Ultra fast recovery diode specification, testing and typical part number
Switch mode power supply requires ultra fast recovery diode at the secondary side to convert the ac voltage to dc. If
you use a general purpose diode to replace the ultra fast recovery diode, it will burn in less than few seconds. Why?
Check from the philips ecg master replacement guide, an ultra fast recovery diode have the speed of 75 nanosecond (ns) or faster while the general purpose diode are quite slow which in the microsecond! Even the replacement diode that have 50 ns different may cause the diode to burn not in 1 hour time but in few seconds. Replacing the diode with original specification make the equipment last longer.
Ultra fast recovery diode specification are quite the same as the general purpose diode which is the volt(prv) and amp. Always replace the ultra fast recovery diode with a spec that is the same or higher. The speed is very important and if possible find the exact speed or faster. For example, you can replace a 75ns diode with a 50ns or 35ns diode but
you cannot substitute with a 100 ns or 150 ns diode. The diode will not last long and sometimes burnt immediately.
Checking the ultra fast recovery diode is just the same way as you check a general purpose diode. Set to time 10 kohm and it should have one reading using analog multimeter. Typical part number for ultra fast recovery diode are UF5404, HER308, 31DF6. The general purpose diode part number are 1N5408, 1n4007 and etc. When you want to buy ultra fast recovery diode from your local electronic parts supplier, be careful that they might give you a wrong part. Most of the time when i ask for UF5404 they will give me 1N5404. Both diode have the same voltage and amp except the speed!