Help on over-heating Op-Amp LM2904 - Page 1
Forum section: electronics / repair

author: Rat_Patrol
author_rank: Frequent Contributor
timestamp: February 14, 2020, 09:24:41 pm
content: I have a 20 year old industrial controller I'm trying to repair. It is responsible for running high voltage solenoids.When just sitting idle, the LM2904 will run about 15c hotter than a reference board (luckily, I have access to a working controller), and the diode that comes off the DC/DC converter that supplies the VCC is also running warm in comparison, confirming the higher current.If I replace the LM2904 with a new one, it will work just sitting at an idle (powering some micros that run the whole thing), but as soon as I have it operate some solenoids, it seems to damage the LM2904. I replace it, its GTG again until I ask it to run solenoids.The chip seems to be responsible for the 80v boost converter circuitry for the solenoids, and also the 5v that runs the micros and other ICs, as that is what I loose when the chip fails and that is what I have traced it out to.Link to the LM2904 datasheethttps://www.onsemi.com/pub/Collateral/LM358-D.PDFThoughts?

author: george.b
author_rank: Frequent Contributor
timestamp: February 14, 2020, 09:31:51 pm
content: Hard to say without schematics.

author: Rat_Patrol
author_rank: Frequent Contributor
timestamp: February 14, 2020, 09:41:54 pm
content: Wish I had schematics.

author: fzabkar
author_rank: Super Contributor
timestamp: February 15, 2020, 02:25:05 am
content: I'm intrigued as to how an op amp can be involved in power supply generation. :-?Could we see a photo of the PCB?

author: Rat_Patrol
author_rank: Frequent Contributor
timestamp: February 15, 2020, 06:21:33 pm
content: Unfortunately no photo, boss would probably fire me if I started putting photos of the board out there, as we had to sign some rock solid NDAs since a lot of what we do is sub-contract work for other companies.Here is what I've mapped out of the IC's duties so far. It appears the other half does something similar with the 80v supply.

author: TerminalJack505
author_rank: Super Contributor
timestamp: February 15, 2020, 06:51:01 pm
content: My suspicion would be that the cap sitting between the MOSFET's source and the op amp's inverting input is causing the voltage at the input to be pulled beyond the op amp's power supply rails. I don't think that part is susceptible to latch-up but it can source current from its inputs when they are below the negative supply. The maximum allowed current is 50mA.See note 6 on page 4 of the datasheet.

author: fzabkar
author_rank: Super Contributor
timestamp: February 15, 2020, 08:09:23 pm
content: Quote from: TerminalJack505 on February 15, 2020, 06:51:01 pmBut there is a relatively big 10uF capacitor at that point. Perhaps it has dried out?

author: TerminalJack505
author_rank: Super Contributor
timestamp: February 15, 2020, 08:14:22 pm
content: Quote from: fzabkar on February 15, 2020, 08:09:23 pmGood point. The equipment is 20 years old so that sounds reasonable.

author: Rat_Patrol
author_rank: Frequent Contributor
timestamp: February 15, 2020, 09:57:13 pm
content: I've already replaced the 10uF cap, it was electrolytic, no joy.I've actually been replacing these components one-at-a-time all day, still keep overheating and popping.

author: Rat_Patrol
author_rank: Frequent Contributor
timestamp: February 15, 2020, 10:01:59 pm
content: Also was able to map out the other half of the that op-amp today.I guess it doesn't do anything directly with the 80v converter circuit, but once it fails, it drops the VCC to about 2.8v and that must affect the 80v circuitry, which is apparently controlled by a LM2901. Haven't mapped that out yet, but I don't think its the culprit.

author: fzabkar
author_rank: Super Contributor
timestamp: February 15, 2020, 10:27:54 pm
content: Would it help to measure the resistances between Ground and each of V-, V+ and Vout, and between Vcc and each of the same pins? You could then compare the results against a good op amp from the same batch. This might point you to the source of the damage.

