SMPS using UC3842 - And related problems

[vantusaonho]

Hi all!
I am designing a smsp using UC3842, but first I test this IC.
The IC work fine with load as Led, but I put smps windings transformer, the IC turn off output, I sense pin has 0 volt. Please explain to me this phenomenom.
Thanks

The schematic


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[FTL]

I'm afraid that I have to start off with the standard answer: You have not given anywhere near enough information for somebody to help you.

Please provide a schematic of how you have it hooked up. You could just be missing some small thing, but without all the details it is impossible to help.

[enzine]

In the flyback converter (your circuit), the energy is stored in the transformer inductance during the "on" phase and released to the load during the "off" phase.
The diode D3 in the off phase is in conduction and "block" this energy.
You must use a voltage clamp snubber instead od D3 (see application note of UC3842 or similar).

Ciao!

[vantusaonho]

In the flyback converter (your circuit), the energy is stored in the transformer inductance during the "on" phase and released to the load during the "off" phase.
The diode D3 in the off phase is in conduction and "block" this energy.
You must use a voltage clamp snubber instead od D3 (see application note of UC3842 or similar).

Ciao!

Exactly! The circuit work when I removed D3 diode. But Output voltage is 25VDC meanwhile output voltage not rectified is 8VAC. I think it should be 8V*sprt(2) =11.3 VDC



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[enzine]

... I think it should be 8V*sprt(2) =11.3 VDC

NO!
This formula is valid only for sinusoidal waveforms.
You must also take into account the duty cycle.
I suggest you study a little bit theory of switching converters

Ciao!

[vantusaonho]

Hi everybody!
I made a smps successfull. My smps has 33VDCt input and 12VDC output 1A. I checked the output about 12.66V. This is the schematic.


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But there is a problem that is the 56K 3W resistor of snubber circuit is too hot and the mostfet too. I measured voltage at pin 3 (Current sense), it is about 0.143V. That proofed that is the current through primary windings about 0.143A. Please explain to me why the resistor and mosfet heating.
Thanks in advance

[Mr. Spock]

Check leakage inductance of transformer.

[Gallymimu]

Check with a scope, you might have a lot of ringing.

[vantusaonho]

Check with a scope, you might have a lot of ringing.

Yes, please tell me how to remove ringing

[Gallymimu]

Yes, please tell me how to remove ringing

Tell us what is ringing. Where are you measuring.

Be sure to check the gate and the drain of your MOSFET.  I am not a snubber expert but have designed a few RC snubbers before.  Not as sharp at the RCD style one you are using.

Typically you can put a series RC across the mosfet such that it is tuned to maximally dampen the ringing frequency.  That might be redundant with the RCD though, not sure without thinking about it harder.

I've used these equations with success for inductive switching placing the snubber across the load or across the mosfet depending on the circuit:
http://www.ti.com/ww/en/analog/power_management/snubber_circuit_design.html.

something seems odd though, why would a 56K resistor be getting hot unless your ring voltage was REALLY REALLY high?  If you were applying 100VRMS to that resistor it would only be 200mW.

What is your peak ring voltage???

[vantusaonho]

Voltage at Capacitor snubber is 317V, I don't understand why it is high like that. I rewound flyback transformer with less turns than, primary winding only wound 50% length bobbin. The rest of space, I wound secondary winding and bias winding. The result that the resistor not overheat more, but when smps work with load, mosfet overheat.

[xenix]

Energy stored in the leakage inductance of the transformer charges C1 when MOSFETis turned off. Snubber resistor R1 discharges C1 until next switching. If you can decrease leakage inductance of the transformer, you can decrease capacitance of C1 and the heat on R1 will be decreased.

If you have a scope, decrease C1 value and check Vds. As you decrease C1, Overshoot voltage on Vds will increase, stop at the point where MOSFET is safe.

If you can share transformer design details, we can help to improve it.

What is the switching frequency? if it is high, you should decrease 100Ohm gate drive resistor to decrease switching losses on the MOSFET.

