Flyback Converter

Thread Starter

fieryfire

Joined Feb 14, 2017
129
Hello Could anyone please help me understand the signficance of the diode D3 and maybe D2 in the below application circuit. I have a circuit that has D3 at 15V and the position is moved after D2, but the direction is the same. Also i have two secondary windings for a bipolar +-12 v output power supply. My Zener diode D3 seems to heat up above 220C. I know its clearly a problem but i cant figure out why. Could someone please explain?

1602489979856.png
 

Marley

Joined Apr 4, 2016
369
Diodes D3 and D2 form a "snubber" circuit. They dissipate excess flyback energy.
They prevent the voltage on pin Vsw from going too high and damaging the LT1170.
The fact that D3 is getting hot suggests a fault elsewhere.
Normally, flyback energy should pass through D1 and charge up C1 and not be dissipated in D3.
When C1 gets to about 5V, the feedback circuit should stop the LT1170 from switching.
Is D1 working? Is the secondary winding of the transformer connected and not internally open-circuit?
What voltage do you get across C1?
If D3 has got to 220deg C it is probably damaged!
Don't power the circuit for more than a few seconds!!
 

ronsimpson

Joined Oct 7, 2019
1,017
Several thing could be wrong.

The coupling (primary to secondary) of the transformer is not good.
The turn ratio of the transformer is such that the flyback part of the wave form is too high a voltage.
What type of diode for D2? Part number for D3? Is D3 a 1 watt part?
Do you have a scope so we can see the voltage at "SW"?
 

Thread Starter

fieryfire

Joined Feb 14, 2017
129
hi ff,
Please post a circuit diagram of the your project, we could use simulation to check for the problem.

E
Hello Eric,

Always the saviour :). Here is the circuit. I have done some simulations on this as well. But not really sure if im doing it correctly on LTSPICE.


1602508852761.png
 

Thread Starter

fieryfire

Joined Feb 14, 2017
129
Several thing could be wrong.

The coupling (primary to secondary) of the transformer is not good.
The turn ratio of the transformer is such that the flyback part of the wave form is too high a voltage.
What type of diode for D2? Part number for D3? Is D3 a 1 watt part?
Do you have a scope so we can see the voltage at "SW"?
Hello Ron, I have posted the circuit with choice of component of diodes as well above. I have a scope to measure. but i removed VR5 at the moment. I am not sure if its safe to turn on the circuit once again since LT1172 could be damaged. The windings on primary are 51 turns and on secondary is 33T. I am not completely aware what does FB and VC have to do in the circuit as well.
 

Thread Starter

fieryfire

Joined Feb 14, 2017
129
Diodes D3 and D2 form a "snubber" circuit. They dissipate excess flyback energy.
They prevent the voltage on pin Vsw from going too high and damaging the LT1170.
The fact that D3 is getting hot suggests a fault elsewhere.
Normally, flyback energy should pass through D1 and charge up C1 and not be dissipated in D3.
When C1 gets to about 5V, the feedback circuit should stop the LT1170 from switching.
Is D1 working? Is the secondary winding of the transformer connected and not internally open-circuit?
What voltage do you get across C1?
If D3 has got to 220deg C it is probably damaged!
Don't power the circuit for more than a few seconds!!
Thank you for the response marley,

Could it be that my circuit using +-12V is has a flyback voltage of about 12V and hence it activates the 15V diode, the VR of this diode is at about 11.4, and it could be this reason perhaps?
 

ericgibbs

Joined Jan 29, 2010
11,122
hi ff,
Whats the Vin filtered Voltage.?
What typical load currents on the +/-12Vout.?

If you have a LTSpice asc sim file please post.

E

BTW: is D8 the wrong way around.?


Update:
Do you know the transformer inductance values.?
 

Thread Starter

fieryfire

Joined Feb 14, 2017
129
hi ff,
Whats the Vin filtered Voltage.?
What typical load currents on the +/-12Vout.?

If you have a LTSpice asc sim file please post.

E

BTW: is D8 the wrong way around.?


Update:
Do you know the transformer inductance values.?
I ve been wondering why D8 is in that direction. ive no idea why its the other way round. Vin_filtered is 30V..I have attached the file. But please see that , it may not be accurate. for ex: ive only approximated some diodes in this circuit. and it could be wrong.

