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Discrete circuit which can detect whether or not an output is pulsing or not?

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treez

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Hello,
We need a discrete circuit which can detect whether or not an output is pulsing or not. (Pulse is 5V/0V) The ambient is 170degC so proprietory IC’s are not likely to be suitable. By “detect”, I mean say give a 0V output when there is pulsing and say a 5V output when not pulsing.

The application is a high duty cycle buck converter where if the gate drive duty cycle suddenly goes to zero, then the FET blows up. This is a common problem with DC restored, transformer coupled FET gate drives and is explained below fig 3 in the following…

Transformer coupled FET gate drive:-
https://www.eetimes.com/document.asp?doc_id=1273338

Article is based on "Gate drive design tips" by Ray Ridley.
 

treez said:
Hello,
We need a discrete circuit which can detect whether or not an output is pulsing or not. (Pulse is 5V/0V) The ambient is 170degC so proprietory IC’s are not likely to be suitable. By “detect”, I mean say give a 0V output when there is pulsing and say a 5V output when not pulsing.

The application is a high duty cycle buck converter where if the gate drive duty cycle suddenly goes to zero, then the FET blows up. This is a common problem with DC restored, transformer coupled FET gate drives and is explained below fig 3 in the following…

Transformer coupled FET gate drive:-
https://www.eetimes.com/document.asp?doc_id=1273338

Article is based on "Gate drive design tips" by Ray Ridley.
Click to expand...

I always wonder why people locate sensitive circuits in a crazy environment like 170 C in your case.
To detect pulsed signals (frequency?) I would recommend to use a simple comparator, plus a detector giving a DC output if pulses are present.
It should be possible to use a cable to get your pulsed signal out of the 170 C , to a temperature allowing to use the above circuit.
 
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Hi,

Use two capacitors C1, C2, two schottky signal diodes D1, D2 and a resistor R1.
Nodes:
* Pulsed output to c1
* other side of C1, cathode of D1, Anode of D2
* one side of C2, cathode of D2, one side of resistor (=output)
* anode of D1, Gnd, other side of R1, other side of C2

When there pulses, then you will see about 4V signal at output
When there are no pulses, then there will be zero voltage at the output.

Without frequency information it is impossible to calculate the values.

Klaus
 
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Thanks, its a 200khz signal

- - - Updated - - -

Thanks, the attached shows ytour kindly supplied circuit....now just going to find a comparator capable of 170degC and it will act on the lack of DC output which comes when there are no pulses
 

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  • NoPulse detector.txt
    1.5 KB · Views: 40
  • NoPulse detector.pdf
    8.1 KB · Views: 93

Hi,

Change C1 to 10n and R1 to 100k.

Klaus
 
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Here is another idea with a fast response when the duty cycle goes to 100%.

- - - Updated - - -

The circuit needs a diode added to cope better with high duty cycles like in this requirement. This gives more defined logic levels.
 

Attachments

  • missing_pulse1.png
    missing_pulse1.png
    22.7 KB · Views: 110
  • missing_pulse.png
    missing_pulse.png
    21.7 KB · Views: 117

If the collector circuit is tied to +5 it should also work for duty cycle dropping to 0%. The output capacitor should be increased to 2.2nF or so.
 

Attachments

  • zero_duty.png
    zero_duty.png
    19.1 KB · Views: 115
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