How to Protect Low-Speed Interfaces and Power Supply Circuits
In this installation of the "Protect Your Ports! Top Design Tips to Keep Your Communications Connected" series, we'll discuss low-speed interfaces and power supply circuits—including audio/video ports, DC power supplies, and more—as well as various methods of how to protect them.
This is the third article in the “Protect Your Ports!” series on protecting communication interfaces. Earlier articles have discussed high data rate interfaces such as power-over-ethernet, USB, HDMI, DisplayPort, and eSATA. This article will present protection schemes for low-speed interfaces. In addition, the discussion will extend to power line frequencies and DC to present protection methods for AC and DC power supplies.
The low-speed interfaces discussed in this article include digital audio, analog video, and a keypad communication protocol. Table 1 lists the data rates for these interfaces.
These low-speed interfaces are susceptible to voltage transients, particularly electrostatic discharge (ESD); whereas the power supplies are susceptible to power-line induced overcurrent conditions and voltage transients. Developing reliable interface circuits that can withstand transient strikes requires the use of protection methodologies to protect the circuitry. This article provides recommendations for protecting these interfaces and the power sources that energize them.
Protecting Digital Audio Ports
The audio codec in a digital audio circuit, shown in the block diagram of Figure 1A, is susceptible to damage from fast transient surges and ESD strikes. Fortunately, designers need only one component to provide protection for both the left-side output and the right-side output.
Figure 1. Transient voltage protection for the ports on audio and video interfaces
A transient voltage suppressor (TVS) diode array consisting of a pair of back-to-back Zener diodes is recommended, as shown in Figure 2.
Figure 2. Bi-directional diode array pair for ESD protection of audio circuits
Advantages of TVS diode arrays include:
- Ability to absorb up to 40 A from an electrically fast transient
- Safe absorption of an ESD strike as large as ±15 kV
- Fast 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 to ensure that the protection diodes will have minimal impact on the integrity of the transmitted signal
In addition, versions of these TVS diode arrays are AEC-Q101-qualified for the automotive market. The AEC-Q101 standard refers to 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 package. Thus, one small, surface-mount package can help create a robust port for audio lines.
Protecting Video Ports
As with the audio ports, the video ports are susceptible to ESD and other transients. Figure 1B shows an analog video port with its four output lines: Y, C, video, and RF. A single component can provide the necessary protection for a video analog-to-digital converter. In this case, the recommended component is a 4-channel TVS diode array chip, as shown in Figure 3.
Figure 3. 4-channel TVS diode array with Zener diode protection
The 4-channel TVS diode array component offers:
- Absorption without damage of up to a 40 A electrically fast transient
- ESD protection of up to ±15 kV through-the-air and ±10 kV from direct contact
- Typical leakage current as low as 10 nA
- Typical pin-ground capacitance of a very low 0.3 pF
Versions of this type of protection component are also AEC-Q101-certified for automotive applications. This component is available in a SOT23-6 surface mount package for minimal consumption of pc board real estate. A 4-channel TVS diode array chip provides the necessary protection with minimal impact on the video signals.
Protecting Keypad Ports
Keypads are the data entry and control devices for numerous industrial and consumer products. Figure 4A shows a block diagram of a keypad port.
Figure 4. Protection schemes for keypads and battery packs
Activation requires direct contact, which can introduce ESD onto the keypad circuitry. For keypad lines, consider using multi-layer varistors (MLVs). MLVs provide a high level of protection with:
- Absorption of a surge current pulse up to 500 A or surge energy of 2.5 J
- Wide operating temperature range of −40° C to 125° C
- Extensive range of operating voltages, starting as low as 3.5 V
MLVs help achieve compliance with EMC standards such as IEC 61000-4-2. In addition, MLVs are available in compact 0402 surface-mount packages.
Protecting a Battery Pack Control Port
The battery pack supplies power, so the controller IC needs overcurrent as well as voltage transient protection. Figure 4B shows the block diagram for the battery signals and the battery controller IC. As the battery sources DC power, a fast blow fuse for overcurrent protection is best.
A fast blow fuse provides:
- Fast trip times of 0.2 s to a 300% overload
- Low current fuse ratings of 0.25 to 5 A
- Current interrupt ratings of as much as 35 A at voltages ratings of up to 32 V
For transient voltage protection, a TVS diode array, such as the 4-channel chip (recommended for protecting the ports of a video interface), will protect the controller IC from ESD and other transients. As previously stated, this component carries the AEC-Q101 qualification.
