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General power supply design information

    General information

    A power supply is a device for the conversion of available power of one set of characteristics to another set of characteristics to meet specified requirements. Typical application of power supplies include to convert raw input power to a controlled or stabilized voltage and/or current for the operation of electronic equipment. Power supplies belong to the field of power electronics, the use of electronics for the control and conversion of electrical power.The simplest unegulated power supply consists of three parts: the transformer, the rectifiers, and the capacitors. This kind of power supply is simple, but the output voltage is not verystable (there can be noticable ripple with the output, and the output voltagechanges with load changed and mains voltage changes).The output voltage can be made more stable by using a more complicatedregulated power supply.The regulated power supply technology can really be divided into two distinct forms; firstly, the linear or series regulator and, secondly, the switched-mode conversion technique. Switched-mode technology is multi-facetted with a wide variety of topologies achieving the end result of providing a regulated DC voltage. The main differences between the linear and switched-mode regulator are in the size, weight and efficiency. The linear regulator utilises simple techniques of controlled energy dissipation to achieve a regulated output voltage independent of line and load variation. It is, therefore, inherently inefficient, especially when a wide input voltage range has to be catered for. Sometimesa power supply is a buffer circuit that provides power with the characteristics required by the load from a primary power source with characteristics incompatible with the load. It makes the load compatible with its power source.A power supply is sometimes called a power converter and the process is called power conversion. It is also sometimes called a power conditioner and the process is called power conditioning. Power supplies should be well designed and protected.Power supply failures can be frustrating, expensive, and time-consuming events because many power supplies are highly complex circuits, with many components operating near the edge of their envelope, and when they fail, they tend to destroy most of the failure evidence with them.There are many different voltage you can encounter in electronics systems and different applications. This list lists some of most commonly used DC voltages you might encounter:

    • 1.5 V: This is the voltage you get from one battery cell.
    • 3 V: This is a voltage you get from two normal battery cells. This voltage is usually used in small devices that operate at two AA batteries.
    • 3.3 V: This is the voltage used in many modern logic IC designs. This voltage is used in the most modern computer digital electronics.
    • 4.5 V: This is the voltage you get from three battery cells in series.
    • 5 V: This is the voltage used in TTL logic ICs. This voltage is found inside most computers.
    • 6.0 V: This is the voltage you get from four battery cells in series. Many small electronic battery operated appliances operate at this voltage.
    • 9 V: This a voltage used in many small electronics applicances, for example majority of multimeters and other small handled test instruments. The power for this kind of devices is normally taken from 9V battery. Also very many small "electronics gadgets" are powered from 9V DC "wall warts".
    • 12 V: This is the voltage used in typical car electrical and boat system. This is also used inside computer and such for powering high power loads like disk drives. Also very many small "electronics gadgets" are powered from 12V DC "wall warts".
    • 15 V: Bipolar +-15V power supply is a very common voltage used in audio electronics circuits that use operational amplifiers.
    • 24 V: This is the voltage used in truck electrical system and for powering industrial automation systems. In industrial autiomation systems devices like relay coils, process controllers, PLCs and current loop interfaces are powered with this voltage. Some distributed power system use this voltage as the distributed voltage.
    • 36 V: This voltage is used by some battery powered small moving devices. The power is taken from three series connected 12V batteries. 36V battery systems are also coming to new cars.
    • 48 V: This is the classical telecom world woltage. Battery backed up -48V (negative compared to ground) powers the telephone exhanges and other telephone network devices. This is also the voltage supplied to a normal telephone line by the telephone central. +48V is used in audio world as the standard voltage for phantom power supply that is supplied by many professional audio mixers to power microphones and DI-boxes through mixer microphone wiring.
    • 60 V: This is the highest "safe" DC voltage (this is the Safety Extra Low Voltage limit). A system/wiring that carries anything higher than is considered to carry "dangrous voltage" and should be constructed in very safe ways (like mains voltage wiring). Some telecom systems use 60V voltage (60V is quite rarely used, 48V is far more common in telecom world).
    • 90 V: This is a voltage used in telecom world in applications where quite a bit of power needs bo be transpowerted through long thin wires (through telephone loop). This voltage is used by some telecom signal amplifier, repeaters and some line powered ISDN system components.
    • 120 V: This is used in some automation systems for controlling heavier loads that can be controlled with 24V DC.
    • 320 V: This is the typical average voltage (can vary somewhat) found in mains powered switched mode power supply after the incoming 230V mains voltage is rectified and filtered (many switchable 230/110V AC input switchers multiply the incoming voltage by two when switched to 110V mode to get this 320V voltage as result).
    You do not usually encoutner much higher voltages than that in typical electronics device.

