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TL494 DCDC converter wide range input issues

Zac1

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With a help of few people I managed to design fairly wide range DCDC converter using TL494

I confirmed it works in range of 30-190V DC
Has fairly stable 12V output with ~40mV peak to peak noise
Can deliver up to 3A.

However it has one fatal flaw that haunts me in my dreams. It kills mosfet Q5 and diode D2 if you suddenly apply 160V+ (high dV/dt)
It does work properly if slowly raising voltage across 40-190V. Also works up until 140V (sudden turn on).

I have managed to capture the problematic moment on thermal camera


Basically there is a full short on Q5 and D2 for a moment, then fuse pops and both components have dead short. I have tried to remedy this situation with help of PNP clamp (Q1) but it doesn't do much. I am out of ideas how to fix this... perhaps the driver is at fault? Or maybe diode is too slow?


1710670474787.png
 
Solution
it needs to be speeded up / higher gain - until it becomes unstable - then back off a bit
I think this can be done after I order PCB, right now i need to order asap to have something to play with.

both soft start and/or dc gain can be adjusted with just cap/resistor values.


Why is the switcher driving high current into D3 even in steady state although the nominal output voltage is said to be 12V? There seems to be a problem with voltage feedback operation point.
I am sorry, i did not understand question? D3 after startup should see a max of 52micro amps.


As already stated, soft start time constant should be much higher, I'd test with 5 or 10 uF C5.
Will do, right now i think i have complete PCB that should...
Hi,

Why Q1?
Why R8 = 1k and much much higher?
Why no bigger C12?
What´s the switching frequency?
What´s the duty cycle range (min / max)?

Show the PCB layout (especially the destroyed parts and all around them)

Did you check signals with a scope?

Klaus
 
Hi,

Why Q1?
Why R8 = 1k and much much higher?
Why no bigger C12?
What´s the switching frequency?
What´s the duty cycle range (min / max)?

Show the PCB layout (especially the destroyed parts and all around them)

Did you check signals with a scope?

Klaus
Q1 to act like strong clamp
R8 is actually 3.3k, i had issues with 1k res
C12 because calculated this value according to LTspice, can't really answer the question why not bigger.
Switching frequency is 200kHz
Duty cycle is irrelevant, from what I can tell, mosfet died before TL had a chance to even boot.

There was no time to scope anything, as soon as power supply got turned on, mosfet got toasted and entire circuit got changed to big short circuit.

I do however have a comparison how normal boot looks like (VIN <= 140V):
https://cdn.discordapp.com/attachme...691016a53ac88f1c501cc185e0340d0638747d1ec97f&

And here is video what happens at VIN 160V:

https://cdn.discordapp.com/attachme...a747eb6398a285a313d5948025c3c36ca281fd4d2b8f&

So my current guess
 
Can you export a schematic suitable for simulation?

You might need NTC thermal protection for booting then bypass.
I tried to simulate this in LTSpice but it is so finicky that small changes causes LTSpice to throw errors, nevertheless i attach ltspice and probably all its models in zip file (may need to change path to IRS21271.lib as it is full path) :
1710699430921.png
 

Attachments

  • pwmdcdc.zip
    31.5 KB · Views: 53
What is the actual output diode ? a 300V rated part or greater ? ( it should be ),

this issue is common with the design approach you have chosen here,

the main fet turns on briefly for high dV/dt applied at power up,

a common solution is a separate circuit to hold the mosfet off ( < 1 ohm G-S )

during power up, this could even be a very small relay, ( or opto and xtor )

when relay is energised, G-S is free to operate, then allow gate drive.

Also your mosfet, at 200V - this is sailing too close to the wind - applying 150 - 190VDC could well be damaging it if there is any type of L-C ckt in the input - if there is - the V can easily ring up to higher than 200V ( basic LC physics ) = poof !

