Innoswitch switching frequency when in QR mode?
Hi, The innoswitch CE datasheet says that switching frequency will end up increasing as load gets heavier, due to the way the current limit gradually falls following the previous FET turn OFF incidence.
But then what happens when in QR Mode (BCM)? For this mode, the switching frequency usually gets less as the load increases, since the current needs more time to build up to the current limit which is higher for heavier loads.
Comments
Surely when operating in BCM mode, the switching frequency must decrease as load is made heavier?
Hi treez,
Thanks for the response. It will only move from CCM to BCM/DCM if you increase the input line with a heavy load. In that case, you will need a lower primary switching current and in effect, the switching frequency will be lower.
You may refer to figure 6 on page 4 of the device datasheet to see the relationship between the primary switch current limit and switching frequency.
Regards, Gavin
Hi treez,
Good day. Yes, you are correct that the switching frequency usually increases with heavier loads.
The same is true whether the circuit is working in CCM, BCM or DCM. The controller monitors the secondary SR FET in each switching cycle, and at heavier loads, it tends to make the current through the SR FET goes to zero more frequently. This make the voltage drop across the switch the falls at approximately 0V.
In discontinuous conduction mode of operation, the SR FET switch is turned off when the voltage drop across it falls below a threshold of 0 V. This will consequently request the primary switch to turn on.
I hope this helps you with your question. Thanks.
Regards, Gavin