[vantusaonho]

Energy stored in the leakage inductance of the transformer charges C1 when MOSFETis turned off. Snubber resistor R1 discharges C1 until next switching. If you can decrease leakage inductance of the transformer, you can decrease capacitance of C1 and the heat on R1 will be decreased.

If you have a scope, decrease C1 value and check Vds. As you decrease C1, Overshoot voltage on Vds will increase, stop at the point where MOSFET is safe.

If you can share transformer design details, we can help to improve it.

What is the switching frequency? if it is high, you should decrease 100Ohm gate drive resistor to decrease switching losses on the MOSFET.

Your explain is easy to understand. The flyback transformer include: EE25 core, primary winding 19 turns, wire 0.4mm - secondary winding 9 turns wire 0.6mm - Bias winding 10 turns, wire 0.3mm. All windings are wounded follow clockwise. switching frequency is 109KHz. Please help me

[xenix]

Heat on the MOSFET is related with the high gate drive resistance, try decreasing it to a lower value like 33 ohms. The gate driver inside UC3843 can deliver ±1A maximum, that means lowest gate drive resistor value is 12 ohm @12V IC supply voltage.

Primary and secondary windings should be as close as possible. In the below figure, prefer the winding arrangement like the transformer at left of the figure. The transformers on the right of the figure has separated primary and secondary windings. In this configuration, leakage inductance is high, so avoid this design.

[vantusaonho]

I find out that diode D1 is overheat, too. I wonder FR207 have break voltage 1000V, current forward 2A meanwhile the current in secondary winding only 1.5A (peak), so why it overheat?
I think that air-gap large (is about 0.75mm, meanwhile is required 0.16mm) is the cause to high leakage inductance, so Vsn is high (317V). I fixed air-gap and result that R1 not overheat. The resistor for gate drive, I replace with 10 Ohm, mosfet seem less overheat but it still is hot.

[zab]

Mosfet heat is result of normaly two things,
1 on resistance of mosfet
2. insufficient drive volt.
The mosfet used here has RDS(ON) = 3.8Ω@VGS = 10V. on resistance is high you can select some other transistor.I have used mosfet having on resistane in m ohms.You are using  high volt 600v mosfet for 33v. bring it down to 100v to get  good on resistance mosfet drive volt are coming from ic. so ensure volt for ic more than 10v all the time.

[vantusaonho]

so ensure volt for ic more than 10v all the time.

When smps have a load, drive volt is about 7.2V (45%*16V -Vcc), normal without load drive volt is about 3V. How to ensure it more than 10V? Please

[Gallymimu]

At 100KHz switching losses are going to dominate over Rds losses.  Be careful reducing the gate drive resistor.  It will likely increase the ringing as your switch edges will be sharper.  This may actually increase your switching losses rather than reducing them.

You should be able to measure your leakage inductance.  Do you have an LCR meter?  To a crude estimation you can measure the primary leakage inductance by shorting the secondary and measuring the inductance of the primary.  This PDF gives a pretty good explanation:
http://0071c19.netsolhost.com/support/articles/55/Measuring%20Leakage%20Inductance%20(104-105).pdf

with 300V ringing you have set up a really nice resonant tank!

For leakage inductance reduction.  No air gap if you have an air gap in the core.  primary and secondary overlaying will help, winding in hand will also help.  Winding in hand means having your primary and secondary wound together at the same time as a pair.  For a 2:1 transformer wound in hand it is a bit of a pain but you can wind 3 wires together and use 1 of them to make the primary and 2 of them to make the secondary.  This helps a lot with coupling but your mileage may vary depending on other design constraints.  I hope that makes sense.  I don't think I explained it well.

Core material can also affect leakage but most ferrites should be pretty good.

