INN20x3-20x5 Current Limit
In looking at the Innoswitch-CH datasheet (rev. G 11/15), page 18, there are two specs I have a question on.
First, the Output (IS Pin) Current Limit Voltage Threshold - typical 33mV, and
Second, the Constant Current Regulation Threshold - typical 2.2A
I do not see different constant current regulation thresholds for the INN20x3, INN20x4, or INN20x5 parts. If I use the INN20x5 part for a 5V 20W power supply, do I need to add a external shunt (between GND and IS) of 15 milliohms to get the part to enter CC mode at 4.4 amps typcial?
I have designed mixed singal ASICs for 14 years now and am designing an off-line switcher for a client. Please, if you do not understand the internals of the innoswitch-CH family ASICs please don't answer.
I also have a question about tweaking the Synchronous Rectifier Pin Threshold voltage but will wait for a true PI engineer to answer my first question.
-EJ
Comments
The threshold of -24mV (typ.) is quite a large negative value. If I use an NMOS with 18mohms as recommended (datasheet, page 10, 4th paragraph under SR MOSFET Operation and Selection), then the SR MOSFET will turn off at an Id of approx. 1.5 Amps. That is still a significant current. I am thinking of using the secondary "flyback" to run a negative charge-pump at which I can add negative bias through a resistor to the FWD pin. The bias would be adjusted such that when the SR MOSFET drain voltage is about -3mV - and rising, the FWD pin is at -24mV - and rising.
By the way, in the RD-420 eval kit, the SR MOSFET installed is a Vishay SI7478DP-T1-E3. It has an Ron of 6 milliohms - 1/3 that called out by the Innoswitch-CH datasheet. I guess that explains why it is getting as hot as the INN2023K on the RD-420 eval kit - it is turning off way too early.
I am hoping to bias the FWD pin negative to keep the SR MOSFET on longer through the transformer "discharge" phase.
-EJ
We don't recommend any modifications to do it on the FWD Pin.
Yes You have to add a External Shunt of 15 milliohms to make the part to enter into CC Mode for 4.4 A , then the power supply can be able to deliver 20 W of output power.What exactly you mean tweaking the Synchronous rectifier Pin Threshold voltage?