IRFP064 gets burnt

[xyz9915]

Please view the attachment of SMPS circuit which was modified by me to fulfill my requirements. I made three PCB's of the above SMPS (Output about 220V/1.5A DC). The problem is that one PCB is working fine and delivering my desired voltage & current (i.e. 220V/1.5A DC). But in the remaining two PCB's, power MOSFET (IRFP064) gets burnt within 2-3 minutes.
My question is that when the three PCB's are identical, so why one board is working satisfactorily & other two boards are not functioning properly? I thoroughly checked everything in all PCB's but remained unsuccessful. Hence please help in this respect.

Further another question is that can I use power transistors (e.g TIP35) despite using MOSFET's? By providing sufficient driving to the transistors?

Note: The IC is TL494
Best regards

 

[stefanodel]

Please view the attachment of SMPS circuit which was modified by me to fulfill my requirements. I made three PCB's of the above SMPS (Output about 220V/1.5A DC). The problem is that one PCB is working fine and delivering my desired voltage & current (i.e. 220V/1.5A DC). But in the remaining two PCB's, power MOSFET (IRFP064) gets burnt within 2-3 minutes.
My question is that when the three PCB's are identical, so why one board is working satisfactorily & other two boards are not functioning properly? I thoroughly checked everything in all PCB's but remained unsuccessful. Hence please help in this respect.

Further another question is that can I use power transistors (e.g TIP35) despite using MOSFET's? By providing sufficient driving to the transistors?

Note: The IC is TL494
Best regards

One possible reason could be the the flux too high in the core of your transformer. If the core is very close to saturation, the current in your mosfet will be very high and consequently the power dissipated by the mosfet very high.
You could be check if the frequency of your first power switching mode power supply is higher than the other two power supplies. If you decrease the frequency, you get a higher flux in the core.
You could also check by calculation, the number of primary turns of the transformer in order to verify if the flux in the core is correct.
I think that the use of mosfet is the best choice for your smps.

stefano

 

[xyz9915]

After receiving suggestions, I tested all three boards with frequency counter and found that even the one board is working satisfactorily but the frequency at the gates of MOSFET's is displaying randomly (ranging from 200KHz to 500KHz), when I disconnected the gates driving resistors (10 Ohms), the frequency counter displayed the actual frequency of the circuit (which is about 36KHz). I then removed the driving circuit (consisting of BD139/BD140) & also removed the diodes D2/D3 (1N4148) with 220 Ohms resistor and then again connected the gates. The frequency counter displaying 36Khz in one side & other side the frequency is showing as 64KHz (just double) but the frequency remained stable. I also tried with different values of resistor but both channels are showing different frequencies (36KHZ & its multiple) but the frequency remains constant in all cases, also tried the circuit function with different frequencies (from 20KHz to 60KHz) but same situation. The transformer (2+2turns/50 turns) I again rewound but the situation still remained unchanged. I also connected two capacitors of 2200uF/25V for decoupling, & snubber circuitry (0.01uf with 4.7 Ohms resistor in series) is also attached.
So, I think that this is the problem within gate driving section, so please help me for resolving the trouble.

Note: transformer is EDT44
Operating frequency: 36KHz (approx)

Furthermore, Someone informed me that since the circuit is running in voltage mode... a push-pull running that way can be subject to "flux walking".
Therefore, my question is that how I prevent the core from reaching saturate in this push pull mode?

 

[stefanodel]

well, I need an other information from you, the nominal voltage of your battery.

Stefano

 

[xyz9915]

The battery is lead acid battery & its voltage will be 12-13.2

 

[stefanodel]

The battery is lead acid battery & its voltage will be 12-13.2

at nominal voltage (12V) the maximun flux in the core of your transformer using the push-pull topology is about 240mT. You can calculate this value using the following formula:
Bac_max=(V*10000)/(4*Ae*Fsw*N)=(12*1000)/(4*1,73*36000*2)=0.241
where:
V is the voltage applied to the primary in Volt
Ae is the the effective area of core in cm^2
Fsw is the frequency of switcing in Hz
N is the number of primary turn
Bac is the maximun ac flux in Tesla

he maximun value of flux depend on the type of ferrite, on the loss in the core , on the temperature of the core.
The shape of your transformer is ETD44, the type of ferrite I don't know, you should find something print on the core like 3C90, N67 tis is the type of ferrite.
The value of 240 mT should work but I would prefer to use a lower value of AC flux, this could be useful also in order to reduce the effect of the DC bias due to the problem of staircase saturation (flux walking) in push-pull.
To reduce the ac flux you can increase the primary turns for example 3+3 turns instead of 2+2 or increase the switching frequency.

If the average volt*second applied to the transformer when the first transistor is on is not equal to that applied when the second transistor is on, then there will be in the core of the transformer a DC component. More the DC current more the current switched by transistor and more the possibility to have the core saturated.
To limit this problem you could introduce a little gap in the core ( using the gap the magnetizing current is increased) or control the peak current, the TL494 has the possibility to do this.
Remove the diode 1n4148 I think isn't a good idea, this diode forces the pnp in off state when the gate of mosfet is driven.
To analize the waveform across the gate it is better to use a scope, a frequencymeter it is not suitable for this job.
If you want to increase the output current delivered from TL494 you should use a mosfet driver like a MIC4427.
Do you have tested the deadtime of the drivers?

stefano

 

[xyz9915]

To reduce the ac flux you can increase the primary turns for example 3+3 turns instead of 2+2 or increase the switching frequency.
stefano

Your tip for increasing the turns ratio has worked. I increased the turns ratio by experimenting & I found with the help of Oscilloscope that with 5+5 turns in the primary side, circuit works fine (MOSFET's remain cool, thus my problem solved).
In this connection I rewound the remaining two power supply's transformer with same turns ratio but they both transformer are not working. when I connected oscilloprobe with the gates of MOSFEt's, I found distorted waveform despite the square wave (with the working transformer, waveform remain almost square wave).
So, please guide me the how I best wind the transformer.

