AC Constant Current Source (2Apk-pk, 10~20kHz) - Possible? - Page 1
Forum section: electronics / general

author: onemilimeter
author_rank: Frequent Contributor
timestamp: January 05, 2011, 03:00:33 pm
content: I wish to have an AC constant current source, which should be capable of supplying a constant 2Apk-pk AC current in the frequency range of 10~20kHz.The load is an inductive load with impedance (0.5 + 2*pi*f*L) ohm, where L is 4mH.Do you it's possible to achieve such current source?Kindly advise if you know any useful online resources.Thanks.

author: Zero999
author_rank: Super Contributor
timestamp: January 05, 2011, 07:23:48 pm
content: It makes sense to me, it's an AC current source which always gives a peak current of 2A regardless of the load impedance.I don't understand what the impedance is though, the inductance is 4mH but what's the resistance? 0.5 Ohm if I've interpreted what you've written correctly but why didn't you just say in the first place?The impedance of a 4mH inductor at 20kHz is 503 OhmThe voltage would need to be 1006kV peak or 711V RMS.This is not an easy task.I think the most sensible way of doing it would be to build an audio amplifier and use a transformer to boost the voltage. The feedback could be taken via a hall effect sensor or a current sense transformer.

author: NiHaoMike
author_rank: Super Contributor
timestamp: January 05, 2011, 10:03:01 pm
content: If only one frequency is needed, make a series resonant circuit driven by a H bridge. Regulate the current by varying the frequency.

author: Zero999
author_rank: Super Contributor
timestamp: January 05, 2011, 10:16:40 pm
content: Yes that will work but you'll need some feedback to keep the current constant if the load impedance changes.The frequency is variable though.If you use a 28nF capacitor and tune it to resonance at 15Hz. Now the impedance should always be <126Ohm so the voltage can be reduced to 252V peak, 178VRMS.

author: onemilimeter
author_rank: Frequent Contributor
timestamp: January 07, 2011, 06:19:39 am
content: Quote from: NiHaoMike on January 05, 2011, 10:03:01 pm"Series Resonant Circuit driven by a H bridge" is something new to me. In my application, I need to vary the frequency from 10Hz to 20kHz.

author: onemilimeter
author_rank: Frequent Contributor
timestamp: January 07, 2011, 06:23:23 am
content: Quote from: Hero999 on January 05, 2011, 10:16:40 pmWould you please advise how did you get "<126Ohm"? By the way, why did you select 15Hz?Thanks.

author: Zero999
author_rank: Super Contributor
timestamp: January 07, 2011, 06:21:35 pm
content: Oh, I thought you said 10kHz to 20kHz, you need to go down to 10Hz which makes it more difficult.I selected 15kHz (15Hz was a typo) because I thought it was half way between the two frequencies you need.126 Ohm was calculated as follows:First I worked out the impedance of the inductor at 15kHz:XL= 2pi*FL = 2*3.142*15000*0.004 = 377 OhmFor resonance at 15kHz the capacitor connected in series with it needs to also be 377 OhmC = 1/(2pi*FXC) = 1/(3.142*15000*337) = 0.0028 = 28nFThe impedance at 20kHz will be:Z = XL- XCXL= 2pi*20000*0.004 = 503OhmCC= 1/(2pi*20000*0.0028) = 284 Ohm503 - 284 = 219 OhmThe impedance at 10kHz will be:Well we don't need to bother doing the calculations again, the impedance of the capacitor will be doubled and the impedance of the inductor will be halved:XC= 2*284 = 568 OhmXL= 503/2 = 251.5 OhmZ = 568 - 251.5 = 316.5Oh well looks like I was wrong LOL, the maximum impedance is 317 Ohm I obviously didn't think it through, never mind you've chosen 10Hz to 20kHz so power factor correction won't work. You need an amplifier capable of putting 2A into a 503 Ohm load which won't be easy to make.

author: onemilimeter
author_rank: Frequent Contributor
timestamp: January 12, 2011, 09:36:54 pm
content: Thanks Hero999...I've decided to reduce the frequency to 10kHz and the pk-pk current to 1.0A.From calculation, the impedance at 10kHz is [0.5ohm + 2*pi*(10kHz)*(4mH)] = 251.83ohm.The required pk-pk voltage = 251.83 * 1.0 = 251.83V. Or, RMS voltage = 251.83 * 0.3536 = 89.05Vrms.Do think it is easy to construct this "reduced" spec variable frequency constant pk-pk AC current source? Using a transformer?Thanks.