      Power distribution in circuit boards and systems

      Power distribution from the power source to the place where it is consumed is a challenging task in modern electronics devices which take low voltage (3.3V or 1.8V) and high currents (easily up to tens of ampreres). Distributing high current and low voltages efficiently for long distances is not a good idea. The losses in wiring will be too much. For this reason a distributed power distribution is becoming popular. The idea on this system is that one major power supply generates some intermediate voltage (typically 12 to 40 volts DC) which is distributed around the system. The lower voltages are generated from that power source using local DC/DC converters. The benefir of this approach is that the losses in the power distribution are minimized (higher voltage means less current needed which means less losses in wiring) and it is easy to get stable voltage to output even when there are losses (every local DC/DC converter regulates the outout voltage, so the input voltage for the converter cna vary somewhat with no effect on the DC/DC converter output voltage). In applications where higher voltages are needed, those can also generated with a DC/DC step-up converter. Most typical intermediate voltage system in use are propably 12V DC system used for car electronics and 24V DC system used in industrial electronics. Telecommunication industry has for long time used 48V or 60V battery backed up DC supplies for telecommunication equipment. 12V voltage is becoming popular also in telecommunication systems as the intermediate voltage within the circuit boards. Most computer systems use one of two dc-distribution voltages: 48V and 12V. Those voltages are isolated from the AC line, and both are reasonably well-regulated (typically +-5%). The breakpoint between 12V and 12V usually falls at 1000-1500W power range. Below the breakpoint, 12V dominates. Above the breakpoint, 48V dominates. A few large, high-end systems distribute 400V DC. In general telecom equipment use unregulated 48V power supply (typically 36 to 75V) or 24V (18 to 36V). Typical power levels are 50-100W per card. In telecom applications it is common to use "brick" modules to generate 3.3V or 5V for distribution within the card. Local regulators then produce lover voltages as needed (for example 2.5V is becoming popular). High power telecom cards can use a 12V intermediate bus. Manufacturers have also discussed on 7V or 8V intermediate-bus voltages.

      Mains power connection

      The mains voltage is alternating voltage which can vary from country to country somewhat. For example USA uses 110-120V AC power (plus minus some tolerance) at 60 Hz frequency. Europe uses 230V AC +/- 10% at 50 Hz frequency. One must design the power supply so that the nominal device will accept a wider band. Then one must control the manufacturing spreads so that the limits of no device enter the band, or one must weed out bad ones in testing forrejection or rework, or one must have plans for dealing with the after-effects of non-compliant devices entering the marketplace.

      The mains power supplies in practically all equipment are very well insulated from the casing (there are very few exception to this). In Europe all mainspowered equipment has to comply with the Low Voltage Directive which has minimum mains to casing insulation resistances. USA has it's own regulations on this insulation.

      For example VCRs are low voltage machines and their circuits are supplied via a linearpower supply with a big mains transformer or a switched mode power supply that isolates from the mains. The metal case is floating.For example a typical PC is a low voltage machine which is supplied with a switched mode power supply that isolates the output from the mains input.

      The PC metal case is connected to themains ground with a ground connector on the mains cable and ground connector on the mains input connector. In PC the output 0V is connected to the PC ground (thus to mains ground).

      No isolation is perfect. The isolation must be good enough to be safe (high enough reistance and withstands enough voltage rating). The demans vary somewhat from country to country and equipment type to another. Testing the quality of mains isolation is a necessary test for making sure that the equipment operating safely (mains does not leak too much to the case). One way to test the isolation quality is to use an insulation tester, which supplies few hundred volts to the main input and measures the resistance to the case at that voltage. Many equipment manufacturers make usualy also a hi-volt testing (test voltage usually in 1.5-2 kV range AC or DC voltage).

      One way to check that the insulation in equipment is OK is to perform a "leakage test". Usea 1.5 K, 10 watt resistor with a .15 mfd 150 volt cap in parallel with theresistor. Take an AC voltmeter with at least 5000 ohms-per-volt sensitivity& measure the voltage drop across the resistorcap network. With one end ofthe network at earth ground and the other end attached to all exposed metalparts the voltage shouldn't exceed .3 volts RMS. This is the industry standard test in the U.S.

      Power supplies typically have a high inrush current when they are connected to mains power. This inrush current can be seen on both traditional transformer based power supplies and switch mode power supplies. In ac/dc power converters above a few watts, a large inrush current flows when the input capacitors are suddenly charged during the initial application of power. In traditional linear power supplies the initial surge is caused by both the magnetization of the transformer core (usually takes one mains half cycle) and then the main capacitor charging time (time depends on cpacitor size and transformer current output capability). The initial surge can be very high on some toroidal transformer. In switch mode power supplies the initial power surge (that can be a very large on some cases) is caused by the mains voltage power storage capacitors getting charged quicly. Universal switch mode power supplies are particularly subject to high inrush current since their input capacitors must be large enough to handle line voltages as low as 110 Vac, as well as voltages as high as 240 Vac at turn-on. Designers of commercial, industrial, and medical systems need to pay special attention to inrush current. High inrush current can damage power supply sooner or later and can also cause circuit breaker to trip.

      It quite possible to get an inrush-current spike of 50 A or more on a nominal 120-Vac line (170 V peak). In countries where the nominal line voltage is 240 Vac, the inrush current can exceed 100 A. This large inrush current degrades the performance and lifetime of the power supply in a number of ways: sparking of the switch contacts, thermally overstress the input rectifiers, stressing the capacitors, slowly degrade the fuse, etc..

      Power-supply manufacturers use one of several inrush-current-limiting techniques to avoid these problems:

      • For very small power supplies (a few watts at most) adding a resistor in series with the line is a simple and practical solution to limit the inrush current, but will cause loss in efficiency.
      • Many power-supply manufacturers use a negative temperature coefficient (NTC) resistor in series with the line. An NTC resistor offers tens of ohms of resistance when cool, dropping to less than one ohm as its temperature increases. If the power supply is cool when turned on, the NTC provides good inrush-current limiting (but not when user turns the system off and then immediately switches it back on).
      • Placing a relay or electronic switch in parallel with either a resistor or NTC can offer high impedance only at startup. This kind of circuit must be designed in such way that the resistor does not burn up during brown-outs.
      • A control circuit can use a zero-crossing techniques to monitor the ac line and turn on the power supply only when the input line is low.
      • Many larger power switch mode supplies use an active power factor correction circuit in front of the main switcher circuit. Some active power factor correction circutis can be designed in such way that they provide a soft-start.

      Higher inrush-current specifications equate to greater stress on the rectifier and lower reliability.

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