author: Rat_Patrol
author_rank: Frequent Contributor
timestamp: February 15, 2020, 10:29:29 pm
content: You know, looking at that first pic, it is not unlike a convoluted voltage regulator, doesn't it?Since VCC is 5 volts, and I know the sensors out to the compressor are supposed to be 5v, is it not just regulating the 24v source (Yeah, its 24v DC, it's for controlling a mobile hydraulic crusher), it appears to just be making a 5v reference for the sensors.Or am I looking at this wrong?So maybe half of this op-amp is for making a 5v sensor reference voltage, the other half I'm not completely sure of yet.

author: TerminalJack505
author_rank: Super Contributor
timestamp: February 15, 2020, 10:56:35 pm
content: I can't quite make sense out of that first schematic. Is the node that runs between the MOSFET's source and the 47k resistor in parallel with the 0.01uF cap actually GND? It doesn't seem correct the way it is shown.Have you checked (with a scope) the output of that op amp to see if it is oscillating? The 0.001uF cap between the output and the inverting input may be bad in that case.

author: Rat_Patrol
author_rank: Frequent Contributor
timestamp: February 15, 2020, 10:59:04 pm
content: It should all be accurate. GNDD is ground. There are 2 grounds: regular battery(-) ground and just a static grounding type ground (mounting screws "pads" have a ground circuit that grounds to the case).

author: Rat_Patrol
author_rank: Frequent Contributor
timestamp: February 15, 2020, 11:00:52 pm
content: Out of Q? (FQP19N20) pin 3 is 5 volts.

author: fzabkar
author_rank: Super Contributor
timestamp: February 15, 2020, 11:10:31 pm
content: IC304A in the second diagram is attempting to maintain its V- input at 2.5V, ie the same as the voltage at V+, which is set by the 2.5V precision reference IC (LT1009ID). Since V- is derived from Vcc via a 1:1 potential divider (1K:1K), then IC304A is operating as a 5V regulator for Vcc.The regulator's error voltage drives TP1005 via Q1000 and Q1001. I don't understand why Q1001 would be powered from the same Vcc voltage that it is trying to control. :-?The regulator's error voltage drives the Gate of Q1000 via TP1004. Therefore Q1000 must be the pass transistor for the linear regulator. Its Drain pin (TP1005) must be the raw DC input, while the Source pin is the regulated 5V output (Vcc).Which voltage powers the op amp? Is it powered directly from the battery (24V)?

author: TerminalJack505
author_rank: Super Contributor
timestamp: February 15, 2020, 11:19:31 pm
content: Quote from: Rat_Patrol on February 15, 2020, 10:29:29 pmYes, this seems right.

author: fzabkar
author_rank: Super Contributor
timestamp: February 15, 2020, 11:27:19 pm
content: Is Q1001 drawn correctly? ISTM that it should be a P-channel MOSFET, otherwise its body diode would be shunting the raw DC supply in forward biased mode. Edit: Even if it were P-channel, I still don't see the point of connecting its D & S directly across a supply rail, unless it is intended as a crowbar. :-?

author: fzabkar
author_rank: Super Contributor
timestamp: February 15, 2020, 11:49:48 pm
content: Quote from: TerminalJack505 on February 15, 2020, 11:19:31 pmI agree.The op amp is comparing Vcc against V_sensor, so the target voltage for V_sensor must be 5V. The error voltage drives the Gate of FQP19N20 (the pass transistor) via the PNP/NPN pair.The 1.8 ohm resistor is a current sensor which turns on the upper PNP transistor when the current reaches 333mA (= 600mV / 1.8 ohms). This then turns on the NPN transistor, which in turn reduces the gate drive to the pass transistor.

