Demag Ac 80 Specifications

[Erminia Scharnberg]

Highside switches commonly control industrial loads and are usually known as digital output devices. Industrial digital output loads are generally inductive in nature. The high-side switches are used in applications such as drive control of lamps, controlling relays, and driving actuator loads/solenoids. Turning the loads on or off as fast as possible enables efficient power utilization. However, it also creates unwanted voltage spikes, which is detrimental to the switch and controller if the components are not selected correctly.

The MAX14922 is a simple and effective way to control and drive an external n-channel MOSFET device, reducing component count, yet providing the efficient and reliable performance needed for industrial and safety applications.

Figure 1 shows a simplified application circuit with the MAX14922 connecting the system power input to a typical industrial load. The electromagnetic load operates by consuming power from the input to convert electrical to mechanical energy. The requirements of the application dictate the power handling capabilities of the load, which determines the system operating current.

The application dictates the working voltage and current, which is determined from the load demand of the load/actuator. It is prudent to choose the external FET with the lowest RDSON. This enables efficient power delivery from the source to the load. The total drop across the high-side switch function is:

The n-channel FET's maximum drain-source voltage (VDSS) or VBR must be equal to or greater than the VCLAMP voltage + VDD. The VCLAMP is the negative clamped voltage during surge or load turn-off, which is determined by the TVS clamp voltage. See the sections on inductive demagnetization and surge protection for more information. The MOSFET's threshold voltage (VTH) must be greater than 0.8V and able to handle VGS voltages up to 16V. The normal functional load current and the fault currents in the application must be within the Safe Operating Area (SOA) of the n-channel FET. The SOA is temperature dependent. So, the FET must be temperature derated to the desired operating temperature to ensure the switch is within the nominal operating conditions.

Miswiring errors during installation or maintenance, load wear/tear over time, or sudden damage to the actuator are the typical reasons for load failures. The series sensing resistor RS, MAX14922, and external FET work together to regulate the set current for a period in cases of over-current faults or short circuits. Then the FET is forced off if the over-current scenario persists, only for it to turn back on after the auto-retry time. This way, the switch/power supply and component blocks are protected, and the fault condition is relayed over to the system controller to intervene.

Shorting the tBLANK input to the ground enables a continuous maximum set current regulated output during over-current events if the blanking time and auto-retry mode are not preferred. Care must be taken during such instances as the external FET is forced to be in the resistive or linear mode, which increases the power dissipation. The FET is damaged when the continuous power dissipation is more than the n-channel FET's thermal dissipation specification, hence compromising the application.

The SOA curve of the selected FET, for example for Si7322DN (Figure 2), provides critical information on the maximum drain current it can handle as a function of VDS and with the duration. The drain current here is associated with the maximum RS current limit.

Figure 2 shows that for a given VDS (maximum supply voltage at the load, which is VDD) as the load current increases along the y-axis, its on-time decreases on the second y-axis to the right. This information is used to determine the appropriate CBLANK capacitance, which sets the blanking time and auto-retry function to ensure the FET is kept within the safe operating area. More information about derating the SOA over temperature can be found in the reference link (derating of the MOSFET safe operating area - Toshiba).

The MAX14922 turns the external switch (FET) on or off, which either delivers power or cuts off power to the load. If the load is inductive or connected with long cables that generate inductance along the load line. Abruptly turning off the power generates unwanted back EMFs, where the energy stored at the inductive load must be dissipated safely back to the power return path. External TVS protection is required for inductive load demagnetization at the source of the FET to dissipate the energy safely. The TVS limits the negative voltage excursion at the source terminal to keep the VDS of the FET lower than the VDSS. The energy stored in the load inductance (EL) is defined as:

The load inductance (L) and the effective DC resistance (R) are parameters based on the load. The higher the VCLAMP voltage, the faster the TVS takes (time = tDEMAG) to dissipate the stored energy. The FET's VDSS must be greater or equal to VCLAMP + VDD of the application.

