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Smart ripple canceller provides near-zero dropout

17 Mar 2014  | Louis Vlemincq

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Less obvious stability problems can also occur when the input supply is not "stiff" enough. That can happen at light loads, when the internal resistance becomes higher. When the circuit tries to compensate for a drop in the input voltage, the amplifier draws momentarily more current, and if this causes the input voltage to drop further, it results in a positive reaction and instabilities, causing motor-boating. The phenomenon is akin to a right half-plane zero and is almost impossible to compensate away using conventional methods without ruining at the same time the performances of the circuit.


Figure 2: An extra filter may be required in some cases.


In moderate cases, a simple filter in series with the primary may be sufficient (figure 2A). In difficult cases, an active circuit is required (figure 2B). This circuit works by duplicating the current drawn by the error amplifier: R2, R5, R6, & R7 are arranged in a bridge, and since R2 is about half the value of R7, Q1 and Q2 try to make R2's current double R7's. But C2 delays the action, which means that short-term, the input 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 magnetizing 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 magnetizing inductance Lm, which should be more than twice the minimum.


Performance
Figure 3 shows the rejection and output impedance curves. Rejection is greater than 40dB for the pertinent frequency range, with a maximum at 100Hz exceeding 46dB. 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.

Figure 3: Output impedance and ripple rejection.


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 2VRMS ripple. With the 5A current, this results in a 10VA rating. But since the ripple is at twice the mains frequency, this means that 5VA 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 bread job after graduating. He has been involved in almost every field of electronics: audio and video design and maintenance, automotive, industrial control, lasers, telecommunications, etc He currently works as a physical layer specialist for DSL technologies (copper) at Belgacom, the main telecom operator in Belgium.


To download the PDF version of this article, click here.


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