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| Mysterious FET destruction on high-power H bridge |
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| Siwastaja:
--- Quote from: cur8xgo on June 21, 2019, 10:03:03 pm --- --- Quote from: Mechatrommer on June 21, 2019, 09:27:45 pm ---if you rate your mosfets properly, current limit will not be necessary, it will complicates your circuit further --- End quote --- Yes I agree with this. A current limit shouldnt be used to protect the fets. --- End quote --- No, as usual, he's spewing total and utter bullshit, not having the slightest idea what he's talking about. Current limit can't be avoided in this power level converter. Every single one on the market has it. "Proper" MOSFET rating without a current limit would be completely impractical. Sidestepping the issue is remotely possible, but much more complicated and still risky. Every proper motor controller has this feature. Complication? Maybe, life is such. Without it, it's impractical, expensive, and VERY complicated to get work reliably. And a motor controller without a torque limit, capable of saturating the motor iron indefinitely and blowing the brushes and windings, it just sucks when you can get a nice torque limit with a few dozen cents, and about two components. Mind, "current limit" here means measuring and quickly reacting to overcurrent as a part of your basic design, not some separate circuit at the input. Even one integrated in the gate driver, based on Rds(on)-based (approximate) measurement could do, and could be named "desat protection" for a double purpose. But it must exist in some form or another. The cost is approx. 50 cents for a shunt and amplifier. All motor PWM controllers (an MCU with three-phase PWM generation, most often) support this input signal. (Break input, overcurrent input, whatever it's called depending on controller.) It's a few lines of code to configure properly. Mechatrommer's "proper" MOSFETs without active current limit would be something rated to 1000A continuous, with like 0.5ohm or less Rds(on), bare minimum, and I'm being really conservative here. With proper 10-20A gate drivers, the cost would be in tens of dollars, and the layout would be massive. And of course, this would be all futile, since now you could easily blow the motor next, and would like to have torque and - tadah - current limiting anyway. Oh, with such 10-100x overengineering, you could be able to do the protection with a traditional fuse, and let the user change the fuse every time it blows. How about an automated, motorized fuse cartridge system which changes a new fuse on the fly after the motor has spun up from an initial stall? To avoid all the complications of current sense! |
| Siwastaja:
duak has the correct answers above as well. This means, once you have the current limit set up and working, you must set it to something considerably less than 100A. Your FETs just aren't up to the task. Start with a 10A current limit... (The 80A maximum rating in the datasheet means that this DC current is possible, if you mount the device to an infinite heatsink perfectly. Basically directly soldering it to a large copper liquid cooling block could come close. This number still neglects the switching losses you additionally have.) Always do a proper thermal analysis. It can be a simple quick&dirty Excel sheet. I do my initial "napkin" Excel calculation like this: 1) Multiply the 25degC worst case ("max") Rds(on) by 1.5, to transform it to somewhere around 100 degC value, 2) Calculate I^2*R loss at your desired current, 3) Assume you have 50% Rds(on) and 50% switching loss -> hence multiply the I^2*R loss by two (This is likely the case with 100kHz switching, if not even worse. Motor controllers sometimes allow you to almost ignore switching losses by using a low frequency (think about 10kHz)) 4) Try to figure out the junction-to-ambient thermal resistance. For a typical 5x6mm SMD device, or DPAK, it would be around 40 degC/W with a basic footprint, around 20 degC/W with added thermal vias and a lot of heatsinking planes, and maybe down to 10 degC/W when you add thermal pads and metal heatsinks to the game. Collecting information from thermal tables of similar package device datasheets and appnotes helps if the particular datasheet doesn't show the actual thermal resistances (junction-to-ambient). 5) Now calculate the junction temperature rise over the ambient. Example calculation for this particular FET. Let's pick 50A as our current: Rdson max 12mOhm *1.5 -> 18mOhm I^2R loss: 50^2 A^2 * 18 mOhm = 45W Assume total loss 2*45W = 90W Assume Rthj-a of 10 degC/W with a bunch of nearby thermal vias and heatsinking from the bottom of the board Temp rise = 900 degC -> not gonna cut it. Put this in Excel and play around. What's your actual max DC link voltage, and your max motor current you are designing for? In an SMD design, you need to select the parts quite carefully (forcing you to aim for good efficiency), because the amount of heatsinkin is limited, unless you pay premium for aluminum core PCB, or copper filled vias under pad. Finally, a practical design example for reference: The last time I did this, I picked a MOSFET rated with 5.6mOhm of Rds(on) for a 25A continuous, 35A peak motor controller, in a 5x6mm SMD package. Voltage rating is Vds(max)=40V for a 25.2V max DC bus. I use a lot of thermal vias around the FETs, connecting to copper pours on the bottom layer (electrically still connected to the FET drains) and heatsink the PCB from the bottom through a thermal silpad, to the aluminimum case of the product. Current sensing is with 1210 size, 1mOhm shunt resistors on bottom phase legs (between the bottom FETs and GND), amplified by TI INA something current sense amplifier. DC link bypassing is done by a few 4.7uF, 35V X7R MLCCs, and a bunch of 330uF bog standard aluminium electrolytics. This has proven to work reliably. |
| cur8xgo:
I always thought current limiting in motor controllers was more to protect the motor...? I dunno I don't do motor controllers....I guess it makes sense they would protect the switch too. |
| Siwastaja:
--- Quote from: cur8xgo on June 21, 2019, 10:56:43 pm ---I always thought current limiting in motor controllers was more to protect the motor...? I dunno I don't do motor controllers....I guess it makes sense they would protect the switch too. --- End quote --- Well, the semiconductor switches die in tens of microseconds, with minuscule energy easily stored in a small capacitor; motors die in tens of seconds or even minutes, million times slower, and take millions of times more energy to die (of course, a semiconductor die weighs milligrams, and the motor windings and brushes weigh kilograms). So it's an absolute must for switches. And for switches, it needs to be really quick. Motor protection is of course equally important if you think about the usability of the whole system. Motor protection and torque limiting could use slower current sense mechanisms. But, if you do it quickly, these two protections come at the same time, with no extra cost or complexity, using just one measurement path - the one which protects both the switches and the motor - and the fuses as well, by the way :). |
| bigamps:
I have not gone through the details and sorry if someone has already mentioned it but do you know if the diodes are sized for the operating conditions you are describing? When reversing the power flow the diodes will take longer conduction times and higher losses. I am more familiar with power transistor modules of much higher power ratings but one of the decisions for their designers is how much real estate within the package should be devoted to the FET or IGBT and how much to the diode. Depending on each particular aplication a module may have perfectly suitable transistors but it may be discarded just because the diodes would not take some operating condition required and blow up. When a module is aimed at covering motor applications it will usually have a fair diode to transistor ratio so that it can operate any direction at similar power levels and also take nice rectified current overloads. Same goes to battery interface converters and other applications. But I am saying usually... I have had some surprises and issues in the past and had to rule out modules last minute because of a diode sizing much poorer than expected. Even once a team of module designers argued it had to do with pressures from their marketing guys : usually the transistor capabilities are easier to sell first glance! An example of a power module doing fine with a diode sizing much smaller than the transistor would be one for a brake chopper stage. Anyway you probably have enough info to figure out whether the problem is related to the diodes looking at the failure modes you are witnessing. Enviado desde mi Aquaris X5 mediante Tapatalk |
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