Burning of resist. R154 of PM8834 auxiliary power to MOSFET driver of PFC of STDES-3KWTLCP ref dsgn

[DSHAR.31]

I have assembled PFC and LLC circuit of STDES-3KWTLCP reference design. I followed the procedure as described in the document to assemble whole circuit. When we gave AC voltage to PFC, resistance R154 gets heat up and eventually burn out. I double checked all the components value and their placement but couldn't find the cause for this behavior.

Can anyone please assist us in finding which part of the circuit or component is creating this issue so that it would help us in rectifying the cause?

Thanks.

[Peter BENSCH]

Well, if R154 gets too hot, there will obviously be too much current flowing through it. Please check C161 and its insulation. Then also check the signals on PWM1, PWM2 and OUT1, OUT2 on U105.

Regards
/Peter

[DSHAR.31]

I changed C161 and checked PWM1, PWM2,  OUT1 and OUT2 of U105 with No Load connected. All these 4 pins have PWMs on them. Still, R154 is heating too much.

[AScha.3]

Check generated voltages : VCC_+ 18A + 18B + 12 V  . ok?

[DSHAR.31]

Voltages are also OK.

[DSHAR.31]

Is there anything I can check to find the cause of this issue?

[AScha.3]

What i would do (at work) : look with a IR-camera , to see, whats getting suspicious "warm" ...

If you dont have this : the small transformer makes 3 outputs: VCC_+ 18A + 18B + 12 V ;

(minimum) one of them using too much power, so R154 gets hot.

Find out, where something gets hot, or where the power is going to.

[DSHAR.31]

We already checked with the IR-camera. Only  R154 gets suspiciously warm (temp. reaches to around 100 deg celsius). Rest all circuit remains cool. We also disconnected the MOSFET driver IC (U101 and U102) which runs by these 3 outputs. The purpose of removing the connection to U101 and U102 is to check whether R154 gets heated despite not being connected to any load. After doing this, only R154 gets too hot when looked by IR-camera.

[AScha.3]

Great...you have camera. 🙂

But still no obvious reason found - right ?

Remains : STM design anyway is this way - 100° dont kill a resistor, but not over limits. (But no "nice" design, if this is normal state .)

So only thing you could look for : is pwm/clock correct for this smps supply ? (maybe its not given in documents)

Because if frequency a bit too low, more current to the transformer -> R154 getting more hot.

Otherwise..try it working . (Just if big inverter driving , do same as i do : wear glasses and ear protection. )

Sometimes a big bang blowing around some parts .

(We make at company inverters up to 120kW - so the bang might be big...)

[DSHAR.31]

We are using the firmware code provided in the documentation so the pwm frequency is already set. 

But, we noticed an interesting thing that when VCC_+18VA , VCC_+18VB and VCC_+12VB points are kept open (i.e. not connect to any load ahead). We thought that keeping these 3 points open means less current drawn in secondary side of transformer, hence less current drawn on primary side where R154 is connected. But, still no change and R154 gets hot and burns out. 

To investigate further, when removed Schottky bridge diodes of VCC_+18VA , VCC_+18VB and VCC_+12VB one by one, which led to decreasing current, eventually R154 remained cool when all Schottky bridge diodes are removed.

This is expected because now no current being drawn on secondary side which led to less current on R154. 

But what confuses me is that this behavior was expected when only VCC_+18VA , VCC_+18VB and VCC_+12VB points are kept open with all bridge diodes connected. 

Is this behavior due to those Zener Diodes D112, D117, D122 which may be drawing too much current even when all 3 VCCs are open?