Rectification basics

[K O'Connor]

Hi Guys

Rectification is the process of converting AC into DC.

AC by definition is "alternating current", which means that for half the cycle current moves in one direction and then for the second half of the cycle the current reverses direction.

DC is "direct current", which moves only in one direction, as with a battery power source.

Transformers must have alternating or at least pulsing current. Mains AC is a nice clean sine wave as generated, although noise may be added along the way, but we will consider the ideal for now. A linear supply uses the mains frequency AC at 50Hz or 60Hz. In a switching supply, the transformer uses chopped DC applied either to one end of the PT winding or to both ends out of phase, at a high frequency which allows the PT to be smaller than the low-frequency PT. In both cases, the PT isolates the mains from the audio circuit to be powered and can provide output voltages of widely differing values if required using multiple secondary windings. The rectification of the output has the same basic requirement and uses the same circuits regardless of the PT operating frequency.
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If you have a transformer winding with one end grounded and a single diode as a rectifier, that diode blocks half the wave and conducts for half the wave. During conduction, the diode is 'on' and has practically zero resistance, so can pass current from the PT to the load. This is obviously called "half-wave rectification". The diode can be oriented to produce a positive or negative output.
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If we configure four diodes into the standard "bridge" arrangement, the winding can no longer be tied to ground. The AC terminals of the bridge tie to the ends of the winding; the DC terminals of the bridge provide current to the load for both halves of the AC wave, so we have "full-wave rectification" AND we are using a "full bridge".

Inside the bridge, two diodes conduct at a time and you could look at this as being an "overlap" of four half-wave rectifiers. There are two negative-output half-wave rectifiers and two that are positive-output. Each end of the winding has a positive-output half-wave rectifier and a negative half-wave rectifier tied to it, with the positive DC outputs tied together and the negative DC outputs tied together. For each half of a cycle the positive-output rectifier at one end of the winding works with the negative-output rectifier at the opposite end of the winding, so two pairs of half-wave rectifiers working alternately to steer the current from the winding to produce full-wave DC.
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We can have a winding with a centretap and use a single diode connected to each end with their outputs joined and the CT grounded. For one half of the AC cycle, one diode conducts current to the load. For the other half cycle the second diode conducts current to the load. The load receives current over the full AC cycle, so we have full-wave rectification again BUT we have a "half-bridge rectifier".
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In the case of a tube amp that requires a high-voltage positive plate supply and a moderate-voltage negative bias supply, we can add a half-wave rectifier to one end of the CTed winding with its diode reversed. This produces a negative DC voltage output BUT uses only half the AC wave. TUT readers know that this is far from ideal, and we can add another half-wave reversed-diode from the other end of the winding and join the two negative outputs to achieve a bias supply derived from full-wave AC. The bias supply is now more reliable and easier to filter.

We now have a CTed winding with a centretap and a full-bridge rectifier producing plus and minus DC rails. This is the most common type of supply for solid-state amplifiers.
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With any of these connections, the DC output is either negative nor positive until we decide to ground one end or the other, as RSG explains.

[K O'Connor]

Hi Guys

In post-1 we described the difference between the linear PT powered from mains frequencies and the type of PT used in switching supplies. The frequencies used by each allow conveniences or introduce complexities to the rectifier choice.

Solid-state power diodes were developed to work efficiently at 50-60Hz or lower, and can handle immense currents in applications such as for electric railway locomotives. of course there are other diode applications where currents and voltages are very low but the switching frequency is much higher, as for radio circuits or computers. Nowadays there are many types of diode offering performance to suit various applications.

Most linear tube amp power supplies use the ubiquitous 1N4007 rectifier, rated for 1A continuous, 30A pulsed and 1kV reverse blocking. It is not particular fast in its switching, nor is it slothy, it is inexpensive and widely available. Slightly faster diodes (UF4007, "ultra-fast") are preferred to reduce switching noise. TUT3 discusses diode noise abatement, which is generally over-blown in MI discussion groups. Hifi hobbyists go a step further in the fast diode upgrade to devices based on mosfet technology.

The fact is that by adding one or two low-value resistors to a stock power supply, and maybe adding a low-value plastic cap, all diode noise is stamped out without changing the diodes. truly, without these seemingly insignifanct passive elements the expensive diode upgrades are wasted.

In a switch-mode power supply (SMPS) the diode should be very fast as the "AC" is not a sine wave; rather, it is a distorted square wave. Even with the low-inductance of the high-frequency PT, the sharp input current pulses cannot come out quite as square BUT they still have copious noise content. In any case, very fast rectifiers are needed at the output and these are usually the Shottky type.

SMPS is everywhere and it is generally done in the cheapest way, resulting in the highest possible noise output. In computers, the high-current windings are all at very low voltage, so to minimise diode-caused voltage loss, half-wave rectification is often used for these outputs. Fortunately computer circuitry does not require a high signal-to-noise ratio as the logic circuits simply need to see an input that is "close to zero" or "close to the rail voltage".

With technological advances and highly integrated circuitry for widely-used circuits, one can find synchronous full-bridge controllers that do not require auxiliary windings on the PT. High-current low-on-resistance mosfets are used as the rectifiers and the control circuitry makes them behave like a very low-loss full-wave bridge. This alleviates some of the noise compared to half-wave rectifiction, but is still not used in the low-end product ranges, including lighting, appliances, or other common consumer items.

Shottky diodes switch extremely fast, so their own noise contribution is low on that account. However, they tend to be "leaky" which means that when they should be 'off' they still pass a small amount of current, which adds to noise. Is the reduction of switching noise greater than the leakage noise? In most cases, yes. Shottkys are essential in the power supplies for class-G/H power amplifiers for best performance.