[vantusaonho]

I replaced resistor for gate mosfet down to 10 Ohm, Mosfet is less hot than (with a led as load, no overheat), but It still hot when I put load as fan 12V 0.7A. The diode for rectifier at secondary side is overheat too. I dont know why it behave like that although the load is only a led with 10mA in current. I don't have oscilloscope even LCR meter because I am a newbie in smps and they are so expensive.
I designed a new circuit with 110-220VAC, output 12V 1A. I hope it work fine

[Gallymimu]

I replaced resistor for gate mosfet down to 10 Ohm, Mosfet is less hot than (with a led as load, no overheat), but It still hot when I put load as fan 12V 0.7A. The diode for rectifier at secondary side is overheat too. I dont know why it behave like that although the load is only a led with 10mA in current. I don't have oscilloscope even LCR meter because I am a newbie in smps and they are so expensive.
I designed a new circuit with 110-220VAC, output 12V 1A. I hope it work fine

Check and see if the ringing voltage has gone up since reducing the gate drive resistor.  It is important to monitor that.  If your ring voltage is 300V and your input voltage is 30V I'd be nervous about using the same/similar design at 100V-200V input until I really understood why had had such high overshoot voltages.

You are playing a tough game working on this without at least a scope.  Like playing basketball with one eye covered.  Good luck.  We'll help as we can.

[zab]

When smps have a load, drive volt is about 7.2V (45%*16V -Vcc), normal without load drive volt is about 3V. How to ensure it more than 10V? Please

Have you check these volt with muti meter on out of ic,If yes then you are mistaken by the values. what is important is the amplitude of the drive pulse.this can be checked only with oscilloscope.
with multimeter you can check the vcc of 3842 if the value is more than 10v it means gate drive pulse will also be of more than 10v . that is ok. if your vcc come down than it need correction.

[vantusaonho]

I only have Volt Ohm meter, no multimeter, no oscilloscope. Poor me!

[zab]

No problem the method will remain same for your meter.So go ahead. Please make clear the point of checking 300v and method as well. check and  share the result.

[Faros]

you can disconnect the UC power pin (pin 7) and connect it to external power supply of 12V, make sure to share the power supply GND with your circuit GND. check your MOSFET for heating. if it is lower in temp this means that the MOSFET gate voltage was under driven, if not then alter the frequency by changing C5 to 10n and R7 to 1.5K (should give 100 kHz), recheck and feed us back.

[vantusaonho]

I will check this. Please tell me how much do I should coefficient and duty cycle with input 110-220 output 12V? And what equation say this? Thanks

[Checksum8]

I think R2 and D5 may be part of your problem. It is only allowing about 15ma to to power UC3842 and drive the mosfet. Replace R2 and D5 with  a LM7815. Do what Faros suggested and that will verify if this is your problem.

[Faros]

the UC3842 is not powered through the R2 / D5 branch, it is powered mainly by the auxiliary winding on the transformer ...

[vantusaonho]

the UC3842 is not powered through the R2 / D5 branch, it is powered mainly by the auxiliary winding on the transformer ...

UC3842 is powered through the R2/D5 branch for start up because if UC3842 is not powered, auxiliary winding will not conduct.

This is a schematic for input 110-220VAC, output 12V


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There are a problem, the voltage at R2/D7 branch is 18.5V when unplug UC3842 but is 10 -14V according sin wave when plug UC3842. Please explain this

[xenix]

This is a flyback circuit. There must be airgap in the transformer.

[vantusaonho]

This is a flyback circuit. There must be airgap in the transformer.

Yes, there is a air-gap about 0.25mm in flyback transformer but I don't know why happen the phenomenon like that

[Faros]

There are a problem, the voltage at R2/D7 branch is 18.5V when unplug UC3842 but is 10 -14V according sin wave when plug UC3842. Please explain this

When the chip is out D3 is reversed and the voltage at pin 7 socket is equal to zener voltage ~18V, when the chip is inserted, the R2/D7 branch provides >1mA (enough for UC3842 to start oscillating), once oscillated, it will drive the MOSFET gate through the 10 ohm resistor and hence consume more current, if the circuit couldn't oscillate correctly for any reason , the auxiliary will not be able to deliver the required voltage and the Vcc of the chip will drop... did you try using an external PS? 

