Power Management Basics: Power Supply Characteristics

Jun 11, 2009 3:57 PM

A power supply’s characteristics influence the design of a power management subsystem. Two major characteristics are efficiency and performance over its specified temperature range, which may require cooling. Also, there are important characteristics that protect the power supply and its load from damage, such as overcurrent, overtemperature, and overvoltage, etc. Then, there are operating parameters that describe a power supply’s performance, like drift, dynamic response, line regulation, load regulation, etc.

Efficiency determines the thermal and electrical losses in the system, as well as the amount of cooling required. Also, it impacts the physical package sizes of both the power supply and the final end-item system. Plus, it affects the operating temperatures of system components and the resultant system reliability. These factors contribute to the determination of the total system cost, both hardware and field support. Power supply data sheets usually include a plot of efficiency vs. output current, as shown in Figure 2-1. This plot shows that efficiency varies with the power supply’s applied voltage as well as the output load current.

Efficiency, reliability, and operating temperature are inter-related. Power supply data sheets usually include specific airflow and heat sink requirements. For example, the ambient operating temperature affects the output load current that the power supply can handle reliably. Derating curves for the power supply (Figure 2-2) indicate its reliable operating current vs. temperature. Figure 2-2 shows how much current the supply can be safely handle if it is operating with natural convection, or 200 LFM and 400 LFM.

Protecting the Supply

There are several other characteristics that impact power supply operation. Among these are those employed to protect the supply, which are listed below.

Overcurrent: A failure mode caused by output load current that is greater than specified. It is limited by the maximum current capability of the power supply and controlled by internal protection circuits. It can also damage the power supply in some cases. Short circuits between the power supply output and ground can create currents within the system that are limited only by the maximum current capability and internal impedance of the power supply. Without limiting, this high current can cause overheating and damage the power supply as well as the load and its interconnects (p.c. board traces, cables). Therefore, most power supplies should have current limiting (overcurrent protection) that activates if the output current exceeds a specified maximum.

Overtemperature: A temperature that is above the power supply’s specified value must be prevented or it can cause power supply failure. Excessive operating temperature can damage a power supply and the circuits connected to it. Therefore, many supplies employ a temperature sensor and associated circuits to disable the supply if its operating temperature exceeds a specific value. In particular, semiconductors used in the supply are vulnerable to temperatures beyond their specified limits. Many supplies include overtemperature protection that turns off the supply if the temperature exceeds the specified limit.

Overvoltage: This failure mode occurs if the output voltage goes above the specified dc value, which can impose excessive dc voltage that damages the load circuits. Typically, electronic system loads can withstand up to 20% overvoltage without incurring any permanent damage. If this is a consideration, select a supply that minimizes this risk. Many supplies include overvoltage protection that turns the supply off if the output voltage exceeds a specified amount. Another approach is a crowbar zener diode that conducts enough current at the overvoltage threshold so that it activates the power supply current limiting and it shuts down.

Soft Start: Inrush current limitation may be needed when power is first applied or when new boards are hot plugged. Typically, this is achieved by a soft-start circuit that slows the initial rise of current and then allows normal operation. If left untreated the inrush current can generate a high peak charging current that impacts the output voltage. If this is an important consideration, select a supply with this feature.

Undervoltage Lockout: Known as UVLO, it turns the supply on when it reaches a high enough input voltage and turns off the supply if the input voltage falls below a certain value. This feature is used for supplies operating from utility power as well as battery power. When operated from battery-based power UVLO disables the power supply (as well as the system) if the battery discharges so much that it drops supply’s input voltage too low to permit reliable operation.

Power Factor Correction (PFC): Applicable only to ac-dc power supplies. The relationship between the ac power line voltage and current is called power factor. For a purely resistive load on the power line, voltage and current are in phase and the power factor is 1.0. However, when an ac-dc power supply is placed on the power line, the voltage-current phase difference increases and power factor decreases because the process of rectifying and filtering the ac input upsets the relationship between voltage and current on the power line. When this occurs it reduces power supply efficiency and generates harmonics that can cause problems for other systems connected to the same power line. Power factor correction (PFC) circuits modify the relationship between power line voltage and current, by making them closer to being in phase. This improves the power factor, reduces the harmonics and improves the power supply’s efficiency. If power line harmonics are important, choose a supply with PFC that has a power factor of 0.9 or higher.

Electromagnetic Compatibility (EMC)

Manufactured power supplies must employ design techniques that provide electromagnetic compatibility (EMC) by minimizing electromagnetic interference (EMI). In switch-mode power supplies, a dc voltage is converted to a chopped or a pulsed waveform. This causes the power supply to generate narrow-band noise (EMI) at the fundamental of the switching frequency and its associated harmonics. To contain the noise, manufacturers must minimize radiated or conducted emissions.

Power supply manufacturers minimize EMI radiation by enclosing the supply in a metal box or spray coating the case with a metallic material. Manufacturers also need to pay attention to the internal layout of the supply and the wiring that goes in and out of the supply, which can generate noise.

Most of the conducted interference on the power line is the result of the main switching transistor or output rectifiers. With power factor correction and proper transformer design, connection of the heat sink, and filter design, the power supply manufacturer can reduce conducted interference so that the supply can achieve EMI regulatory agency approvals without incurring excessive filter cost. Always check to see that the power supply manufacturer meets the requirement of the regulatory EMI standards.

Regulatory Standards

Compliance with national or international standards is usually required by individual nations. Different nations can require compliance with different standards. These standards attempt to standardize product’s EMC performance with respect to EMI. Among the regulatory standards are:

• Electromagnetic disturbance characteristics - Limits and methods of measurement.
• Electromagnetic compatibility - Requirements for household appliances
• Radio disturbance characteristics - Limits and methods of measurement for the protection of receivers except those installed in the vehicle/boat/device itself or in adjacent vehicles/boats/devices.
• Specification for radio disturbance and immunity measurement apparatus and methods

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