Super flyback driver schematics

Small PC power supply encasing, now
contains 80VA transformer and flyback
driver. White disc shaped mini-flyback
is on the right.

Download better better quality schematics for Eagle design-cad (shareware, Includes preliminary PCB-layouts that have been tested and do work, but are quite bad layouts because no ground plane is used and tracks are a bit thin... You should do your own PCB routing and proper layout...

Properties: frequency set, duty cycle set, power adjust, audio interaction and interaction level adjust, and of course plenty of watts of output power. Requires minimal heatsink or even none at all. It is best to mount the driver and mains transformer inside an old PC power supply metal encasing, and use connectors to plug in a TV flyback externally.

Note: At maximum settings the output of the flyback will be very high power, and is definitely not something to touch as such nor as in a plasma/lightbulb globe. At low settings, it should be "safer" to use in plasma globes.

Grounding: of the driver metal encasing. As the output is power RF, the mains ground might not be enough. For use in places where the floor is covered with plastic, the mains ground should be sufficient. But, especially on concrete or tile or wooden floor, an additional copper strip lead to a very large metal object is necessary to prevent zaps. Don't forget that the mains wall plug must also be grounded - otherwise the driver case is not properly grounded for low frequencies and you will feel 50Hz, hefty jolts, and get very nasty RF burns when the flyback is running and you try to touch the case!!

Do not use the driver with car ignition coils!

Some components here are a bit over-rated voltage wise... But, on the other hand, even TVs use 1500V transistors to do the flyback primary switching...

Improvements and comments are welcome!

Section for the TV flyback transformer:
  • two female plug/banana connectors on the driver encasing

  • the primary of the external flyback goes to these connectors

  • across the connectors is one 160VAC varistor (VDR i.e. MOV), then follows a diode + resistor + capacitor snubber network

  • snubber: around 47nF or 22nF 400VAC polypropylene, 390 Ohm and at least 5 Watt, and one fast recovery diode like FR107 or BY229 or ultrafast recovery TV diode like in the BYW3x series, voltage rating at least 200V.

  • yes, the diode is the right way round. The marker stripe points to the +35V rail.

  • top part goes directly to the +35V supply (>1mm diameter, Litz wire, keep short)

  • lower part ("L") goest to the MOSFET section (>1mm diameter, Litz wire, keep short)

What it does: kills out some of the voltage spikes caused by the flyback.


Section for the power supply:
  • car battery charger transformer, at least 70VA, with two 12V secondaries (both have to be high-current, >5A)

  • two diode rectifier bridges, at least 60V and 10A

  • a 7812 voltage regulator plus storage caps (47uF 16V)

  • on the +35V line, a 100uF elko for storage and two 100nF polypropylene (FKP1 or MKP1) capacitors for pulse discharge

  • the transformer primary i.e. mains side MUST be fused, and must have an RF filter towards the mains. A mains switch is also good. 

  • to power a 12VDC brushless motor fan (like the built-in one of a PC power supply) to cool any components that might get hot (like the snubbers),, the 7812 has to be extended to allow more current. See 7812 chip datasheet for how to do this.

What it does: outputs regulated 12VDC for the drive sections and unregulated 35VDC for the flyback.

Note: normally TV flybacks are driven from 97VDC and thus the current drawn decreases by 2/3, but, high voltage spikes increase proportionally, so if you have a 90V transformer you would have to replace the MOSFETs used here with high voltage switching transistors plus a base resistor. These transistors are S2000AFI, BUH315, and others, all rated 1500V and fast-switching.

Audio sense section:
  • 3.5mm mono audio connector mounted on the metal case, for the external mic & preamp
  • case mount potentiometer for audio react % setting

What it does: input signal is amplified to about 200 times. The amplified waveform goes to a low-pass style level-detect circuit. The potentiometer adjusts speed of level decay (lowest setting => no audio reaction, max setting => frequency goes up fast and drifts back to the base frequency very slowly).

Section needs a further mic preamplifier to react properly! Otherwise only knocking on mic will show effects.

(This circuit needs some improvement - that is, a further preamp with adjustable gain)

This is a slighly newer and improved version of the audio section. It uses a single LM324 op-amp.