author: Rat_Patrol
author_rank: Frequent Contributor
timestamp: February 15, 2020, 11:50:53 pm
content: Yes, all N channel MOSFETS

author: fzabkar
author_rank: Super Contributor
timestamp: February 16, 2020, 12:00:27 am
content: Would it make sense, for troubleshooting purposes, to add Schottky rectifiers between ground and each of the V- and V+ inputs, and between those same inputs and the op amp's supply pin? This would help to eliminate latchup as a failure mode.You might also like to consider powering the op amp from clean DC source to eliminate that as a point of failure, if only for troubleshooting purposes.

author: fzabkar
author_rank: Super Contributor
timestamp: February 16, 2020, 12:04:35 am
content: Quote from: Rat_Patrol on February 15, 2020, 11:50:53 pmBut then there would be a forward biased body diode across the supply. That can't be right.https://www.st.com/content/ccc/resource/technical/document/datasheet/66/f2/4e/c4/ee/dc/45/e4/CD00002848.pdf/files/CD00002848.pdf/jcr:content/translations/en.CD00002848.pdf

author: TerminalJack505
author_rank: Super Contributor
timestamp: February 16, 2020, 02:06:21 am
content: I can't really say what the problem might be. Here are my suggestions...Check that resistor on the output of the op amp in your first schematic.Check the cap going from the output to the inverting input.Put a scope on each of the op amp outputs and check to see if they are oscillating. So far as I can tell from your schematics, they shouldn't.Put a scope on each of the inputs and make sure they don't exceed the rails of the op amp.Test one of the failed ICs out of circuit to see if just one of the op amps is bad. If so then focus on the parts of the circuit that affect that op amp.

author: SMdude
author_rank: Frequent Contributor
timestamp: February 16, 2020, 09:39:02 am
content: Perhaps there is nothing wrong with that section of the circuit at all as when you replace the opamp it works nicely and regulates the voltage just fine.Then when you engage the 80v circuit, perhaps there is a voltage transient on the opamps supply that damages the opamp. The only way to catch this is with a storage scope monitoring the opamp supply. Perhaps there is a bad diode somewhere?Given the high resistance of the resistors at the opamp output and inputs, I don't think it would be likely that the opamp is being damaged on its input/output pins.

author: duak
author_rank: Super Contributor
timestamp: February 16, 2020, 08:12:36 pm
content: What happens to the LM2904s? Do any of the inputs or outputs short to GND or Vcc or go open?Here's a link to some design considerations for the LM2904:http://www.ti.com/lit/an/sloa277/sloa277.pdfTwo things come up on a quick perusal:p.4 -1.4: Vcc max might be as low as 26 V - is this exceeded?p.8: if the inputs are brought much below 0 V, the output can experience phase reversal. I don't see how that can happen here unless V_SENSOR goes way negative.

author: Rat_Patrol
author_rank: Frequent Contributor
timestamp: February 17, 2020, 04:53:59 pm
content: I took a failed 2904 and put it on a breakout board, and tested it on the bench.I tied both inverting and non-inverting inputs to ground, gave it 5 volts on Vcc.Side A worked fine, 4.9 something volts. Side B only gave me .6 volts, so side B is what has the issue.That means its somewhere on the first schematic, I would think.

author: fzabkar
author_rank: Super Contributor
timestamp: February 17, 2020, 07:13:28 pm
content: Quote from: Rat_Patrol on February 17, 2020, 04:53:59 pmI don't think that's a valid test.You are amplifying the input offset voltage in open loop mode. The output could hit either rail, even if the op amp were working correctly.

author: james_s
author_rank: Super Contributor
timestamp: February 17, 2020, 07:27:21 pm
content: I suspect you're frying the IC due to transients on the power supply. The first thing I'd do is use a DSO to look at the power rail, set it to trigger on a rising edge just a little above the normal supply rail voltage and then activate the solenoids. Do the solenoids have internal freewheel diodes or are those part of the controller? I'd probably check those to make sure none of them have gone open. Then look at the power supply and see if you've got bad electrolytics, the power supply in my mom's TV blew up a while back because the bulk filter capacitor had gone open circuit, I repaired it and powered it on the bench and it seemed fine but blew up immediately when I tried it in the TV. I had missed that capacitor being bad and under load that was causing the input rail to bounce all over the place.