[xenix]

Yes, there is a air-gap about 0.25mm in flyback transformer but I don't know why happen the phenomenon like that

Transformers do not store energy, they just transfer it.
Inductors store energy. In this manner, flayback transformer is more like an inductor rather than a transformer. They should store energy when the transsistor is turned on. When turned off, they should deliver the energy to secondary winding.

Magnetic energy stored in unit volume is E=B²/(2µ). Relative magnetic permeability of ferrite is in the range of 50-500. That means you cannot store energy in ferrite. Therefore you need airgap. Nearly all the energy is stored in the airgap. it can be formulated as
½×L×I²=AirGapVolume×½×B²/µ

[vantusaonho]

Bum!!!!! That is the sound have just happen.
I replace R2 with 27K to increase more current through pin 7 UC3842, the voltage not vary anymore but the frequency at pin 6 only 4Khz meanwhile it should be 100KHz. When I remove current of auxiliary winding from pin 7, the circuit have burned after 2 seconds.

[vantusaonho]

I noticed that Resistor snubber and Mosfet overheat. I feel that flyback transformer have not transfer all energy cause overheat on mosfet and snubber resistor

[vantusaonho]

I make a smps input 33VDC output 12V
I wound a smps in order as follow: primary winding 15 Turns wire 0.4mm, bias winding 11 Turns wire 0.4mm, secondary winding wire 0.6mm
All winding are wounded as clockwise


* With first scheme:
- Output voltage 18V, rectifier diode at secondary side overheat. bias voltage 60V



* With second scheme:
- Ouput voltage 10.5V, bias voltage 18.5V, mosfet and snubber resistor overheat


Please let me know which scheme right

By the way please confirm to me which schema for flyback transformer. I read many app note, I saw both schema so I don't know what is right


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Thanks in Advance

[Faros]

I am afraid that I didn't understand that this BUM sound was related to which component?!! ...

[Faros]

you may find this site helpful ...

http://schmidt-walter.eit.h-da.de/smps_e/smps_e.html

[vantusaonho]

I just find out that the rectifier diode at secondary side is very important. In my schematic, I use FR207 (Fast recovery diode) which I thought it can be work fine in high frequency. But it seem not like that. The diode overheat due to not  conduct quickly and block quickly. I think it should be replaced by a schottky barrier rectifier diode as: SR340, MBR1060....
The mosfet also overheat due to diode. I think so.
What do you think? Please let me know. Thanks

[pickit2]

Bum sound = bum note. or maybe even a fart....

I hope he not going into full production anytime soon.
How many are you planing on building? 1 or 1 million?

Note: My place of work buy this Item for $1 in lots of 10.

[alexxx]

I just find out that the rectifier diode at secondary side is very important. In my schematic, I use FR207 (Fast recovery diode) which I thought it can be work fine in high frequency. But it seem not like that. The diode overheat due to not  conduct quickly and block quickly. I think it should be replaced by a schottky barrier rectifier diode as: SR340, MBR1060....

I agree that a schottky diode should be used for that purpose.

If your amperage is below 1A, then MBRS1100 could be an option. I used it several times as the diode after the secondary winding, without having any overheating problems up to 250mA.
http://www.alldatasheet.com/datasheet-pdf/pdf/3176/MOTOROLA/MBRS1100.html

For higher currents, you could also look into SS34. I have never used it for such purpose, but I think it could do the job if the output is 18V as mentioned.
http://www.alldatasheet.com/datasheet-pdf/pdf/34915/ZOWIE/SS34.html

[Gallymimu]

I just find out that the rectifier diode at secondary side is very important. In my schematic, I use FR207 (Fast recovery diode) which I thought it can be work fine in high frequency. But it seem not like that. The diode overheat due to not  conduct quickly and block quickly. I think it should be replaced by a schottky barrier rectifier diode as: SR340, MBR1060....
The mosfet also overheat due to diode. I think so.
What do you think? Please let me know. Thanks

I can't think of a time that you WOULDN'T use a schottky in an SMPS.  You really can't do better than a SCHOTTKY for fast switch times and minimal switching and conduction losses.

