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Thursday, February 19, 2009

Off Line-UPS Offers (100 -5000 Watts)

With increased dependency on electric power for various domestic, commercial purposes and the seemingly declining capacity of power utilities in many countries, the need for additional backup power sources is on the rise. Various modules are already available to address these different needs. However, most modules are too expensive, too bulky, or too rigid in their power capacity, capability, and flexibility.

The circuit described here is an off-line uninterruptible power supply. It has an expandable power stage design that can be easily modified for use with power ranges from as low as 100 W to as high as 5000 W with forced cooling.

The design is based on the LM3524D, a popular industrial-grade, pulse-width-modulation (PWM) controller. This device is fully self-contained and has all of the necessary logic built-in to ensure a compact and cost-effective product. The controller offers:

  • Complementary power drive output stage with 180° out-of-phase switching.
  • Stable oscillator with shutdown capability and noise suppression.
  • Current-sense and voltage-sense circuitry.
  • Dead time between the two complementary switching stages to prevent cross conduction and subsequent core saturation.
  • Low voltage cutoff.

The on-chip oscillator frequency is controlled by the RC pair R2 and C1, and the required frequency is calculated as fOSC = 1/(R2 × C1). Depending on the country, fOSC = 50 Hz or 60 Hz. The low-power drive stage, containing transistors Q1 and Q2, is driven by the outputs at pin 12 and 13 of the controller; out-of-phase switching is handled by its own internal logic.

The final switching stage is comprised of transistors Q5 and Q6 wired across the main step-up transformer. These transistors are switched on in tandem to drive current through the two halves of the primary winding of transformer T2, independently.

Power diodes D2 and D3 are included to protect the power-stage transistors against reverse currents. Switching pulses from the drive transistors are transferred to the power stage via transistors Q3 and Q4, which serve as current amplifiers.

Resistor R10 is used to feed back the current to the current-sense monitor via pin 5. The current-sense logic monitors this feedback and triggers shutoff in event of either overload or power-stage failure.

The value of R10 is calculated as IMAX = 200 mV/R10, where IMAX is proportional to the power rating of the load, and 200 mV is the minimum required potential drop across R10 to trigger a shutdown by the current-sense logic.

The transformer T1 is connected to the standard mains supply and the rectified dc is fed to the shutdown pin (pin 10). This pin serves to turn off the LM3524 during the presence of the standard mains supply. If the mains supply fails, the bias on the shutdown pin is removed, thus starting the oscillator and hence the drive stages.

The power stages and transformer T2 must be appropriately rated for the required output power. The power transistors (Q3, Q4, Q5, and Q6) can be paralleled with similar devices for increased power-handling capacity. In addition, forced cooling can be used to achieve an even higher output power rating. However, for low- or medium-output power ratings, these transistors must be mounted on a large heat sink. Transformer T2 is a standard 12 V: 0:12 V to 230-240 V step-up winding, while T1 is a small 230-V to 12-V step-down transformer rated for 50 mA on the secondary windings. Additional filter and shaping circuitry can be added on the secondary winding of T2 to obtain a near sine-wave output. The RC network comprising R3 and C2 is included for compensation.

Thursday, February 12, 2009

13.8V 40A Switching Power Supply By LM3524,LM324

This article was originally published (in a slightly modified form) in the QST magazine, December 1998 and January 1999, and in the Radio Amateur's Handbook, 1999. Visit the American Radio Relay League for information on these publications, and a world of ham radio related things!
Design decisions
There are several different topologies for switchers in common use, and the first decision a designer must take is which of them to consider. Among the factors affecting the decision are the power level, the number of outputs needed, the range of input voltage to be accepted, the desired tradeoff between complexity, quality and cost, and many more. For this power supply I decided to use the half bridge forward converter design. This topology connects the power transformer to a bridge formed by two power transistors and two capacitors. It is reasonably simple, puts relatively low stress on the power transistors, and makes efficient use of the transformer's magnetic capabilities.The second basic decision is which switching frequency to use. The present trend is to use ever higher frequencies. But by doing so it becomes more difficult to filter out the RF noise inevitably generated by the switching. So I decided to stay at a low switching frequency of only 25 kHz for the full cycle, which due to the frequency doubling effect of the rectifiers results in 50 kHz on the output filter.

For the main switching elements, bipolar transistors or MOSFETs can be used. Bipolars have lower conduction losses, while MOSFETs switch faster. As in this design I wanted to keep the RF noise at an absolute minimum, very fast switching was not desired, so I used bipolar transistors. But these tend to become too slow if the driving is heavier than necessary. So, if the transistors have to switch at varying current levels, the drive to them must also be varied. This is called proportional driving, and is used in this project.

The half bridge converter is best controlled by pulse width modulation. There are several ICs available for this exact purpose. I chose the 3524, which is very simple to use and easy to find. Any 3524 will do the job. It can be an LM3524, SG3524, etc.

This basically ends the big decisions. From now on, designing the circuit is a matter of calculating proper values for everything.

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Wednesday, February 4, 2009

High Current Regulated Supply By LM317

The high current regulator below uses an additional winding or a separate transformer to supply power for the LM317 regulator so that the pass transistors can operate closer to saturation and improve efficiency. For good efficiency the voltage at the collectors of the two parallel 2N3055 pass transistors should be close to the output voltage. The LM317 requires a couple extra volts on the input side, plus the emitter/base drop of the 3055s, plus whatever is lost across the (0.1 ohm) equalizing resistors (1volt at 10 amps), so a separate transformer and rectifier/filter circuit is used that is a few volts higher than the output voltage.

The LM317 will provide over 1 amp of current to drive the bases of the pass transistors and assuming a gain of 10 the combination should deliver 15 amps or more. The LM317 always operates with a voltage difference of 1.2 between the output terminal and adjustment terminal and requires a minimum load of 10mA, so a 75 ohm resistor was chosen which will draw (1.2/75 = 16mA). This same current flows through the emitter resistor of the 2N3904 which produces about a 1 volt drop across the 62 ohm resistor and 1.7 volts at the base. The output voltage is set with the voltage divider (1K/560) so that 1.7 volts is applied to the 3904 base when the output is 5 volts. For 13 volt operation, the 1K resistor could be adjusted to around 3.6K. The regulator has no output short circuit protection so the output probably should be fused.