Protection for Power Source Circuitry
The power circuits are subject to both overcurrent and overvoltage transients that originate on AC power lines. Thus, protecting an AC circuit requires both a fuse for overcurrent protection and an MOV for overvoltage protection. Some applications may also consider other technologies such as TVS diodes, SIDACtors, or GDTs. (Further discussion on this topic can be found in the IEC 62368-1 reference document.)
Figure 5 shows the suggested protection network.
Figure 5. AC input recommended protection
For AC-DC power supplies, consider using a slow blow glass body fuse. The slow blow fuse will prevent opening due to inductive current surges on the AC line. Other features of this type of fuse are:
- Current ratings from a low of 10 mA to a maximum of 30 A
- Voltage rating to 250 VAC and higher
- Current interrupting ratings as high as 10 kA for 120 VAC circuits
These fuses are UL- and CSA-component recognized for fast and simplified approval by a standards body.
An MOV with substantial power handling capacity across the input to the AC-DC supply will safely absorb transients that can be propagated on an AC line and keep the harmful transients out of the circuitry of the supply. Look for an MOV that can withstand up to 10 kA of peak pulse current or 400 J of pulse energy. Also consider a rugged MOV that can operate over a wide temperature range, such as −55 to 125° C. As with the fuse, ensure the MOV is UL- or CSA- component recognized.
Protecting the Various Types of DC Power Supplies
Depending on the voltage of the circuit and on the application, there are different protection schemes for the inputs to DC power supplies. There are different recommendations for each type of circuit.
For protecting a 12 V or 24 V DC supply, an MOV or TVS diode (similar to what was previously suggested for an AC input power supply) is recommended. Figure 6A shows the MOV across the DC supply input. This component will protect the circuitry from peak surges of up to 10 kA and can operate up to 125° C. An MOV offers good long-term reliability for power circuits.
Figure 6. Recommended protection configurations for the input to various DC supplies
For higher voltage DC supplies, such as 48 VDC supplies, a varistor or TVS diode across the input should be used, as should a gas discharge tube on the ground line (see Figure 6B). The gas discharge tube can withstand current surges as high as 20 kA and protect the circuit from floating to a dangerous level above ground when a surge occurs. With insulation resistance of 10 GΩ, the gas discharge tube draws less than 10 nA during normal operation.
A DC power supply that includes a power factor correction circuit should have a series fuse and a parallel MOV for overcurrent and overvoltage protection. See Figure 6C. Consider a slow blow fuse to avoid the fuse opening due to a startup current surge from a switching power supply. Slow blow, ceramic fuses with 10 to 30 A ratings are available with interrupting ratings of 20 kA DC at DC voltages of 500 V. Versions of these fuses consume minimal space with 6.3 mm by 32 mm packages. The same type of MOV is suggested for the 12/24 V and the 48 V DC circuits.
Protecting a DC supply in a portable device has some additional considerations. Both overcurrent and overvoltage protection are still required; however, you should also consider a polymer positive temperature coefficient (PPTC), resettable fuse. See Figure 6D. PPTC fuses provide the convenience of not having to replace the fuse if an overcurrent condition occurs. They have low resistances, typically in the tens to hundreds of milliohms (mΩ), and fast trip times that are under 5 s. PPTC fuses come in 0402 surface mount packages to minimize space in small portable devices. Instead of an MOV, a TVS diode for overvoltage protection of the DC circuitry should be considered. A TVS Zener diode provides circuit protection from ±30 kV of large through-the-air or direct contact ESD strikes. Also, the TVS diode can safely absorb up to 80 A from a lightning strike.
The output of your DC supply should also have proper protection. A TVS diode, as shown in Figure 7, will protect sensitive downstream circuitry by clamping transients to low voltages. These TVS diodes are available in either uni-directional or bi-directional models.
Figure 7. Recommended component for the protection of the output of a DC supply circuit
Protected Ports Are Robust, Reliable Ports
You can develop reliable and robust circuits by incorporating protection components into your designs. You do not need many components and the component configurations are simple. A wide range of protection components is available. To ease the task and the time involved in component selection, take advantage of a manufacturer’s expertise. The manufacturer’s application engineers can assist you with component selection and guide you through the numerous standards that apply to the product you are developing. Some manufacturers offer testing capabilities—both to verify the performance of your design for protection against overloads and to perform pre-compliance testing to appropriate national and international standards. The added protection for your design will reduce your 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.
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