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Question why one of the MOSFET burn shorted in my SS relay

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I was simply assuming that the solid state relays will never be turned on while a larger output voltage is present by working of the protection circuit. If this happens though with low impedance load, there can be a much higher energy dissipated due to the relative slow turn on and SOA exceedance is more likely.

Hi FvM

Lowering the R4 from 5.1K to 2.5K is the best I can do. The recommended current through the diode is 10mA. With 2.5K and +33V supply, assume voltage drop across the R4 is 30V, that's 12mA already. Do you have any better Photovoltiac isolator better than the ASR-V621, I can't find any? OR do you have a better circuit for SS relay for this purpose ( linear and below 2mohm on resistance).

Mechanical relay doesn't even come close to this good.

Thanks
 

If, you are using fets from Ali-express - you need to allow that failures will happen ...
 

Finally received the new MOSFET and 100ohm small resistors.

New FET.JPG

I bought 50 of this MOSFET, it's from Mouser, one of the two biggest electronic components distributors.

https://www.mouser.com/ProductDetail/757-TK2R9E10PLS1X

Now I've put in the modification and see.
 

I am reading the SOA graph, I want to confirm we are worry about the SATURATION region when the MOSFET is turn ON ( or OFF) where the MOSFET is going through the CONSTANT CURRENT. I think I ordered the wrong MOSFET. I attached the SOA graph of both the TK2R9 which I just ordered and the CSD19536. CSD19536 is 24A at 1mS pulse width. The TK2R9 is only 4A at 1mS pulse width.

SOA CSD19536.JPG
This is CSD19536
https://www.mouser.com/ProductDetail/Texas-Instruments/CSD19536KCS?qs=sGAEpiMZZMshyDBzk1%2FWiw3ktwnhg7wCabYHfJTF4q7FiwdiMsVDFQ%3D%3D



SOA TK2R9.JPG
This is TK2R9
https://www.mouser.com/ProductDetail/Toshiba/TK2R9E10PLS1X?qs=%2Fha2pyFadujl0vlHrjpaUCsY6lLeLXrf30DrgrlhSws%3D

I think I should have bought the CSD19536 even though it's a lot more expensive.

Please take a look and let me know. I have to contact Mouser to see how can I return the TK2R9.

The reason I choose 1mS is because of looking at this scope picture:

SS relay drive fast.JPG

You can see the picture is 20mS/div and 5V/div. So it is about 4mS to go from 5V to 6V. I just assume this is the window where the MOSFET goes from OFF-->SATURATION-->LINEAR(ohmic Rdson). I assume the transition from OFF to LINEAR is about 1mS.

Thanks

Alan
 
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Hi,

So it is about 4mS to go from 5V to 6V. I just assume this is the window where the MOSFET goes from OFF-->SATURATION-->LINEAR(ohmic Rdson).
I see no miller plateau, thus I assume this is "V_DS = 0 switching".
V_DS = 0 also means "now power".

If so, then please show a scope picture with V_DS = your max expectable signal voltage. (No need for very low load resistance, moderate load resistance is good). Then you will see true switching times.

Klaus
 

Hi,


I see no miller plateau, thus I assume this is "V_DS = 0 switching".
V_DS = 0 also means "now power".

If so, then please show a scope picture with V_DS = your max expectable signal voltage. (No need for very low load resistance, moderate load resistance is good). Then you will see true switching times.

Klaus

What is a "miller plateau"?

Yes, the picture is taken on only the switch exactly like the circuit shown in Post#1. The input and output of the SS relay is left open and just the scope ground at the S and probe at the G.

In the real circuit is shown in post#22 and the simplified block diagram is shown in post#31. It is driven by the output of the amp, so the voltage is varying.


I expect the max voltage from the amp is +33V and lowest is -33V.

Can I just set the input of the SS relay to say +33V and the output to ground with either a resistor or a capacitor like this?

SS relay1.jpg





I think I know what you are looking for. The waveform scope picture was taken with the drain D open, no load and no drive. With real voltage and real load, you expect when the Vds collapse or build up, there will be a kink on the Vgs wave form at the transition and you want to look at the kink. Am I right?

