A couple of years ago, when I was working with interfaces in the industrial market, the size of the device package was never a showstopper. Fast-forward a few years, and device package size is sometimes a deciding factor as to whether you can use it in your application.
Industrial application functionality is increasing while size is decreasing, similar to the consumer market, but on a smaller scale. The challenge is now with semiconductor manufacturers to provide smaller package solutions without losing performance. This is true in the isolator space as well; but with isolators, there are certain dimension requirements for high-voltage creepage and clearance demands. But still smaller is better.
In my last post, I discussed a new device from Texas Instruments, the ISOW7841, which integrates isolated data and power in a single package and can replace the discrete solution (dotted section) shown in Figure 1.
Figure 1: Isolated data and power subsystem
This integrated solution is smaller than the discrete solution which incorporates a transformer driver, transformer, rectifier and regulator (as shown in Figure 2).
Figure 2: Size comparison of discrete and integrated isolated data and power subsystem board layouts
On the left side of Figure 2, you can see the board layout with the discrete components needed to build an isolated data and power solution. On the right side, the design is simplified by just one component with two decoupling capacitors. The size of the design drops from 35mm by 28mm to 22mm by 15mm, around 33% of the discrete solution’s board space.
But is that the only savings you can expect from a single-chip solution? No. Here are some additional benefits:
Figure 3: Discrete solution for isolated power with a 4.10mm-thick isolation transformer
Figure 4: ISOW7841 evaluation module with planar transformers in a 2.65mm-thick 16-pin SOIC package
Another feature that is nice to have in isolated power supplies is the soft-start mechanism, which ensures controlled in-rush current and avoids any overshoot on the output during power up. Figure 5 compares the VCC and output voltage VISO waveforms of the ISOW7841 with a similar device on the market. Any overshoot at the output could potentially damage the load circuits. The soft-start scheme in the ISOW7841 limits primary peak currents drawn from the VCC supply and charges the VISO output in a controlled manner, avoiding overshoots.
These features (thermal shutdown, current limiting, soft-start) in the ISOW7841 device reduce the amount of external protection circuitry in the design, thereby simplifying it.
Figure 5: VISO power-up oscilloscope capture compares VISO waveforms of the ISOW7841 with a similar device on the market
The next time you are looking for space-saving isolation devices, look at more than the device’s XY dimensions. Additional device benefits may make your design smaller and your time to market faster.
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