As the high-side switch switching rates are increased, the power dissipation increases proportionally in the clamp. The inductive clamp must be selected to allow dissipation of the power dissipation during the highest switching rates.

The inductor discharge current during demagnetization (Figure 4b) can be approximated to a linear model to calculate the peak and average energy dissipated from the TVS. The area under the triangle (Figure 4b) of discharge current across the clamp for 17ms provides the average energy across the clamp.

The surge level requirements are defined by the end equipment manufacturer, and it is critical the system can handle protection for these levels. Typically, digital outputs (DO) have specified surge limits with a 42? tester source impedance characteristic. Table 1 shows the peak current observed at the source terminal of MAX14922 for specific surge levels.

Figure 5a shows the typical protection for a 24V application. The TVS diode at the S terminal of MAX14922 absorbs the surge energy through it. The level of surge capability of the application is dependent on the choice of the TVS characteristics (clamping current, power rating). The protection diode at the supply input (VDD) protects the supply side of the application.

Figure 5b shows the typical protection for a 60V application. A negative surge transient is dissipated by the TVS diode at the S terminal of MAX14922. The energy of a positive surge conducts through the body diode of the external FET and is absorbed by the TVS protection diode at the supply side (VDD).

It is important to consider the tolerances and temperature coefficient for the series sense resistor RS and the current limit voltage threshold VCL. The variations caused by the tolerance and temperature coefficient add margin on the range of operating current as:

where ITRIP is the maximum current allowed in the application. When the load current exceeds the set current limit, the MAX14922 actively controls the MOSFET VGS to regulate the load current to ITRIP.

The VCL tolerance is 10%, with a sense resistor RS with tolerance of 1% and a 300ppm temperature coefficient. The trip current maximum and minimum limits are calculated as following with 2A as the working current:

The nominal rated load current level must be less than the ITRIP_MIN level to ensure proper function. The temperature coefficient of the sense resistor (TCR) contributes less than 1mO change from 25C to 125C temperature variation for sense resistor values of less than 20mO if it is kept under 300ppm.

TVS diodes with the steady-state derating curve provide information on temperature, which is useful to determine the right type of TVS (in this case SMC33A from Bourns, Inc.). The average power must be within the derated curve of the TVS diode characteristics.

The above curve provides a steady-state power dissipation of the clamp with respect to its lead temperature. The junction to lead thermal resistance and junction to ambient thermal resistance are available for the SMC package. It can be determined from the available information if the clamp used is within safe operating specifications for the entire ambient temperature range.

The junction temperature of the SMC package during steady-state dissipation is 139C with the maximum ambient temperature at +85C. The lead temperature is 128C with the junction temperature determined.

For a clamp, when subjected to demagnetization with a constant 0.5Hz switching cycle, it dissipates a steady-state power of 0.72W from the clamp and is within the derated curve of Figure 6 at maximum operated temperatures.

Clamp consideration with lower switching rates: When the load is seldom switched (once per hour or day), the peak power dissipation for a non-repetitive pulse vs. pulse duration aids in selecting the clamp. The steady-state derated curve is used when the switching is repetitive, more often it is a worse case design target as compared to the reduced/ lower rate of load switching (Including a Zener diode like the SMCJ60A at the VDD input supports the surge current requirements. Use a fast recovery epitaxial diode to avoid TVS clamp turn-on during the +60V normal operation. Figure 5b shows typical surge events and clamping action of the application. Figure 5b shows the current path in the event of a surge and the clamping action of the TVS diode.

Before this Court are the parties' cross-motions for partial summary judgment pursuant to Federal Rule of Civil Procedure 56. Plaintiff filed this action on October 13, 1988, alleging that defendant has infringed its patent for "snubbers" in violation of 35 U.S.C. 271, and engaged in unfair competition. Plaintiff seeks a permanent injunction and damages. Defendant has asserted counterclaims for a declaratory judgment of invalidity and non-infringement, libel and/or slander per se, and tortious interference with contract. This Court has jurisdiction of the subject matter of this action under 28 U.S.C. 1331, 1332, and 1338.

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