zener recovery

[mikec]

Hi
When the voltage applied to a zener diode in reverse (regulating) setup goes below the zener voltage does it take the zener diode a finite time to stop conducting? If so what is the typical recovery for a zener diode?


Thanks

[Bertho]

When the voltage applied to a zener diode in reverse (regulating) setup goes below the zener voltage does it take the zener diode a finite time to stop conducting? If so what is the typical recovery for a zener diode?

The recovery is basically limited by the capacitance of the diode. That should be quite small. Unless you are switching at very high frequencies, you can ignore it altogether,

[SeanB]

It is going to be a problem with high power diodes, but generally with low power ones it will be about the same as that of a signal diode, typically a few ns to lower the current to a low level, but they typically are going to be quite small. Generally if this is a problem then you change to using the diode at a constant current and switch the current into it with PIN diodes instead.

[mikec]

Thank guys,
The reason I asked is to do with a problem I am dealing with at the moment.
I have a circuit for an invertor welder see attached picture for part of the schematic. I have also attached scope pictures.

In the scope picture the yellow trace represents the input (high and low) to the L6386(U50_1) and the blue line represents the PIN13 of this chip. The first issue I have with this is that when the input is low I believe pin13 should be low also. Now if I connect the scope to PIN14 which is the bootstrap input, I see a similar trace to PIN13 except is has a bit of an offset (around 5Volts), see the second scope picture attached.

This firstly tells me that PIN13 is tracking PIN14, which I think is wrong when the input is low.
Secondly in the interval where the input is low the bootstrap capacitor is supposed to be charging, which initially it appears to be doing but at some point in this period it suddendly starts to discharge at around 17-18Volts. It than continues to discharge until around 10volts and begins charging again until the input goes high.

This tells me that at some point some component begins to conduct and therefore discharges the cap. I was suspicous of diode D05_3 because it so happens to be 16Volt Zener diode (BZV55C16), this led to my original question which is why would the zener continue to discharge until 10Volts or for about 2uS if indeed it was the zener kicking in around the 16-18Volts?

Just for information, PIN9 which is the low-side output works absolutely fine and tracks the input perfectly.
The High and Low outputs drive IGBTs (G10N60A)on another board which I don't have a schematic for.
I have replaced the L6386 and also the IGBT driver board which was the original fault.
The welder is not connected to the mains during these tests, i am following manufacturers recommended setup for testing various components which basically involves putting 15Volts DC between Vcc and GND.
Datasheet for L6386 is here: http://www.st.com/web/en/resource/technical/document/datasheet/CD00001383.pdf


Appreciate any suggestions.