Typical current at the load is about 9mA and max at 150mA. But there is about two loads connected in parallel. So lets say a max at about 300mA?
 

Attachments

crutschow

Joined Mar 14, 2008
25,660
The windings on primary are 51 turns and on secondary is 33T.
With that turns ratio the flyback voltage across the primary is a minimum of 51/33 * 12V or 18.5V.
It's no wonder your 15V Zener is conducting and dissipating a lot of power.
That's why you need at least a 25V Zener for protection.

So why did you think a 15V Zener would be sufficient?
You need to brush up on your transformer theory. :)
 

Thread Starter

fieryfire

Joined Feb 14, 2017
129
hi ff,
You have a 15V Zener dumping 46V at SW into a 30V supply .!!!!
The design is flawed.
E
YEs indeed, but do you think a 25V zener or 50V Zener replacement would prevent the 46V from hitting the 30V power supply?.. Also, i see that your circuit is missing the bidirectional diodes which is the P4KE12CA.

here is the ltspice model of that




1602512293506.png
 

Attachments

Thread Starter

fieryfire

Joined Feb 14, 2017
129
With that turns ratio the flyback voltage across the primary is a minimum of 51/33 * 12V or 18.5V.
It's no wonder your 15V Zener is conducting and dissipating a lot of power.
That's why you need at least a 25V Zener for protection.

So why did you think a 15V Zener would be sufficient?
You need to brush up on your transformer theory. :)
that makes a lot of sense actually. i havent designed this circuit. it was designed by a 40 years experience electrical engineer. Im just the new guy who is trying to figure why it was designed like this in the first place.
 

Thread Starter

fieryfire

Joined Feb 14, 2017
129
With that turns ratio the flyback voltage across the primary is a minimum of 51/33 * 12V or 18.5V.
It's no wonder your 15V Zener is conducting and dissipating a lot of power.
That's why you need at least a 25V Zener for protection.

So why did you think a 15V Zener would be sufficient?
You need to brush up on your transformer theory. :)
Could you by any chance, figure out why the circuitry around the Vc and VB pins? i still dont understand it
 

ericgibbs

Joined Jan 29, 2010
11,122
hi ff,
This is the basic circuit with a 25Vz.

How did you decide on the transformer ratios/inductances.?

I used UF4007 diodes and a GP zener, use a 1W zener.

EAAA 617 15.42.gif
 

Thread Starter

fieryfire

Joined Feb 14, 2017
129
hi ff,
This is the basic circuit with a 25Vz.

How did you decide on the transformer ratios/inductances.?

I used UF4007 diodes and a GP zener, use a 1W zener.

EView attachment 219425
Hello Éric. The transformer inductance were chosen 51 turns to 33turns.it was basically a copy paste from an older circuit. Adapted to 12v..there is also the missing transient suppressor didoes between line and ground
 

crutschow

Joined Mar 14, 2008
25,660
figure out why the circuitry around the Vc and VB pins?
Vc is the compensation input.
I'm not sure what the rest of the circuit is for, unless it's supposed to provide a soft-start or shut-down function.

What pin is VB?
The FB pin is the the feedback pin to regulate the output voltage.
 

Thread Starter

fieryfire

Joined Feb 14, 2017
129
With that turns ratio the flyback voltage across the primary is a minimum of 51/33 * 12V or 18.5V.
It's no wonder your 15V Zener is conducting and dissipating a lot of power.
That's why you need at least a 25V Zener for protection.

So why did you think a 15V Zener would be sufficient?
You need to brush up on your transformer theory. :)
Would you think swapping the 15V Zener for a 25V should be sufficient. I understand that, the Feedback pin is the one that controls the Voltage outputs. I still don't understand what the VC does, is it an output pin or input pin?
 

crutschow

Joined Mar 14, 2008
25,660
Would you think swapping the 15V Zener for a 25V should be sufficient.
Because the transformer primary flyback voltage for a 12V output is about 18.5V for the transformer turns ratio you have, and the Zener circuit is connected across the primary.
Do you still not understand how transformers work?
I still don't understand what the VC does, is it an output pin or input pin?
It not really either.
It provides RC compensation for the internal circuit to stabilize it for the large LC output filter, which creates a large phase-shift between the output and the feedback pin.
 
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