Multiple conversion redundant power supply system with current limiting

The Advanced Telecom Computing Architecture (ATCA) standard provides a modular approach to designing telecom equipment. The adoption of this industry standard not only speeds up product design but also simplifies field upgrades. In the ATCA architecture, each Carrier Blade device includes up to 8 AMC modules, all of which require hot-swap protection. The carrier board provides two main power supplies for each AMC module: a 3.3V management power supply and a 12V effective load power supply. To help designers meet these requirements, we have designed a full-featured dual-slot AdvancedMC™ controller, the TPS2359, to provide all the protection and monitoring circuits necessary to support two AMC modules. The controller fully integrates management power surge control, overcurrent protection, and FET ORing functions. Adding two external power transistors can provide all of these same functions for each effective load power channel. Figure 1 shows a simplified block diagram of a dual-channel AMC application, with both channels powered by the same power supply. Independent power supplies can also be used to power the two channels.

Figure 1 Simplified block diagram of an application using the TPS2359 dual-channel controller to power two AdvancedMCs This unique controller integrates the active load power and the precise current limit functions of the management power channels. The active load power current limit uses three external resistors per channel to meet the 8.25A +/– 10% ATCA specification. The management power current limit uses one external resistor per channel to meet the 195mA +/– 15% specification.

High Accuracy Current Limit

A simplified block diagram of the controller's active load current limiting circuit is shown in Figure 2. Amplifier A1 monitors the load current ILOAD by sensing the voltage across a sense resistor. A similar circuit is used to manage the power channel, except that resistors RSENSE and RSET are integrated.

Figure 2 Schematic diagram of the effective load supply current limit circuit The current limit circuit includes two amplifiers: A1 and A2. Amplifier A1 makes the voltage across the external resistor RSET equal to the voltage across the external resistor RSENSE. The current flowing through RSET also flows through the external resistor RSUM, generating a voltage at the SUMA pin as shown below:

Amplifier A2 senses the voltage on the SUMA pin. As long as the voltage remains below 675mV, the amplifier sources 30µA of current to PASSA. When the voltage on the SUMA pin exceeds 675mV, amplifier A2 starts to sink current from PASSA. The gate-source voltage of the pass FET MPASS decreases until the voltages on the two inputs of amplifier A2 are balanced, and the current flowing through the channel is equal to:

To set a current limit of 8.25A as required by the MicroTCA™ specification, RSET = 417 is chosen. The latest EIA standard 1% resistor is 422. This resistor allows the system to power an 80W AMC module. Both the active load and management channels have their own programmable default timers. These timers are turned on whenever the respective channel enters current limit. If a channel is in current limit for too long and the default timer times out, the channel turns the pass FET off and reports an error condition. The management channel incorporates an Over Temperature Shutdown (OTSD) feature. If the management channel remains in current limit long enough due to the die temperature near the internal on-resistance exceeding approximately 140C, the OTSD feature is tripped. Once this occurs, the management channel operating in current limit is turned off. This feature prevents damage to the internal pass transistor from a fault that lasts too long. The current limit feedback loop has a specific response time. Severe faults such as shorted loads require a fast response to avoid damage to the pass FET or voltage rail voltage sag. The TPS2359 includes a fast-trip comparator that turns off the channel under these conditions (although this is not discussed in this article). In addition, the controller supports ORing using an additional blocking FET. This feature blocks reverse conduction when the output-input differential voltage exceeds 3mV.

Multiple conversion redundancy

ATCA systems often require redundant parallel power supplies. The MicroTCA specification advocates a redundancy technique that requires a microcontroller to independently limit the current of each power supply. The current drawn by the load cannot exceed the sum of the individual power supply current limits. An alternative operating mode is available, multi-conversion redundancy. It limits the load current to a fixed value regardless of the number of active power supplies. Removing or inserting power supplies in a multi-conversion system does not affect the load current limit. This technique does not require a microcontroller, making it simpler and faster than the redundancy method described in the MicroTCA standard above. This is an attractive approach for AMC applications that do not require full compliance with the MicroTCA power module standard.

Figure 3 Active Load Supply Current Limiting Application Using Multi-Conversion To implement multi-conversion redundancy, connect the SUM pins of the redundant channels together and tie an RSUM resistor from that node to ground. Unlike the MicroTCA redundant structure (where each power supply has its own resistor RSUM), RSUM needs to reside on the backplane of the multi-conversion structure. Figure 3 shows an application of the active load supply using the multi-conversion function. The current limit threshold will now apply to the sum of the currents provided by the redundant supplies. When implementing multi-conversion redundancy on the active load supply channels, all channels must use the same RSENSE and RSET values.

in conclusion

ATCA is the first open standard to address the power requirements of telecommunications equipment. For ATCA, the power management challenges faced by system designers include limited current limiting, high-availability redundant power supplies, hot-swap requirements, fault protection, and complex status monitoring. These issues are solved after the TPS2359 implements high-precision current limiting circuits, unique multi-conversion features, and integration of all necessary protection and monitoring circuits in a compact 36-pin QFN package. The TPS2358 has all the same functionality, but occupies the area of ​​a 48-pin QFN package, which supports designs that use external control and indication (rather than the I2C™ interface).