Also test your boot strap supply circuit separately with 190VDC suddenly applied - it may be putting out too may volts very briefly, also the dv/dt may be affecting the gate driver - giving a short ( or longer ) output pulse that over-currents the mosfet ( charging output caps )

EP.
--- Updated ---

Also - 200kHz is optimistic - if your gate drive is not perfect the switching losses go up with Vin and current - if the mosfet is not super well connected to a suitable heatsink ( much larger than just the pcb pad ) - then it will overheat at some point and go bang

The diode can also contribute to sw loss heating - depending on what type it actually is (?)

100kHz will halve the sw losses

The video of the temp shows the above all too well.

A screen shot of the source to gnd volts at 20nS / div would be instructive for turn on and off !
--- Updated ---

Also - as the fet gets hotter - its R-ds-on goes up ( at least doubles here ) - and for constant current waveform - then its heating goes up ( at least doubles ) and you get thermal run-away ( seen in video )

also the Vgs-thres falls at the temp goes up - and this can be a bad thing for imperfect gate drive,

please note carefully the graphs from the data sheet:

1710713113610.png
 
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What is the actual output diode ? a 300V rated part or greater ? ( it should be ),
Every part is as in schematic in first post, so SS520F, 200V, you think this could be a problem?
To be perfectly clear, LTSpice sim has approximate parts, first post has real schematic that i soldered onto PCB.

during power up, this could even be a very small relay, ( or opto and xtor )
Do you have example of such circuit? Unfortunately relay is probably not going to fit. I just have some space for couple smd components at most.

Also - 200kHz is optimistic - if your gate drive is not perfect the switching losses go up with Vin and current - if the mosfet is not super well connected to a suitable heatsink ( much larger than just the pcb pad ) - then it will overheat at some point and go bang
I did check it with scope, it is 200kHz
1710730181447.png


This is the output of TL494, driver mirrors that pretty well.
100kHz will halve the sw losses
But switching is really not a problem, once it is running, even at 190V, output is stable and whole circuit stays relatively cool (32 degree)

Take a look at first video, mosfet isn't even remotely getting close to being warm let alone hot. There is a little flare on output zener but that goes away quickly.
If you compared that to second video (failure) zener D5 does not light up, which means output is not getting close to 12V.

I am not sure how can i explain it better, but in simple terms, whole circuit behaves completely different when running at 140V vs 160V. And when it is working (<=140V) nothing gets hot.
 
Also - is D1, the bootstrap diode 300V or greater and ultra fast ? ( Oh - I now see it is only just 200V rated - too low )

Also - the BSS126 at 700 ohm rated appears too small for the work required of it, you want a 1 amp part at least, also well heatsunk.

I see the SS520F main buck diode is 200V rated - this is too low for an 190V application - also it needs to be very well heatsunk.

you can see in the video - the diode heats 1st - because its reverse current goes up at it gets hotter, as do its sw losses - this causes the fet to heat and it goes too:
1710732789736.png
 
Last edited:
Also - is D1, the bootstrap diode 300V or greater and ultra fast ? ( Oh - I now see it is only just 200V rated - too low )

Also - the BSS126 at 700 ohm rated appears too small for the work required of it, you want a 1 amp part at least, also well heatsunk.

I see the SS520F main buck diode is 200V rated - this is too low for an 190V application - also it needs to be very well heatsunk.

you can see in the video - the diode heats 1st - because its reverse current goes up at it gets hotter, as do its sw losses - this causes the fet to heat and it goes too:
What could you recommend instead of D1 ? Would ST5L300 work?

BSS126 is depletion mode transistor, it is suppose to deliver enough current for driver and TL494 to start switching, then it turns off seeping only like 150nA.
It does that but but i must say, there is a possibility it may be a little underpowered, however how can you explain it works just fine up to 140V?

