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Hi
Why did you use RC snubber across primary of transfermer? For Flyback configuration you need to use RCD / ZCD type snubber circuit. This will suppress the transients to some extent. Also, keep RCD snubber(configuration is differ from earlier) across Drain-Source of the switching device.
also, remember that as C value increases, the peak overshoot decreases, but the power dissipation increases. accordingingly you need to select the wattage of the resister and capacitor.
just go to google search for RCD snubber+flyback. You will get more information on it.
The RC snubber configuration is doest work for fly-back topology. if you give the same arragement like what you shown in schematic, the voltage in C is keep on increase in one direction, and voltage stress at DS of switch will increase. this will destroy the switching device.
hi megtech
yes this RCD snubber is best suitable for flyback converter. If you understood the concept of energy transfer from primary to secondary of the Fly-back converter, it is easy for you understand the RCD snubber operation. ZCD snubbers are used to supress high transients, and costlier as well. make sure the wattage of the devices you selected should be atleast 1.5 times the what you get in theoritical calculation(for safety purpose). In addition to this also provide proper heat sinks to the switching device(MOSFET/Transister).
The selection between transister and MOSFET is purely based on Voltage and current. For high current application, MOSFETs are the good choice.
1)higher the Rds(Drain-Source) resistance gives higher loss for high current application. so choose Mosfet, which has low Rds.
2) Also the rise time and falll time of the device place major role in high frequency operation. make sure that high frequency at lower dutycycle are should not affected by the rise time of the device.
Yes it is advised to not exceed duty cycle to 48%. If it exceeds, the energy transfer from primary to secondary will get affect. All the stored energy in primary magnetic field when Switch is ON, will not transfer to Secondary when Switch is OFF. Hence transformer will get saturate.
In your application, what is primary voltage and desired secondary output voltage?
If i were you i would prefer using pwm ic's but if you want to use the 555 the you must reduce you duty cycle as low as 25-30 percent then you must control pin 5 to do that.
seems it's a open loop system. So, you can not stable the output.
And make sure the transformer will not be in saturated state, that's will make transistor overheat.
If you want to generate gate pulse less than 50% duty cycle, better go for UC 3525. Its an totem pole configuration. It can able to drive the mosfet directly. If you want to have 100% duty cycle(97% actual), better you choose UC3524. Its open collector configuration. Here you should use driver IC. These are the commonly used IC's to drive the mosfet.
The RC across the transformer is not good, it will cause high losses in the transistor (should be a diode off the collector and then an RC in parallel to the top of the transformer - but not 1.1uF this is far too high), driving a transistor direct from the 555 is no good as there is no limiting resistor for the base current, the transistor is not very efficient in this circuit, better to use a mosfet (logic level to accept the 5V drive), What is the value of Vcc? how much power do you want to get on the output side? What is the primary inductance of the transformer? What peak current has the primary been designed for? (peak flux density?) If the transformer is saturating due to the low frequency then you will get additional losses, also the diode in parallel with the transistor serves no purpose. Regards, Orson Cart.
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