stube40

Thanks, gave them a call. Nice girl on the phone, but not too good with Allegro stuff (their computer system couldn't find the ACS758 part at all). They are investigating and will give me a call back soon (hopefully) with the details - will post back when I know more.

Any good suggestions where I can get hold of the Allegro ACS758 hall effect current sensor in low quantities with short lead times? I'm in Australia, but shipping costs aren't important. I've already tried RS, Farnell and Digikey - all out of stock and long delays before new stock.

OK, thanks - I will adjust the circuit so the load is no longer inbalanced

As shown in the attached schematic, I have a resistor/LED load across the +/- 15V terminals of the DC/DC to meet the minimum load requirements of the device, however it is the 0V/+15V terminals that I am using to power the VO3120 MOSFET driver. Schematic.pdf

DOH - my bad. Its the NMA1215SC, data sheet attached. Murata_DC_DC.pdf

I am pulling my hair out since I keep destroying the Murata NMC1215SC DC/DC converters in my CPU controlled H-Bridge design. There is no obvious signs of damage such as heat or smell, they simply stop outputting +/- 15V. Testing out of circuit against a reference device shows they truly are dead. The H-bridge design uses the 4x Murata device to provide an 4x isolated power rails for the 4x Fairchild IGBTs and the Vishay VO3120 opto-isolated MOSFET drivers. I have attached a pictures that shows the general idea of the schematic, as well as a detailed schematic with every component. The design is for switching a 150V / 40A high-voltage power source, but so far I've only been bench testing with 12V into a 27 Ohm resistor. I suppose I must be damaging the Murata's though inadequate protection, although so far I haven't been able to catch any suspicious spikes on the oscilloscope. At $20 a pop though from Farnell, these parts aint cheap and it's proving to be an expensive detective project. My only other thought is that I'm over-loading the output of the Muratas which are only rated to 1W. I'm beginning to run out of ideas - can anyone suggest something to try?

Any ideas who might be able to supply a high voltage isolation switch capable of handling a constant 150V @ 40A?? The Jaycar SF2245 ones I selected weren't up to the job and began to melt (I thought they might!!) http://www.jaycar.com.au/productView.asp?ID=SF2245&keywords=sf2245&form=KEYWORD I've tried all the usual suspects RS, Farnell etc. I live in QLD, Australia, but worldwide shipping is no problem.

I've recently been doing some work with lead-acid batteries. As most of you know, these things are nuts in terms of the amount of power they can deliver - effectively they are a power supply that can deliver hundreds of amps in a small space of time. The upper limit is only defined by the limits of the chemical equation going on inside, coupled with the melting point of the matterials used to make the battery. I'm wondering if there's any existing or new battery technologies that have an inherently more stable chemical equation that results in some sort of integral virtual current limiting or current control? It's been a while since my high-school chemistry lessons, but maybe there are some chemical equations that cannot be speeded up? The assumption here is that the speed at which the chemical equation takes place is proportional to the power delivery. Can anyone can shed any light on this?

Anyone know a good way of getting hold of a decent transformer oil? We have RS and Farnell here in Oz but neither of them stock any (RS used to but discontinued it)

Yes, absolutely

It's amazing that sometimes the most obvious things sometime escape me - an old electric heater is a great source of cheap wirewound resistors!!

OK, the design has changed a little since I last posted - I've gone for the Vishay VO3120 opto-isolated driver powered by dedicated isolated DC/DC converters. I've attached 2 JPGs - one which shows the general layout and one which zooms in showing how the VO3120s are powered and connected. I'd be intrigued to get some feedback on what people think of the circuit. Also, I can't decide on whether to use a bi-polar DC/DC to provide the Vcc / Vee pins of the VO3120 with +/- 15V, ot whether to use a uni-polar DC/DC and provide 0V/15V - any thoughts?

I have some resistors to cool that I presume are going to get rather hot!! I am using 6x wirewound resistors in parallel to create a 3.75 Ohm load for a 150V source, hence generating 40A and 6kW. Later I may up the voltage to 200 so I need to spec the cooling for a max of 10kW. Each wirewound resistor is 28mm dia and 350mm long. I was thinking of a open perspex box filled with some oil (maybe olive oil). Then creating a custom lid for the box out of a thermally conductive material that is an electrical insulator. Then mounting the resistors on to the bottom side of the lid on long legs so that they are completely submerged. On the top side of the lid I would put a large heatsink and fan. The maximum length of time the power will flow through the load is 1 minute, so this will help reduce the overall specs. If anyone likes this idea then my next challenge is to spec the size of the box, amount of oil and the size of the heatsink and fan. This is the tricky bit obviously and I'm not really sure where to start.

OK, I've been pointed to the existence of linear optocouplers to solve this problem! You learn something new everyday.... Having decided to go down the opto-isolator route, I'm struggling with the logistics of powering the IGBT-side of the opto-isolator. To elaborate, there are actually 4x IGBTs and drivers in an H-bridge formation. They are routing the 150V / 40A power source through are large superconducting coil. We have found from previous experiments that all sorts of strange things happen when the coil is being charged and switched including zero voltage and negative voltages. Hence, this cannot be the source of the 15V for the IGBT side of the opto-coupler and the TC247 driver. Yet, to turn the IGBT on I need Vge to be 15V, but I'm worried about connecting the ground of the 150V PSU to the 15V PSU. Does this make any sense at all, or should I upload a diagram?