 

[stefanodel]

Your tip for increasing the turns ratio has worked. I increased the turns ratio by experimenting & I found with the help of Oscilloscope that with 5+5 turns in the primary side, circuit works fine (MOSFET's remain cool, thus my problem solved).
In this connection I rewound the remaining two power supply's transformer with same turns ratio but they both transformer are not working. when I connected oscilloprobe with the gates of MOSFEt's, I found distorted waveform despite the square wave (with the working transformer, waveform remain almost square wave).
So, please guide me the how I best wind the transformer.

My advise is to wind the two halves of the primary windings in bifilar-wound.
I attached a photo of an examples of primary winding, I used isolated coloured wire to see better the windings, these wires are not suitable for windings transformer. Wires D1 and D2 are connected to the mosfet drain and the two wire + to the positive voltage of battery.
Could you post a photo of voltage waveform of the mosfet gate with respect to source?

Stefano

 

[xyz9915]

Please see the attached waveforms taken from oscilloscope. The pattern (A) shows the waveform of MOSFET's gates when the positive supply (12V) not connected to the push-pull transformer. When it is connected, the waveform (B) produces (too much noise/spikes) even the snubber is connected. After changing frequency from 36KHz to around 50KHz, waveform (C) appears which seems better but the MOSFET's gets very hot within one minute at 30% load. I changed the circuit with SG3525 (totem pole output) but the same result. So, the most suspected thing is transformer. changing the turns ratio or frequency slightly change the waveform. So, what is the solution?

Tomorrow I will wound the transformer as bifilar as per your advise.

 

[stefanodel]

Please see the attached waveforms taken from oscilloscope. The pattern (A) shows the waveform of MOSFET's gates when the positive supply (12V) not connected to the push-pull transformer. When it is connected, the waveform (B) produces (too much noise/spikes) even the snubber is connected. After changing frequency from 36KHz to around 50KHz, waveform (C) appears which seems better but the MOSFET's gets very hot within one minute at 30% load. I changed the circuit with SG3525 (totem pole output) but the same result. So, the most suspected thing is transformer. changing the turns ratio or frequency slightly change the waveform. So, what is the solution?

Tomorrow I will wound the transformer as bifilar as per your advise.

I saw the waveforms, but unfortunately the waveforms B and C are not useful, the traces are too confused probaly due to the noise.
Could be useful to see , using a more little time base for example 1us or 2us, only the rising edge of one gate and the following edge of the other one. This measure to evaluate how much is the dead time of the driving signals. A too short dead time could produces high dissipation in the transistors.
To avoid noise, using the probe of the scope, you should make one or two turns using the wire of the clip-on ground lead, over the probe tip.
There is a good application note of Tektronix regarding the scope probes:
**broken link removed**
Do you think that the layout of your converter is good or do you have some doubts about it?
Stefano

 

[xyz9915]

The attached file contain waveforms taken from oscilloscope. Circuit work fine under 30% load. The problem is that about 70% of the load, the MOSFET's get suddenly hot and if the power supply is not disconnected, they burn within 3-5 minutes. I increased MOSFET's from 1 to 5 each side but the result is same, MOSFET's gets burnt. So, please help me is this respect.

EDIT:- Furthermore, although TL494 has a 45% maximum duty cycle. Increasing deadtime can fix the problem?

 

[stefanodel]

The attached file contain waveforms taken from oscilloscope. Circuit work fine under 30% load. The problem is that about 70% of the load, the MOSFET's get suddenly hot and if the power supply is not disconnected, they burn within 3-5 minutes. I increased MOSFET's from 1 to 5 each side but the result is same, MOSFET's gets burnt. So, please help me is this respect.

EDIT:- Furthermore, although TL494 has a 45% maximum duty cycle. Increasing deadtime can fix the problem?

I don't think that the problem is the deadtime, I saw the photos of gate signals, the dead time is present. It is not a problem of mosfet current rating, someting changes in the circuit when a higher power is delivered to the load.
In the third row (photo: gate waveform both channel 70% load) there is something strange, the amplitude of upper trace is higher than the lower trace, the two trace must have the same amplitude. The amplitude of lower trace is about 10Volts. This value may not be enough to drive the mosfet in on state.
Your battery has a nominal voltage of 12V (six elements), when the current load increase, there is a voltage drop in the battery ( this also depends on the size of the battery). I don't know if you power the tl494 using the battery. Which is the function of the regulator 7812 and what is the value of vcc?
Have you tried, using the oscilloscope, to measure the voltage of across the battery when the load is at 70%?.

Could be useful to measure the current flowing in mosfet at different levels of output current, to try to understand the behavior of the push-pull.

Stefano