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06-26-2024, 12:08 PM
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.
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.
