Introduction
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.
Figure 1: RWS50B output adjustment
potentiometer (left side)
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.
Figure 2: CCG30 DC-DC converter
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.
Figure 3: Adjusting the output of a CCG
converter
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.
Figure 4: TDK-Lambda’s i7C 300W
buck-boost DC-DC converter
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.
Figure 5: i7C trim circuit
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.
Figure 6: Typical application circuit
for digital output voltage adjustment
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.
Power Guy
