power line voltage rise when plugging power from AC->DC adapter into device

[Valentin Tihomirov]

Background:



Hello, I've developed two devices. They have absolutely different purposes
and voltage regulation systems. The first is based on linear regulators the
second is based on switching regulators. Both systems are based on the same
MCUs (TUSB3210). They have an external crystal and internal oscillator. The
oscillator starts oscillating very stably when I power the device from:

- DC supply (6v .. 40v)

- AC/DC adapter with stabilized output (5v)

- USB hub (5v)



The Problem:

However, there are two types of AC/DC adapters that cause problems with
start up. The oscillator does not start (it seems that it starts for several
milliseconds and becomes dim), positive polarity signal is coming from rs232
driver causing errors to connected systems. This problem occurs only when I
connect output of the powered adapter into power socket of my device. If I
change the order plugging the output of adapter into my device and only then
insert the adapter into outlet the system starts OK!



I have done some homework and additionally figured out that the DC supplies,
which do not cause problems at start up, charge the power line of my device
smoothly. That is, according to theory of phisics, the capacitors (2x300uF,
filtering input and output of switching power regulator) should be charged
exponentially. This (smooth characteristic) is what I see using correct
adapters. The picture is quite different using the problematic types of
AC/DC adapters. I clearly see a segment of linear growth of voltage of power
line during power-on.



I have solved the problem by adding a coil to the input of power regulator.
This has improved the characteristic of power line voltage raise. For the
time being the characteristic is purely exponential. Have anybody faced the
same problem and what could be the cause of such a strange behavior?

 

[Tim Shoppa]

I have done some homework and additionally figured out that the DC supplies,
which do not cause problems at start up, charge the power line of my device
smoothly. That is, according to theory of phisics, the capacitors (2x300uF,
filtering input and output of switching power regulator) should be charged
exponentially. This (smooth characteristic) is what I see using correct
adapters. The picture is quite different using the problematic types of
AC/DC adapters. I clearly see a segment of linear growth of voltage of power
line during power-on.

It's not the segment of linear growth that's causing your problems. It's
almost certainly the region outside the linear segment.

Regulated AC/DC adapters with low current limits will go into
current limiting if you hook a large capacitive load onto them. This is
probably what's causing your problems. A constant current into a
capacitor will give a perfectly linear voltage rise. If the regulator
goes into foldback current limiting then all sorts of bizarre behaviors
will result.

The regulator inside your device isn't helping things either - you told
us it's a switching regulator, which means it will draw the most current
when the input voltage is low. This will make the chance of the AC/DC
adapter's regulator going into current limiting even greater.

I have solved the problem by adding a coil to the input of power regulator.

Yeah, the inductance will counteract the large capacitive load.

One general solution is to not string regulators together in series.

A more specific solution is to not start your switching regulator until
the input voltage to your device has stabilized. Most switching regulator
IC's have a pin just for this purpose.

Tim.