SCR Phase control - Single pulse or Multiple Pulses?

I am Trying to control the DC voltage for a Battery charger using phase control. Basic circuit here.
In the firing position should I send Single Pulse or should I send a pulse train till the end of cycle?
To my Knowledge one the SCR is on, it will remain on till the next zero cross.
I am using a microcntroller to drive the Gate through transistor, but in another analogue circuit when checked in a scope There seems to be a pulse train. I was not able to check it properly.
What is the ideal method in this case?

Thanks

Hi,

To my Knowledge one the SCR is on, it will remain on till the next zero cross.

Click to expand...

only partly correct. More precise is:
* it will remain ON as slong as the load current is > then the hold current of the SCR (read datasheet)

And exactly this is the problem.

In case you just have one short ignition pulse then the load current may not reach the trigger current before the ignition pulse goes low --> the SCR stays OFF
This especially applies to inductive loads.

In case you have one long ignition pulse until the voltage zero cross then the load current may not reach the trigger current before the ignition transformer gets into saturation. --> the SCR stays OFF
This especially applies to inductive loads.
To avoid ignition transformer saturation usually one uses pulse trains. Mind to allow fast decay.

The next is that the load current may include some ringing, the load current may temprarily drop below the hold current and the SCR may release (before the zero cross)
Additionally - especially with inductive loads - the load current is phase shifted. The SCR doesn´t release at zero cross of voltage but at zero cross of current.
Thus the SCR may be ON a dedicated time after voltage zero cross. During this time you can´t trigger the SCR for the next half wave.

***
With your application I recommend to use pulse train. Stop the pulse train before the voltage zero cross.

Instead of ignition transformers you may consider to use MOCs to trigger the SCR. Read application notes.

****
The SCR trigger problem is not a new one, thus there are many discussions, solutions, schematics in the internet.
I recommend to look at the semiconductor manufacturers for application notes.

Klaus

Thanks for clarifying the key issues, exactly on my requirement. Now very clear about the problem.
Regarding the pulse train, what is the basis of On Time and Off Time? which parameter of SCR I need to look for in that case?

1- gate trigger current
2- Maximum turn-on time
3- latching current
4- holding current (may or may not be identical to latching current)

EDIT:
Something that you do not show in your schematic is the pulse transformer's phasing. That is important.

Hi,

Regarding the pulse train, what is the basis of On Time and Off Time? which parameter of SCR I need to look for in that case?

Click to expand...

Sorry for not being clear enough.

The SCR does not need a pulse train.
You may drive the gate with DC as long as you don´t overload the gate.

BUT the ignition transformer needs a pulse train.
As soon as it saturates there is no gate drive anymore. Thus you need to switch OFF the pulse.... the ignition transformer will relax .. then you may switch ON the next pulse.
--> For the pulse timing you need to look at the ignition transformer´s datasheet. Especially for the Vt integral specification.


Klaus

KlausST said:

Hi,
BUT the ignition transformer needs a pulse train.
--> For the pulse timing you need to look at the ignition transformer´s datasheet. Especially for the Vt integral specification.
Klaus

Click to expand...

Got it. Thanks. As you have suggested if I can use a MOC opto, then no pulse train needed, just Steady DC dc drive will be Ok.

Hi,

Correct.

Mind to use a MOC with immediate signaling (no zero cross type)
Follow the MOC datasheet recommendations.
Use wide creepage and clearance distances.

Klaus