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reducing input voltage for flyback ic

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Arpit Gupta

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I have implemented a flyback transformer using LT8302 whose input voltage range is from 2.8 to 42 V. now In my system the in the input voltage is going to 44-56v.

Is there any practical way to reduce the input voltage below 42v without using a buck converter?

I dont want to use another ic to step down the input voltage, would zener diodes of 40,30V work to clamp the voltage at 40,30v respectively?
 

FvM

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I don't see a simple way. You'll preferably use a high voltage capable flyback converter/controller IC.
 

Arpit Gupta

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Would using a voltage divider using resistances help?
I cant redesign my system now
 

FvM

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Surely not a resistor divider. If efficiency isn't important, a linear pre-regulator (Z-diode + series transistor) could be used.
 

Warpspeed

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I agree, a pre regulator is called for.

How about an LM317HV ?
https://www.ti.com/lit/ds/symlink/lm317hv.pdf
These have a max input output differential of 57 volts.
If you set the output for 40V the maximum safe input voltage would then be 97 volts.
Minimum input to maintain 40v output would be about 43 volts.

I know you don't wish to use another IC, but it will perform much better than a zener and won't be any different efficiency wise.
 

mtwieg

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Problem is that the Vin pin is also used as the reference for detecting the output voltage via the flyback voltage. So a preregulator would completely screw up the output voltage regulation. I don't think there's any way to do it with particular IC.
 

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Problem is that the Vin pin is also used as the reference for detecting the output voltage via the flyback voltage. So a preregulator would completely screw up the output voltage regulation. I don't think there's any way to do it with particular IC.
The suggestion was to supply the reduced voltage to the complete flyback converter, including the transformer. That's no good solution efficiency-wise, but the only way to operate the particular converter.
 

Arpit Gupta

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I am considering lm317hv to decrease the input voltage and have already ordered the samples, but cant mess with the regulation
How exactly will it affect the regulation?
 

SunnySkyguy

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Drop Vin below 40V Iq=0.38mA max.
Switch Vmax= 60V, which may need a zener + diode clamp to V+.

Vin just powers all internal Vref and regulators used for Vfb so voltage and current are both much lower than the driver limit. (0.38mA) to limit self heating of Bandgap reference diode inside.

So output regulation is not affected. But driver Vds max should not exceed 60V peak on flyback.
 

mtwieg

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Vin just powers all internal Vref and regulators used for Vfb so voltage and current are both much lower than the driver limit. (0.38mA) to limit self heating of Bandgap reference diode inside.

So output regulation is not affected. But driver Vds max should not exceed 60V peak on flyback.
Read the datasheet for the controller. It uses back emf from the flyback transformer for output voltage detection. This is measured relative to the Vin pin so it will affect regulation.

- - - Updated - - -

The only way I can see to make this work without greatly lowering efficiency is to put a zener diode between the DC input and the controller's Vin pin. That way, the Vin pin will still track with the DC supply, but with a fixed offset of Vz. But you will have to modify the equations for Rref and Rfb in order to get the correct voltage. Also the minimum input voltage of the converter will also be increased by Vz.

But then you will still have to deal with the 65V limit on the internal FET, which means you have almost no headroom there. You should find another control IC.
 

SunnySkyguy

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Read the datasheet for the controller. It uses back emf from the flyback transformer for output voltage detection. This is measured relative to the Vin pin so it will affect regulation.
... You should find another control IC.

I agree.

I overlooked the current mirror and stand corrected.

Reducing Vin by an offset would degrade supply sensitivity. Reducing Vin by some R ratio and also reducing Rfb by same R ratio with a shunt to ground and adjusting Rref might compensate for gain from Vref.
9490042000_1436739521.jpg

Good luck.
 

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