Column: DC/DC converters
Selecting external components for a DC-DC converter
By Lars Foerster, DC/DC Business Development Manager EMEA, TDK-Lambda T
here are many benefits to using standard off-the-shelf DC-DC converters. Tey reduce the design time, risk and cost oſten associated with discrete converters.
For optimal operation and performance, however, a standard converter usually requires added external components. Simple yet comprehensive instructions on which parts to use should be available in the manufacturer’s installation manual, which should also give guidance on where to position these for best possible performance. It is highly recommended to draw a schematic first, then build and test the prototype before starting PCB layout. Tere are a few commonly-used external components for standard converters:
Input capacitor A low ESR electrolytic capacitor, placed as close to the converter as possible, will minimise the impedance of the input wiring from the DC source to the converter. Ensure that the capacitor’s voltage rating is sufficient. Another point to consider is whether the end equipment will be used in below-zero temperatures. If it is likely that the capacitor will be used in environments below -10°C, then its capacitance value may need increasing to compensate for the drop in its ESR.
Input fuse Te installation manual or datasheet will instruct whether an external fuse is required. If a fuse is necessary, then the rating and type will be recommended. Failure to follow these instructions will result in a safety hazard.
Reverse polarity protection If the converter’s input polarity could conceivably be reversed, a protective diode must be inserted across the input; see
Figure 1: Input connection to a standard DC-DC converter
impedance in the wiring or PCB traces between the DC-DC converter and the load. To avoid any start-up issues, follow the
maximum capacitance values specified by the manufacturer.
Figure 2: Adjusting the DC-DC converters output using the ‘trim’ connection
Output voltage adjustment Te DC-DC converter may feature a ‘trim’ connection if the output voltage is adjustable; see Figure 2. To trim the output voltage up, connect a .
resistor from the ‘trim’ connection to -Vout
Don’t exceed the maximum output power of the converter. To trim the output down, the resistor . To avoid noise
is connected to the +Vout
pickup, the trim resistor should be as close as possible to the DC-DC converter. Te manufacturer should provide the relevant equations to calculate the required resistance.
Figure 3: On/Off control using the remote control (RC) terminal
Remote on/off Most DC-DC converters offer a function to turn the output on or off remotely, using a relay, switch or resistor; see Figure 3. Since there are two types of remote on/
Figure 1. Te diode should be rated for the maximum input voltage and, of course, have a higher surge rating than the input fuse.
Output filter Fitting an electrolytic and/or ceramic capacitor across the output terminals will improve the converter’s response to sudden load changes. It will also reduce output ripple and noise, and compensate for any
off polarity, care must be taken. For ‘negative logic’, the RC terminal must be pulled down (shorted) to -Vin
to activate the output
voltage, or pulled high (disconnected) to inhibit the output. On the other hand, ‘positive logic’ requires the RC terminal to be open to enable, or shorted to -Vin
to inhibit. If the installation manual does not provide
enough details, contact the manufacturer’s sales or technical support team.
www.electronicsworld.co.uk March 2021 11
Page 1 |
Page 2 |
Page 3 |
Page 4 |
Page 5 |
Page 6 |
Page 7 |
Page 8 |
Page 9 |
Page 10 |
Page 11 |
Page 12 |
Page 13 |
Page 14 |
Page 15 |
Page 16 |
Page 17 |
Page 18 |
Page 19 |
Page 20 |
Page 21 |
Page 22 |
Page 23 |
Page 24 |
Page 25 |
Page 26 |
Page 27 |
Page 28 |
Page 29 |
Page 30 |
Page 31 |
Page 32 |
Page 33 |
Page 34 |
Page 35 |
Page 36 |
Page 37 |
Page 38 |
Page 39 |
Page 40 |
Page 41 |
Page 42 |
Page 43 |
Page 44
