September 30, 2021
AC-DC power supplies and DC-DC converters often feature an output voltage adjustment, the range of which is stated in the datasheet. An AC-DC power supply may have a potentiometer accessible to set the output voltage from the original factory set point. (See Figure 1). Adjusting the output voltage may be needed to compensate for voltage drops in the load cables, or to optimize the load’s performance or efficiency.
Board mount DC-DC converters may also have an output voltage adjustment range, usually achieved via the “trim” terminal. The trim connection is either pulled high or low via a resistor depending on the product.
Isolated DC-DC converters
An isolated model typically has a narrow adjustment range. For example TDK-Lambda’s 30W rated single output CCG30 models (Figure 2) can be set within + or -10% of the nominal output voltage.
A resistor is fitted between the trim pin and either the -Vout or the +Vout terminal depending if the voltage needs to be increased or reduced (Figure 3). If no resistor is fitted, the output will be the nominal value stated on the datasheet.
Non-isolated DC-DC converters
Non-isolated DC-DC models can have very wide adjustment ranges because of the topology used in the design. TDK-Lambda’s i7C4W008A120V converter (Figure 4), for example, has an output voltage range of up to 9.6 to 48V. This enables one part number to be used for several different system voltages and help inventory management.
The trim connection for the i7C, and other non-isolated models like the i3A, i6A and i7A, is slightly different. If no trim resistor is fitted, the output voltage will default to the low limit. From Figure 5, the trim resistor is always fitted between trim and –Vout. The full product specification provides the values and formula. As the resistance is lowered, the output voltage increases.
Adjusting a converter using a digital-to-analog converter
With the growing number of autonomous applications for battery powered robots and drones, there are demands for the output voltage to be electronically adjusted using a programmable digital voltage. A lower voltage might be used to extend battery life, enabling the robot to return to its charging station for example.
One method is to use a D/A converter (DAC) connected to the trim terminal of the DC-DC converter (Figure 6). TDK-Lambda recently posted an application note on how to use a DAC to adjust their buck and buck-boost non-isolated series. A link can be found here.
The application note details the type of DAC needed and shows the test results of an i7C converter being digitally controlled.
The DAC must be connected with short, direct traces to trim and ground terminal of the DC-DC power module. To avoid regulation errors from voltage drops, the ground paths need to be kept separate from the power traces carrying load currents. Any noise or voltage drop at the DAC output will cause unexpected output voltage variation.
It should also be noted that when adjusting the output up in voltage, the maximum power rating of the power supply or converter is not exceeded. Likewise the maximum current must not be exceeded when the voltage is below the nominal set point.