Failed Opamp - Page 1
Forum section: electronics / projects

author: Upballoon
author_rank: Contributor
timestamp: October 30, 2020, 02:14:22 pm
content: I made a Electronic Load to test my DIY reflow oven. The E-load uses a stm32f103 and a DAC to to control the current. As i was testing the max current through the load, my Opamp that's supposed to be driving the mosfet gave up. And I'm not sure why. My current theory is that it got too hot? But i'm not sure.Further most of the opamps on Digikey say that their operating temps are sub 200°C. But then How do you solder these?

author: Upballoon
author_rank: Contributor
timestamp: October 30, 2020, 02:15:37 pm
content: Here (attached) is the PCB layout too. The opamp is on the top left. Mosfet in the middle

author: fcb
author_rank: Super Contributor
timestamp: October 30, 2020, 02:45:39 pm
content: Possibly inductive load resistors leading to spiking the opamp inverting input. But a few k of resistance on the input to the opamp. Also, 50ohm is quite low for the MOSFET, you really only need low values in the gate drive when you have to overcome significant gate capacitance (i.e. when switching fast).

author: wraper
author_rank: Unknown
timestamp: October 30, 2020, 02:51:26 pm
content: Quote from: Upballoon on October 30, 2020, 02:14:22 pmMost? Where did you see any opamp which can operate at 200oC?Operatingtemperature range has nothing to do with soldering temperature.

author: Upballoon
author_rank: Contributor
timestamp: October 30, 2020, 02:56:59 pm
content: Quote from: fcb on October 30, 2020, 02:45:39 pmYa I was initially thinking of using PWM and low pass filtering it but it added to much unecessary complexity. The 50 Ohm is a remenant from that design.

author: langwadt
author_rank: Super Contributor
timestamp: October 30, 2020, 02:58:38 pm
content: Quote from: Upballoon on October 30, 2020, 02:14:22 pmsure it wasn't the mosfet that gave up first? that's tiny mosfet is designed for switching

author: Upballoon
author_rank: Contributor
timestamp: October 30, 2020, 02:59:45 pm
content: Quote from: wraper on October 30, 2020, 02:51:26 pmSo what's the soldering temperature? How do I know i'm not causing the opamp to deviate from its properties?

author: Upballoon
author_rank: Contributor
timestamp: October 30, 2020, 03:05:46 pm
content: Quote from: langwadt on October 30, 2020, 02:58:38 pmI'm not sure, The drain to source resistance when not powered reads 118 Ohms. I feel like it's supposed to be higher. Either way, The output of the opamp reads 0.2V when the non inverting lead is at 2.5V. So the opamp is definitely bad. It could be that the MOSFET failed creating a current spike that caused a parasitic inductance in the resistors to create a kick causing the inverting input to fail. But that's just speculation. Dont' think the resistors are that inductive.

author: wraper
author_rank: Unknown
timestamp: October 30, 2020, 03:18:16 pm
content: Quote from: Upballoon on October 30, 2020, 02:59:45 pmReflow temperature when the whole package is heated. Soldering iron can be hotter than that.

author: Upballoon
author_rank: Contributor
timestamp: October 30, 2020, 03:28:04 pm
content: Quote from: wraper on October 30, 2020, 03:18:16 pmOh that's what the packaging information table is for. I see. I've never looked at the table. Thanks for pointing it out.

author: exe
author_rank: Unknown
timestamp: October 30, 2020, 03:30:05 pm
content: Quote from: fcb on October 30, 2020, 02:45:39 pmWhy it can't be low? I don't think it needs to be high as higher value will create more phase lag and lower bandwidth or create instability.

author: Kleinstein
author_rank: Super Contributor
timestamp: October 30, 2020, 04:11:56 pm
content: In the given circuit a low resistance value is Ok, a larger resistor would make things only worse with higher probability for oscillation. If the MOSFET is broken, the gate may be conducting an pulling the OP low.The simple circuit is a bit prone to oscillation, especially if the power source is inductive or high impedance. When oscillating the DMM readings may also look odd, as they are only some average of some type. So the the cards could be still ok just working in AC mode.For stable operation it usually needs a slightly different circuit with some direct FB from the OP. Than a larger resistor in front of the gate can help, though 50 Ohms are not that bad.Quote from: langwadt on October 30, 2020, 02:58:38 pmsure it wasn't the mosfet that gave up first? that's tiny mosfet is designed for switchingThis is a good point ! Think more about a TO247 or similar case and higher voltage type to have at least a chance for a acceptable SOA. The MOSFET will have to consume most of the power.

