We're using a total of 18 of the ADuM4471 four-channel isolators to support 9 Integrated Power Modules in a magnet control system. We were originally using the recommended Coilcraft CR798x transformers.
Everything works just fine. However, during the open-loop startup sequence of the ADuM4471, the chip overvoltages its secondary side before the beginning of closed-loop operation and this causes the pass transistor of the internal 5-volt regulator to permanently short-circuit. The external 5-volt load is a constant 20mA, well within specifications.
We can solve the problem by using fewer secondary turns on the transformer, but because the IPM will go into undervoltage lockout at 13.5V, we need *at least* 15.0V from the ADuM4471. We are using the recommended circuit for +15V regulated output (figure 48 in the datasheet).
After a half dozen of these failures, we are now instead powering the ADuM4471 secondary side from a separate isolated regulated +15V supply. After a few startups, another internal regulator failed.
These are production-quality circuit boards with careful layout and isolation. Perhaps the isolator chip is too-lightly loaded at startup?
After reading through the ADuM4471 datasheet several times, it appears the breakdown voltage of the secondary VREG (Pin 20) may be very low, perhaps less than 20 volts which may be causing this problem.
We have decided to clamp VREG to 16.5 volts using a 16V zener and a 2-ohm resistor to protect the ADuM4471. Our feedback network is calibrated to 15.1 volts.
The overall systems require extreme reliability for safety reasons. Will the zener clamp solve our problem? Perhaps an application engineer can suggest an alternative isolator which is better suited to the job, although we've browsed your product selection extensively.
Each IPM needs 6 forward channels and 2 reverse channels at no higher than 20kHz, and requires +15V at up to 50mA to run the IPM internals. However because of our difficulties we are considering not to use the +15V output from the ADuM4471 to power the IPM.
Can you suggest a better part, or a better solution? When it works, it works great. The parts only fail on startup, they have never failed while in operation.
Thanks for your consideration
It is possible to add the zener diode clamp to protect the VREG pin, depending on the tolerances of the zener.
Looking at your application, the recommended circuit for 5V to 15V would be figure 49, which uses a doubling circuit and transformer turns ratio of 3, from the datasheet equation (2):
Ns/Np = (Viso/2)/(Vdd1(min)*D*2) = (15/2)/(4.5V*0.3*2) = 3.0.
This circuit has less voltage stress by using Viso/2 to power the VREG pin.
If you want to discuss the use of figure 48 circuit further, can you send me your schematic and the spec for the transformer used?
Thank you Brian for your prompt and helpful reply.
We used figure 48 not realizing the maximum output voltage shown might stress the part. We used equation (1) giving a transformer turns ratio of 5. This no doubt made our problem worse, since it made the overvoltage higher.
Ns/Np = (Viso + Vf diode) / (Vdd1(min)*D*2) = (15+0.2) / (5.0*0.3*2) = 5.0
As of the time of this writing, neither Halo nor Coilcraft (nor any distributors) have any of the 1:3 ratio transformers in stock, but we now have received the ADuM4471 Evaluation Board and will start experimenting next week when we receive our sample 1:3 transformers from Halo.
We agree, it's much better to power the secondary side with a lower voltage. If we use the voltage-doubler circuit in figure 49, will this decrease the available output current at 15 volts, compared to figure 48 or will it be the same? One option we're very interested in is powering two separate ADuM4471 chips from the output voltage of only one transformer. This would be a nice improvement but because it isn't mentioned in the datasheet, the idea did not pass design review.
We'd like to keep using the ADuM4471 for the next revision of the IPM Driver board. I'd like to send you our schematic along with some notes, for your review. What is the procedure for sending?
The ADuM4471 used in figure 49 will provide 100mA output current when used at 2Mbps data rate for 5V to 15V as specified in Table 4 of the datasheet.
Powering the VREG pin from an external power supply may effect the output state of the secondary data channels in startup.
You can send me the schematic for review. I'll send you an email.
Thank you Brian for discovering the mistake in our schematic.
I am quite sure this is the reason our chips failed, because we connected Pin 20 (VREG) to the input side of the filter inductor, instead of the smooth filtered side as in the datasheet schematic.
I'm very confident this error we made caused the problem, as you explained in your email.
It is great you were able to review our circuit. We will be making the changes you suggested.
Our sincere thanks for solving this so quickly. We'll breadboard and test the changes to make sure we get it right.