Ultra Fast Recovery Diode Specification, Part Number & TESTING

Switch mode power supplies requires ultra fast recovery diode at the secondary side of power supply to convert the ac to dc voltage. If you use a normal diode to replace the ultra fast recovery diode, it will burn in less than few minutes and sometimes in seconds. Why? Check from the philips ecg master replacement book, an ultra fast recovery diode have the speed of 50 nanosecond (ns) or faster while the normal diode are quite slow which in the microsecond! Even the replacement diode that have 25ns different may cause the diode to burn not in 1 hour time but in few seconds. Replacing the ultra fast recovery diode with the original specification will make the equipment that you repair last longer.

Ultra fast recovery diode specification are quite the same as the normal diode which is the volt(prv) and amp. Always replace the ultra fast recovery diode with a specification rating 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 100ns diode with a 75ns or 50ns diode but you cannot substitute with 150 ns diode. The diode will not last long and sometimes will burnt immediately when you switch on the equipment. A large or big diode doesn't mean that the diode specification is bigger, one always have to refer to databook or search the internet for actual data.

Measuring the ultra fast recovery diode is just the same method as you test a general purpose diode or normal diode. Set to the range of time 10 k ohm and it should have one reading using analogue multimeter. If you get two readings then the diode is consider shorted. Some example of  ultra fast recovery diode part number are UF5408, HER302, 31DF4 and etc. The general purpose diode part number are 1N5406, 1n4005 and etc. When you want to buy ultra fast recovery diode from your local electronic parts distributors, be careful that they might give you a wrong part number. Most of the time when i ask for UF5408 they will give me 1N5408. Both diodes have the same voltage and ampere except the speed! That's why experience count and hope you get the knowledge of this secrets.