How to Protect Communication Ports from Unwanted Interruptions – Part 3: Protecting Low-Speed Interfaces and Power Supply Circuits
November 24, 2021
Previous articles have discussed several high data rate interfaces such as power-over-ethernet (PoE), USB, HDMI, DisplayPort, and eSATA. This article addresses protection schemes for low-speed interfaces. The discussion will also extend to the power line frequencies and present several protection methods for the AC and DC power supplies.
Low-speed interfaces include digital audio, analog video, as well as keypad communications. See Table 1 for the data rates of these interfaces.
Low-speed interfaces are susceptible to voltage transients, especially electrostatic discharge (ESD), frequently resulting from end-user contact with the device. The power supplies used with these low-speed interfaces are vulnerable to power-line induced overcurrent conditions as well as voltage transients. To develop reliable interface circuits capable of withstanding transient strikes, electronics designers must utilize protection methodologies to protect the circuitry. This article highlights several recommendations for protecting these low-speed ports and their power sources.
Digital audio ports protection
In a digital audio circuit (Figure 1A), the audio codec is susceptible to damage from ESD strikes and fast transient surges. Fortunately, the circuit design will only need one component to protect the left- and right-side outputs. A transient voltage suppressor (TVS) diode array consisting of a pair of back-to-back Zener diodes is recommended (Figure 2).
Figure 1. Audio and video interface protection recommendations
TVS diode arrays include several advantages:
- Ability to absorb up to 40 A from an electrically fast transient.
- Safe absorption of an ESD strike as large as ±15 kV.
- Quick response to a transient of under 1 ns.
- Low leakage current of 0.5 µA to consume a negligible amount of power under normal operating conditions.
- A maximum capacitance of 6 pF ensures that the TVS diodes will have minimal impact on the transmitted signal’s integrity.
In addition, there are AEC-Q101-qualified TVS diode arrays available for the automotive market. 1 The AEC-Q101 standard references a series of stress tests that a component must meet to be classified as an automotive-grade component. These TVS diode arrays are available in an SC70-3 surface mount design (SMD) package. Thus, one small SMD package can help create a robust port for audio lines.
Figure 2. Bi-directional TVS diode array for audio circuit ESD protection
Video ports protection
Video ports are also susceptible to damaging ESD and other transients. An analog video port includes four output lines, Y, C, video, and RF (Figure 1B). One component can provide the necessary protection for the video analog-to-digital converter. A 4-channel TVS diode array is recommended (Figure 3) for the following reasons:
- Absorption without damage of up to a 40 A electrically fast transient.
- Up to ±15 kV ESD protection through-the-air and ±10 kV from direct contact.
- Typical low 10 nA leakage current.
- Typical very low 0.3 pF pin-ground capacitance.
AEC-Q101-certified versions of this type of protection component are also available for automotive applications. In an SOT23-6 surface mount package, this component uses a minimal amount of printed circuit board space. A 4-channel TVS diode array provides the necessary protection with minimal impact on the video signals.
Figure 3. 4-channel TVS diode array incorporating Zener diode protection
Keypad ports protection
Many industrial and consumer products use keypads as data entry and control devices. Keypad activation through the keypad port (Figure 4A) requires direct user contact, which can introduce ESD onto the keypad circuitry. Consider using multi-layer varistors (MLVs) for keypad lines. MLVs provide a high level of protection, including:
- Surge current pulse absorption of up to 500 A or surge energy of 2.5 J
- Operating temperature range of -40° C to 125° C
- Range of operating voltages beginning as low as 3.5 V.
MLVs help electronics designers achieve compliance with EMC standards such as IEC 61000-4-2.2. Also, MLVs are available in compact 0402 surface-mount packages.
Figure 4. Keypad and battery pack protection recommendations
Battery pack control port protection
The battery pack’s controller IC needs overcurrent as well as voltage transient protection. See the battery signals and the battery controller IC block diagram in Figure 4B. Since the battery sources DC power, consider using a fast blow fuse for overcurrent protection. Fast blow fuses provide:
- Fast trip times: 0.2 s to a 300% overload
- Low current fuse ratings: 0.25 – 5 A
- Current interrupt ratings: up to 35 A @ voltages ratings up to 32 V
A TVS diode array, such as the 4-channel chip, is recommended for transient voltage protection. Just as it will protect the ports of a video interface, it will protect the battery pack’s Controller IC from ESD and other transients. As stated previously, this TVS diode array carries the AEC-Q101 qualification.
