555 Timer Flyback Driver

Overview

Progress on building a 555 timer-based flyback driver, using the 555 timer to drive a MOSFET for high voltage generation.

Flybacks typically operate between 15 kHz and 150 kHz. The exact frequency depends on the specific electronic device they were designed for, often a CRT television, where the frequency is related to the horizontal scanning rate.

Sources / Inspiration

I’ve been a fan of Uzzors2k’s work for a long time - all credit to him for his designs.

• http://uzzors2k.com/index.php?page=flybacktransformerdrivers

Schematics

Circuit Operation

The 7812 provides a stable 12V supply to the 555 timer. The 555 timer produces a square wave (frequency to be confirmed with oscilloscope) to drive the main switching MOSFET.

Timing Calculations

The 555 timer in astable mode produces a square wave with frequency determined by:

With our values (R₁ = 10kΩ, R₂ = 16kΩ, RV₁ = 0-50kΩ, C₂ = 1nF):

Minimum frequency (RV₁ = 50kΩ):
$f_{min} = \frac{1.44}{(10k + 2(16k + 50k)) \cdot 1nF} = 10.1\text{ kHz}$

Maximum frequency (RV₁ = 0Ω):
$f_{max} = \frac{1.44}{(10k + 2 \cdot 16k) \cdot 1nF} = 34.3\text{ kHz}$

The duty cycle is:
$D = \frac{R_1 + R_2}{R_1 + 2R_2} \approx 62\%$

Bill of Materials

Component	Value/Part Number	Qty	Notes	Cost	Source
IC1	NE555 Timer	1	Requires DIL8 socket	$2.25	ZL3555
Socket	DIL8 Socket	1	For NE555	$0.38	PI6500
Q1	IRFP450 IRF540N	1	Rated 100V, 33A (substitute)	$5.95	ZT2466
Heatsink	TO-220 Heatsink	1	For IRF540N	$1.45	HH8504
T1	Flyback Transformer	1	Salvaged from CRT	$0.00	Salvaged
R1	10kΩ Resistor	1	1W	$0.33	RR2798
R2	16kΩ Resistor	1	0.5W, timing (18kΩ specified)	$0.11	RR0601
R3	10Ω Resistor	1	1W, gate resistor	$0.33	RR2526
R4	470Ω Resistor	1	1W	$0.33	RR2766
RV1	50kΩ Potentiometer	1	Frequency adjust (11-42 kHz)	$3.95	RP3616
C1	100nF Ceramic	2	50V rated	$1.10	RC5360
C2	1nF Ceramic	1	50V rated	$0.45	RC5336
C3	220µF Electrolytic	1	50V rated (470µF specified)	$1.20	RE6180
IC2	7812 Voltage Regulator	1	12V output, TO-220	$0.95	ZV1512
Q2	2N3904 NPN Transistor	1	Small signal	$0.75	ZT2326
Q3	2N3906 PNP Transistor	1	Small signal	$0.75	ZT2328

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First Iteration - Breadboard

The original circuit specified an IRFP450 (avalanche-rated 500V, 14A N-channel MOSFET). I initially built using an IRF540N (avalanche-rated 100V, 33A MOSFET) - below the author’s recommendation of “any MOSFET with breakdown voltage above 200V and avalanche rated.” Planning to upgrade to IRFP250 when moving to PCB.

The breadboard prototype has been current-limited by contact resistance. Short run cycles drawing 3-5A have produced impressive results (and a little smoke)!

Component substitutions:

• 16kΩ resistor instead of specified 18kΩ
• 220µF capacitor instead of specified 470µF

Video of initial build output at 15V, drawing ~3-5A under load:

Measuring the circuit between pin 3 of the 555 timer and circuit ground I found a square wave output of between 20 and 35KHz depending on the configuration.

This was the circuit under load, interestingly it seems to sag at the start of each signal

and this is it with no primary coil attached

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Second Iteration - PCB

Moving to a PCB build with the following improvements:

• Hot-swappable DIL8 socket for 555 timer
• IRFP250 MOSFET (200V, 30A) for better voltage headroom
• Proper heatsinking for 7812 voltage regulator
• Provisions for isolated audio modulation circuit

Component	Value/Part Number	Qty	Notes	Cost	Source
IC1	NE555 Timer	1	DIL8 socket	$2.25	ZL3555
Socket	DIL8 Socket	1	For NE555	$0.38	PI6500
Q1	IRFP250	1	Rated 200V, 30A	…	…
R2	18kΩ Resistor	1	0.5W, timing	…	…
C3	470µF Electrolytic	1	50V rated	…	…

PCB by http://uzzors2k.com/index.php?page=flybacktransformerdrivers

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The first gulp from the glass of natural sciences will make you an atheist, but at the bottom of the glass God is waiting for you. - Werner Heisenberg