flyback using NE5555

Thread Starter

mtLg

Joined Oct 16, 2013
8
Hello,

I've been a reader of this forum for a while now, thanks to everyone who makes all the information in here available!

Now to my problem, I'm trying to build a wireless battery charger based on a flyback converter design. I've designed the modules and separately everything works, but when together, they dont:

1) DC power supply as source
2) Oscillator circuit - NE555 - tested and behaves I wish on a frequency close to what I want, about 43 kHz.
3) "flyback" converter - Coupled inductive coils + Ac-DC converter - tested and works successfully from 40-50 kHz with square, triangle, sine, offset, etc waves.

When I try to power up circuit (3) with the output of the 555 timer, nothing works, I have no idea why.

I also attempted driving a mosfet (irf5400 - no success at all) and a bjt(2n5401 - drives the transistor, but again as soon as the load is connected, nothing happens).

I think the problem might be related to waveform spikes in voltage generated at the receiving circuit (see oscilloscope photo).

I'm stuck at the moment, not sure what to do to successfully get the square waveform at some voltage to my flyback converter circuit.

All helps is appreciated.
Thanks
 

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THE_RB

Joined Feb 11, 2008
5,438
The 555 oscillator circuit is faulty. When the discharge pin 7 goes low it can only discharge C2 to half of Vcc!

It needs to discharge C2 to less than 1/3 of Vcc to trigger the internal comparator which oscillates between 2/3 Vcc and 1/3 Vcc.
 

crutschow

Joined Mar 14, 2008
34,285
For that circuit to work the transformer primary must be connected between the power supply and the collector/drain of the transistors without any resistors. You can't drive the primary from the output of a collector resistor as you (apparently) show.

A flyback circuit works by turning on the transistor to build up a current in the primary which stores energy in the primary inductance. This energy is then transferred to the secondary by transformer action when the primary current is abruptly terminated by the transistor turning off, which generates current in the secondary.
 

crutschow

Joined Mar 14, 2008
34,285
The 555 oscillator circuit is faulty. When the discharge pin 7 goes low it can only discharge C2 to half of Vcc!

It needs to discharge C2 to less than 1/3 of Vcc to trigger the internal comparator which oscillates between 2/3 Vcc and 1/3 Vcc.
I believe that is a standard astable configuration for a 555.
 

Thread Starter

mtLg

Joined Oct 16, 2013
8
Thanks for the quick replies.

How pure is the signal coming from the power supply??
The power supply I'm using for testing is from an electronics laboratory, so I'm be confident thats not the issue.

The 555 oscillator circuit is faulty.
As crutschow mentioned, the circuit is based on a standard 555 astable configuration. I tested this circuit separately and the 555 outputs an offset (positive only) square wave as desired.


For that circuit to work the transformer primary must be connected between the power supply and the collector/drain of the transistors without any resistors. You can't drive the primary from the output of a collector resistor as you (apparently) show.
Ok, so based on what you recommended, I drew a new schematic for the whole circuit (attached).
I'm curious though, why can't I drive the primary from the transformer straight from the 555 output and ground without the use of a transistor?


I'll test this out as soon as I can access the lab again, in the meantime any other advice or solutions are most welcome.


Thanks for all the help and replies so far, keep advices coming! :D
 

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iimagine

Joined Dec 20, 2010
511
I'm curious though, why can't I drive the primary from the transformer straight from the 555 output and ground without the use of a transistor?
A transistor is needed to provide additional current beyond the 200 mA limit of the 555 timer.

Why did you reversed Q1 CE? :confused:
 

Thread Starter

mtLg

Joined Oct 16, 2013
8
A transistor is needed to provide additional current beyond the 200 mA limit of the 555 timer.

Why did you reversed Q1 CE? :confused:
Hmm, now I remember reading something about the 200mA limit. Thanks.

What do you mean by reversed Q1 CE? It's still the same orientation as in the original diagrams.
As it is at the moment, the inductive coil is connected between the DC power supply and the Collector of the transistor. Isn't that the correct orientation?

Cheers
 

iimagine

Joined Dec 20, 2010
511
Hmm, now I remember reading something about the 200mA limit. Thanks.

What do you mean by reversed Q1 CE? It's still the same orientation as in the original diagrams.
As it is at the moment, the inductive coil is connected between the DC power supply and the Collector of the transistor. Isn't that the correct orientation?

Cheers
That transistor is a PNP, not an NPN, therefore its E (emitter) should be connected to the coil, and C (collector) to ground.
The way that you have it connected right now is what i believe, called reversed bias, it would still function, but it will not provide much gain
 

Dodgydave

Joined Jun 22, 2012
11,285
Your circuit is oscillating at about 49khz, so the transformer should also be tuned with the capacitor to give that resonance to give out max voltage. L1/C3, L2/C4 and drive the transformer with a mosfet for more power
 
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crutschow

Joined Mar 14, 2008
34,285
Your circuit looks okay expect for a couple things.

I didn't notice your circuits show a PNP transistor, which won't work since the voltages are the wrong polarity. You need an NPN.

What is the purpose of C3 and C4?

You may want a add a zener across the transistor collector-emitter to suppress high voltage spikes from the parasitic inductances. Its voltage should be about 75% of the transistor voltage rating.
 

iimagine

Joined Dec 20, 2010
511
You better of using a NPN transistor, because the high output level of the 555 timer is 1.7 volts less than the supply voltage, therefore the need of adding two diodes (or an LED) in series with R3 to increases the forward voltage required for the PNP transistor to about 2.1 volts so that the 1.7 volt difference from supply to the output is not enough to turn on the transistor.

