I wanted to design a versatile driver circuit that could drive a half- or full-bridge of MOSFETs or IGBTs through a gate drive transformer (GDT). This should make a driver that is able to run flyback transformers found in CRT TV sets and computer monitors.
The TL494 IC is designed for maintaining all the functions needed in a switching mode power supply using pulse width modulation (PWM). The output transistors can be run in either single ended mode or push-pull. The pulse width is normally controlled through a feedback signal in the power supply, but for this project we want to control it manually, this is done differently in almost all schematics found.
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Flyback transformers from a CRT TV are typically driven at 15 kHz and flyback transformers from computer monitors are typically driven between 30 to 150 kHz.
The TL494 IC uses a 5% dead time to insure proper switching and at frequencies over 150 kHz this minimum dead time is higher.
The design goals for this project will be a driver with a variable duty cycle from 0% to 45% and a variable frequency from 50 kHz to 150 kHz.
This should make for a efficient driver and one that works out of the audible spectrum. In order to design with components at hand, the frequency span is not going so low as 15 kHz.
|Voltage supply||IRFP250N: 0 VAC to 120 VAC|
|Frequency span||38 kHz to 150 kHz.|
|Duty cycle span||0% to 43%|
Design and calculations
The output control on pin 13 is set high from the 5 Volt reference voltage on pin 14, this makes the two output transistors work in push-pull mode, which will be used to drive each their non-inverted MOSFET driver IC.
For further study and experiments the output control can be tied to ground to enable single end mode, the two output transistors will be in phase and can be paralleled for a higher output driving current. This could be used if only driving one transistor or non-inverted and inverted MOSFET drivers are used.
Its possible to implement features as soft start and over-voltage , -current protection with the dead time control on pin 4, I have chosen to wire this to ground which disables the DTC. This is a versatile and experimental driver circuit and when at some point a final product is going to be made, it would make sense to build in these protective circuits.
The outputs are each connected to the positive rail through a 150R 2W pull up resistor to bring the output signal up in amplitude.
The frequency is determined by Rt (pin 6) and Ct (pin 5) as a normal RC timing circuit.
I choose to use a 1 nF capacitor, so I can calculate the resistor values and find what potentiometer is needed, I aimed to use a 10 kΩ potentiometer as I had them at hand.
The frequency in push-pull operations are f = 1/(2Rt * Ct)
Resistor value for 50 kHz = (1/(50000 * (1 * 10^-9)))/2) = 10K (rounded)
Resistor value for 150 kHz = (1/(150000 * (1 * 10^-9)))/2) = 3K3 (rounded)
So the lowest value will be at Rt = 3K3
The frequency can be lower than 50 kHz without complications, so combined with a 10 kΩ potentiometer the low frequency will be = 1/((2 * 13300) * (1 * 10^-9)) ~ 38 kHz.
The TL494 have two error amplifiers which I for the sake of leaving no inputs floating have paralleled. This is the same method used in the data sheet when making a test circuit.
To adjust the duty cycle I have set up the error amplifier as a voltage follower, the feedback from the op-amp is tied to the – input, that way the output voltage will be just under the input voltage. With a potentiometer and a resistor I can vary the input voltage from 0.5 V to 4.76 V, this voltage span is enough to adjust the duty cycle from 0 to 43%.
What I then have here is variable frequency from 38 kHz to 150 kHz and variable duty cycle from 0% to 43%. This is acceptable in respect to the design goals.
25th May 2009
The breadboard prototype is ready to be tested, the tape is to hold the timing capacitor in place since the legs on it was too short.
In the first oscilloscope shot we see the output waveform without pull up resistors, it is about 38 kHz at 43% duty cycle.
In the second oscilloscope shot we see the output waveform without pull up resistors, it is about 38 kHz at 5-7% duty cycle.
In the third oscilloscope shot we see the output waveform without pull up resistors, it is about 150 kHz at 43% duty cycle.
27th May 2009
PCBs was made for both the driver and half-bridge section. The full bridge rectifier used here in the pictures is only rated for a mere 4 A. This is not enough for running a flyback with low input voltage and high duty cycle. A 25 A bridge with heat sink should be used to ensure some overhead.
29th May 2009
In the oscilloscope shot we see the waveform of the primary side of the GDT driving a MOSFET half-bridge. To test the circuit I first used a old half-bridge I had from an earlier project.
The sturdiness of this new driver shines through when I killed a flyback transformer due to over-voltage on the secondary side. Corona glow can be seen in the center towards the ferrite core.
This universal inverter makes it possible to adjust the output voltage and current exactly to ones needs. It makes a great and much more sturdy flyback driver than many simple drivers with just a single transistor, which is of course no surprise as it implements its own control IC, MOSFET driver ICs and a half-bridge of MOSFETs.
For a final constant voltage or current power supply it will not work, as there is no feedback adjusting the pulse width to a certain load.