The Zero Volt Switching Driver
What is a ZVS Driver?
The Zero volt switching driver popularly known as a ZVS driver is probably the most powerful and efficient driver for induction coils and flyback transformers. It was invented by Vladmiro Mazilli. It uses the technique of resonant zero-voltage switching to drive the flyback transformer or an induction coil. This means the circuit is designed such that the MOSFETs switch (on or off) when the voltage across them becomes zero. The MOSFETs will generate very little to no heat, as the MOSFETs switches on when there is no voltage across them. The only source of heat is the heat produced by the MOSFETs internal resistance. Unlike the simple 555 timer drivers, The ZVS drivers will allow you to run your flyback transformers and induction coil for much longer periods of time before the MOSFET's overheat (if designed correctly).
So, how this circuit works?
The schematic of the ZVS driver
Now at first glance, it may look like it's very complex but if you even kind of understand the basic laws of electricity you can understand how this circuit works so what happens is the electricity flows through L3 and L2 into the L1 where L1 could be the primary of a flyback transformer or an induction coil. Now the electricity is also flowing through R2 into the gates of each MOSFET Q1 and Q2 now what happens is because no two components are perfect one MOSFET is going to turn on before the other so let's say the current flows through L2 into L3 and the electricity is also flowing through R1 and R2 into the gates of Q1 and Q2 respectively.
Now what happens is because no two components are perfect one of the MOSFET is going to turn on before the other so in this case, let’s say that current flows through R1 and turns Q1 on that means this P1 is going to be at ground and P2 is going to be at positive and the current is going to flow through L1 and create a magnetic field around it now what happens is when P1 is grounded it'll also shut off Q2 because the electricity will flow through D1 to ground and besides any electricity that flows through into Q1 will direct all the current so no current will flow through Q2 or very little will. Now what happens is once Q1 is on, and it draws current and creates a magnetic field in L1 the voltage on P2 will actually rise and fall due to C1 and L1 being a resonant circuit that means if you give a little boost of power, it will start oscillating back and forth Now what happens P2 will be positive at first because the positive current flows through it from L2 and P1 is negative.
But as soon as this ringing happens and P2 reaches negative or hits zero then what will happen is the current will be sucked down into Q2 via D2 fast diode, and it will actually shut off Q1 because P2 is ground potential and the positive potential in this circuit will flow to the ground, therefore, shutting off Q1 now as soon as Q1 shuts off then the current will flow through R2 and into Q2 turning it on now as soon as Q2 turns on just a little bit D2 will pull all the current from Q1’s gate to permanently shut it off so Q1 is off and Q2 is on that means the current will be flowing the other way through L3 and into L1 through Q2 making P1 positive and P2 ground now that will continue that ringing or oscillations and so P1 becomes ground again and both transistors switch their roles, So basically this is how this whole circuit. Now, this constant push-pull of current creates a magnetic field which is fluctuating inside L1 that fluctuating magnetic field induces the current into the secondary coil, if it is a flyback transformer which allows it to create a high voltage.
Now, we also see some other components besides D1, D2, Q1, Q2, L1, L2, L3 and C1 the purpose of L2 and L3 is to make sure that not too much current flows and into L1 and into the whole circuit because if we have too much of a high current flowing through here it will literally blow up Q1 and Q2 so what an inductive choke does is as the current flows through here the magnetic field inside the inductive choke slowly builds and that causes the current instead of having a superfast rise time to slowly rise which allows Q1 and Q2 to not explode because when the current Rises slowly it allows the circuit to start switching and not blow up.
Now R3 and R4 going from the gate to the ground is to make sure that Q1 and Q2 don't have parasitic capacitance on the gate because once Q1 and Q2 are charged then they might not turn off so this is to bleed the charge off the gate so that way Q1 and Q2 don’t stay on permanently. Now D3 and D4 are there to make sure that even if you have 80 volts as an input voltage the voltage at your gate will always remain 15 volts. Now this R1 and R2 are here to regulate the current flowing through the gates of Q1 and Q2 so that way you don't have too much current and you don't blow up Q1 and Q2.
So what do you need to make this ZVS Driver?
