Hi!
I have a flyback configuration as a part of a more complicated low power design.
in this design I must disconnect the secondary winding from output capacitor
using a switch or a device funktioning as a switch.
Since this switch must be placed between the secondary winding(which has floating voltage levels)
and output capacitor(which is being charged and has no constant voltage), both sides of the switch have
floating potentials, therefore there is no reference potential to activate the switch.

Requirements:
1: an absolute disconnection must exist(no leakage current)
2: Maximum current flowing through this switch is about 1A
3: reverse voltage on the switch(off status) about 900V (difference of the voltage on the
output capacitor and the secondary winding)
4: switch power dissipation must be very low
5: switch must be very fast, some micro seconds
6: rise and fall time for the switch turn on and off must be in nano second
Important: -MOS transistors are not adequate because current leakage through free wheeling diode
makes the absolute disconnection impossible!
-Relays are too slow and bulky

I do appreciate your idea!
I'll be gratefull if any body can help me with this problem!
thank you very much in advance

 

Just use a fast recovery diode rated for 1kv.

 

the switch must be on and off upon a driving signal.
switching activity on the primary side makes a voltage on the secondary, which turns the diode on and off automatically. I want a switch which is always on or always off independent of how the voltage on the secondary changes.

thanks

 

Then use an SCR.

 

when connected the switch must conduct in both directions

 

That would be silly.

An SCR will conduct in both directions, but when current flow falls to near zero, it turns off.

If it were still conducting after transferring energy to the capacitor, it would continue to conduct until the capacitor were discharged.

I guess you need to re-think your requirements.

 

thanx
but how can I activate a SCR (activation voltage)
when both sides of the SCR have floating voltages?
I think the activation voltage schould be more positive or negative compared to SCR's connection pins

 

actually in my design I have mirrored configuration of the flyback, that means I have another switch in the secondary which discharges my output capacitor afrter being charged by the switch in the primary.Therefore the connection between secondary winding and output capacitor must be able to conduct in both directions.
But after the capacitor being discharged to zero I must disconnect the capacitor from the secondary winding..

 

I don't see a schematic.

What you are describing at the moment makes no sense.

 

I try to find out how can I paste the schematic (*.jpg)

 

A simple schematic from this configuration at:

http://www.4shared.com/photo/ElAXGiDe/question_pic.html

 

That design doesn't make good sense. Why are you switching the low side of the secondary coil, and also want to switch the high side?

Take a look at Ronald Dekker's page, here:
http://www.dos4ever.com/flyback/flyback.html

 

but how can I activate a SCR (activation voltage)
when both sides of the SCR have floating voltages?
I think the activation voltage schould be more positive or negative compared to SCR's connection pins

SCRs can be triggered via pulse transformers, which provide perfect electrical isolation between driver and SCRs. Their physical size are small too, some the size of a sugar cube.

 

SCRs can be triggered via pulse transformers, which provide perfect electrical isolation between driver and SCRs. Their physical size are small too, some the size of a sugar cube.

May I have more information or do you know a good ref. in internet?
thanx a lot

 

That design doesn't make good sense. Why are you switching the low side of the secondary coil, and also want to switch the high side?

Take a look at Ronald Dekker's page, here:
http://www.dos4ever.com/flyback/flyback.html

thanx a lot for your responce!

In my design I try to charge and discharge a capacitive load, demonstrated as output capacitor.
Primary is connected to a low power supply and output capacitor should be charged to a high voltage level. I switch the primary (low side) in order to charge the capacitor and I switch the high side(secondary) to discharge the capacitor.

 

You're currently switching the low side of both the primary and secondary.
Switching the low side of the primary makes sense.
Switching the low side of the secondary does not make sense.
You don't want to discharge the capacitor through the secondary winding.

 

You're currently switching the low side of both the primary and secondary.
Switching the low side of the primary makes sense.
Switching the low side of the secondary does not make sense.
You don't want to discharge the capacitor through the secondary winding.

I'm a little bit confused with what you mean about primary and secondary lower side?
the upper side of the primary winding is connected to power supply and is in phase with the lower side of the secondary winding which is connected to the drain of the switch in the secondary, so in this way we have a mirrored configuration and the capacitor is discharged, what actually happens in the realized hardware.

 

Here is what I am talking about:

The MOSFET switch on the low side of the secondary does not make any sense.

Instead of having a MOSFET on the low side of the secondary, it should be grounded.

Your "switch" (the box on the high side of the secondary between the cap) should be a diode or SCR.

 

Here is what I am talking about:



The MOSFET switch on the low side of the secondary does not make any sense.

Instead of having a MOSFET on the low side of the secondary, it should be grounded.

Your "switch" (the box on the high side of the secondary between the cap) should be a diode or SCR.

thanx a lot for the responce!
in more detail: I want to charge and discharge a load capacitor. During the charging period the switch in primary toggles while the switch in secondary is OFF(therefore free wheeling diode realizes a diode in the secondary as required).
having the capacitor fully charged, now the primary switch is turned off(realizing a diode in the primary) and the secondary switch toggles, till the cpacitor is complitely discharged.
As you see I have the required diode in my circuit but they are realized by switches in OFF mode.
---
The current flows in both directions "to" and "from" the capacitor during charging and discharging phase.That means if I use SCR I most use two of them.
By using SCR I encounter the following questions
1-is it possible to hold SCR in OFF mode while the rate of voltages on the secondary winding is high(high voltage rate may turns the SCR ON, without being triggered)
2-I must trigger the SCRs many times during charging or discharging period because they are OFF as soon as the current flowing through gets near zero(I need a path always closed durind charging and discharging phase).
in other words charging and discharging is performed by sucessive toggling of the primary or the secondary switch, that means for instance , during charging phase, each time the primary switch toggles, the SCR must be triggered to, because it turnes OFF when the secondary current goes to zero( and this happens each time the primary switch toggles).


hopefully I could explain clearly

 

I do not know why you want to switch the low side of the secondary.

If the secondary MOSFET is off, the body diode will still conduct (if the current is flowing from GND), but the power dissipation will be higher due to the Vf of the body diode.

I do not know why you want to discharge the capacitor via the transformer secondary. I am starting to think that you are attempting to make some sort of "free energy" device, which is not possible. All you will wind up with is hot MOSFETs, a hot transformer, and power consumption.