VIPer53 - E

Current mode topology

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Current mode topology

The VIPer53-E implements the conventional current mode control method for regulating the

output voltage. This kind of feedback includes two nested regulation loops:

The inner loop controls the peak primary current cycle by cycle. When the Power MOSFET

output transistor is on, the inductor current (primary side of the transformer) is monitored

power switch is turned off. This structure is completely integrated as shown on the Block

Diagram on page 1, with the current amplifier, the PWM comparator, the blanking time

function and the PWM latch. The following formula gives the peak current in the Power

MOSFET according to the compensation voltage:

Equation 1

The outer loop defines the level at which the inner loop regulates peak current in the power

optocoupler in secondary feedback configuration, see Figure 19 on page 17) and is sets

accordingly the peak drain current for each switching cycle.

As the inner loop regulates the peak primary current in the primary side of the transformer,

all input voltage changes are compensated for before impacting the output voltage. This

results in an improved line regulation, instantaneous correction to line changes, and better

stability for the voltage regulation loop.

Current mode topology also provides a good converter start-up control. The compensation

voltage can be controlled to increase slowly during the start-up phase, so the peak primary

current will follow this soft voltage slope to provide a smooth output voltage rise, without any

overshoot. The simpler voltage mode structure which only controls the duty cycle, leads

generally to high current at start-up with the risk of transformer saturation.

An integrated blanking filter inhibits the PWM comparator output for a short time after the

integrated Power MOSFET is switched on. This function prevents anomalous or premature

termination of the switching pulse in the case of current spikes caused by primary side

transformer capacitance or secondary side rectifier reverse recovery time when working in

continuous mode.

I

Dpeak

V

COMP

V

COMPos

–

H

COMP

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