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Part Number: ISOW7842
we are using the ISO7842 in one of our products to control an isolated switch as depicted in the picture below. "control signal" has logic high voltage level for a maximum of 1.5s and therefore, the switch closes (load: 1A) and then reopens again. On some of our PCBs we observe the following problem:
"control signal" is activated (high) for 1.5s, the switch closes and then reopens again. But after 1.5s, the ISOW7842 heats up caused by a short circuit of Viso/GNDiso (it is definitely the chip itself, after removing it from the PCB the short circuit on the PCB also disappears). In general, the ISOW7842 is not damaged the first time when "control signal" is activated, it needs some trials until the short circuit appears. Is there a mistake in our circuit (maybe the high impedance connection of primary and isolated ground) or a known issue with the isolated supply?
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In reply to Manuel Chavez:
your understanding is correct, the ISOW7842 only supplies the T600-2 gate through IC601-2 (by the way, the 1A through the switch are driven from an external power supply). I will do some measurements and hope to get the moment the ISOW7842 is destroyed. Maybe it will take some time to reproduce but I will let you know as soon as possible.
In reply to Sebastian Reichhold:
here are some screenshots of the measurement. Keep in mind that the device is still working (I did more than 50 tests) which means I do not have a timing breakdown of the failure.
Nevertheless, channel 1 (yellow, 1V/div) is Viso and channel 2 (green, 2V/div) is pin 14, the signal that controlls the gate driver. At first view, Viso voltage looks OK. What about pin 14 signal (There is a short spike up to 6V but this could also be caused by my setup because I had to use wires of 10cm length to contact the measuring point)?
Beside this, I will contact our manufacturer whether something special happend to the ISOW7842 parts during the manufacturing process? Are there any special recommendations about the handling of the ISOW7842?
some more information:
1. The parts were baked (48h @ 120°C) before use and the maximum temperature during the soldering process was 230°C. So moisture and soldering temperature (according to datasheet, 260°C is the peak temperature) can be excluded as the damaged parts cause, correct?
2. There is another PCB with one damaged ISOW7842 where the circuit on the isolated side (gate driver and FET) was never used. The ISOW7842 was broken before the FET switched 1A load for the first time.
3. I did a test where 4 circuits were tested 3000 times (each circuit switched 1A 3000 times). I could not reproduce a broken ISOW7842, they are still working.
I think (because of information 2. and 3.) the circuit on the isolated side is not the reason for the broken ISOW7842. The only thing (I found out up to now) that is common to all damaged ISOW7842 is their part marking which is
Is it possible that we got bad components (for whatever reason)?
Hi Koteshwar, hi Manuel,
to answer your questions:
1. The temperature test (baking) was done with the devices unpowered.
2. Unfortunately, I do not have more details when and how exactly the failure appears. All I can say is that some devices were damaged during the test (control the switch for 1A load) and some devices were already damaged before the test. As mentioned above, I tried to reproduce the failure by testing 4 devices 3000 times, but all devices are still working.
a. Switch T600-2 is used as lowside switch.
b. The switch is used for "nearly static on/off" control. Nearly means, the switch is turned on (= conducting) for 1.5s seconds and then turned off again. During the 3000 times test, the switch was turned on every 10s for 1.5s.
c. Assuming the gate of switch T600-2 is completely discharged, the peak current is about 5V/5Ohm = 1A. I know 1A is too much for ISOW7842 output but as there is nearly no voltage breakdown of Viso (see screenshot above) I reason the current is drawn from the bulk capacitors C600-2, C606-2 and C608-2 (the gate capacity of T600-2 is much less than the capacity of the bulk capacitors).
It is difficult to test a different channel, e.g. B (pins 4 & 13) because it is hardwired to GND. I would have to rip up some via/track of my PCB. But the waveform is nearly the same when tested with another device.
In reply to Koteshwar Rao:
In reply to BO:
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