What if I used an opto-coupler instead?? The switching frequency is only around 10Hz, but reaction time has to be within 1ms - I think a good opto-coupler could achieve this. If I used an opto-coupler, the only remaining problem is what to do with the ground for the resistor-divider voltage measurement feed that allows the CPU to measure the amplitude of the high-power voltage source. Maybe using the CPU's differential ADC inputs?

I am putting together a PCB that has a CPU to control the on-off state of a high power IGBT. There are two power supplies associated with this: 1) A 24 DC lab PSU that is regulated on the PCB to both 5V (CPU) and 15V (TC427 MOSFET driver) 2) A meaty 150V / 40A DC PSU that flows through the IGBT and into a massive inductor The trouble is, I'm nervous about connecting the grounds of both PSUs together due to my suspicion that in certain conditions in my application I'll get large negative currents on the ground plane and/or other nasty stuff that the CPU and related electronics will hate. However, if I dont I have a concern regarding getting the correct 15V gate voltage to turn the IGBT on and also another circuit where I use a 2-resistor voltage divider to downscale the 150V to a meagre 5V so that it can be fed into one of the CPU's ADC inputs to measure the incoming voltage. Can anyone suggest a way forward?

Thanks for your advice - I need 4 of these IGBTs to switch a 150V / 40A DC power source in a H-bridge formation - hence only 2 will ever be on. Effectively, I will switch 2 of them on for approx 20ms then off for 80ms, then switch the other 2 (ie reverse the output polarity) for 20ms then off again for 80ms - and so on in an endless loop. In other words, switching at 10hz with a 20% duty cycle. When I decide to switch one on I would ideally like it to go on within 1ms. By the sounds of it, the IGBT I selected may not be the best choice although I am certainly talking about low switching speeds

Anyone have any suggestions how to drive an IGBT via a microcontroller? I'm trying to drive a STGE200NB60S with an Atmel AVR CPU. The AVR can run at both 3.3V and 5V and the gate threshold of the IGBT is 3V, so in theory I could just connect up an go. But, i cant help thinking that it can't be that simple, can it? Also, if anyone has any suggestions where I can source low numbers (<= 5) of the 64-lead TQFP version of the AVR (id AT90CAN128-16AU or AT90CAN128-16AI) then I'd love to hear them - Digikey dont do the TQFP package.

I've trawled the data sheet but there's no mention of the internal resistance.

I'm expecting to connect the battery array to a simple circuit with a 45mH inductance and current limiting resistor. The connection will be made with a relay and then I'll expect the current to ramp linearly up to 40A in about 13ms, after which I want the resistor to stop the current rising further. The liquid nitrogen is just from the local cryogenics supplier - it's fairly simply to get hold of over here in Oz.

I am setting up a brief experiment that involves a switch an array on twelve 12V lead-acid batteries into a test inductor load at 150V with a limited 40A current. This test will only be conducted for about 10 seconds and I dont want to spend too much time/effort worrying about current limiting methods. Hence, I was going to spec a high-wattage resistor around 4Ohm, connect it in series with the batteries and throw it in a bath of liquid nitrogen to keep it cool. Since the test will run for only 10 seconds, I thought I would get away with this. Anyone disagree or have any better suggestions?

Thanks for your comments. I hadn't considered connecting the MOSFETs in parallel - cost is not a problem and I could put 10 in parallel if needbe (and it was physically possible to do so on a PCB). Are there any disadvantages to connecting MOSFETs in parallel and/or any caveats that you are aware of? You have already mentioned the increased gate capacitance but I'm not 100% sure if that will have a detrimental effect in my application (I'm actually a digital electronics guy who's working on his first power electronics project!).

Thanks for that. It's 150V DC, but a quick calculation I did showed 72W dissapation at 40A and I have to multiply that by 2 for normal H-bridge operation. I wondered if I couldn't use electromechanical relays to achieve the same thing. My time constraints are pretty tight (eg make the circuit at +/- 1ms of the target time), but my switching frequency is low at around 10Hz max and I will know at least 100ms in advance when the switch has to be made. So maybe I can pre-trigger the relays to fire close to the target provided that the relay has reliable response characteristics and that it doesn't bounce on the contacts (which will blow mp PSU). Or maybe I should take the power loss hit on the solid state solutions?

I'm trying to design an H-bridge that can control the output polarity of an electromagenetic coil with inductance of around 50mH and very low resistance. The source voltage is around 150V at 40A and hence the design and the switch selection aren't trivial. The timing constraints make things even tougher - the circuit needs to be able to switch within a few ms of the control signal trigger going high. Once switched, the circuit will stay in the same polarity for around 100ms or so. Hence, it's far more about response time and lag than it is about actual switch frequency. I've considered both solid state relays (IGBTs and MOSFETs) as well as electro-mechanical relays. The solid state stuff seems to have considerable power losses due to the voltage drop between emitter and collector. The electro-mechanical relays suffer from being too slow and/or occasionaly bouncing when driven hard (which is likely to blow my PSU) Does anyone have any suggestions for a good solid state solution that has low power losses, or an electromechanical solution that is fast enough and wont bounce?