author: scrat
author_rank: Frequent Contributor
timestamp: January 12, 2011, 11:22:56 pm
content: Is the frequency to be varied continuously? I mean, could you divide the range into subranges, and move from one to another with some discontinuity, for example moving from one transformer ratio to another?In the hypothesis of changing the ratio at some steps, I think it would be feasible, otherwise I think it would be quite difficult to control a current by means of an actuator voltage that goes across 3 orders of magnitude...

author: onemilimeter
author_rank: Frequent Contributor
timestamp: January 13, 2011, 01:48:51 am
content: Quote from: scrat on January 12, 2011, 11:22:56 pmI wish the frequency of the sinusoidal current to be varied continuously, or in step of 10Hz.I'm not sure if the following works:Audio Power Amplifier (e.g. LM3886)==>Step-Up Transformer==>LoadWith +/-35V supplies, the LM3886 is capable of supplying 50W at 8ohm (i.e. Iout=2.5Arms and Vout=20Vrms).The transformer is designed to have a step-up turn-ratio of 5. Thus, the output voltage of the secondary winding will be 20Vrms*5=100Vrms, which is higher than the required 89.05Vrms. For a secondary output current (i.e. load current) of 0.3536Arms (i.e. 1.0Apkpk), the primary current will be 0.3536*5=1.768Arms, which is still within the output current capability of the LM3886.For frequencies lower than 10kHz, the output Vrms of the LM3886 is decreased to maintain the load current of 1.0Apkpk.The impedance of the transformer should be made as small as possible. The transformer should not be saturated when the secondary current is 1.0Apk-pk (10kHz). What do you think about using toroidal core to build the transformer?Do you think the above idea will work?Please kindly advise if you know a better power amplifier with high supply voltages and high output current.Thanks.

author: NiHaoMike
author_rank: Super Contributor
timestamp: January 13, 2011, 03:28:11 am
content: A MOSFET H bridge using current feedback PWM at 100kHz or so should work nicely for direct driving the load. A 10Hz transformer is not going to be very practical. And unless you need really low noise, PWM techniques will result in a cheaper circuit due to less power dissipation. (If low noise rules out PWM, vary the supply voltage with the peak of the output voltage to reduce dissipation.)Note that most modern motor drives are PWM AC current sources.

author: scrat
author_rank: Frequent Contributor
timestamp: January 13, 2011, 10:24:01 am
content: If steps are allowed, and precision is needed, I'd go for a multiple output transformer, switching between the various outputs depending on frequency (= depending on output voltage). At least for the linear solution.Quote from: NiHaoMike on January 13, 2011, 03:28:11 amA current control on (~100kHz) PWM modulation could be a solution, with a good filtering. Maybe, even here, choosing between multiple outputs of a tranformer would be better for noise reduction and lower voltage components.Otherwise, you could change DC bus voltage according to frequency.As Mike says, with PWM the transformer can be smaller.

author: Zad
author_rank: Super Contributor
timestamp: January 13, 2011, 03:36:22 pm
content: To be honest, it just sounds like a Class B audio power amplifier. If you need less distortion then bias is into Class AB. If you need no distortion at all, and don't mind heating the house then go full Class A.If you need the ultimate efficiency then use a Class D ampl like thishttp://focus.ti.com/docs/prod/folders/print/tas5630.htmlIt will drive two independent 300W loads at 50V (600W in push/pull) and up to 80kHz. Full overload protection and reporting.

author: scrat
author_rank: Frequent Contributor
timestamp: January 14, 2011, 12:41:32 am
content: Quote from: Zad on January 13, 2011, 03:36:22 pmThe only (!) difference I see is the fact you need to control current, not voltage, with an impedance which grows one thousand times.

