IR2127 Smoke - Buck Converter - Page 1
Forum section: electronics / beginners

author: unit998x
author_rank: Contributor
timestamp: May 21, 2014, 03:51:26 pm
content: Hi everyone, this is my first post so hopefully I can explain everything clearly.As shown in the schematic, I am trying to drive the MOSFET from a logic level source, when testing the IN was held at 5V. When I tested this schematic originally, the voltage output rose very slowly, and did not get to the full 24V. I believed this slow rise was caused by the FET only being driven at too low gate to source voltage, and the voltage rose as there is a large capacitor on the output side. This wasn't right as I expected the full power of the batteries to be available on the output.In a moment of incompetency, I decided to remove D14 and connect VB to an external (but same source) 36V supply, I thought this would give the extra gate voltage required, this did work initial but after tampering with 36V (lowering it to about 24v), the IR2127 began to heavily smoke almost instantly.I guess I didn't really know enough to just tinker with it, but I was surprised the device was destroyed as there shouldn't have been hardly any current flow.Would someone be able to explain in more beginner terms how the IR2127 works and what I did wrong, and how I can prevent this happening again.Thanks.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: May 21, 2014, 11:20:43 pm
content: http://www.irf.com/product-info/datasheets/data/ir2127.pdf>Absolute Maximum Ratings>V_B High Side Floating Supply Voltage 625VThat's V_B with respect to COM, mind.Alright, tons of headroom, no problem there...>V_S High Side Floating Offset Voltage V_B - 25V to V_B + 0.3VHmm only 25V output supply though. A 36V source isn't likely to please...If that outboard supply is returned to circuit ground (you didn't mention), then as long as the output is driving high and the switching transistor stays on, you get 36V at V_B, 24V at V_S and the difference, a good 12V, gate drive. As soon as it switches off, current in the inductor drives V_S down below 0V (forward biasing the diode), you get 36V from V_B to V_S and the magic smoke exists stage left.If the supply is returned to V_S, and it's isolated, it's dead right away because, boom, 36V all the time. If it's not isolated, but grounded to the circuit elsewhere, you've just shorted out the switching node. If nothing else, it's going to be one monster of an EMI / interference mess -- maybe not enough to break the circuit, but enough to get some pretty nasty waveforms when measuring almost any other point in the circuit (assuming you're watching this thing with a scope at all..).Tim

author: unit998x
author_rank: Contributor
timestamp: May 22, 2014, 06:15:01 pm
content: Hi Tim, thanks for the quick and comprehensive reply.I have attached a schematic of what the circuit looked like while I was tinkering and the device was destroyed. The IN- and OUT- on the boost converter are connected internally. The output of this boost converter would have varied between 24/36V depending on a trimmer pot.I did look at the datasheet beforehand, but looking at it now it appears like the diode should have been left their and the 36V not introduced in the first place.I hope you can still help with this problem, I would rather not waste another chip tinkering as they are comparatively expensive. The device is powered by lead acid batteries and hence there is a large amount of current available for destroying things on the board.Thanks

author: T3sl4co1l
author_rank: Super Contributor
timestamp: May 23, 2014, 02:33:31 am
content: Easy way... current feedback. What happens if the PWM gets stuck on? Boom. What happens if the output gets shorted? Boom.Na, with current feedback, it turns off when it reaches high current -- every single cycle, all the time. Who needs protection when your circuit is inherently safe?Easiest 'building block' method I think would be, get some UC3843s and current transformers. Use a circuit something like the top half of this:http://seventransistorlabs.com/Images/Mag_Amp_PSU.pngBut drive your transistor with the gate driver chip as before, not right off the controller, because the buck switch is floating, not ground referenced. The CT can be in series with the transistor, "above" (as shown) or "below" (but not in series with the diode or inductor). A snubber (the SB540, 1uF and 10 ohm) comes in handy to control peak voltages, especially in high power circuits. So this way, you get to dive head-in with SMPS controllers (for extra credit, build your own based on the block diagram -- you only need an LM393, enough gates to build a latch, say CD4001 or 4011, or a 4013 straight away, the gate driver, a voltage ref like TL431, and maybe an opamp), current transformers (you'll probably want to read up on them and play around a bit first to make sure you have it the right way around and all), current limiting and yes, even chaos, the real thing in all its simple complicated glory.For breadboarding, try a nice ~1 ohm power resistor in series. Limits current, gets smoking hot to indicate overload. Saves burning out wires, if not necessarily transistors..Tim

author: unit998x
author_rank: Contributor
timestamp: May 23, 2014, 10:52:36 pm
content: OK, thanks for that I have seen the uc3834 used before for switching converters. However I would much prefer to just use the hardware I already have as it cuts cost but also PCB space will be extremely limited.If you think that there is nothing wrong with the schematic I posted second, I would be happy to test it with another ir2127. Perhaps there was a problem elsewhere in the circuit that was causing this issue.Thanks again,

author: T3sl4co1l
author_rank: Super Contributor
timestamp: May 23, 2014, 11:21:50 pm
content: The second schematic answered my questions.. you poofed it. Go back to the boostrap diode thing. It'll still be poof-prone until you have a proper circuit though.Space is hardly an excuse; monolithic controllers come in 2x2mm packages (er... have fun with that), or some ingenious circuits can do it in a couple discrete transistors (though by sheer pin count, it still ends up bigger than using ICs). Spare MCU pins simply do not count as saving space.Tim

author: T3sl4co1l
author_rank: Super Contributor
timestamp: May 24, 2014, 05:17:31 am
content: Also the obvious... what's the load, how much of it, and where's the reaction diode?Tim

author: unit998x
author_rank: Contributor
timestamp: May 24, 2014, 10:53:14 pm
content: The continuous current will be about 25, with surge of up to 50a. The buck diode is instead a MOSFET which switches synchronously with the other so to increase efficiency.