UCC2818: Multiplier: Current Source Compliance

Hello Ryan,

The internal power for the UCC2818 is derived from the reference voltage.

So any model of the part should use 7.5v and not VCC.

The multiplier output provides a current source that is proportional to the error amplifier output and the input line voltage.

The steady state output voltage is zero.

This is because MOUT is connected directly to the inverting input of the current amp and the non inverting input of the current amp is connected to ground.

There is an equation in the data sheet for the proper calculation of IMOUT.

It is not possible to set a 2.5V output

Regards

John

Hi John,

Thank for the detailed reply.

The part I'm confused on is this: "The steady state output voltage is zero."

From what I've read in the datasheet, the MOUT pin will always be sourcing DC current that is proportional to things like the voltage EA output and Vff (Figure 3). Reviewing the typical application circuit (Figure 9) it looks like the only DC path from the MOUT pin is through a resistor R9 to ground. It looks like that resistor is sized to give a full scale voltage when the MOUT current is at its highest.

Reading the below section leads me to believe the voltage on MOUT will always be in the range of 0V-1.25V.

Please let me know where I might be making a mistake, thank you!

Hi Ryan,

Sorry about the confusion.
In a closed loop pfc application the multiplier output is fed into the inverting input of the current amplifier.
The non inverting input of the current amplifier goes to ground. This means that for steady state operating conditions the multiplier output voltage must be zero.
If MOUT wanders above 0V , the inverting nature of the current amp will tend to turn off PWM.
And this in turn will act to pull MOUT low. This is the nature of a stable control system.

The multiplier is still sourcing current into the resistor on its output.(RMOUT in above equation)
One side of this resistor is connected to a negative voltage and the system acts to source just enough current from the multiplier to ensure the V_MOUT = 0V
The equation above is used to calculate the appropriate resistor value.

Regards

John

Thanks for the update.

It seemed that Imout had a compliance of around 3.2 volts, not 7.5 because no matter what I did, that's the most voltage I could get at the multiplier output.

In order to get the system to work I had to scale everything down so that Rmout allowed no more than about 2.5 volts.

In most designs the shunt resistor is sized such that you typically stay below a couple of volts anyway because of power dissipation in the shunt but in my case I have an op-amp buffering a low value shunt resistor so I had designed Rmout to operate at a higher voltage than typical.

The UCC2818 is different than the UC2854 series in that the CEA has flipped polarity to allow for leading edge modulation.  In past designs using the 2854 I would drive the current sense pin directly from the current amp buffer but with the 2818 I had to configure the CEA as a differential amplifier.

Everything works fine so long as I don't try to get more than ~2.5 volts at the multiplier output.

Thanks,

Jeff