For this circuit, you will need the following things:
1) Two IRFP250 or equivalent Mosfets. (Q1 and Q2)
2) Two high-frequency diodes (FR107 or UF4007 will work fine). (I used BA157 as I didn't have FR107 or UF4007 in stock.) (D1 and D2)
3) Two 12V at least 1 Watt rated Zener diodes if you wanna power it below 40V. ( I am using 15V as I want to run this driver above 40 V) ( D3 and D4)
4) Two 470 Ohm 2 Watt rated resistors if you wanna run it below 40V The colour code is yellow/purple/brown (I used 1K resistors as I want to run it above 40V) (R1 and R2)
5) Two 10kOhm resistors, can be 1/4 Watt or more. The colour code is orange/brown/black. (R3 and R4)
6) 0.66µF to 1µF high voltage (above 400) high frequency (above 50khz) Capacitor.( I used two 0.33µF induction capacitors in parallel) (C1)
7) Heat Sink (the bigger the better)
8) Thick wires (the thicker the better)
9) Thermal pads
10) Choke (The value is not critical but it should be 47uH to 200uH rated at 10A or more. You can find an inductor from a computer PSU or you can simply make your own, just wrap 20 turns of 16 gauge of enamelled wire around a ferrite toroid.)
12) Soldering iron, flux and solder
13) Screws
14) Power Supply (12V to 40V if R1 and R2 = 470 OHM, D3 and D4 = 12v zener) (12V to 60V if R1 and R2 = 1K OHM, D3 and D4 = 15v zener) (V1)
Construction of the ZVS
So, gather your components, switch on your soldering iron and get ready to construct this circuit!
Step 1 – Place the thermal pads on the two MOSFETS (Q1 and Q2) and screw them to the heatsink. (Refer to the pictures below)
Step 2 – Bend and trim the legs of the MOSFETS (Q1 and Q2) (as shown in the picture below)
Step 3 – Now twist the leads of the Zener diode (D3 and D4) and the 10k resistor (R3 and R4) as shown in the picture.
Step 4 – Trim the leads of the Zener diode and the 10k resistor combination (D3, D4 and R3, R4) like shown in the picture and solder it between gate (pin 1) and source (pin 3) of the MOSFET. Make sure that the stipe (Cathode) on the Zener diode is connected to the gate (pin 1) of the MOSFET. (Refer to the picture)
Step 5 - Now solder one of the ultra-fast switching diodes (D1 and D2) between the drain (pin 2) of Q1 and gate of Q2 (pin1). Make sure that the stripe (Cathode) of the diode is connected to the drain (pin 2) of Q1 and Now solder the other ultra-fast switching diode between the drain (pin 2) of Q2 and gate of Q1 (pin1) Make sure that the stripe (Cathode) of the diode is connected to the drain (pin 2) of Q2. (Refer to the picture)
Step 6 – Now solder the source (pin 3) of Q1 and Q2 together. Using a wire. (Refer to the picture)
Step 7 – Connect the two 1k resistors (R1 and R2) (in my case as I will be operating this circuit above 40 V, might be 470 OHMS in your case, refer to the part list for details) together like shown in the picture, trim their leads and solder a wire between the connection of the two. (Refer to the picture)
Step 10- Solder the resistors between the gates (pin 1) of Q1 and Q2.
Step 11- Now solder the capacitor (C1) between the drain (pin 2) of the MOSFETS. (refer to the picture)
Step 12 – Now solder the two inductors (L2 and L3) between the positive and the drain of the Q1 and Q2 respectively.
Step 13- Solder the negative wire between the source of the 2 MOSFETS (Refer the pictures)
Step 14 – Wollah! Your ZVS driver is ready! Now you just need to connect L1 in parallel with C1 where L1 could be the primary of a flyback transformer, induction coil etc and just power it up!
Make sure you use thick wire as it will be handling currents up to 10 amps.
If you are going to attach both of your MOSFETs on one heatsink, Use mica insulators! Or other types of insulators to isolate the MOSFET's tabs from each other, otherwise your ZVS driver won't work.
When you first power on your ZVS driver, start with 12v input to make sure everything is working. Then you can increase the input voltage up to 40v. You can power the ZVS driver above 36v, but then you risk blowing up your driver.(12V to 40V if R1 and R2 = 470 OHM, D3 and D4 = 12v zener) (12V to 60V if R1 and R2 = 1K OHM, D3 and D4 = 15v zener)
You may hear a very high pitched squeal from your ZVS driver, don't worry, that is normal.
Also, this ZVS driver circuit does not have to be used just for the flyback transformer or induction heating, you can replace L1 to use a different transformer to charge up your large capacitor banks for coilguns, railguns, etc. It might be even possible to make an SSTC out of this ZVS by decreasing the value of C1.
Best of luck!
Happy making :)
The Terminator over and out!