author: NiHaoMike
author_rank: Super Contributor
timestamp: January 14, 2011, 02:07:44 am
content: QuoteJust like a brushless DC servo motor drive, except at about 50 times higher maximum frequency. (Assuming the same output torque, brushless DC motors have an effective impedance that increases with speed/frequency due to back EMF.) A current feedback PWM should work nicely. It might not even have to be a proper PWM - a Schmitt trigger comparator in a current feedback loop might be good enough.QuoteAt those frequencies and power levels, a PWM design is highly preferred unless there's a good reason not to.A friend of mine has wrote about this before.http://www.diyaudio.com/forums/chip-amps/172570-want-build-300-watt-stereo-power-amp-3.html#post2286906QuoteIn other words, PWM makes more efficient use of your transistors.

author: scrat
author_rank: Frequent Contributor
timestamp: January 14, 2011, 09:40:07 am
content: Quote from: NiHaoMike on January 14, 2011, 02:07:44 amExcuse me...As I'm involved into electric drives, I can't help specifying... Increasing impedance in every motor is due to inductance, not to back-EMF, but you're right about the fact that voltage increases mainly because of flux linkage.I would be careful considering an electric drive inverter as a good example of current source: it can control quite good the average current, not the instant value. By using PWM (either from a modulator or an hysteris comparator) you are switching a high voltage even at low frequencies (since you need it for going at the high end of frequency range). This will likely produce much noise, which COULD be unacceptable if, for example, the application is signal generation or something related to a measure. This is the reason why some instrumentation generators are still working in linear mode.What I'm trying to say is... IMHO, PWM can be better or worse depending on the application, and the same for solutions with or without different DC bus voltage values or transformer ratios for different frequency ranges.

author: NiHaoMike
author_rank: Super Contributor
timestamp: January 14, 2011, 03:22:49 pm
content: Varying the supply voltage depending on output peak voltage can give you a greater dynamic range. Some of the higher powered TI Hybrid Digital amplifiers vary their supply voltage depending on volume setting. It is also done a lot in motor drives where there already is a voltage converter before the inverter.http://techno-fandom.org/~hobbit/cars/ginv/VH.htmlQuote

author: onemilimeter
author_rank: Frequent Contributor
timestamp: January 18, 2011, 06:16:13 am
content: Quote from: NiHaoMike on January 13, 2011, 03:28:11 amThanks for your suggestion. The PWM switching noise may affect my application. However, if the PWM switching frequency is 100kHz or higher, then it should be OK as the frequency range of interest for my application is 0~20kHz.If you read my previous posts, you may find out that in my application, at 10kHz, the impedance of the load will be [0.5ohm + 2*pi*(10kHz)*(4mH)] = 251.83ohm. Thus, to push a 1.0Apk-pk sinusoidal current through the load, the circuit should be able to supply 251.83Vpk-pk or higher. Do you think we can achieve this with the "MOSFET H bridge" you suggested?Cheers

author: onemilimeter
author_rank: Frequent Contributor
timestamp: January 18, 2011, 06:40:34 am
content: Quote from: scrat on January 13, 2011, 10:24:01 amThanks. Is it difficult to design the filtering stage?100kHz MOSFET PWM H-Bridge --> Filtering --> Step-up Transformer --> LoadIn my application, the impedance changes with supply current frequency:10Hz: 0.75 ohm100Hz: 3.01 ohm1000Hz: 25.63 ohm10000Hz: 251.82 ohm20000Hz: 503.15 ohmSince the load, by nature, is lower at low frequency range, the filtered output of the PWM H-Bridge can be directly connected to the load. For higher frequency range, a transformer is used to step-up the filtered output voltage of the PWM H-Bridge. What do you reckon?

author: onemilimeter
author_rank: Frequent Contributor
timestamp: January 18, 2011, 07:06:47 am
content: Quote from: NiHaoMike on January 14, 2011, 03:22:49 pmThanks for this link. Since the current in my application is relatively low compared to those HEV drives, it's possible to switch the power MOSFET at higher frequency, e.g. 100kHz.

author: Zero999
author_rank: Super Contributor
timestamp: January 18, 2011, 05:40:23 pm
content: Anyway why do you want to do this? It seems like a very odd thing to do.Have you looked into class D power amplifier ICs?

author: qno
author_rank: Frequent Contributor
timestamp: January 19, 2011, 02:10:41 pm
content: Looks like you need a IGBT module.Check out this onehttp://www.fujielectric.com/device/semi/products/powerdevices/2pack.html