1. On Board Charger: Error Amplifiers (I and V)
Measure output current
Compare to reference
Output error signal (power demand)
Measure output voltage
Diode ‘or’ errors – lowest error ‘wins’
Automatic CV / CI transition
This is the usual technique
Lowest error ‘wins’ and controls the output
Low side sense at 400Vout
High side sense at 12Vout is possible
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The charger has to switch between CV and CI regulation like the typical characteristic shown at the bottom left. There are two error amplifiers, one comparing the output current against the constant current regulation setpoint and the other comparing the output voltage against the constant voltage regulation setpoint. If the system is in CI mode, the voltage error amplifier is saturated high because Vo is too low, if the system is in CV mode the current error amplifier saturates high because the current is below the regulation setpoint. The outputs from the two error amplifiers are diode ored and the lower of the two errors is fed into the controller. Note that the error amplifier in the UCC28951-Q1 is configured as a voltage follower and other than this it plays no part in the control loop. The on-board reference of the UCC28951-Q1 is not accurate enough for Li Ion battery charging in particular so an external LM4132-Q1 reference is used.
Output voltage control for temperature compensation of the battery voltage and for setting the float voltage in the lead acid battery charger is accomplished by a digital potentiometer controlled by an external microcontroller over an I2C bus
The TPL0401A-10-Q1 device shown here is a single-channel, digital potentiometer with 128 wiper positions.
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