Specifying which low wattage, multiple output power supply to use can be complex if the user does not fully understand how output loading affects the product’s regulation characteristics.
An application powering a single board with fixed loads is fairly simple. The power supply is turned on, and the output voltages are monitored to ensure that they are delivering the correct voltage for the circuit card operation.
If the power supply is being used in a system with multiple cards, particularly when the end product is configurable for different user options, trying to predict output voltage changes requires more care and testing.
The block diagram of a traditional, low cost triple output is shown in figure 1.
The power supply control circuitry senses the output voltage of the +5V output, and if the voltage changes, it will compensate accordingly. The two other outputs, +V and –V, are not part of the control loop and are classed as semi regulated.
There are three types of conditions which will cause the voltages to vary.
This is the change in output voltage due to variations in the AC input, usually specified from minimum to maximum AC (90-264Vac). When the AC input is reduced, the power supply’s control loop will compensate accordingly by increasing the pulse width of the switching section. The actual line regulation on all three outputs will be quite small, just a few mV.
This is the change in output voltage due to variations in output load. A change in the +5V load will again be a few mV as the control loop will compensate. On the +V & -V, because they are semi regulated, the voltage regulation will be much higher, in the order of +/-360mV for the +/-12V outputs. Most manufacturers will also stipulate a minimum load condition for the regulation, sometimes as high as 25%. A point to note is that if the minimum load is not applied, the power supply will not be damaged, but the change in output voltage becomes much greater.
When the 5V load is changed, the control circuitry adjusts the pulse width of the switching section as before, but this significantly alters the +V and –V output voltages. Even with a minimum load of 25%, voltage changes of 700mV can be expected.
If the +5V has a trim potentiometer to adjust the output voltage, then the +V & -V outputs will rise or fall by a similar percentage.
For systems that cannot tolerate such large variations, or if minimum loading requirements are not desired, there is a solution available. TDK-Lambda’s CUT75 triple output power supply utilises a two converter topology, one to power the 5V output and one to power the +12V & -12V outputs. The block diagram is shown in figure 2.
The result is a dramatic improvement over a one converter solution. Measured line and load regulation on the +/-12V outputs is only 300mV for a 0-100% load change.
- The two converter approach also has other benefits:
- No minimum loading requirements
- No cross regulation from +5V to the +/-V outputs
- The 5V can be adjusted up to 5.25V to compensate for cable drops, without affecting the +V & -V output voltage levels
- The +V and –V outputs are electrically isolated (tested to 500Vac) from the 5V, allowing them to be connected in series as one output. For example, the dual +/-12V can be configured to provide an isolated 24V output.
- The two transformer design also allows lower profile magnetics to be used, giving the CUT75 a height of only 38mm.
Traditionally post regulators are used in low power multiple output designs to improve regulation performance. In the CUT75 these have been eliminated with a novel passive circuit. This gives the CUT75 an efficiency rating of 85%, enabling the series to operate without the need for forced air cooling.
More details on the CUT75 can be found on this link www.emea.lambda.tdk.com/uk/cut.