If that's what you are looking for, there's not much I can do about it, it's like that's the best I can do so far with the ASSR-V621. If you have a better circuit for the SS relay, let me know. Or else, this is the end of the road after changing the R4 from 5.1K to 2.5K. Only question remain is to find the MOSFET with the best SOA. Seems like the CSD19536 is the best I can find with 100V and Rdson < 2.8mohm.

Thanks
 
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Hi,

What is a "miller plateau"?
Should be described in every MOSFET application note... and is a well known phrase when talking about MOSFETs.
--> internet search, wikipedia, MOSFET application note....

In short: Flat horizontal line in V_GS chart when V_DS is moving, caused by C_DG --> miller capacitance.

Also important when looking for
* total gate charge... and thus important for
* swtiching times

Klaus
 

Hi,


Should be described in every MOSFET application note... and is a well known phrase when talking about MOSFETs.
--> internet search, wikipedia, MOSFET application note....

In short: Flat horizontal line in V_GS chart when V_DS is moving, caused by C_DG --> miller capacitance.

Also important when looking for
* total gate charge... and thus important for
* swtiching times

Klaus

.............

I think I know what you are looking for. The waveform scope picture was taken with the drain D open, no load and no drive. With real voltage and real load, you expect when the Vds collapse or build up, there will be a kink on the Vgs wave form at the transition and you want to look at the kink. Am I right?

If that's what you are looking for, there's not much I can do about it, it's like that's the best I can do so far with the ASSR-V621. If you have a better circuit for the SS relay, let me know. Or else, this is the end of the road after changing the R4 from 5.1K to 2.5K. Only question remain is to find the MOSFET with the best SOA. Seems like the CSD19536 is the best I can find with 100V and Rdson < 2.8mohm.

Thanks


I know what it is, I just don't know the name. That's what I described last post as the "kink" at the transition when the MOSFET turn on.

Like I said already, given that, unless there is a better Photovoltiac isolator than ASSR-V621 or a better driving circuit, there's nothing I can do to improve that. Only two things that can improve are:

1) Find the MOSFET that has the highest SOA graph. Which in this case is the CSD19536.

2) Find the MOSFET that has the LOWEST Cgd. That's the capacitance that cause the "kink" or Miller Plateau.


I looked at the Cgd ( Crss) of TK2R9 is worst at Vgs<10V, so CSD19536 is over all better. Please double check my finding to confirm and I'll try to return the TK2R9.


Thanks
 
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I have a new analysis that might or might not be a problem.

The source of the MOSFETs are floating, meaning it can float up and down until something stop it from going up or down. Floating up is no problem because as soon as the source S goes beyond +33V, the body diode DQ1 or DQ2 will conduct throught D1 or D2 resp and clamped at +33V.

BUT if the source floating negative, there's nothing to stop UNTIL it goes -ve beyond the Vdsmax of the MOSFET and AVALANCHE break down as shown in the drawing on the right.

SS relay2.jpg

From my experience, it's not a problem. For one, there is NO CURRENT flow, so nothing can be damaged. Just charging up/down. But I want to hear from you guys.

The fix is very simple as shown in the drawing on the left. The added diode in RED is 1N4148 will clamp the SOURCE to -33V. The resistor in red gives the SOURCE 0V reference so it stays at 0V. I don't think the resistor is necessary.

Tell me what you guys think.

Thanks
 

A useless consideration. Why should the source node float up or down without external current injected to it? And you completely forgot the MOSFET substrate diodes which clamp the source node to +/- (33 + 2*Ud) in your circuit. Finally, even without clamping diodes, the MOSFETs have a considerable avalanche energy absorbing capability, you need to drive larger currents to exceed it.

All-in-all it's quite unlikely that avalanche breakdown is the failure mechanism in your circuit.
 

A useless consideration. Why should the source node float up or down without external current injected to it? And you completely forgot the MOSFET substrate diodes which clamp the source node to +/- (33 + 2*Ud) in your circuit. Finally, even without clamping diodes, the MOSFETs have a considerable avalanche energy absorbing capability, you need to drive larger currents to exceed it.

