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I face to some issue with my prototype based on the HV9961 (see picture).enter image description here

I replaced L1 by a 1.8mH power inductor and R1 by a 68k resistor. Those values came from HV9961 datasheet formulas for a 170V led output. Also i added a potentiometer between R2 and the LD pin to adjust the VLD voltage between 0.2V and 1.6V. The output led current depends of the VLD voltage. R6 and R6a have been replaced by a 0.3 Ohm to setup ILED at 920mA max.

After power on, my design works very well. The 170V led string sees a 350mA when VLD is adjusted at 0.6V and brights fine. But after 5-10mn, D1 breaks! I decided to change D1 by the same reference STTH2R06H (fast recovery diode 600V 2A). The problem is the same, D1 is destroyed after 5 min.

Could you help me to find the good recovery diode ? and explain why it breaks ? |


HV9961 datasheet
STD5NM50 datasheet
STTH2R06 datasheet

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  • \$\begingroup\$ Where possible PLESSe provide datasheets eg STTH2R06U datasheet || What is the diode package? How is it heatsunk? How hot does it get? What is its FORWARD voltage . What is its reverse voltage? || V \$\endgroup\$ – Russell McMahon Oct 17 '19 at 8:44
  • \$\begingroup\$ Reverse voltage should be only about 330 VDC. Dissipation steady state should be maybe slightly over 0.5 Watt. (Fig1 page 3, 0.4A, ~~~= 50% on. | I'd not expect dI/dT to be an issue. Maybe recovery losses but ...? || Try a say 10 Ohm resistor n series with D1 - it drops about 3V in operation (so minimal loss %) but may soften the Q1 turnoff. Maybe a small cap across D1. Small. . || HV9961 datasheet \$\endgroup\$ – Russell McMahon Oct 17 '19 at 9:21
  • \$\begingroup\$ Thanks you Russell, i will try all of this. But i still don't understand why this diode failure happens. If someone could share his knowledge too. ;) \$\endgroup\$ – Teddol Oct 17 '19 at 11:58
  • \$\begingroup\$ Can i add 2, 3 or 4 diodes in parallel to limit each one's power ? \$\endgroup\$ – Teddol Oct 17 '19 at 13:23
  • \$\begingroup\$ We don't know either - but, as you say you are using components and design parameters from the data sheet, then your answering the questions that I asked may be a major factor in solving your problems, or not. No amount of our "sharing our knowledge" will help if you have used a high thermal resistance pkg device, heatinking is minimal and it is filing due to overtemperature. Until you answer the related questions so we know what you know then endless speculation on our part is pointless. What are the answers to the questions above. If you don't know you need to. \$\endgroup\$ – Russell McMahon Oct 17 '19 at 14:02
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I said:

What is the diode package?
How is it heatsunk?
How hot does it get?
What is its FORWARD voltage .

Reverse voltage should be only about 330 VDC.
Dissipation steady state should be maybe slightly over 0.5 Watt. (Fig1 page 3, 0.4A, ~~~= 50% on.
I'd not expect dI/dT to be an issue.
Maybe recovery losses but ...?
Try a say 10 Ohm resistor n series with D1 - it drops about 3V in operation (so minimal loss %) but may soften the Q1 turnoff. Maybe a small cap across D1. Small.

If you have used a high thermal resistance pkg device, heatinking is minimal and it is filing due to overtemperature.

You said: I used SMB package for the diode STTH2R06U.
The diode temperature increases very quickly when the design is powered and it becomes very hot before crashing.

SMB Rth-jc is 25C/Watt so you should have no problems IF the diode has adequate heat sinking. BUT fig 11 in datasheet shows Rth-ja of 90 C/W - without any heatsinking you are liable to "have problems" if air access is other than superb.

The datasheet says that 3cm^2 of FR4 PCB copper gives about 60 C/W.

I cannot measure temperature and there is no heatsink for my SMB package. But here I need a quick course about "Thermal resistance junction to ambient versus copper surface under each lead".

If you burn yourself on it you probably need more heat sink.
It COULD run far hotter than that and be OK but ... .
If you are JUST able to keep touching the case it is at about 55 C.
If a drop of water on it boils or if when you lick your finger and dab at it with the wetted finger it hisses then it's over 100 degrees C.
Even that's perhaps OK but also maybe not.
If it DOESN'T ever get hiss test hot it's probably OK.

Below: j=junction, c=case, a=air.
Rth = thermal resistance.
Rth_jc is Rth from junction to case.
Rth_ca = case to air.
With no heatsing Rthja = 90 C/W. ie junction is 90C > air temp with no heatsink.
Rthjc is 25 C/W.
So Rth_ca = Rth_ja - Rth_jc = 90-25 = 65 C/W.
Si if air is 20C and case just sizzles = 100C then
Tj = 100 + 25/65 x (Tc-Ta) = 100 + 31 ~= 130 C.

Tj_allowed is < 175C so IF you can keep the leads below 100C (sizzling hot) the temperature SHOULD be OK. Should.

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