Yes as for SS520F i did have some suspicion since it flared up first.
 
the key elements have been explained for everything above - just read it again but this time much more slowly & carefully.
--- Updated ---

" But switching is really not a problem, once it is running, even at 190V, output is stable and whole circuit stays relatively cool (32 degree) "

" but in simple terms, whole circuit behaves completely different when running at 140V vs 160V. And when it is working (<=140V) nothing gets hot. "

these two statements contradict each other - but they correlate well with the explanation given - and the video.

your heat-sinking appears to be non existent, the voltage ratings of the critical parts is too low, the gate drive transitions are unknown, the peak volts on the circuit due to input filter ring up from step application of Vin is also unknown. 15v would be better as a GD supply voltage. The action of the booststrap supply at fast power up is unknown.

How long does the ckt actually go for at 190Vin ? before it overheats ?

--- Updated ---

p.s. look up what optimistic means on a google dictionary
 
Last edited:
these two statements contradict each other - but they correlate well with the explanation given - and the video.
I did not state myself clearly.

So there are 2 states in which this DCDC operates

Boot and normal operation

In normal operation under light load no matter the input voltage, it does not heat up beyond 35C even after hours.
There is no problem whatsoever in "normal operation".

The only problem is during boot so initial 0.5s after first applying voltage and if VIN >160V it ends up badly for mosfet and diode.

How long does the ckt actually go for at 190Vin ? before it overheats ?
Indefinitely. I did test it extensively while raising voltage from 90->190.

I probably should have mentioned this but this step down is designed to work with circuit that draws absolute max of whooping 4W (0.33A). Which means it has massive headroom. If i wanted to draw 3A continuously, I would need some cooling.

As for the bootstrap you are probably right and BSS126 may not be enough, which could potentially be the cause of extended boot time.

I also simulated bootstrap, attaching ltspice files
 

Attachments

  • boottl431.zip
    7.1 KB · Views: 48
Also - this part can only source 200mA to turn the fet on - this is too low for 200kHz operation, the output needs to be fully buffered by an emitter follower pair;
1710796616170.png

data sheet shows the 200mA limit,

also there is no max dV/dt listed in the data sheet for this part - which is a bad sign, at fet turn off if the falling dV/dt on the source is too great it may affect the ability of the driver to turn the fet off properly. Put a 4u7 MLCC across the high side power pins on the IC as well as your 220nF

EP
--- Updated ---

" I also simulated bootstrap, attaching ltspice files "

simulation does not cut it for the real world - test the real thing - but disable the power fet ( short G-S )
--- Updated ---

Have you implemented the standard soft start on the TL494 ? if not - this would be a good thing to do, otherwise the start up transient current through the fet and diode will be quite high while Cout charges at 95% duty cycle from the TL494 until the feedback loop catches up ( too late ) - and worse at higher Vin.
--- Updated ---

1710797286112.png


a diode across R6, is a good idea.
 
Last edited:
Also the dead time pin should be set to 1.6V this sets the max pwm to ~ 48% which is enough for the 30V in to 12V out ( 41% pwm )

but will lower the start up stresses compared to flat out at hard power up - a proper soft-start will lower them still more
 
The LTspice glitches are from ideal parts PS & Caps with 0 ESR leading to time steps too small and megawatts or kW of pulses.

Too many errors matching simulation to your build.

I guess the std cap. library is too small to match them.
 
To perfect the simulation to match your design, will permit optimizing the design for each part, but that takes more time than a soft-start design.

To choose ESR in supply, you may need the load regulation value to estimate ESR.
For the Caps you need an RLC meter which may be easier.
Guessing ESR will just lead to more phantom simulation issues but 0 ohms is a definite no-no.

But if you don't have 2 sq.in of Cu per watt on board for the FET and diode each, it's going to fry, even with soft-start with a 50W load.
Otherwise it could be (?)(100'C/W without.

While your at it, the VFB dividers are drawing a few watts, so scale up x10R.
 
How does Q2 work?
Why D6?
Q2 has a gate limited voltage that gives 10V out for start up ( it does rely on leakage currents to start - Vbatt is the 30-190Vin ), D6 supplies 12V from the output to turn Q2 off after start, else Q2 would overheat.
--- Updated ---

@Zac1 - Do you really have an 11V zener on the output ? ( from part number ) why not 13V ?
 