All-in-all it's quite unlikely that avalanche breakdown is the failure mechanism in your circuit.

I worked with a lot of high voltage circuits, you'll be surprised any floating conductor can charge up. For example, any electrostatic charge can transfer onto the SOURCE and raise the voltage. Believe me, it can happen. For example, it is a really bad practice for some people to fill the blank pcb surface with little copper square that I've seen. They can be charged and discharged. We had fun looking the arc when any floating conductor in any high static or high voltage surrounding. You turn off the light, you can see the arc and discharge path!!!

That said, I don't think that hurts the circuit as I explained there is no current to destroy the transistors. BUT remember, it it charged the Vds to 100V, the moment the MOSFET turns on, you have 100V to discharge from the Cds, this likely affect the Vgs through the Cdg and worsen the Miller Plateau.

That's the reason I run this by you guys.

Thanks.

- - - Updated - - -






Hi FvM

Can you take a look from Post#45 to Post#49 to see whether you agree that CSD19536 is the best MOSFET? I want to confirm with you so I can return the 50 TK2R9 back to Mouser asap. It's over $100USD.

But one thing though, the only MOSFET that went bad is a CSD19536.
 

Anyone can comment on whether CSD19536 is better than TK2R9 from Post#45 to #49? Just want to confirm so I can return the TK2R9 back to Mouser.

Thanks
 

I worked with a lot of high voltage circuits, you'll be surprised any floating conductor can charge up.
The common source node is not floating, it's clamped by diodes.

- - - Updated - - -

CSD19536 has higher avalanche energy and SOA rating with slightly lower gate charge. In so far I would prefer it.

But as long as you are still guessing about the actual failure cause, the relevance of these transistor parameters isn't clear.
 
The common source node is not floating, it's clamped by diodes.

Going to the negative side, there's no diode clamping on the source until it pass 100V and the D and S start to conduct ( avalanche or break down). If you look at the left circuit in post#50, you'll see. The node labeled "SOURCE" in GREEN can be charged to -100V without being clamped.

Anything with capacitance can be charged up or down. With small capacitance, you cannot use a meter to measure as even if the impedance of the meter is 10Mohm, it will discharge the cap before the meter settle down and you read 0V.

I ordered a bunch of 10,000uF, they came in a box, the top layer where the two contact of the caps are facing, they put an Aluminum foil. Outside the box, the instruction is "before opening the box, turn the box up-side-down and let it rest for a few second. When the box is turned up side down, the contacts of the capacitors touch the foil and discharge the caps.

This is a very common thing, people just don't pay attention. Anything that is floating can be charged up or down from static.

- - - Updated - - -

The common source node is not floating, it's clamped by diodes.

- - - Updated - - -





CSD19536 has higher avalanche energy and SOA rating with slightly lower gate charge. In so far I would prefer it.

But as long as you are still guessing about the actual failure cause, the relevance of these transistor parameters isn't clear.

Thanks

How do you read CSD19536 has higher avalanche energy? I don't know how to read this one.

Yes, I read from the graph CSD has higher SOA rating and lower Cgd, so less charge for the same voltage.

I am going to return the TK2R9 tomorrow. Now I can put the CSD19536 in the circuit. I still have like 24 of this at home.

Thanks
 

Avalanche energy is specified in maximum ratings.

You are right about no diode clamping in negative direction, sorry for adding confusion. Anyway, I keep my point that there's absolutely no risk of uncontrolled charging.
 
Avalanche energy is specified in maximum ratings.

You are right about no diode clamping in negative direction, sorry for adding confusion. Anyway, I keep my point that there's absolutely no risk of uncontrolled charging.

Thanks so much for the help. I really learn a lot from you and others. I experience is not in power electronics. I learn of SOA, but this is the first time I really dig deeper into this and learn. My mistake was just look at the Rdson and Vdsmax and not on the SOA and avalanche energy. Even at that, I was only look in terms of Cgd.

Is the Avalanche energy specified as Eas?

Thanks you. Now I am going to return the TK2R9 and start building.
 

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