Gradually switching a FET ON?

[Flyback]

Hello, i periodically (every 10 minutes) need to switch a fet on (ON for 1 minute) which delivers 400mA current from the secondary of a current transformer.

The primary contains a 50KHz sinusoidal current (2A9 Average), which is produced by a highly resonant LC tank circuit.

Because of the resonant nature, i cannot simply and suddenly switch the switch from OFF to ON (because it would excite resonances in the resonant system)......but instead i must PWM the switch ON/OFF/ON...etc until its ON all the time....so i must gradually increase the dutcy cycle from 0 to 100%, until its ON all the time.

I need the gradual switching interval to be 5ms.

Do you know of a way of doing this?

 

[alec_t]

A FET alone can't switch AC from a transformer. Are you rectifying the AC first?

 

[Flyback]

yes its rectified full wave.

 

[tcmtech]

A bit of added capacitance on the gate and a properly sized resistor would bring the gate voltage up slowly and likely give you the desired soft on soft off switching action you are wanting.

 

[alec_t]

i cannot simply and suddenly switch the switch from OFF to ON (because it would excite resonances in the resonant system)......but instead i must PWM the switch ON/OFF/ON...etc until its ON all the time..

But won't the PWM switching (which is sudden) also cause the same problem? And surely drawing 400mA from the secondary will affect the primary side?

 

[Flyback]

But won't the PWM switching (which is sudden) also cause the same problem?

not to the same extent, because the step ON switching, will not be followed by as much energy flow, which will excite the resonant circuit.

I know its only 400mA, but there are 25 of these on the bus, and together they will add up.

 

[alec_t]

Something based on this?

FET current is proportional to the control voltage, which can be ramped up (e.g. using an RC circuit) as required.

 

[jpanhalt]

The term you need to search for is "soft start"

One approach would be to use a NE555/LM555 in a PWM configuration. Then ramp the control voltage with a resistor/capacitor combination. Probably, any voltage controlled PWM driver would do the same. Here is a link to a popular 555-based PWM generator: **broken link removed**

There are lots of options along similar lines. Here is a Google search

https://www.google.com/webhp#hl=en&...598,d.dmg&fp=897ba9997041d6f1&biw=930&bih=613

And a couple of specific examples:

**broken link removed**

https://www.electro-tech-online.com/custompdfs/2013/03/91093A.pdf

John