Charger Technology

Prepair manufactures battery chargers using two types of electronic circuitry
The following is a simple explanation of how these circuits work:

LINEAR

With the linear type of circuitry, the output of the charger is regulated by a series linear regulator, usually a power transistor, which takes in a higher voltage than the battery being charged, and allows through only that amount of power which has been set by the circuit components. A typical 12V constant voltage charger may have between 17 and 22 volts DC going into the regulator, and less than 12 volts going in to the battery when flat. The transistor can be turned on or off or some in-between stage quite simply, and the control is continuously variable.

The difference in voltage multiplied by the charging current in amps will give the total dissipation in the regulator, which is also wasted power which comes out in the form of heat. The linear charger is suitable for smaller output ratings, and was also used a lot in communications work where the very low ripple and noise levels were needed to keep the lines free of background interference.

Disadvantages are:- The efficiency of the circuit is very low, around 25% -35%, and as the power losses are given off in heat, the system is not used for high power chargers. Protection of the regulator in short-circuit conditions can be a problem in high voltage chargers, and where the battery being charged is over-discharged or totally flat.

Advantages are:- Simple circuitry, especially when using 3-terminal regulators such as the National Semiconductor LM3XX series, reliability through low component count, low output noise/ripple and low cost.

THYRISTOR

The silicon controlled rectifier (SCR) or thyristor is a commonly used device for handling large amounts of power. The device is a relation of the transistor but is far more rugged and can control much higher voltages and currents. The one important feature of the thyristor is that once it is turned on it will not turn off again, even if the gate signal is removed, unless the current through the device falls to zero. As this happens at every cycle of AC power, the controlling of the device becomes fairly simple, and is known as phase-angle firing or control.

The thyristor conducts or allows all or part of the sine-wave AC input through on each cycle, the amount let through is controlled by the firing circuitry. The possible angle of conduction is 0 Deg to 180 Deg of the AC sine wave, and in a fully controlled thyristor bridge, the positive and negative output thyristors are fired in alternate pairs. A half controlled bridge has thyristors only on half of the bridge, the others being replaced by diodes which will conduct as long as their position allows a current path in the bridge.

Because the control can only happen on each cycle, it is not continuously variable in the same way as the transistor example above, but in most applications this is not a problem as electrical power is stored in the smoothing choke and reservoir capacitor and smoothes out the pulses of output current from the thyristor(s).

Disadvantages are:- Mainly the weight of the transformer and smoothing choke, although there is greater complexity of the control and firing circuits compared with a linear circuit. The output is pretty much full of pulses and firing 'mush', so the smoothing choke and capacitor(s) are very much required fitting. Control of the output can become difficult if a very low output ripple is required, and various techniques are used to provide a fast response to load changes, which are not helped by the large L-C time constant of the choke and capacitor on the output. Efficiency in the order of 65%.

Advantages are:-Much more rugged devices than the transistor, the thyristors are available with ratings into the thousands of amps at up to 1800V piv. Most electric power control circuits use thyristors in some form or other, and the power ratings ensure that there is a suitable device available for almost any application. Three-phase fully-controlled bridges have much lower levels of ripple and the frequency is higher than a single phase circuit, and most charger circuits above 3 or 4 KW use three-phase circuitry for this reason. Smoothing is much simpler at 300Hz ripple than 100Hz, so the smoothing choke can be simpler, plus the control of the output is better with overlapping firing of the bridge thyristors.

SWITCHMODE

The other main type of circuit in volume use is the Switchmode Power Supply (SMPS). This has gained enormous penetration of power supply and charger markets in the past 15 years, and is now the predominant mode of operation in the telecomms and computer markets. It is light weight in comparison with Linear and Thyristor systems, but it does have problems with circuit complexity and EMI/RFI emissions.

In operation, the SMPS converts the incoming AC supply to high voltage DC. This is then switched at very high frequencies through a transformer which achieves the step-down to low voltage levels and also the isolation from the AC supply. Because the switching frequency is very high, the output is very easily smoothed, and the speed of response of the circuit is many times faster than the thyristor, as the controlling circuitry can change control levels at very short intervals, probably into micro-seconds while the thyristor system has to wait for the next cycle to come around before it can change anything. The fast switching times of the system mean that power dissipation in the form of waste heat is much lower than the other two systems.

The step-down transformer is very light in weight, and is made of Ferrite material. The volume of iron or steel in a transformer goes down with an increase in frequency, and at 100Khz the Ferrite volume is very small indeed, and also very light. A modern 1KW SMPS transformer is probably less than 3" cube and a few ounces, while a thyristor unit would be closer to 10" cube and weigh many pounds, plus the thyristor unit would also need a choke for smoothing, while the SMPS would not.

Disadvantages are:- Complexity, EMI/RFI radiation, MTBF low in comparison with linear and thyristor (although improving fast) fragility of the circuitry and ferrite transformer, high DC voltages used inside the unit.

Advantages are:- Light in weight for the power, small, low heat dissipation, low cost as volumes very high. Efficiency better than 95% for a modern unit, although they tend to get expensive in the low volume high-power market.

SUMMARY

LINEAR systems are good for low noise and ripple, simple, but are heavy and inefficient.

THYRISTOR systems are more rugged than linear, go to much greater power, but are heavy. Efficiency is more than double that of the linear charger.

SWITCHMODE systems are fast, light, cheap and easily fitted, if a bit fragile and complex. Not able to handle high powers unless many units are put in parallel, efficiency is the best of the lot.

Of necessity, these notes are a great simplification of the real-world operations of the chargers mentioned, and for some specialised applications there may be a good reason to use a particular type of charger where another may be more obviously the right choice. We are always prepared to offer technical advice on charging matters where it would assist a customer with selection of the best type of charger, and we do quite often recommend switch-mode systems, even though we do not make them ourselves.

© Prepair Ltd 1995 - 2005 E&OE

Contact us: Phone: +44 (0) 1933 356666 Fax: +44 (0) 1933 355557 Email: prepair@easynet.co.uk


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