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    USB Type-C电路保护Circuit Protection for USB Type-C.pdf

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    USB Type-C电路保护Circuit Protection for USB Type-C.pdf

    Circuit Protection for USB Type-C ™ Padmanabhan Gopalakrishnan [GP] Systems and Applications Manager Michael Koltun IV Systems Engineer Integrated Protection Devices Texas Instruments Circuit Protection for USB Type-C ™ 2 October 2016 Benefits of USB Type-C Reversible in a small form factor connector The symmetrical definition of the Type-C connector enables reversible plug orientation in a small form factor connector. The Type-C connector supports either host or device mode, and in time will replace various Type-B and Type-A connectors and cables in the market. The 24-pin, double-sided connector is similar in size to the USB Micro-B connector, with a Type-C port measuring 8.4 millimeters 0.33 inches by 2.6 millimeters 0.10 in. Increased USB power delivery The USB Type-C connector supports the USB Power Delivery USB PD standard, which enables higher power transfers than previous USB protocols. On the higher end, power capabilities with the Type-C standard are now extended up to 100 W. This standard is based on new provisions allowing the Why adding proper circuit protection to your USB Type-C ™ design reduces the risk of system damage and field failures. USB Type-C™ is the latest universal serial bus USB standard that combines support for data, video and power delivery into a single, flexible interface. USB Type-C defines a new receptacle, plug and cable standard compatible with all existing USB interfaces. The simplicity and convenience of the end consumer experience is expected to drive rapid adoption of the new connector, but introduces new challenges to system designers as they make the migration. This paper will discuss how the new features and capabilities of the USB Type-C connector can also introduce risk for system damage and field failures, if proper circuit protection is not a key component of the system design. Such a failure can occur independently of the end-equipment, whether it be for personal electronics, industrial or automotive applications. Although the USB interface has long been the work horse of interfaces for high-speed data and up to 7.5 W of power delivery through USB BC1.2, the system use cases demanded by the market, the evolution of technology, and competitive proprietary interfaces have accelerated the need for a more flexible, capable and powerful interface. Adoption of USB Type-C started in 2015. Early adopters were first to see the benefits, as well as the new potential failure mechanisms the new connector may introduce. [1] Figure 1. Mechanical layout of a USB Type-C versus a USB Type-A connector. Source Images courtesy of USB Type-C and USB Type A standards. USB Type-C USB Type A 0.5 mm pitch between pins 2.5 mm pitch between pins 1,2; 3,4 2 mm pitch between pins 2,3 USB Type-C USB Type A 0.5 mm pitch between pins 2.5 mm pitch between pins 1,2; 3,4 2 mm pitch between pins 2,3 Circuit Protection for USB Type-C ™ 3 October 2016 source to dynamically manage current from 0.5 A to 5.0 A. In this case, the nominal voltage on the V BUS can be up to 20 V. This enables end products from laptops to mobile phones and power tools to charge faster. Small pin-to-pin pitch To support the smaller and symmetrical form factor, the Type-C connector has 0.5-mm pin-to-pin pitch as shown in Figure 1. This small pitch is 20 percent of the pin-to-pin distance of the Type-A connector pins, and the proximity of the pins increases the risk for a pin-to-pin short. For example, a short could occur with a twist of the connector, or if the cable is pulled out of the receptacle at an angle. This type of pin-to-pin short failure is easy to replicate in the lab with simple experiments Figure 2. Figure 2. An evaluation module EVM with an interposer board shows evidence of a pin-to-pin short. In addition to risk for a pin-to-pin short from mechanically twisting the connector, a build up of small debris due to connector aging can cause a short with V BUS . Figure 3 shows an example of debris leading to a pin-to-pin short due to a small pitch in the connector. Cable reversibility The 24-pin connector provides four power/ground pairs, two differential pairs D/D– for USB2.0 data even though only one pair is populated in a Type-C cable, four pairs for SuperSpeed data bus TX/RX, two side-band use SBU pins, and two configuration channel pins CC for detecting cable orientation, a dedicated biphase mark code BMC configuration data channel, and V CONN 5-V power for active cables. Connecting an older device to a host with a Type-C receptacle requires a cable or adapter with a Type-A or Type-B plug, or a receptacle on one end with a Type-C plug on the other end. Legacy adapters with a Type-C receptacle are not defined or allowed by the specification because they can create many invalid and potentially unsafe cable combinations. Figure 