boylesg
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The zener diode array on that schematic is a very lossy way to achive snubbing of the turn-off transient. I suggest tying a sunbber from the MOSFET drain to the power rail...diode (anode at the mosfet) to an R&C in parallel.
...I tried the TC4422 connected to the 12/24V rail with a 12V zenner diode in parallel with it to protect it but it appears that 24V results in the power rating of the zenner diode being exceeded followed by over voltage on the TC4422. So it would appear that zener diodes are not all that a robust way to protect the TC4422 if you want to increase the input voltage to the fly back transformer.
On the other hand a voltage regulator appears to be a far more robust way of protecting voltage sensitive components, up to a point, as it is a simple matter of attaching an appropriate size heat sink to it.
So I attached a larger than usual heat sink to my 12V voltage regulator and then a much larger heat sink to the top of this, face to face. So I should have a fairly large heat dissipation capacity for it as long as the heat transfers adequately across the junction between the two heat sinks. There is a pad of some sort between them that was mounted under the transistor that was originally attached to the large heat sink on an old tv circuit board - I guess it is an alternative to the thermal paste normally used between the electronic components and heat sinks.
Probably all very obvious to you experts though......but I am just learning.
Several points occur to me about your design --- but the first & foremost is your use of a Zener as a "protection device".
Let's be clear about this ... a zener is not designed to be a protection device, but rather a simple way to achieve voltage stability. Connecting a 24v supply directly to a 12v zener is almost the equivalent of shorting the supply outputs. You HAVE to use a resistor in series (chosen appropriately) for the zener to do any regulation whatsoever.
Now in your published circuit, you have D2 connected at the output of the 7812xx. It serves no purpose here since the 7812 will never put out a voltage greater than the 1N4745A's 16v limit. You can delete this component entirely.
Next - you regulator should AlSO have a series resistor on its input (an not just a diode). The value of this resistor should be small enough such that when the expected current is being drawn by your 555 & TC4422 - which i suspect will be in the ~10's of mA range, then the voltage drop across the resistor is less than (24v - 12v - 7812 dropout voltage). I.e. you want to design the input voltage at your 7812 to be as low as possible (for cool running), but not so low that it is below the level where the 7812 can do its regulation job.
So lets assume you expect a max load of 100mA. Your input voltage to 7812 should typically be > 12+2. Lets chose 15v. Hence your resistor needs to drop (24v -15v) at a current of 100ma. So chose a resistor of 9/.1 = 90 ohm. The wattage of this resistor should be 2x 9 x 0.1 ~ 2watts. (The 2x is for a safety margin). You can delete the diode - or keep it there if you want some sort of polarity reversal protection.
With the resistor in place, your 7812 will now dissipate ( 15v - 12v) x 0.1 ~ 0.3watts. No need for a heatsink on it anymore !!
Lastly regd your D4..D7. Why do you need these ? Are they for MOSFET protection ? Once again, 4x 16v = 64v, whereas the MOSFET (i can barely read it as an iRF531) has a max Vdss of 60v. The zeners won't be doing much here either. You can remove these too.
Lastly - what sort of transformer are you using. You should know what that primary inductance is, so that you do not exceed the max drain current of the MOSFET. If your 555 frequency is too low, then the main current limiting element is a 1ohm resistor. With a good battery, this implies an Imax of almost 24amps ! Your resistor will surely burn up long before that.
SO, to keep the current in check, you need to have a high enough frequency on the 555.
This is how you can calculate for current --
Lets assume your primary inductance is L. (Measure it/ get it from the spec sheets)
then the current through this will follow the basic equation I = V . t / L
Where V ~24v, and t is the ON-time of your mosfet.
I won't go beyond this, because its not clear how you intend to use your circuit.
1. Are you trying to generate high voltage on the flyback's output ?
2. Why do you think you need a snubber (as mentioned in the other posts here) ?
OK, here's a comment:
Shouldn't D4 go to the drain of Q1 ?
It should also be a much faster diode than a 1N4007.
Brian.
I suppose I could use a schottky diode as it only has to withstand 36V in reverse bias, while the kick back voltage of several hundred V will forward bias a schottky diode.
I suppose, the assumption is wrong. C2 will be charged up to the flyback voltage and the diode should be able to block it.
Thanks rohitkhanna but there have been some changes to my circuit as follows:
I have added a voltage pre-regulator to cut down 36V (3 x series car batteries) to 16V which then feeds into the LM317 voltage regulator. I probably don't need it for 36V but it would allow me add another series car battery to bring the voltage supply up to 48V for the flyback transformer. I will need to be careful how far I go adding car batteries as at some point my 5W / 16V zener in the voltage pre-regulator will fry.
.....
My understanding of this website is that you use one or the other type of snubber, not both at the same time.
I still haven't figured out the ultimate objective of this project.
Is it to generate a very high voltage from the flyback secondary.... or is it not ?
If so, then there is really no need to have a huge (24v... 36v.. 48v) input voltage which is causing your design to get more & more complicated. The kickback voltage (if that IS what you are trying to transformer-multiply) does not depend solely on the voltage. It depends ALSO on the ratio of your PWM T-On/ T-Off times.
In fact, since T-Off is in the denominator, then focusing on better switch-off speeds will result in higher kick-back at even lower voltages.
Next - your TC4422 may be CAPABLE of 9A pulsed, but that doesn't mean it will actually NEED this current in your present design. In fact, with your chosen MOSFET (is it IRF531? IRF631? still can't tell...) input capacitance is < 800pF and Vgs(on) is 2-4v. You can switch this On/ Off in less than 25nS with just a 100mA drive !! I don't get it... why the overkilll overdesign ?
Lastly - I see you've added "audio modulation".
So is this some sort of class-D amplifier finally ?
confused... and signing out.
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