An LC filter for supplying a line stage
Posted 19th February 2016 at 01:33 AM by abraxalito
Updated 22nd February 2016 at 01:51 AM by abraxalito
Updated 22nd February 2016 at 01:51 AM by abraxalito
I've designed LC filters for classAB amp power supplies before - for those applications iron powder toroids work fine for the Ls. However for the power supply in my latest DAC design I wanted more supply rejection and this calls for higher value inductors - in the tens of mH. Creating a 10mH inductor on a toroid takes way too long and is hugely fiddly as the wire length needed is substantial and I don't have any specialized winding machine. So toroids are really out of the question at such values.
I have some largeish inductors in the right range wound on bobbin cores but when I checked the DCR it was a little high, 20ohms or so. As I might need up to 100mA, a 2V drop is too great. In any case, in LTspice this resulted in rather an overdamped response - what I really needed is something in the range 1 to 2 ohms. The solution seemed to be use ferrite cores of the kind normally used to make transformers. Which means breaking a kind of informal rule I made for myself about not using ferrite cores for PSU filtering.
MnZn ferrites have very high mu meaning few turns are needed to reach relatively high values of inductance. All well and good as I'm the lazy type, the fewer the turns the better. The trouble is at such high values their current carrying capacity is limited - ferrite saturates around 0.35T and rather abruptly too. With very few turns on a core its also extremely hard to optimize the available resource, which is copper in the winding window. Few turns means thick wire. With a too low DCR inductor an external resistor is needed to hit the target (below target means ringing), rather sub-optimal.
The optimal solution is to insert an air gap between the two core halves - it allows thinner wire to hit the DCR target spot on and also permits higher currents without saturation.
The approach I adopted to get the value wasn't theoretical as right now I feel too lazy to learn the ins and outs of magnetic reluctance - I think one day I'll wrap my brain around that. But for now I proceeded by trial and error.
First up I worked out what gauge of wire would fill the winding window and give me between 1 and 2 ohms DCR. This turned out to be 0.33mm diameter wire on a PQ20/20 bobbin and air-cored it measured in the region of 500uH. Putting the cores around it raised the inductance to around 300mH. Since I was planning to gap the cores I then tried the effect of a single layer of yellow transformer tape. Bingo - the result came out around 40mH, pretty much bang-on where I wanted to be. But what I didn't know was whether this core had sufficient current carrying capacity before saturation. Which meant building the filter to see.
The design being a 4th order I made another inductor similarly and wired them up to the capacitors and the rectifier. A test load was made up of 1W 100ohm resistors. I was gratified to see that the input ripple - almost 1V peak-peak was reduced after the second L to under 2mV p-p. And it looked like a fairly pure sinewave at 100Hz - so the inductors couldn't have been saturating.
Ever curious, I wondered how close to saturation I was actually running - to optimize the design I'd be better off with more inductance as that would give me better ripple rejection. So I swapped out the transformer tape for sellotape and measured the cores again. This time the inductance was a little higher, a shade below 50mH. On firing up, the ripple voltage decreased a shade so they're still not saturating at my test current (about 120mA). But I don't know if I have any thinner material to gap the cores with, I'll have a hunt around....
Update :
Having various incarnations of this inductor ready wound I didn't stop at just 4th order, I wanted to try getting rid totally of the 100Hz component (at least visually) so I slapped on a third 2nd order section. This time the choke's gapped with some even thinner material. Trafo tape measures 60um thick, sellotape a little thinner, about 50um, the stuff I'm using is self-adhesive on both sides so I've not tried to measure it but I believe its about 40um. The resulting inductor measured close to 60mH. I found its saturation is somewhere between 80 and 160mA from observing the residual ripple under those two loading conditions. The 100Hz has totally vanished on the scope now, the noise trace at HF looks no different when the power supply is on, compared to off. But LF-wise, there are relatively sudden fluctuations of 1 or 2mV on the trace. To get rid of these the next stage will need to be active methinks. Pic of the 6th order filter attached.
I have some largeish inductors in the right range wound on bobbin cores but when I checked the DCR it was a little high, 20ohms or so. As I might need up to 100mA, a 2V drop is too great. In any case, in LTspice this resulted in rather an overdamped response - what I really needed is something in the range 1 to 2 ohms. The solution seemed to be use ferrite cores of the kind normally used to make transformers. Which means breaking a kind of informal rule I made for myself about not using ferrite cores for PSU filtering.
MnZn ferrites have very high mu meaning few turns are needed to reach relatively high values of inductance. All well and good as I'm the lazy type, the fewer the turns the better. The trouble is at such high values their current carrying capacity is limited - ferrite saturates around 0.35T and rather abruptly too. With very few turns on a core its also extremely hard to optimize the available resource, which is copper in the winding window. Few turns means thick wire. With a too low DCR inductor an external resistor is needed to hit the target (below target means ringing), rather sub-optimal.
The optimal solution is to insert an air gap between the two core halves - it allows thinner wire to hit the DCR target spot on and also permits higher currents without saturation.
The approach I adopted to get the value wasn't theoretical as right now I feel too lazy to learn the ins and outs of magnetic reluctance - I think one day I'll wrap my brain around that. But for now I proceeded by trial and error.
First up I worked out what gauge of wire would fill the winding window and give me between 1 and 2 ohms DCR. This turned out to be 0.33mm diameter wire on a PQ20/20 bobbin and air-cored it measured in the region of 500uH. Putting the cores around it raised the inductance to around 300mH. Since I was planning to gap the cores I then tried the effect of a single layer of yellow transformer tape. Bingo - the result came out around 40mH, pretty much bang-on where I wanted to be. But what I didn't know was whether this core had sufficient current carrying capacity before saturation. Which meant building the filter to see.
The design being a 4th order I made another inductor similarly and wired them up to the capacitors and the rectifier. A test load was made up of 1W 100ohm resistors. I was gratified to see that the input ripple - almost 1V peak-peak was reduced after the second L to under 2mV p-p. And it looked like a fairly pure sinewave at 100Hz - so the inductors couldn't have been saturating.
Ever curious, I wondered how close to saturation I was actually running - to optimize the design I'd be better off with more inductance as that would give me better ripple rejection. So I swapped out the transformer tape for sellotape and measured the cores again. This time the inductance was a little higher, a shade below 50mH. On firing up, the ripple voltage decreased a shade so they're still not saturating at my test current (about 120mA). But I don't know if I have any thinner material to gap the cores with, I'll have a hunt around....
Update :
Having various incarnations of this inductor ready wound I didn't stop at just 4th order, I wanted to try getting rid totally of the 100Hz component (at least visually) so I slapped on a third 2nd order section. This time the choke's gapped with some even thinner material. Trafo tape measures 60um thick, sellotape a little thinner, about 50um, the stuff I'm using is self-adhesive on both sides so I've not tried to measure it but I believe its about 40um. The resulting inductor measured close to 60mH. I found its saturation is somewhere between 80 and 160mA from observing the residual ripple under those two loading conditions. The 100Hz has totally vanished on the scope now, the noise trace at HF looks no different when the power supply is on, compared to off. But LF-wise, there are relatively sudden fluctuations of 1 or 2mV on the trace. To get rid of these the next stage will need to be active methinks. Pic of the 6th order filter attached.