3. Illustration of debris in a small pitch aiding easy shorts. Circuit Protection for USB Type-C ™ 4 October 2016 Figure 4. Illustration of a full-featured Type-C plug-in pin out. Since the introduction of Type-C, over 2000 manufacturing companies have introduced Type-C cables to the market worldwide. Texas Instruments developed the industry’s first integrated Type-C and a USB Power Delivery PD controller. As an early adopter of this new standard, TI has seen the issues encountered by early adopters and in the field due to non-compliant cables. Protecting against non-compliant cables Amazon conducted a survey of Type-C cables and found that a large amount of cables surveyed were non-compliant with the USB-IF specification, making the risk of an end user purchasing a non- compliant Type-C cable real [2]. The system design must include circuit protection to ensure that a faulty or non-compliant cable does not damage the system. In addition to non-compliant cables, there are many power adaptors that are also non-compliant with the Type-C standard. These adaptors could deliver up to 20 V of power to V BUS before the PD negotiation begins to support this high voltage. This high voltage can damage Type-C ports that are designed to support only 5 V on V BUS . Additionally, if the CC pin is pulled up to the V BUS rail in these faulty adaptors, the system will see a short-to-V BUS failure. If R P is pulled up to V BUS in these adaptors, greater than 5.5 V can be exposed to the CC pin. If a standard 3A R PULLUP cable is used Figure 5, 7.43 V can be exposed on the CC pin. Figure 5. CC pin exposure for regular and faulty wall adaptors. CC2 CC1 V BU S RD RD VBUS Power Switch Battery Charger PD Controller SINK AC/DC VBUS Power Switch CC2 CC1 V BU S RP RP Wall Outlet 20V Faulty Wall Adaptor AC/DC VBUS Power Switch CC2 CC 1 V BU S RP RP Wall Outlet 5V - 20V after PD Wall Adaptor PD Controller LDO 3.3V 3.3V Circuit Protection for USB Type-C ™ 5 October 2016 With the aim to develop low-cost accessories to support the fast-growing market for Type-C, cable manufacturers may introduce problems that impact the end application. If the cable is mis-wired or improperly soldered, the small pin-to-pin pitch increases the probability that a short can occur. As also mentioned, even if a cable is compliant to the USB-C standard, mechanical twists during usage and removal of the Type-C cable can still cause these shorts; so whether a cable is compliant or not, shorts to V BUS on adjacent pins can occur. Therefore, the end-equipment manufacturers need circuit protection to avoid this risk. In addition to the potential for these cables to expose the system to an over-voltage condition, the cable could also carry up to 5 A of current. If this should occur, any small damage caused by the cable could be detrimental to the end product. Now we will review the different types of configurations and the potential circuit protection considerations for each. Challenges Circuit protection While the advantages we just mentioned are a benefit of this standard, they also pose a challenge to system designers who need to ensure that the downstream circuitry can withstand 20 V. Currently, all USB PD controllers in the market are only 5-V tolerant or less on the CC, SBU, and transceiver/ receiver TX/RX pins. The CC and SBU pins are directly adjacent to the V BUS pins, so a short to these pins can expose 20 V to the downstream circuitry and be destructive to the system. Combining IEC and short-to-V BUS Any external connector will require system-level IEC 61000-4-2 ESD protection. Some end products require 8-kV contact. When we need to combine OVP and IEC ESD protection, it is critical to have a clamping voltage low enough to protect the system. Note that the type of diode required when using Type-C is not a conventional transient-voltage suppression TVS diode, but a high-voltage, DC- tolerant TVS diode. Many options available in the market today clamp at voltages too high to protect the downstream controllers in the event of an IEC ESD strike. An integrated protection solution for OVP and IEC ESD combined ensures robust protection for the end system. Short-to-V BUS events We have identified various causes of the short-to-V BUS event; however, how does a system designer protect from such short-to-V BUS faults Next we will discuss the unique characteristics of the short-to-V BUS fault, which makes implementing a proper protection solution from this fault event complex. Figure 6. Short-to-V BUS system set up without a protection device. Circuit Protection for USB Type-C ™ 6 October 2016 Short-to-V BUS model Figure 6 shows a system configuration as it is performing a short-to-V BUS to a CC line without a protection device. There are two high-level use cases of the short-to-V BUS event short-to-V BUS through a cable and short-to-V BUS without a cable. If a Type-C cable is present and a short in a Type-C connector happens on the connector where V BUS power is sinking, then a cable exists between the