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802.11 systems aren't synchronous, are seldom planned, use unlicensed spectrum, can experience significant interference from multiple wireless networks and other non-WLAN devices, and are generally unpredictable at the microsecond level even though they may be robust overall. Talk time, i.e. battery life, is an important point of comparison between cellular telephony and VoIP-over-WLAN. When 802.11 subsystems are added to cellular handsets, they're constrained to use the existing battery system and will be compared directly with the cellular implementation. A well-designed 802.11 subsystem can deliver talk times and power budgets comparable to cellular systems only by placing the 802.11 subsystem into sleep mode between transmitting voice packets, just like the cellular systems.

CLSA2

802.11 systems aren't synchronous, are seldom planned, use unlicensed spectrum, can experience significant interference from multiple wireless networks and other non-WLAN devices, and are generally unpredictable at the microsecond level even though they may be robust overall. Talk time, i.e. battery life, is an important point of comparison between cellular telephony and VoIP-over-WLAN. When 802.11 subsystems are added to cellular handsets, they're constrained to use the existing battery system and will be compared directly with the cellular implementation. A well-designed 802.11 subsystem can deliver talk times and power budgets comparable to cellular systems only by placing the 802.11 subsystem into sleep mode between transmitting voice packets, just like the cellular systems.

Linear Current Limit

The SOA problem does not prohibit one from using a hot swap system in a linear current limit mode. It highlights the fact that an unstable thermal condition will occur at some gate voltage. This can be a serious problem for hot swap controllers using external FETs. One way to get around this problem is to integrate the FET and controller on a monolithic die.

CLSA2

The use of a monolithic controller and FET allows for direct thermal sensing of the FET die area. The die temperature is the one variable that can protect the FET against all combinations of thermal overstress. Individual controllers and FETs can not sense the die temperature, and instead must take into account the range of drain-to-source voltages, the approximate drain current, time duration of pulse, and thermal path of the FET in an attempt to protect the power device from thermal overstress. This solution can protect the FET, but necessitates a much larger FET than would otherwise be required due to the number of variables and inexact knowledge of each.

Using a hybrid solution, such as the NIS5101, the device can be made to accommodate any set of voltages and currents (not to exceed the maximum device voltage rating) and still remain within its SOA. Figure 4 shows a typical die layout for such a device. Several forward biased diodes are used as temperature sensing devices. These diodes have a negative temperature characteristic. The forward drop is measured and compared to a reference voltage to determine when the device has reached its maximum temperature of 135°C.

CLSA2

This circuit requires several micro-seconds to respond to a fault. The thermal time constant for a D2PAK, which is the package for this device is approximately 200 ms. The thermal shut-down propagation delay allows the maximum temperature to exceed the trip point by only a few degrees at the most, depending on the power being dissipated in the device. There is a margin of 40 degrees between the trip point and maximum rating of the die, so there is never any danger of damaging the die. A maximum temperature of 135°C was chosen to hold the maximum lead temperature at 105°C or less. This assures that during a prolonged fault, the board will not be damaged due to heat.

This circuit can be made to limit the temperature in a hysteretic manner ” switching between 135°C and 95°C indefinitely (auto-retry mode), or it can latch off when the 135°C limit is reached.

CLSA2

A sample of 40 units was tested while shorting the devices for 20 seconds and removing the load for 15. After 10,000 shorting cycles, there were no failures. Since the auto-retry devices were used, the circuit thermal cycled multiple times during the 20 second short, since the power delivered to the FET was approximately 250 watts. This resulted in a total of 800,000,000 thermal cycles among the units tested.

Slow turn-on or turn-off

The size of the initial investment, lack of familiarity with robot technology and past failed attempts are all reasons that people sometimes shy away from using robotics technology. To improve productivity and gain lasting competitive advantage, however, it's important to look beyond misconceptions such as these. While it's true that robotics are not the answer to every single productivity improvement, robots can certainly help in many situations.

Time to market, increased productivity, operating simplicity, flexibility, reusability, dependability, precision, controls capability, and the long term cost of ownership are all strong reasons to use robotics technology.

The second 10

The previous top ten mistakes represent a selection of mistakes that I've seen most frequently in the field. But there are many others that I could have very easily included in the top ten:

Failure to Consider Future Applications for the RobotChoosing a Robot Solely On PriceNot Understanding the Full Capabilities of the Robot before ImplementationNot Fully Utilizing the Robots CapabilitiesBelieving Robotics Are Too ComplicatedBelieving There Is a Perfect Robotics System

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