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designideas Figure 3. Output impedance and ripple rejection current is invariable, eliminating therefore the RHP zero. The double-current-law ensures the circuit is never starved dynamically. Note that most of the time, this circuit will not be required. Implementation notes The maximum ripple rejection capacity is set by T1’s ratio: n = 200/(% ripple P-P). This in turn sets the ratio of R2||R3 to R5. The transformer’s magnetising inductance must be large enough to allow the amplifier to develop its full swing. This mandates Lm > (n•VIN) / (2πf•IOUT). It is advisable to provide some margin with respect to these values, in particular the magnetising inductance Lm, which should be more than twice the minimum. Performance Figure 3 shows the rejection and output impedance curves. Rejection is greater than 40 dB for the pertinent frequency range, with a maximum at 100 Hz exceeding 46 dB. The output impedance too is impressive: the gain of the amplifier helps not only the ripple rejection, but it also actively reduces the output impedance. The only losses are caused by R5 and the resistance of the transformer’s secondary. They may not be zero, but they are so low that the circuit has a negative drop-out for almost 50% of the time! This remarkable performance only requires a moderately sized transformer. Let us take an unfavourable example: a 50V/5A supply having up to 10% ripple. The secondary/core must have a high enough V•s product to accommodate about 2 VRMS ripple. With the 5A current, this results in a 10 VA rating. But since the ripple is at twice the mains frequency, this means that 5 VA is in fact sufficient for this heavily rippled 250W supply. About the author Louis Vlemincq started electronics as a young teenager, first as a hobby, then as a regular job after graduating. He has touched to almost every field of electronics: audio and video design and maintenance, automotive, industrial control, lasers, telecommunications. He currently works as a physical layer specialist for DSL technologies (copper) at Belgacom, the main telecom operator in Belgium. High-power shunt regulator uses BJT and reference IC by Chris Toliver  Some voltage references can be used as shunt regulators, either by tying VIN and VOUT together or by leaving VIN open. These devices are limited to low currents, however, with a typical limit of around 10 mA. Standard Zener diodes serve the same function, and can operate up to about a watt, but they suffer from high series resistance. The AD584 voltage reference, along with an external pass transistor, such as the NTE-244, allows the synthesis of shunt regulators that can handle as much as 50W. The circuit shown can be used to clip high-current, long duration, over-voltage pulses – or it can be used as a floating series voltage drop. The AD584 voltage reference combines a precise band-gap reference cell, an error amplifier, and a feedback network. Direct access to the band-gap’s input allows the internal feedback network to be overridden via the external pass transistor and feedback network. The result is a stable voltage between the V+ and V– terminals. R13 and REB are included to properly bias the AD584. The 10 nF and 0.1 μF bypass capacitors are included for stability. RSCALE can be calculated from: VOUT = V+ – V– = VBG × (RSCALE/RBG + 1) where VBG = 1.22V. From the data sheet, the internal feedback resistors in the AD584 are 36 kΩ and 12 kΩ, and the desired voltage on pin 3 is V– + 2.5 V. Thus, R13 can be calculated from: 12kΩ/(12kΩ + (36kΩ || R13)) × (V+ – VEB) = 2.5V 28 EDN Europe | MAY 2014 www.edn-europe.com


EDNE MAY 2014
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