Pre-biased circuit for synchronous rectified power converters
The present invention relates to voltage converters and especially to a control circuit with an input from the voltage converter output and arranged to control the voltage level on the voltage converter output. The problem addressed relates to the situation when there is a pre-bias voltage on the converter output at the moment it is switched on. The object of the control circuit is to increase the voltage on the converter output fast and avoiding any drain of voltage or current from the output at the start up sequence. This is performed by a comparator in the control circuit that is arranged to compare the reference voltage with a division of the output voltage and if the reference voltage is lower that the divided output voltage the reference voltage is increased at the comparator output. The comparator circuit includes an OP-amplifier.
1. A control circuit for controlling the output voltage V 1 +−V 1 − of a connected voltage converter, comprising:
a connection to said voltage converter output for receiving a first voltage V 1 +−V 1 − produced by the voltage converter;
impedances arranged to divide the first voltage V 1 +−V 1 − into a first signal V 2 and a second signal V 3 , wherein V 2 >V 3 ;
a compensator network comprising a first amplifier with a first signal input for receiving said first signal V 2 , a second signal input for receiving a reference voltage Vref, and arranged to compare the first signal V 2 with the reference voltage Vref and produce at an output a comparison signal for controlling the voltage level V 1 of the voltage converter;
a capacitor having a first side connected to the lowest potential of the circuit, having a second side connected to an a resistor for receiving a charging current via the resistor, further on the second side connected to the first signal input of the first amplifier, and being arranged for stabilizing the reference voltage Vref;
a comparator circuit comprising a second amplifier with a first signal input receiving the reference signal Vref and, a second signal input receiving the second signal V 3 , and arranged for comparing the second input signal V 3 with the reference signal Vref and increase the reference signal Vref at its output if Vref<V 3 .
2. The control circuit of claim 1 wherein;
said connection comprises a first and a second input respectively for receiving the positive voltage potential V 1 + and negative voltage potential V 1 − of the voltage converter output, and wherein,
said impedances for voltage division are resistors R 7 and R 5 coupled in series for producing the first signal V 2 such that V 2 =(V 1 +−V 1 −) R 7 /(R 7 +R 5 ).
3. The control circuit of claim 1 wherein said compensator network is an OP-amplifier with an inverted RC-feedback loop.