructure: to protect human operators and low-voltage circuitry from high voltages, to improve noise immunity, and to handle ground-potential differences between communicating subsystems.
In most cases, an isolated power supply is required in conjunction with data isolation. These power supplies may be generated using flyback, fly-buck or push-pull DC/DC converters to drive an isolation transformer, followed by rectification on the secondary side. However, compact, efficient and fully integrated solutions are now available that combine both data and power isolation in one chip. Figures 1 and 2 show a conceptual block diagram and example pinout of such a device, respectively.
Figure 1: Fully integrated solution with data and power isolation.
Figure 2: Single-chip power and data isolation.
Traditionally, transformers along with drivers are used to transfer power from side one to side two. Scaling them to a single IC solution, these chip-scale transformers make the integration of an isolated power converter in a small outline package possible. These transformers are very small in size, use only a few turns (to reduce series resistance) and often do not use a magnetic core. Consequently, the DC/DC converter driving these transformers needs to operate at a very high frequency: several tens of megahertz and beyond. At the same time, since the primary and secondary windings of the transformers are very close to each other inside the package, a large parasitic capacitance forms between the two coils of the transformer.
The fast transients in the DC/DC converter couple through this parasitic capacitance, creating a common-mode current between side 1 and side 2 (Figure 3). Since the two sides are completely isolated, the current forms a large return loop through board-level parasitic capacitances. It is this large current loop that causes radiated emissions in isolated systems. Another way to look at it is that the two isolated parts of the board form a dipole antenna transmitter.
In discrete implementations with on-board transformers, radiated emissions are lower due to the use of high-inductance magnetic-core transformers and much lower switching frequencies.
Figure 3: Common-mode currents across the isolation barrier form a large return loop.
In this article, we will discuss several methods for lowering radiated emissions in systems using fully integrated data and power-supply solutions.
Step 1: Choosing the right integrated device
Existing integrated isolated power and data solutions are not all equal in terms of electromagnetic emissions. Through careful circuit design and clock management, it is possible to reduce electromagnetic emissions at the device level. Figure 4 shows an example of a device that meets Comité International Spécial des Perturbations Radioélectriques (CISPR) 22B emissions with a simple evaluation board, side by side with a device that does not. It is important to make this comparison at the chip level, and choose the device with lower emissions.
Figure 4: Radiated emissions of the ISOW7841 and a competitive device at 5V input and 80mA load.
Step 2: Lower input-voltage operation
Isolated power devices support a wide input range (usually 3V to 5.5V) for compatibility with both 3.3V and 5V standard supplies. Compared to 5V, at 3.3V operation the slew rates of the internal DC/DC converter are lower due to the lower voltage drive available to the power transistors. This reduces the common-mode current across the isolation barrier, leading to lower radiated emissions. As you can see in Figure 5, the emissions with a 3.3V supply are much lower than with a 5V supply.
Figure 5: Radiated emissions of ISOW7841 at 5V and 3.3V inputs.
Step 3: I/O decoupling capacitors, input ballast resistor, ferrite beads
As we mentioned earlier, devices with fully integrated isolated power use high switching frequencies to compensate for low transformer inductance. These devices use some form of power-converter duty cycling to provide the required output DC load while maintaining regulation. Whenever the converter is on, there is a high current draw from the input supply VCC (figure 6). This current has low-frequency content (roughly proportional to the closed-loop regulation bandwidth) and high-frequency content at the DC/DC converter switching frequency and harmonics.
Figure 6: Supply decoupling capacitors and input ballasting resistor.
An input capacitor bank of different capacitors (C1 = 100nF, C2 = 1µF, C3 = 100nF) at the input of the integrated circuit (IC) filters out a lot of the high-frequency content and prevents it from propagating to the backplane supply routing. It is critical to place these capacitors as close to the IC as possible to limit the area of loop 1. Place the smallest-value capacitor closest to the IC. A similar bank of capacitors placed on the output side filters out switching current on the secondary side of the DC/DC converter. It is important to minimise the area of loop 2.
In spite of the decoupling capacitors, the backplane supply routing can still draw some high- and low-frequency current (peak currents from 100mA to 500mA lasting several microseconds) depending on the output impedance of the supply network. If the input supply routing is long, a ballasting resistor (RS) and an additional larger capacitor (CF) can prevent current from going into the input supply routing, leading to lower radiated emissions. This helps reduce the current in loop 3.
The recommended values of RS are 1Ω for full loads (130mA) to 5Ω at light loads (<10mA) on VISO. The recommended value for CF is 100µF. You can adjust these values or remove noncritical components based on the radiated emissions results. Including a ferrite bead (L1) in the design can prevent high-frequency noise from reaching the backplane.
Figure 7 shows an image of an example layout, showing the placement of decoupling capacitors. In this layout, stitching vias along the board edge that connect all ground planes create a Faraday shield. This prevents any radiation from noisy inner traces or planes.
Figure 7: ISOW7841 evaluation module.
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