author: Rat_Patrol
author_rank: Frequent Contributor
timestamp: February 17, 2020, 07:29:49 pm
content: I was measuring voltage on the outputs. Since I got a difference, why would that not be a valid test?I should say that my normal job is assembly and PCB layout. This PCB came in from one of our good customers, and he can't get another (obsolete product), and its about $500k to replace this machine that runs fine, sans a PCB fault. Push come to shove it would be worth his money and time to have us design a new PCB and just take the micro off this faulty one and drop it on the new one full of new general components (the micro is an obsolete Motorola). A new PCB may be the only solution if there is some internal trace short anyway (but I don't think there is a trace short).

author: fzabkar
author_rank: Super Contributor
timestamp: February 17, 2020, 08:14:25 pm
content: Quote from: Rat_Patrol on February 17, 2020, 07:29:49 pmThe datasheet states that the typical input offset voltage is 2mV, and the typical open loop gain is 100V/mV.This means that, even when the inputs are tied together, there is still a voltage difference of +/- 2mV which, when amplified, becomes +/- 200V. That is, Vout could hit either rail.

author: Rat_Patrol
author_rank: Frequent Contributor
timestamp: February 17, 2020, 08:21:18 pm
content: When I tie the non-inverting pin on side A to Vcc, I get 4.98v out of output A.When I tie the non-inverting pin on side B to Vcc, I get 4.2v out of output B.

author: fzabkar
author_rank: Super Contributor
timestamp: February 17, 2020, 08:26:50 pm
content: Quote from: Rat_Patrol on February 17, 2020, 08:21:18 pmThat isn't really a valid test, either. This op amp is not a "rail-to-rail IO" type. It has an input common mode voltage range of 0V to Vcc - 1.7V (for Vcc = 30V). That is, the op amp doesn't operate like a real op amp if the input voltage is within 1.7V of the positive supply rail.I would configure the op amp as a simple voltage follower and set the input to Vcc/2 by using a 1:1 potential divider.

author: Rat_Patrol
author_rank: Frequent Contributor
timestamp: February 17, 2020, 09:05:55 pm
content: Setup the op-amp as a voltage follower, both sides simultaneously on the same Vcc and GND, and 1/2 Vcc .Used a pair 22k resistors for the voltage divider (first ones I found of THT resistors), gave it independent 2.2k dummy loads on the outputs.Side A: 1.8 volts on the output. Nearly no oscillation on my DMM.Side B: 2.3 volts on the output. Few hundreds (3-7) of a volt oscillation on my DMM.I did have to grab another blown 2904, as I accidentally blew this one (accidentally cranked the voltage knob on my power supply) and I didn't want to skew results.

author: Rat_Patrol
author_rank: Frequent Contributor
timestamp: February 17, 2020, 09:15:56 pm
content: Just took a look with the thermal, and the IC is running at ambient room temp. Been powered for about 10 minutes. Power supply shows zero amps draw (not very precise meter), so that is interesting.

author: Rat_Patrol
author_rank: Frequent Contributor
timestamp: February 18, 2020, 09:54:40 pm
content: I took off multiple resistors on both sides of the op-amp one at a time, even when my 5 volt and 80 volt supplies were killed off, the IC was still hot.This one is very puzzling to me.

author: fzabkar
author_rank: Super Contributor
timestamp: February 18, 2020, 10:46:32 pm
content: I would be worried that a failure of the op amp might lead to the Vcc and V_Sensor voltages rising rather falling, resulting in failures to other components.I would follow the advice regarding the possibility of open freewheel diodes across the solenoid coils. I would also consider a TVS diode across the op amp's supply rail to protect it from spikes.