Possible fet based soft start circuit

[GregH2]

Hi all,

I have been thinking about a simple FET based circuit for use as a possible soft start mechanism and would appreciate your feedback on this design. Go easy on my as I am a definite noob. It is designed to bring the transformer up to voltage in several tens of seconds and can be seen in the attached pdf.

Essentially we have two back to back fets used as a bi directional solid state relay. It slowly turns on when supplied with 15v from a dedicated transformer. The turn on time is determined by the RC combo. I also need to devise a simple way to drain the cap on turn off, but that will be easy.

The only problems I can see with the circuit is a minor voltage drop and a little switching noise created when the mains cycle is between -0.6 and 0.6 volts due to switching of the recovery diodes.

So what do you think? Too noisy for use? Too lossy? I'd like to hear your thoughts.

Greg.

[Ian Finch]

Hi Greg
I assume your design can be refined to work well but needing an auxiliary transformer and supply to operate it may be an issue with, for example, simple DIY amps. More ambitious integrated amps and Pro. gear could be suitable applications, where this could act as a standby. In these, using a separate trafo for auxiliary circuits, preamp etc. is probably no big deal.

Beware, an amplifier in which the rail voltages rise very slowly (10s of seconds...any reason for the long time?) may also be acting as a strong oscillator or other mode and it is unwise to power up an amp in this way, connected to speakers or not.

[GregH2]

Thanks Ian. It's nice to have an encouraging comment as I expected this to be shut down pretty quickly given its elegant simplicity.

I think in this case the slow increase in voltage will not be detrimental as the amplifier is a very simple and stable single ended class a design (Pass labs Aleph 3). In any case the start up time is easily reduced by varying the RC combo.

Given that I have a 650VA main transformer and 300 000uF of capacitance the cost incurred by a small 15v transformer would not be a big deal, and certainly worth the soft start functionality.

What are your thoughts on noise generated by the switching? I guess in reality it can't be much worse than those in the main rectifiers, and should be easily snubbed with some 0.1uF mains caps??

Thanks again for your comments.

Greg.

[VivaVee]

This circuit is lethal. Please do not use it.

Relays or optically coupled triacs are required to provide sufficient isolation for safe operation.

Do a bit of reading on how to turn on a mosfet and you will see that your control circuit is also a non-starter.

[GregH2]

Alan,

Thanks for your reply.

I am happy to use an optoisolator if necessary for safety. Good point.

As for the non starter part, would you mind enlightening me? Other than the fact that I may need some resistance going from the source pins to ground to ensure the gate voltage is always positive relative to the source I can't see the problem. It could also use a high value gate to gnd resistor toi ensure turn off and an off state on startup. But, as I said, I am a solid state noob so please help.

As I understand it the gate pins will always be positive wrt to ground and hence also the source pins (if I have a resistor from source to ground).

On positive half cycles the upper mosfet will conduct and the bottom bypass diode will also conduct, on negative half cycles the bottom fet will conduct and the upper diode will conduct. Ultimate current passed will be controlled by the degree to which the conducting fet is switched on, hence the gradual gate control using an RC network.

Could you please explain why it won't work? I knew there would be problems and I want to learn.

As for safety, you were right about the isolation, and that is exactly why I posted here for advice before attempting anything.

Any info you can give would be great!

[GregH2]

Investigating further I can see that the gate driver circuit may need to be floating. But as I understand it this is easily achieved by using a dedicated transformer and not grounding the secondary. Thoughts?

[tangmonster]

I have a question:

What are your reason for not wanting to use a thermistor with a relay bridging the thermistor after 5 or 10 seconds? Using a simple lm555 circuit the 5 second timer can easily be done. A CL60 should do fine with a 650VA. For all those capacitance i might leave it in for 10 seconds. OR even use 2 x CL60's in series.

I agree with using a secondary transformer to power this circuit (they are cheap) compared to the rest of most amps and can be used for all sorts of other uses.

[Mooly]

Originally Posted by VivaVee

This circuit is lethal. Please do not use it.

Relays or optically coupled triacs are required to provide sufficient isolation for safe operation.

Do a bit of reading on how to turn on a mosfet and you will see that your control circuit is also a non-starter.

Agreed... it's lethal.

Not trying to be negative, but the fact you post such a design without even mentioning isolation shows you aren't familiar or understand the issues, and others won't too, possibly with disastrous consequencies.

[VivaVee]

Originally Posted by swordfishy

Alan,
Could you please explain why it won't work? I knew there would be problems and I want to learn.

If, and only if, you promise to never build this lethal circuit. This may sound harsh, but I feel an ethical responsibility to the children and small animals that you may live with

[GregH2]

Firstly a pic from a previous hobby, and yes that is a 10kVA 11kV pole transformer in the background.... I made this when I was 15, and without the help of the internet, so I am capable of learning and doing things safely under adequate instruction.

I posted this circuit as a conceptual idea, and yes I did not consider the need for isolation. It seems obvious now. I would never have built the circuit without first discussing it here with everyone and working towards a suitable and safe incarnation.

I can see now that perhaps posting an unfinished circuit was a bad idea and may mislead others. However, I would still like to try and make it work with the help of forum members, either publicly or discreetly, if you are willing.

I think it offers a neat way to reduce inrush current in a predictable way with no moving parts. It works both as the suppressing resistor and as the relay. I also think it will generate less noise than the usual SSRs that are out there due to the better suitability of the bypass diodes for this purpose. If you know of a commercial mosfet based SSR then by all means let me know.

In any case, if you are not interested I will go back to my other idea of using a 74HC14 schmitt trigger and a relay, but I really like the pure solid state option.