Have you ever had difficulty getting a stable voltage on a AC to DC regulator with multiple outputs? The ratings for an output say they can provide low current, but you try this and it seems to drop the voltage. Why is this happening? Well, there may be some limitations on your AC DC converter here.
Example
An example would be our part number 1866-3960-ND (QP-320D). A customer informed us that line 2 was not regulating properly. They were trying to power a device that took very little current on channel 3 and only use a device that drew 0.5A on channel 2. However, this was causing the output voltage of channel 2 to drop to 9V. They had already adjusted the dial for this and wondered if this was normal behavior. So, is it defective? Not quite.
Main Line Regulation Feedback Loop
I was perplexed about this too because the datasheet isn’t very clear about minimum loading. The manufacturer informed me that there is a feedback loop for this particular design. They told me you have to minimally load Channel 1 to ensure the regulation will work properly for the other channels. This makes more sense if you take a look at the block diagram on page 2, see below:
V1 should be the same thing as channel 1 in this instance. This is the only line showing a feedback loop to a PWM control that is fed back to the rectifiers, PFC, and power switch devices before the transformers for each voltage channel. You can consider channel 1 as a “Main Line” here. This particular design is not uncommon. It is actually more expensive to have dedicated regulators for each channel. The design here could be considered a type of cross-regulation where the other channels depend on the load of the main channel.
Conclusion
If you are experiencing a dropout on one of your lines, check the block diagram if there is one. You may need to provide some kind of dummy load on one of the lines to ensure that regulation is consistent for your other channels. This is done to make a design more compact and efficient.
When a power supply requires a minimum load, the manufacturer is supposed to provide a specification giving the minimum load current required for regulation.
Without that spec you can’t design your system accurately to operate over many production lots with a fixed dummy load. Instead you’ll need to measure every unit during production runs to verify your self determined minimum load is applicable to the newer individual units.
If a manufacturer can’t or won’t provide an accurate minimum load value, and the product containing it will be mass produced, then it’s best to use a different manufacturer that does provide the specification.
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They do provide details in this datasheet about minimum loads for each channel. However, it wasn’t made very clear that the other channels (2 through 4) were dependent on channel 1. I should have been more specific. I just wanted to point out that some supply designs may require analyzing the block diagram. If I recall, there have been other instances of supplies not calling out details like this before where it can lead to similar situations.
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