Electret mic capsules are likely not to work with this circuit - use a standard coil microphone.

The microphone signal is referenced around 1/2*12V=6V, which means that a negative opamp supply is not needed.

The signal is amplified twice (due to lm324 gain*bandwidth and slewrate restrictions) with IC1A and IC1B, then buffered in IC1C, and finally filtered using a single amplifier biquad setup around IC1D.

The "volume" adjustment is a 1kOhm pot. The output is fed into the pulse width modulator section shown below.


Pulse width modulator section:
  • use a large surface ground track/fill for this circuit, otherwise the LM3524 will not work reliably!

  • mount the two potentiometers through the enclosure, as shown in the picture on the start of this page (these pots are used to control the frequency and duty cycle)

  • the driver is based on the LM3524 PWM chip, adaptation to TL494 or some newer chip should be easy (just compare pin-outs). Equivalents: CA3524, UC3524, SG3524. These are classic PWM ICs, and should be available just about anywhere, for cheap.

  • to keep oscillator noise low and eliminate errors (those comparators inside LM3524 are sensitive...), use a large ground plane on the PCB

  • timing capacitor C1 should be between 1nF and 22nF, 10nF is a good start. The cap must be immediately next to the chip, and connect directly to the ground plane.

  • it would be a good idea to place a 3-pin switch between the power section 12V out and the 12V input for this section, together with a two-color LED (red+green) and series 6.8k resistor, to indicate whether this driver section is running or not - i.e. the flyback is running or not.

  • the MIC-AMP lead to the BC547B transistor comes from the audio sense section

  • I-SENSE is the current sense feedback signal from the MOSFET section and is only a few 100 millivolts, so you should use a thin shielded audio cable for signal interconnection

What this section does: the driver outputs a square wave just like the 555 timer chip. The maximum frequency is 300kHz as given in IC specs. The BC547B transistor pulls down more current from pin 6 (="timing resistor") according to the audio sense section output signal, and thus can increase the frequency. The I-SENSE signal changes the output square wave pulse width and reduces it as MOSFET drain current increases, so it protects against overcurrent and short-circuit failure. The maximum for peak currents can be set in the current sense / mosfet section.


MOSFET section:
  • PWM signal (called "FETs" in previous picture) goes to the totem pole circuit (one NPN 1A 20V transistor like 2N2222 and one PNP 1A 20V transistor like BC640, or others)
  • a 17.5V 1W zener diode protects against gate overvoltage
  • the varistor is rated 120VAC and protects the MOSFET against destructive drain<->source overvoltages. Note that the mosfet must be rated at least 200V for the varistor to have any protection effect!
  • MOSFET: this could be two IRF740 mosfets in parallel (each 500V and 10A, less than 1 Ohm R_ds_on), IRF460 (500V 20A) or some other 10A >200V mosfet.
  • using a heatsink for the mosfet(s) is adviseable - it doesn't have to be very large, though
  • the current sense resistor (0.05 Ohm) should be low inductance type, not wire-wound. Otherwise you'll end up with inductive, "false" voltage spikes on the current sense signal.
  • the trimmer next to the current sense resistor is really intended to be a trimmer, on the circuit board, i.e. not a potentiometer for free user adjustment! Maybe 10kOhm. With the trimmer, the overcurrent triggering point can be cranked up higher than 5A. Just be careful not to kill the mosfets on overcurrent=>overheating.

Keep all connections on the +35V/flyback side as short as possible and use broad copper traces!! A large ground plane / ground copper fill is essential! (see halfbridge pages to get the idea)

Don't place any of the +12V gate drive side components inside the area of the +35V & large current side!
Keep the leads to the mosfet(s) short! The ground in the schematic should go via a broad copper strip/braid to the metal encasing of the driver box (remember the metal casing also connects to the power supply section).

What it does: the totem pole circuit enables faster turn-on and turn-off by being able to source and sink 1A from the mosfet gate capacitance, so the mosfet is switched is faster and switching losses are much lower. The zener diode keeps the mosfet gate from frying and blocks reverse voltages.



(C) 2001 Jan Wagner