Recently I was try to rectify the output of an “electronic transformer” using an ordinary rectifier diode 1N4007. The output of this device is bursts of frequency around 50kHz, with an envelope of 100Hz and RMS 15Volt measured by scope.

Under these conditions, why does the diode become extremely hot even with a pure ohmic load that requires just 120mA?

Also, what is the effect is we are use a fast-diode to rectify a low frequency waveform i.e. 50~160Hz.

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    \$\begingroup\$ What is your question, exactly? \$\endgroup\$ – alex.forencich Jan 27 '14 at 8:49
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    \$\begingroup\$ The question is why a normal diode heats when passing high frequency waveform \$\endgroup\$ – GR Tech Jan 27 '14 at 9:22
  • \$\begingroup\$ @GRTech - See the way in which your question has been reworded so that it IS a question. As it was it read like a statement and so was confusing to readers. \$\endgroup\$ – Russell McMahon Jan 27 '14 at 15:14
  • \$\begingroup\$ @Russell McMahon Thanks for edit. Sorry but English is not my language \$\endgroup\$ – GR Tech Jan 27 '14 at 16:08
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    \$\begingroup\$ @GRTech - Yes - I realised that. I made the edit so that others could better understand the question, and I commented to you so that you could see why people were wondering what the question was. Communications across a language "barrier" can be interesting :-). \$\endgroup\$ – Russell McMahon Jan 27 '14 at 16:48

Every diode needs some time to recover its reverse resistance after voltage polarity changes. During the recovery time there is high reverse current through the diode. This reverse current makes the diode to produce some heat. The higher the frequency is, the more time is spent in the recovery state and so the more heat is produced by the reverse-recovery process. Low-frequency diodes (like 1N4007) have relatively long reverse recovery time, and you've seen what it leads to. Higher-frequency diodes (like HER or FR series, for example) have much shorter recovery time which allows them to operate at about 300 kHz. Using a high frequency diode to rectify a low-frequency waveform will produce even less heat by reverse-recovery than in case with a low-frequency diode, but in both cases it will be negligable to heating by forward-current. So the only drawback of using a high-frequency diode to rectify a low-frequency waveform is the diode's price.

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  • \$\begingroup\$ What is "reverse resistance"? \$\endgroup\$ – Phil Frost Jan 27 '14 at 12:16
  • \$\begingroup\$ @motoprogger Not only price but also the low revesre voltage is a limitation in using Schottky diodes for example \$\endgroup\$ – GR Tech Jan 27 '14 at 17:54

You can use fast rectifier diodes for slow AC waveforms. Just make sure that the reverse voltage and current rating is suitable and that any heat-sinking requirements are implemented.

Slow diodes won't work very well as the frequency rises because the diode's reverse recovery time is usually quite long - this makes them unsuitable.

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  • \$\begingroup\$ I see. So the maximum frequency that can handle the 4007 is around 10kHz, right? \$\endgroup\$ – GR Tech Jan 27 '14 at 17:04
  • \$\begingroup\$ @Andy - for interest - see note 2 here dugi-doc.udg.edu/bitstream/handle/10256/7455/… \$\endgroup\$ – Russell McMahon Feb 3 '15 at 10:01
  • \$\begingroup\$ @RussellMcMahon - this is the method by which the reverse capacitance is measured - this doesn't indicate that the device's reverse recovery time is appropriate. \$\endgroup\$ – Andy aka Feb 3 '15 at 10:46
  • \$\begingroup\$ @RussellMcMahon - see this data sheet - reverse recovery time is 30uS google.co.uk/… \$\endgroup\$ – Andy aka Feb 3 '15 at 10:48
  • \$\begingroup\$ @Andy - Och aye. I couldn't find a reverse recovery time but found that test frequency while looking. That would make it 'somewhat soggy' at a few kHz and downright bad at 10 kHz. \$\endgroup\$ – Russell McMahon Feb 3 '15 at 11:24

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