author: Conrad Hoffman
author_rank: Super Contributor
timestamp: October 30, 2020, 04:59:21 pm
content: Most successful constant current sinks will have a resistor in series with the negative input of the opamp, say 1k or so, and a small capacitor directly from the opamp output to negative input, say 100 to 3000 pF. You often get regions of instability without doing that.

author: Vovk_Z
author_rank: Super Contributor
timestamp: October 30, 2020, 06:21:24 pm
content: Quote from: Upballoon on October 30, 2020, 03:28:04 pmA usual rated soldering temperature for all semiconductor (and not semi) parts is about 260-270 C.With a hand-soldering we solder usually with that temperature or a bit higher (up to 300 C), it depends on a soldering station quality and else).

author: Vovk_Z
author_rank: Super Contributor
timestamp: October 30, 2020, 06:23:35 pm
content: Quote from: Conrad Hoffman on October 30, 2020, 04:59:21 pm+1.

author: mathsquid
author_rank: Frequent Contributor
timestamp: October 30, 2020, 07:03:29 pm
content: Maybe you just got a bad op-amp?

author: Upballoon
author_rank: Contributor
timestamp: October 30, 2020, 08:30:50 pm
content: Quote from: Conrad Hoffman on October 30, 2020, 04:59:21 pmCould you explain how adding the capacitor helps?

author: Conrad Hoffman
author_rank: Super Contributor
timestamp: October 30, 2020, 08:43:00 pm
content: The small cap just provides local feedback around the opamp, assuming it's unity gain stable. The original circuit, AFAIK, was in an old Siliconix MOSPOWER application book. Efforts to remove those seemingly minor parts usually result in poor performance or instability. The resistor also isolates the input from any bad things that might happen with the load.

author: Doctorandus_P
author_rank: Super Contributor
timestamp: October 30, 2020, 10:20:40 pm
content: In addition to filtering the the opamp-input (which also works as a current limit for spurious input signals), a (Darlington) transistor is probably a better fit for current sink / source.With a darlington (or Szishalsky pair) the drive current is small enough to not be a problem (unless you want 6 digits accuracy), and a BJT is easier to get the control loop stable.

author: S. Petrukhin
author_rank: Super Contributor
timestamp: November 02, 2020, 02:27:17 am
content: Quote from: Upballoon on October 30, 2020, 02:14:22 pmI think you built the generator because there is no load. When 3.3V is applied to the + OP input, its - input is grounded, the OP starts to set the output (3.3-0)*K ~12V, the transistor starts to open, and as soon as the voltage on the shunt (set of resistors) reaches 3.3V, the OP turns off, closes the transistor, and the whole story repeats to the maximum frequency that the OP is capable of. This is a high enough frequency that the OP output is affected by the MOSFET capacity.For the current limiting circuit to work correctly, a load that will drop the entire voltage connected in the drain circuit is required. The shunt is only used as a current-to-voltage converter and a small part of the voltage is lost on it.And in general, linear regulation at this power seems unreasonable to me. Why don't you use PWM?

author: wraper
author_rank: Unknown
timestamp: November 02, 2020, 03:06:09 am
content: Quote from: S. Petrukhin on November 02, 2020, 02:27:17 amPWMing MOSFET in electronic load, really? What no load, it's electronic load by itself, it loads something, not the other way around.QuoteIt defeats the purpose of electronic load. It's electronic load which uses MOSFET in linear mode. Not a switch with current sensing.QuoteThere is 0.3 Ohm shunt resistor. At 3.3V it corresponds to 9.9A current which is quite reasonable as max range for electronic load. Though you will need more MOSFETs with individual feedback loops and large heatsink if it is intended to drop significant voltage at such current.

author: S. Petrukhin
author_rank: Super Contributor
timestamp: November 02, 2020, 05:20:16 am
content: Quote from: wraper on November 02, 2020, 03:06:09 amPerhaps I did not understand something and the control voltage is applied in proportion to the permissible range of voltages on the shunt...

author: Upballoon
author_rank: Contributor
timestamp: November 02, 2020, 03:08:36 pm
content: Ya I don't think that will happen. Since there is no source the MOSFET gate will read 12V but the Source of the FET will have a 0.3Ohms pull down so it will never go above well 0V as long as no source voltage is applied.