Here you are: FET DRIVER V2

- It is the very same circuit diagram but. I tried to visualize the exact wiring.
- The PSU for driver (or Socket charger) shall be connected to the 555/transistor cluster first.
- Make wiring as short as possible.
- The blocking capacitors store local energy for 555 and transistors in order to deliver sudden current. The PSU will deliver energy somewhat delayed.
- Put the 3 wires from transistor stage and FETs close together and use stranded wire of about 1mm diameter. Twist them whenever possible.
- It is advisable to put 2 FETs back to back on a heat sink.
- Cut the gate pins down to some mm and solder the respective resistor and diode DIRECTLY to the gate. Use metal film resitors or SMD at gate (10 Ohm). The carbon ones are inductive. Induction is toxic to gates.
- Solder source and gate to a massive stranded wire or bus bar respectively.
- If you intend to use more than 2 FETs add another identical cluster of 2 and connect it by 3 twisted wires as well. All triple wires shall be made of SAMAE length.
- Chose FETs to have maximum current (summed up) twice than your intended application because they derate when heating up and much current.
Those measures above help making the driver fast and free of ringing. This reduces heat generation and performance considerably = cool design + more radiant.
TEST:
- Start with low voltage first and increase after checking your assembly.
- For initial test apply the RC assembly R11/C6 in order to check for voltage spikes first. If you are sure to not experience overvoltage disconnect it and check again.
- If you have no scope available use neons in parallel to FETs for voltage check. One neon lights up at 70...100V. You can use neons in series in order to double the indication of voltage.
- Another check can be done with a voltage meter. (see D3 / C16) Those components act as voltage peak detector. Discharge cap before, run your setup, chek meter increasing to stable value -> that's your peak voltage of the spikes.
- For first tests use a car bulb (head light - H7 / H4) as additional resistor in series with your coil (overcurrent protection). Thus you protect the FETs in case of malfunction. Disconnect the bulb if you are sure your assembly is safe for the FETs.
BTW: The 555 will not activate the FETs while opto inactive. You shall activate the opto by 10mA. Account 1.6V voltage drop for LED input at opto. Calculate the resistor: (U-1.6V)/ 0.01A = resistor (Ohm)
Please ask if you are in doubt.
JohnS




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