Tech Forum

The maximum reverse voltage across the diode will be 36 volts. A conservative rule of thumb is to double that, so choose a diode with a rating of at least 72 volts. The maximum current will be whatever your PWM controller delivers to the motor. When the PWM controller shuts off at the end of each cycle, the voltage across the motor inductance will instantaneously reverse polarity and begin to flow through the diode. Using the same rule of thumb, pick a diode that is rated for twice this current. You also need to be sure the diode can dissipate the power while keeping its junction temperature below TJ max (150 deg C is typical). The power dissipated is 1/2 L I^2 (the energy stored in the motor inductance) multiplied by the frequency of your PWM controller. Depending on this result, you may need to put the diode on a heatsink. Once you know how much power you need to dissipate, check the diode data sheet to get the thermal resistance Rth(j-c) in degrees C/W. Multiplying your calculated watts by Rth tells you how much the junction temperature will rise above its case temperature. There will also be a rise from the case to whatever the case is mounted on (probably a heat sink) but this will be small so you can probably ignore it if you follow the diode manufacturer’s recommendations on how to mount the diode. There will also be a temperature rise of the heat sink above the ambient air. Calculate this using the heat sink data sheet and your calculated power dissipation. Add all of the temperature rises to whatever your maximum ambient temperature is expected to be and if this is less than Tj max, you are OK thermally.