voltage source and shorted CC or SBU line. If a Type-C Cable is present, but a short happens on the connector where V BUS is sourcing power, then even though a Type-C cable is present, the voltage source is applied directly to the CC and SBU pins, bypassing the cable. Both the cable and non-cable use case for short-to-V BUS present their own challenges and both use cases need to be accounted for to ensure robust protection for the system. Short-to-V BUS through a cable The short-to-V BUS generator shown Figure 7 is capable of generating a short both with and without a cable. The waveform was generated assuming a one-meter USB-C cable. RD 5.1 kΩ, and CC_Cap 200 pF 1 Figure 7. Simulated short-to-V BUS waveform through a one-meter USB-C cable. Figure 8 shows the capture of a short-to-V BUS event with a one-meter Type-C cable in the lab. RD 5.1 kΩ, CC_CAP 220 pF 2 Figure 8. Lab-performed short-to-V BUS waveform through a one-meter USB-C cable. As the waveform shows, when a cable is present during a short-to-V BUS event, enough inductance exists in the resistor-inductor-capacitor RLC circuit relative to the resistance and capacitance to generate a peak voltage during the ringing that is nearly double the value of the settling or final voltage. If the CC line capacitors de-rate greatly over their voltage range, then the peak voltage that is present during the ringing can be more than double the setting or final voltage. This means for a 22-V short, up to 44 V can be seen on the CC or SBU lines during a short-to-V BUS event. What was initially thought to require only 22-V protection ends up needing to be 44 V. Short-to-V BUS without a cable The short-to-V BUS waveform shown in Figure 9 is configured to simulate the short-to-V BUS tester when a cable is not used. RD 5.1 kΩ, CC_CAP 200 pF 3 Circuit Protection for USB Type-C ™ 7 October 2016 Figure 9. Simulated short-to-V BUS waveform without using a USB-C cable. Figure 10 is a capture of a short-to-V BUS event in the lab without a cable. RD 5.1 kΩ, CC_CAP 220 pF 4 Figure 10. Lab-performed short-to-V BUS waveform without using a USB-C cable. When not using a cable, a short-to-V BUS event is especially challenging to protect from. Because the inductance is so low without a cable, the rise time of the short event can be less than 10 ns from 10 percent to 90 percent of the rising edge. This is extremely fast. This rise time is so fast it makes using only a discrete field-effect transistor FET to protect the line useless. Also, in this use case, the total resistance of the path is so low, the amount of current that can be introduced on the CC and SBU lines increases substantially, when compared to using a cable. Other system requirements ESD protection A consumer-interfacing product needs to have some level of the International Electrotechnical Commission IEC 61000-4-2 standard for electrostatic discharge ESD protection. This standard more accurately approximates the types of ESD events that end-products may encounter when a user operates them. However, with the possibility of the CC and SBU lines being exposed to 22 V of direct current VDC from the connector side, the IEC solution becomes more challenging. Solutions exist on the market today with breakdown voltages greater than 22 V. However, the issue is that the majority of these devices that also have low-clamping performance to minimize voltage to the downstream PD Controller have deep snapback technology. Figure 11 shows a transmission-line pulse TLP curve from this type of device that is currently on the market. Figure 11. TLP curve of a deep snapback ESD protection diode. This device’s TLP curve shows that the trigger voltage is well above 22 V, so it would seem that this diode should be able to withstand a 22-VDC short. For IEC ESD strikes, the deep snapback technology makes the clamping voltage very low, allowing for a much lower voltage tolerance for the system and/or total protection circuit. The key issue Circuit Protection for USB Type-C ™ 8 October 2016 is that this does not provide protection when a short-to-V BUS event occurs. If a short-to-V BUS occurs, the voltage can ring much greater than 22 V, even up to 44 V as previously discussed. Therefore the trigger voltage of this ESD cell can be surpassed. Once the diode is triggered, it begins conducting in its high-current region. Since we are applying a 22-VDC source to this line, this diode can conduct in its high-current region indefinitely. This leads to over heating and causes permanent damage to the diode, and also introduces an over-voltage condition to the downstream system circuitry. Since deep snapback technology clearly has issues in the short-to-V BUS system, non-snapback diodes must be investigated. Therefore, we tested a non-snapback diode and collected its TLP curve, shown in Figure 12. Figure 12. TLP curve of a non-snapback ESD protection diode. As Figure 12 shows, its trigger voltage is near 30 V. Since it does not have

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