current control circuit for transfomer

[icemetal]

I'm building a dc to dc circuit with a transformer, I'm pulsing the transformer 12v at 20khz
the problem the FET are getting too hot cause is taking too much current 1.4A, I'm trying to reduce the current going to the transformer and be able to adjusted with out having to change the frequency, any suggestions?

 

[Tahmid]

First of all, you must specify the output power. Then, we can figure out the expected current draw.

What is the expected current draw? Is 1.4A much much higher than the expected current draw? Which MOSFET are you using?

Your MOSFET could heat up excessively due to spikes when switching off or due to incorrect gate drive or excessive voltage stress when operating or other reasons.

Which topology are you using?

It would be easier to help you if you uploaded the circuit diagram and provided circuit specifications.

 

[icemetal]

First of all, you must specify the output power. Then, we can figure out the expected current draw.

What is the expected current draw? Is 1.4A much much higher than the expected current draw? Which MOSFET are you using?

Your MOSFET could heat up excessively due to spikes when switching off or due to incorrect gate drive or excessive voltage stress when operating or other reasons.

Which topology are you using?

It would be easier to help you if you uploaded the circuit diagram and provided circuit specifications.

that could all be true, but do you know how to build a current control circuit to be able to adjust the 1.4A?

 

[Tahmid]

Without knowing your exact specifications, one way would be to increase transformer primary inductance to reduce the 1.4A.

If you want to vary the 1.4A and want a current controller for that, you'd have to specify why 1.4A is being drawn. What is the load?

To reduce the drawn current, reduce the duty cycle.

 

[icemetal]

I did adjust the duty cycle and it does drops the current but it drops the voltage to the output also. that's why I'm looking for a way to limit the current to about .5A but still get the full voltage.

 

[Tahmid]

If you want to limit current to 0.5A, you must ensure that this is enough to provide enough power at the output. If, due to the load at the output, the current increases, you can't limit current and get required output power at the same time.

 

[icemetal]

I can see you just like to keep the post counting going up, cause you reply 3 times without answering the post.

 

[Tahmid]

Perhaps you haven't understood that the perfect answer can not be provided without knowing the transformer output load - voltage and current. If all you want is a current limiter, you may just use a linear current regulator made with LM317 or LM350. However, you don't know why the current being drawn is 1.4A and you're not sure if limiting the current to a lower value will give you the required output.

 

[BradtheRad]

the problem the FET are getting too hot cause is taking too much current 1.4A

Are you talking about a mosfet? The great thing about switching current on and off through the converter, is that you are pulsing a mosfet fully on and fully off. That way it wastes little power and dissipates little heat. To achieve this, the gate needs to be driven by a volt level which is high enough to turn it on fully, and low enough to turn it off fully.

The mosfet should not heat up so much when you switch 1.4 A through it in the above manner. I believe it is in linear mode some of the time, therefore it is dissipating substantial heat.

It may be due to insufficient volt levels coming from your control circuit, or stray capacitance somewhere, or indefinite gate turn-on caused by a high impedance block in the return path.

 

[crutschow]

If it is drawing 1.4A with no load on the transformer output then the transformer is likely saturating so you need to increase the switching frequency or otherwise modify the circuit to avoid this saturation.

If the 1.4A is due to the transformer load then there is nothing you can do about that since a given output load always requires a given input current. There's no free lunch.

We have been trying to answer your question but you have been ignoring our questions so don't get snippy. It's a two way street. Also post a circuit diagram so we can see what you are talking about. You may be wanting to fix the problem the wrong way. We want to understand the cause of the problem so we can fix it properly.