High frequency average current sensing - Page 1
Forum section: electronics / projects

author: CM800
author_rank: Frequent Contributor
timestamp: May 05, 2016, 06:04:55 pm
content: I'm working on developing a full-bridge driver for a large high-frequency transformer.f = 20kHz400Vdc max in.I'm gonna use some 600V or 1200V IGBTsPart of this means I need to measure the average current for active current limiting (through a comparator and EN of the gate drivers)The current range I want to measure is between 0A and 100A peak and scale it down to 0-5V.For such an application, I believe it would be best to go with a current transformer, however I'm also considering Hall-effect current sensors and shunt resistors with some form of isolation.Accuracy isn't too important but I want to be able to represent the current for what it is on the GUIThe main reason I need to focus on this design is that the transformer inductance and max power rating is actually unknown (It's being used with surplus X-Ray transformers)Advice and suggestions greatly appreciated.

author: Richard Head
author_rank: Frequent Contributor
timestamp: May 05, 2016, 06:30:38 pm
content: I've used both over the years and I'm inclined to use a CT. It gives you galvanic isolation also which can be handy if your control is on the secondary side. Also, you can scale the output voltage to a convenient value without having to add an op-amp. I've always had them custom wound rather than use off-the-shelf units, to keep cost down. Just don't use polyurethane coated wire or you'll end up with random inter-winding shorts. Unfortunately a CT will be quite large at 20Khz. Use a high mu core material and keep the flux density below about 50mT. This low flux density will give you flux headroom for any small temporary volt-second imbalance in the power transformer that may occur during transient load conditions. You never want the CT to saturate or you lose current feedback.A drawback of IGBTs in a full bridge configuration is the lack of built-in anti-parallel diodes. You will have to add them externally unless your devices have them built-in. With MOSFETs they come for free. I know losses are probably higher with MOSFETs but they can also be turned off much faster than IGBTs under short circuit conditions.

author: CM800
author_rank: Frequent Contributor
timestamp: May 05, 2016, 07:24:31 pm
content: Quote from: Richard Head on May 05, 2016, 06:30:38 pmfor high voltage high current, I would say IGBTs are really the only way. It also helps I have literally hundreds of IGBT bricks (600V 600A and 1700V 800A) from them being thrown away due to a re-design from some local company. They all work too. I have also checked, the do have anti-parallel diodes too (as do most / all IGBT bricks)Do you have any useful information about the high current CT's I have no really experience with magnetics. My main area is really microcontrollers and the actual power bridge themselves (I haven't really looked at closing the current loop before at this higher power)

author: langwadt
author_rank: Super Contributor
timestamp: May 05, 2016, 08:19:29 pm
content: Quote from: Richard Head on May 05, 2016, 06:30:38 pmthe anti parallel diode in mosfets can be a mixed blessing, the can have very slow reverse recoveryand many IGBTs are robust enough to handle a short longer than it takes to turn off

author: Phoenix
author_rank: Frequent Contributor
timestamp: May 06, 2016, 05:25:30 am
content: Quote from: TCWilliamson on May 05, 2016, 06:04:55 pmDo you need to measure down to DC, that's a major benefit of the hall effect approach over a CT. Some hall effect have the burden resistor/amplifier internally and output a voltage (see LEM's range).It sounds like you'll be using it for fast acting overcurrent protection - you should also consider using gate drivers with IGBT desaturation detection (e.g. HCPL-316J).

author: floobydust
author_rank: Super Contributor
timestamp: May 06, 2016, 05:46:17 am
content: Maybe look at Avago opto-isolation ICs, they have a delta-sigma A/D and 0-200kHz bandwidth. The only hassle is a high-side DC-DC converter to power it, like aPEM1-Sand a sense resistor.I useACPL-79because I need the 5kV isolation rating for UL approvals.

author: Ian.M
author_rank: Super Contributor
timestamp: May 06, 2016, 06:56:01 am
content: If your IGBT bricks can survive a 5us severe overload, take a look athttp://www.allegromicro.com/en/Products/Current-Sensor-ICs/Fifty-To-Two-Hundred-Amp-Integrated-Conductor-Sensor-ICs/ACS758.aspx4800V~ isolation rated for 990V working voltage, linear voltage output, single 5V supply to output side, 120KHz bandwidth, 3us step response time, and a choice of unidirectional and bidirectional sensors in a range of sensitivities including 100A and 150A FSD.It doesn't get much simpler . . . .Worst case you might need to add a series air-cored high current coil as a choke to slug the current risetime enough for the sensor and H-bridge shutdown circuit to react within the limits of the IGBT pulsed SOA curve if the transformer primary is a dead short. N.B be careful with inductor position and orientation with respect to the hall sensor!

author: Marco
author_rank: Super Contributor
timestamp: May 06, 2016, 07:05:24 am
content: Ignoring Hall and normal high side current sensors for a moment. Why not use a shunt and a small voltage transformer? Seems cheaper and more compact than the current transformer even with a sizable shunt (high frequency and high current is a bad combination for current transformers).

author: CM800
author_rank: Frequent Contributor
timestamp: May 06, 2016, 08:44:07 am
content: Quote from: Ian.M on May 06, 2016, 06:56:01 amHmm, I like the idea of these a lot! It looks like it will be the perfect solution for my isse.Thanks for the link. That will plug straight into the Arduino Due I'm using to base this project off without any real problem.I could mount this directly on the bus bar too by cutting then soldering it to each side of the copper plate.