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Posted on 10 July 2019

Power Modules in High-End Market Segments

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A low output ripple is mandatory as well when supplying analogue systems

The new SIMPLE SWITCHER® Power Modules from National Semiconductor reach the highest possible level of “Ease of Use” by integrating an inductor and a monolithic synchronous regulator within one power package. Professional power designers have far more requirements than “Ease of Use” related to design efforts, including: high ambient temperatures, requiring guaranteed performance and electrical specs; electrical emission (EMI) within international standards, reliable soldering; and vibration robustness.

By Ralf Regenhold, Technical Marketing Manager Power Management Europe, National Semiconductor

 

This article describes how these high performance targets can be achieved in the following market segments: Industrial, medical, avionics and military, broadcast video and communication infrastructure. All the market segments mentioned above are targeting highly reliable applications. The requirements for the power supplies are similar. The following sections focus on performance aspects which are specifically critical in those segments but are actually applicable for all high-end systems.

Industrial Market

Power supplies for industrial applications use nominal input voltages of 24V but require full functionality during transients up to 36V or 42V. National’s new power modules are designed to support full operation up to those voltages. The loads often require several different voltages from 5V down to 0.8V. The best approach to find the optimum solution for such requirements is to use a two stage converter. National Semiconductor’s WEBENCH® Power Architect enables the analysis of the total system efficiency using different interim bus voltages. Several combinations of regulators can be compared based on total efficiency, footprint size and solution cost. The initial list of combinations is limited and shows the best compromise between cost and efficiency. Additional solutions are offered if the optimisation tuning dial is moved.

A specific solution is chosen, using only one regulator to generate the 5V bus voltage. All second stage regulators work from the 5V, generating several point of load voltages. In a project optimisation step alternative devices for the low load currents are selected in order to reduce the solution cost. The final design is displayed in Fig.1 (Block Diagram). A total efficiency of 82% is achieved.

 Block Diagram

Industrial applications need to work up to an ambient temperature of 85°C without an external heatsink or fan. This capability of National’s power modules is demonstrated at the LMZ14203 in the first stage of the design and represents a very common requirement. Vin=24V, Vout=5V, Iout=2.52A.

Under these conditions the LMZ14203 can perform with 87.7% of efficiency. This generates a power loss of PIC=1.75W. The package of the power module has been optimised for very low thermal resistance and evenly distributed heat flow on the PCB by using a solid exposed pad on the back of the module (see Fig.2).

Heat flow on PCB

With the thermal resistance ΘJA=19.3°C/W the rise of the junction temperature can be calculated as ΔTJ= ΘJA x PIC = 33.8°C. This means that at an ambient temperature of 85°C the junction reaches a maximum temperature of TJ operating =119°C. This is fully within the maximum rating of TJ max =125°C specified in the datasheet. The actual distribution of the heat on a realistic PCB can be simulated with WEBENCH ”Thermal simulations”, see Fig.3 (Thermal Image). During the selection phase, the module datasheets help the designer to estimate the maximum output current versus ambient temperature by providing derating curves. Only modules designed for the industrial market offer full load current at ambient temperatures of 85°C and higher, see Fig.4 (Derating Curves). The LMZ10504 is capable of supplying full load of 4A at up to 106°C ambient temperature at Vin=5V and Vout=3.3V.

Thermal Image

Electro Static Discharge (ESD), burst and surge, current injection requirements according to IEC 61132 PLC are not discussed here. The design of the power supply can be done independently of the protection circuitry needed for the system ruggedness towards above mentioned tests.

LMZ10504 Power Derating Curves

Medical and Telecommunication Markets

Applications in these segments require a high system integrity. Sensitive signal path electronics need to be immune to Electromagnetic Emission (EMI) from the power supply. Linear regulators are usually used to supply analogue amplifiers and A/D converters. In addition to this, higher power for digital processing units is also required. Only switching regulators are able to support these as high efficiency and low heat generation are mandatory.

National’s Power Modules are specifically designed for minimum possible emission by the integration of all switching power elements and the fully shielded inductor. Fig.5 shows the critical paths. Switching regulators have paths with high dI/dt switched currents. These act like an antenna and cause high electromagnetic emissions so they must be kept as small as possible.

Sensitive Noise Paths

They can be identified by two steps:
1. Draw the current loop during ON (Loop 1) and OFF-Time (Loop 2). Note that both loops on their own are NOT the critical paths and are NOT the ones to be minimised.
2. The branches of Loop 1 and Loop 2 which do NOT overlap contain the critical high di/dt current. Branches that carry currents during both cycles are considered as DC currents (with a small ripple on top) and do not create EMI. So only the area indicated in yellow as high dI/dt must be minimised for lowest radiation. Therefore the input capacitor (Cin1) is placed as close as possible to the LMZ10504 VIN pin and GND exposed pad.

The switching node area is critical as well and is minimised by the integration of the inductor in the regulator package. With all these design optimisations the complete family of National Semiconductor Power Modules can guarantee within the datasheets the compliance to the international norm CISPR22 (Class B) and EN55022, see Fig.6 (EMI graph LMZ10504 radiated emission).

LMZ10504 Radiated Emission

Avionics and Military Markets

These market segments gain the most advantage from the integration of the regulator, the inductor and even capacitors at critical nodes. Device and system failures are reduced to a minimum. In addition this offers the best thermal protection against over-temperature of each Point-of-Load regulator. The RoHS compliant TO263 package has been designed to have standard leads and a single exposed pad in order to guarantee high vibration ruggedness. Certifications of compliance to international standards are in progress.

High efficiency is a key for low heat generation and reliability and can only be achieved by synchronous regulators. Nevertheless, efficiency is heavily dependent of the difference between Vin and Vout. National’s Power Modules reach efficiencies of 90% and above.

Examples:
LMZ10504 Vin=5V and Vout=3.3V Iout=3A Efficiency(η)=95%
LMZ10504 Vin=5V and Vout=1.8V Iout=2A Efficiency(η)=93%
LMZ14203 Vin=24V and Vout=5.0V Iout=1.5A Efficiency(η)=90%

Fast transient response is needed to supply high end digital processing units like FPGAs or DSPs as they can unpredictably change their processing mode. This requires special care of the regulation loop inside the module. Fig.7 (LMZ10504 Transient Response) shows the deviation of the 2.5V output during a huge load transient from 0.4A to 3.6A.

LMZ10504 Transient Response

A low output ripple is mandatory as well when supplying analogue systems. All National Semiconductor Power Modules work stably with ceramic as well as low ESR electrolytic or tantalum output capacitors.

The high switching frequency of up to 1MHz and the low capacitor ESR leads to a very low output ripple in the range of only a few mV - see Fig.8 (LMZ10504 Output Voltage Ripple).

LMZ10504 Output Voltage Ripple

Summary

Several features of the new SIMPLE SWITCHER Power Modules from National Semiconductor address specific high performance markets. The company is in the process of releasing more voltage and power options of the modules including additional features that will significantly add more design flexibility.

 

 

 

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