Is it possible and economical to wind a Toroid Power Transformer with significant Leakage Inductance?
Conventional PSUs on Class B amps, but also other stuff rely, on Transformers with low series resistance and huge zillion uF electrolytic caps for good regulation.
Good regulation means the voltages, on the first caps after the rectifiers, sag less. On low level devices, it means you can use a lower voltage transformer and that means less heat in any Regulator ICs (LM317, 7815 etc). When a power amplifier overloads, the PSU ripple is superimposed on the clipped signal and this sounds really nasty.
Of the 2 methods of getting good regulation, a transformer with low series resistance is better than having another zillion uF. For a low power device, this is easy. You simply specify a power transformer with higher VA for the same output windings. eg 50VA instead of 30VA. But this is an expensive approach for Power Amps.
Toroids have MUCH smaller series resistance and hence better VA for the same size/weight compared to old EI type cores.
But both methods have a problem. With less sag between charging cycles, the charging spikes become huge as the caps have to be recharged in a smaller period of time.
[list type=decimal]
[*]These short sharp current spikes have harmonics up into the low RF frequencies and were a worry in the early days of EU certification and their obfuscating EMI specs.
[*]The spiky currents are responsible for most of the "buzz" in Low Noise circuits via 2 mechanisms.
On a Class B amp, this noise is worse when there is no signal cos the spikes are then shorter and nastier. At full power, there is more sag, the transformer takes more time to recharge the caps and there is a lot of signal drowning out the noise too.
In all these cases, it is beneficial to lengthen the period of the charging spikes. This makes the spikes fatter and rounder for the same average current. In the early days of EU certification, simply lengthening the spikes so they were at least 1ms long meant you passed the "EMI on the mains" specs. Dunno about today cos I last looked at this nearly 2 decades ago.
You can do this with an Choke in series with the transformer. This is not a full Choke Input PSU where the Choke must be big enough to ensure the current never stops and is completely wrong for a class B amplifier. Here we only use the Choke to stretch out the charging spikes slightly. You can even get away with poor saturation characteristics for this choke.
You can wind an EI core transformer with enough Leakage Inductance so you don't need a separate choke but these have much poorer series resistance compared to a good Toroid.
Do yus gurus with experience designing or specifying transformers know if it is economical (or even possible) for a Toroid Power Transformer to have significant Leakage Inductance? I know this sounds like an oxymoron.
How can we keep the low series resistance advantage of a large toroid while reducing the charging spike noise?
Conventional PSUs on Class B amps, but also other stuff rely, on Transformers with low series resistance and huge zillion uF electrolytic caps for good regulation.
Good regulation means the voltages, on the first caps after the rectifiers, sag less. On low level devices, it means you can use a lower voltage transformer and that means less heat in any Regulator ICs (LM317, 7815 etc). When a power amplifier overloads, the PSU ripple is superimposed on the clipped signal and this sounds really nasty.
Of the 2 methods of getting good regulation, a transformer with low series resistance is better than having another zillion uF. For a low power device, this is easy. You simply specify a power transformer with higher VA for the same output windings. eg 50VA instead of 30VA. But this is an expensive approach for Power Amps.
Toroids have MUCH smaller series resistance and hence better VA for the same size/weight compared to old EI type cores.
But both methods have a problem. With less sag between charging cycles, the charging spikes become huge as the caps have to be recharged in a smaller period of time.
[list type=decimal]
[*]These short sharp current spikes have harmonics up into the low RF frequencies and were a worry in the early days of EU certification and their obfuscating EMI specs.
[*]The spiky currents are responsible for most of the "buzz" in Low Noise circuits via 2 mechanisms.
- poor earthing arrangements, wiring and layout leading to IR noise voltages
- if the conductors and wiring for these spiky currents are not twisted together, they are efficient loop aerials inducing spiky "buzz" noise in poor layouts for sensitive input stages.
On a Class B amp, this noise is worse when there is no signal cos the spikes are then shorter and nastier. At full power, there is more sag, the transformer takes more time to recharge the caps and there is a lot of signal drowning out the noise too.
In all these cases, it is beneficial to lengthen the period of the charging spikes. This makes the spikes fatter and rounder for the same average current. In the early days of EU certification, simply lengthening the spikes so they were at least 1ms long meant you passed the "EMI on the mains" specs. Dunno about today cos I last looked at this nearly 2 decades ago.
You can do this with an Choke in series with the transformer. This is not a full Choke Input PSU where the Choke must be big enough to ensure the current never stops and is completely wrong for a class B amplifier. Here we only use the Choke to stretch out the charging spikes slightly. You can even get away with poor saturation characteristics for this choke.
You can wind an EI core transformer with enough Leakage Inductance so you don't need a separate choke but these have much poorer series resistance compared to a good Toroid.
Do yus gurus with experience designing or specifying transformers know if it is economical (or even possible) for a Toroid Power Transformer to have significant Leakage Inductance? I know this sounds like an oxymoron.
How can we keep the low series resistance advantage of a large toroid while reducing the charging spike noise?