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Thank you guys for all replies so far.

Let me start with one important thing about this step down converter. Main focus is reliability.
Which means, I am very open to changing parts if there is even slight suspicion that one of them may not be sufficient.
Which is why I designed it to be able to output 3A while using max 0.3A. I want maximum headroom possible with reasonable circuit size.

So I have compiled a list of changes that would implement based on your suggestions
  • D2 -> ST5L300, Schottky, 5A, 300V
  • Q2 -> CPC3980, Depletion nfet, Bigger package (SOT223), a lot smaller RDSon (40ohm), 5x the current, better cooling than current BSS126
  • Q5 -> IRFS4227, Bigger package, better ciss/crss ratio (less prone to self turn on), 10x lower RDSon, better power dissipation
  • U3 -> ??? no candidate for now, single drivers are fairly rare, can I use in this scenario common half bridge driver?
I have on hand IR2181 which can output a lot more current than IRS21271 but I am not sure if it will work with just one mosfet, I know it should be technically possible but I have not tried it..

I did not implement soft start but that is idea worth considering.

By the way if you guys suggest some changes can i please ask for quick paint over the original circuit? It would make it easier for me to wrap my head around it and keep track of suggestions.

How does Q2 work?
Why D6?
Still miss the PCB layout..
Q2 is depletion mode nfet, which means it is initially ON without driving gate, and stays on until D6 pulls up voltage above the output of bootstrap circut. It is a nifty circuit that can generate 12V out of 15-500V with low current and turns itself off once the TL494 takes over. As for PCB layout I made sure to keep both loops very tight. I sincerely doubt it is the cause of this issue.

To perfect the simulation to match your design, will permit optimizing the design for each part, but that takes more time than a soft-start design.
I will be honest, I am not that good with ltspice. This design is a combination of many people knowledge. I tried my best to make sim better but there is always some catch.

But if you don't have 2 sq.in of Cu per watt on board for the FET and diode each, it's going to fry, even with soft-start with a 50W load.
Otherwise it could be (?)(100'C/W without.
While it is designed to output 3A, the circuit that is suppose to work with it consumes maximum of 0.3A so ~4W.

While your at it, the VFB dividers are drawing a few watts, so scale up x10R.
Good catch, will do.

@Zac1 - Do you really have an 11V zener on the output ? ( from part number ) why not 13V ?
3SMB5928B is nominal 13V zener unless im missing something?

I will compile a new schematic and will post it tomorrow.

Very very important note: DCDC dies regardless of load, even without any load whatsoever as soon as VIN is at 160V applied suddenly.
 
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Q2 is depletion mode nfet
I missed that.... now it makes sense.

***

What you call "Bootstrap" indeed is the power supply for the drivers.
It may lead to problems on forum discussions, because usually Pin8 of U3. With C12 to be the bootstrap capacitor.

Also "-Batt" is misleading. Because one may assume there is a negative voltage. But indeed it is used as your system_GND (0V). Missing GND symbols make the schematic more difficult to read.

***
I just spent some time on imagining how the whole power supply and real_Bootstrap looks like at power up.
The power supply for sure has a current limit on power up.
C12 relies on empty output capacitors and can only be charged with (negative) current through L1.

There will be a time where the ouput voltage is 6V.
This is not high enough to supply your_bootstrap_node via D6 ... while it is also not low enough to charge C12 to a proper level.
Not high enough to work peoperly and not low enough to work properly .... means: what happens then?

So maybe you are doing several tests .. with increasing input voltage. But the fault you see is not caused by the "increasing input voltage" but indeed the root cause may be the only partially discharged output voltage. As said: maybe.

***
For the third time: PCB layout!
You want to focus on reliability. In many discussions here we see that the PCB layout has big impact on reliability. Causing unstable situations, causing voltage spikes due to stray inductance, ground bounce due to currents ...
--> there is now way to talk about reliability .. without being able to check the PCB layout.

I will not come back until we see the PCB layout.

Klaus
 

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