[vantusaonho]

I replaced FR207 rectifier diode with MBR1060. But there is still a problem. The ouput voltage is 18V meanwhile the steady voltage is 12V. Perhap I biased for TL431 not properly. I read some paper about bias for TL431 but I don't understand because they present so complicated. I need a simple equation to calculate Rup, Rlow, Cf, Rled and Rbias. it would be good if people could help me in this problem.
 Thank in advanced

[Faros]

trying to design an SMPS without a scope is like trying to put together a 1000 pieces puzzle with your eye blindfolded.
you will keep having signs that you will not be able to interpret simple because you don't see what is going on.
take your circuit to a friend who has a scope and then you will be able to judge for your self what a 42 reply so far couldn't .
 

 

[Ichan]

All winding are wounded as clockwise

I have a batch of EE16 transformer which is useless because the maker wound the the windings wrongly in the same direction, where it should be in opposite direction - the result is the similar with yours, hot everywhere and i do not know why. I know a little about transformer things, but from that experience maybe a worthy try for you to wound the secondary winding in CCW.

@Xenix, looks like you know transformer well - on EE core, where should the gap made: on center leg, on the two side leg, or on all three legs? What is the use of copper foil that sometimes found on hf transformer?

-ichan

[vantusaonho]

Yes! Mosfet and snubber resistor overheat due to wrong direction, rectifier diode overheat due to wrong diode. All fixed now, I replaced FR207 by 1N5822. It' ok fully. I made successfull a smps input 33V output 12V 1A
And I now am making a smsp with input 110V - 220VAC ouput 12V 1 A

[zab]

Mosfet and snubber resistor overheat due to wrong direction

Wrong direction of what ? Please make it clear. so that the whole problem may understandable.

[vantusaonho]

Wrong direction in windings flyback transformer

[vantusaonho]

Hi everybody!
I have succeed with a smps input 220VAC output12VDC. But I still have a problem. In my design, I must use Mosfet 25N120 (25A, 1200V), the board work fine. If I use 7N80 (7A, 800V) , 4N60 (4A, 600V), the mosfet and fuse will be burned. I wonder that why some smps use mosfet IRF840 (500V, 8A) work fine?
Please explain to me.

[Ichan]

First make sure that you use "original" mosfet, not a "fake" one - they are too many on the street market.

-ichan

[Gallymimu]

Hi everybody!
I have succeed with a smps input 220VAC output12VDC. But I still have a problem. In my design, I must use Mosfet 25N120 (25A, 1200V), the board work fine. If I use 7N80 (7A, 800V) , 4N60 (4A, 600V), the mosfet and fuse will be burned. I wonder that why some smps use mosfet IRF840 (500V, 8A) work fine?
Please explain to me.

Based upon your previous issues with high ringing voltages you might check to see if you are ringing beyond the voltage capacity of the MOSFET.  Other factors such as gate capacitance, switching loses, Rdson could also be sources of problems if they are not specified correctly for your alternate mosfets.

[vantusaonho]

I think it due to core saturation. Because with the same board, I tried with 110VAC, it work fine. But for 220VAC, it burned fuse and mosfet. Do you think same to me?

[vantusaonho]

Job done Thanks All

[Faros]

Job done Thanks All

@vantusaonho
You owe members who tried to help you some explanation of "how your job is DONE?!!" , this way you may find someone answering your request next time.

[vantusaonho]

Oh! I wind another transformer again with less than primary winding in turns. And note that with 220VAC, the DC voltage after rectifier will be 220*sprt(2) - 10% margin - 20V (ripple and drop diode voltage) = 260V. From that, the turns for primary winding will be decrease as a result.
More turns more flux flow. So core saturation will reach quickly and leading to smoke mosfet and fuse. That is real problem
I hope you get success as me.