In other word, even if you correct the polarity of the PNP, It will stays on forever without the 2 diodes or LED mentioned :)
 
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Thread Starter

mtLg

Joined Oct 16, 2013
8
That transistor is a PNP, not an NPN, therefore its E (emitter) should be connected to the coil, and C (collector) to ground.
The way that you have it connected right now is what i believe, called reversed bias, it would still function, but it will not provide much gain
Hmm, I'll look into this. Thanks

Your circuit is oscillating at about 49khz, so the transformer should also be tuned with the capacitor to give that resonance to give out max voltage. L1/C3, L2/C4 and drive the transformer with a mosfet for more power
Based on calculations of the inductance and capacitor, they were meant to resonate at around 48 kHz. The thing is all the components errors are giving me a lot of "distortion", during testing the 555 circuit output was at about 43 kHz. I also didnt do much investigation the resonance itself, just relied on the face values of inductance and capacitance.
I just tested the resonant circuit from 40-50 kHz and power was going through ok.
I guess its something to keep in mind when optimising the circuit.

I'll keep the mosfet idea in mind next time I visit an electronics shop.

Thanks
 
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Thread Starter

mtLg

Joined Oct 16, 2013
8
What is the purpose of C3 and C4?
C3 and C4 I'm using to increase the coupling of the inductors, define a resonant frequency and run the circuit at around that frequency. Im not using a real transformer, just two co-planar coils at a short distance. Did I do something wrong here?


Your circuit looks okay expect for a couple things.

I didn't notice your circuits show a PNP transistor, which won't work since the voltages are the wrong polarity. You need an NPN.

You may want a add a zener across the transistor collector-emitter to suppress high voltage spikes from the parasitic inductances. Its voltage should be about 75% of the transistor voltage rating.
[/QUOTE]

You better of using a NPN transistor, because the high output level of the 555 timer is 1.7 volts less than the supply voltage, therefore the need of adding two diodes (or an LED) in series with R3 to increases the forward voltage required for the PNP transistor to about 2.1 volts so that the 1.7 volt difference from supply to the output is not enough to turn on the transistor.

In other word, even if you correct the polarity of the PNP, It will stays on forever without the 2 diodes or LED mentioned :)
Honestly it has been a while since I really studied all of this, a few mistakes to correct I guess. I'll keep the idea of the NPN transistor as an improvement and something to test later in the week, however I wanted to have this running by Monday before I have the chance to visit an electronics shop again.


I have a spare N-channel MOSFET, VN10KM (the name on the diagram attached is wrong), as a bonus this mosfet already seems to have a zener diode across it. Would replacing the transistor with this mosfet yield the desired results? Is the diagram below the correct way of going about this?

Again, thanks for all feedback!
 

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iimagine

Joined Dec 20, 2010
511
It should work, Although I would add a pull down resistor at the gate of the MOSFET to help it turns off quicker, a value of 1k would do
 

crutschow

Joined Mar 14, 2008
34,285
C3 and C4 I'm using to increase the coupling of the inductors, define a resonant frequency and run the circuit at around that frequency. Im not using a real transformer, just two co-planar coils at a short distance. Did I do something wrong here?

...........................

I have a spare N-channel MOSFET, VN10KM (the name on the diagram attached is wrong), as a bonus this mosfet already seems to have a zener diode across it. Would replacing the transistor with this mosfet yield the desired results? Is the diagram below the correct way of going about this?
I didn't know you were using resonant coupling and not a regular transformer. That's a different animal.

That MOSFET is connected correctly but its maximum current rating is only 0.31A. Is that sufficient for your circuit?

The zener in that MOSFET protects the gate, not the drain-source. But a MOSFET is more tolerant of drain-source voltage spikes so likely will be okay in your circuit without additional protection.
 

Thread Starter

mtLg

Joined Oct 16, 2013
8
That MOSFET is connected correctly but its maximum current rating is only 0.31A. Is that sufficient for your circuit?
For the purposes of the experiment, yes. It is only meant to take a very light load.
I'm hoping the 10k resistor on the secondary is enough to reduce the current through the mosfet. Do you think I need to add anything to circuit to ensure the current remains low?

Off to the lab in a couple of hours, lets see how everything goes!

Thanks
 

iimagine

Joined Dec 20, 2010
511
For the purposes of the experiment, yes. It is only meant to take a very light load.
I'm hoping the 10k resistor on the secondary is enough to reduce the current through the mosfet. Do you think I need to add anything to circuit to ensure the current remains low?

Off to the lab in a couple of hours, lets see how everything goes!

Thanks
You got it all wrong. What you'd get through the MOSFET everytime it turns off is 'voltage spike', not current, google 'flyback diode' :D
 

Thread Starter

mtLg

Joined Oct 16, 2013
8
You got it all wrong. What you'd get through the MOSFET everytime it turns off is 'voltage spike', not current, google 'flyback diode' :D
Shocking!

So from what I read, a Schottky diode will do the trick (or am I better off with a normal diode?).
I happen to have a 40V rated one. Added to the diagram attached. The model I have is actually the 1N5819. I do have normal diodes available too in case it fails.

I guess this would also help reducing the voltage spikes I used to see on the oscilloscope, right?

Thanks
 

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