Power source circuitry protection
The power circuits are subject to both overvoltage and overcurrent transients that originate on AC power lines. So, protecting the AC circuits requires both an MOV for overvoltage protection and a fuse for overcurrent protection. Depending on the application, other technologies such as TVS diodes, SIDACtors, or GDTs might be worth considering. (For more information, see the IEC 62368-1 reference document.2) For AC-DC power supplies protection, consider using a slow blow glass body fuse (Figure 5). Slow blow fuses prevent opening due to inductive current surges on the AC line. Other features of slow blow fuses are:
- Current rating: as low as 10 mA to a maximum of 30 A
- Voltage rating: 250 VAC and higher
- Current interrupting rating: as high as 10 kA for 120 VAC circuits
These fuses are UL- and CSA-component recognized for fast and simplified certification approval by a standards body.
Figure 5. AC input protection recommendations
To safely absorb transients that can be propagated on an AC line and keep the harmful transients out of the supply’s circuitry, use an MOV with substantial power handling capacity across the input to the AC-DC supply. Select an MOV that can withstand up to 10 kA of peak pulse current or 400 J of pulse energy. Consider using a rugged MOV that can operate over a wide temperature range, such as -55 to +125° C. As with the fuse selections, ensure that the MOV is a UL- or CSA-certified component.
DC power supplies protection
Depending on the application and the voltage of the circuit, there are different protection schemes for the DC power supply inputs. Depending on the type of circuit, there are additional protection recommendations.
Figure 6. DC input protection recommendations
For protecting a 12 V or 24 V DC supply, an MOV or TVS diode is recommended (Figure 6A). This component protects the circuitry from peak surges up to 10 kA and can operate up to 125° C. The MOV will provide good long-term reliability for these power circuits.
For higher voltage DC supplies, such as 48 VDC supplies, a varistor or TVS diode across the input and a gas discharge tube (GDT) on the ground line should be used (Figure 6B). The GDT can withstand current surges up to 20 kA as well as protect the circuit from floating to a dangerous level above ground when a surge occurs. With insulation resistance of 10 GΩ, the GDT draws less than 10 nA during normal operation.
If a power factor correction circuit is a part of the DC power supply, it should have a series fuse and a parallel MOV for overcurrent and overvoltage protection (Figure 6C). To avoid the fuse opening due to a startup current surge from a switching power supply, consider a slow blow fuse. Ceramic slow blow fuses with 10 A to 30 A ratings are available with interrupting ratings of 20 kA DC at DC voltages of 500 V. These fuses are available in small form factors that consume minimal space with 6.3 mm x 32 mm packages. The 12/24 V and the 48 V DC circuits should use the same type of MOV.
There are additional considerations when designing DC supply protection in a portable device. While both overcurrent and overvoltage protection are still needed, also consider a polymer positive temperature coefficient (PPTC) resettable fuse (Figure 6D). Because they are resettable, PPTC fuses provide the convenience of not having to replace the fuse if an overcurrent condition occurs. They have low resistance, typically in the tens to hundreds of milliohms (mΩ), and fast trip times under 5 s. PPTC fuses come in 0402 SMD packages to minimize space in small portable devices. Consider a TVS diode instead of an MOV for overvoltage protection of the DC circuitry. A TVS Zener diode provides circuit protection from ±30 kV of large through-the-air or direct contact ESD strikes. The TVS diode can also safely absorb up to 80 A from lightning strikes.
The DC supply output should also have proper circuit protection. A TVS diode (Figure 7) will protect sensitive downstream circuitry by clamping transients to low voltages. Uni-directional or bi-directional TVS diodes are available.
Figure 7. DC output protection recommendations
Protected low-speed data ports are robust, reliable ports
Electronics engineers can develop reliable and robust circuits that last by incorporating the appropriate protection components into product designs. A lot of additional parts aren’t required, and the component configurations are pretty simple. There is a wide range of circuit protection components available. To help simplify the task and reduce the time involved in selecting the right parts, take advantage of a manufacturer’s expertise. The manufacturer’s application engineers can assist with component selection and guidance through the numerous standards for the product’s development. Some manufacturers also offer multiple testing capabilities to verify the design’s performance for protection against overloads and perform pre-compliance testing to appropriate national and international standards. The added protection for next-generation products will reduce potential in-warranty costs and enhance your company’s reputation for high-quality products.
To learn more, download the Circuit Protection Products Selection Guide, courtesy of Littelfuse, Inc.