Transformer VA when feeding large choke input filter - Page 1
Forum section: electronics / projects

author: Circlotron
author_rank: Super Contributor
timestamp: June 17, 2016, 12:48:50 am
content: Say you have a transformer rated for 100VA. With a resistive load it can deliver 100 watts / 100VA. If it feeds a bridge rectifier and large smoothing capacitor it can deliver approximately 60 watts / 100VA. What about if the bridge rectifier instead feed a very large inductance series choke, such that the current through the transformer secondary during each half cycle was essentially constant, that is to say, the transformer secondary current was a square wave in phase with its sinewave voltage output. If a 100 watt dc load was applied to the far end of the choke what VA would be demanded of the transformer? Would the transformer be able to deliver more than its rated current seeing as the transformer winding losses vary as the instantaneous square of the current and in this case the current would be a constant level? Imagine please that the inductor is lossless and free.

author: uncle_bob
author_rank: Unknown
timestamp: June 17, 2016, 12:57:10 am
content: Quote from: Circlotron on June 17, 2016, 12:48:50 amHiSimple answer: no, you don't get a free lunch. There are core losses along with winding losses.More complex answer: you can Google choke input filter and come up with the factor for a standard choke input.Bob

author: Kleinstein
author_rank: Super Contributor
timestamp: June 17, 2016, 07:05:31 am
content: You can't get more power than the rated 100 VA. The sine current with a ohmic load is already the best to get the highest possible power out.With a choke the DC current will be higher than with just the rectifier. How much one could check with a simulation. This will also show how much choke is needed and how much the voltage drops.As a first estimated the DC current might reach the rated RMS current, which would be the case with a square wave current.There is also the alternative position of the choke between rectifier and capacitor.

author: uncle_bob
author_rank: Unknown
timestamp: June 17, 2016, 11:31:46 am
content: HiJust to put a number on things:The traditional numbers for capacitor input are in the 50 to 60% of VA range. The traditional numbers for choke input are in the 70% to 80% range. There are things like core vs winding loss and mutual coupling that impact the numbers.Bob

author: MagicSmoker
author_rank: Super Contributor
timestamp: June 17, 2016, 11:51:06 am
content: Max power factor for the case where a very large DC choke is before the filter capacitor is around 0.9, so you could get about 90W from a 100VA transformer. In no case can you get more current from the transformer (without overloading it, of course, and there will still be an ohmic drop across the windings, as well as a [mostly fixed] reactive drop across the leakage inductance).

author: SeanB
author_rank: Super Contributor
timestamp: June 17, 2016, 04:57:18 pm
content: A first approximation for the choke size is that it will be at least as big as the power transformer, and will likely also be a custom wound component, as it needs to be wound with a nice air gap in the core ( often implemented as an angled slot in the middle leg and thus made not from the common EI laminates but from a less common EE laminate with the centre legs cropped slightly) and also has to have a low DC resistance, thus is wound with a thicker wire.You can do a choke filter with a SMPS and have it come out as small as the regular transformer, and gain a good amount of efficiency from lower diode losses in the rectifier on the output, but on a 50/60Hz transformer you will be always better off ( aside from one application) with a bigger transformer, beefier diodes and living with the ripple. It does reduce the diode peak current, and increases conduction angle, and in some designs is actually an energy store that provides power to the load as well during half the cycle.The only time that it makes sense is in high voltage supplies, where you are limited in component values either by the diode ratings, or the ripple current ratings on the capacitors and thus have to use lower value capacitors than you want, and then use a LC smoothing choke to reduce the noise ( you can use RC smoothing, but this only is useful at very low power as the resistor is a hot component) on your high voltage supply. This is common in valve equipment, especially high power amplifiers, that need a low noise ( but not terribly well regulated) power rail for low noise operation, and where the complexity of having a choke capable of withstanding the high voltage from the coil to the core is easier than trying to fit a bathtub sized capacitor in the housing along with the inrush limiting for turning it on. You can get a lower value capacitor ( 22uF or 47uF 630VDC) and have a lower turn on inrush, along with much lower diode peak currents.