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Current Sense Transformer for Synchronous Buck (non symetrical AC in the CST Primary)

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treez

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Hello,
The attached Current Mode Synchronous Buck converter has a Current sense transformer (CST) to sense the hi-side FET current.
Because this is a Synchronous Buck, the current in the primary of the CST is sometimes AC..(eg when the Buck is lightly loaded)
With no-load on the Sync Buck output, it is “symmetrical” AC current in the CST primary, but when lightly loaded, it is non-symetrical AC current in the primary of the CST.
Do you know of a topology of CST that can ensure equal volt.seconds across the CST secondary through all these conditions of current flow in the CST? (ie to ensure reset of the CST under all conditions of current flow in the CST ..eg symmetrical AC, non-symmetrical AC, Unidirectional)
(When the Buck is fully loaded, the CST primary obviously has “unidirectional” current flow in its primary)

(the Buck is actually a Dual cascaded Buck, but that’s not central to the discussion)

Please find schematic and lightly loaded LTspice simulation attached.
 

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  • Dual sync buck with CST_1.pdf
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  • Dual sync buck with CST_1.txt
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The attached Sync Buck simulation (schematic also attached), has non symmetrical AC current in the CST , but there is very little build up of magnetising current in the secondary of the CST, due to the use of the diode bridge at the CST secondary. Therefore, would you agree that this circuit is fine?
-The secondary magnetising current merely has a 2mA DC level in it. (its saturation level is 52mA).

In fact, even a unipolar CST setup is fine (with only single rectifier diode in the CST secondary), but just has up to 20mA DC level in the secondary magnetising current, which is unwanted, but doesn’t saturate the EP13 (3C96) core, which, with 50 turns, would need the secondary magnetising current to go up to 52mA before saturation was caused.

Anyway, in the attached simulation , the secondary magnetising current is seen by putting “I(L28) + I(L27)/50” into the waveform window.

- - - Updated - - -

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In relation to the use of CST’s with synchronous Buck’s, the following article has called it totally wrong…………

Article:
**broken link removed**

Her is the duff quote from the above article (which concerns CSTs in sync Bucks)….
Alternatively, a current sense transformer can be exploited, but only if the current sense location is such that the current waveform is zero for part of the switching period to allow transformer reset, e.g. in series with the high-side FET.[unquote]

As I have prooven here, the synchronous buck has AC current in the CST when its in that location, and there is no zero current interval, and yet the CST operates perfectly fine, as long as a diode bridge is used in the secondary of the CST
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Attachments

  • Dual sync buck with CST_3.pdf
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  • Dual sync buck with CST_3.txt
    25.5 KB · Views: 44

You can't put a B rect on a CT driven this way, for unidirectional current in the CT, (one way buck converter) you have to use a resistor and diode so that the CT can reset when the input side switch is off

- - - Updated - - -

p.s. if you put the CT in the drain of the fet you will get less dv/dt induced noised into your CT circuit...
 
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You can't put a Bridge rect on a CT driven this way, for unidirectional current in the CT, (one way buck converter) you have to use a resistor and diode so that the CT can reset when the input side switch is off

The thing is that this CST is in a "Synchronous" Buck converter , and it does not always have unidirectional current in the CST primary.
If i use the standard unidirectional CST setup with the single rectifier diode, then there exists a too high magnetising current DC level in the secondary of the CST when the CST primary current goes bidirectional (during lighter loading)
Therefore the bridge rectifier is essential..if you run the simulation (in post #2) under any loading, you can see that the secondary magnetising current does not go anywhere near saturation level, so it is fine with the bridge rectifier.

The CST must be resetting fine because otherwise the secondary magnetising current would "staircase" high, and it does not...the simulation shows this well
 

Actually my mistake , Easy Peasy you are correct, it doesn't work after all with the rectifier bridge, just worked it with higher duty cycle and there isn't enough volt.seconds to reset the transformer when on heavy load. So we need to find a solution for a sync buck high side current sense, remembering that sometimes the current in the CST primary "can go back the other way", and indeed, when in light load, the current in the CST primary will not have that zero interval, because current is going backwards and forwards, rather than forwards for a bit , then stop etc etc.
 

Comparators used to sense the voltage across the lower fet and the buck inductor will tell you whether the FET should be on or not...
 
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