Hydraulic pressure supply system of automatic transmission

A hydraulic pressure supply system of an automatic transmission for a vehicle, comprising:

• a low-pressure hydraulic pump receiving oil from an oil pan through an input line, generating a low hydraulic pressure to the oil and supplying the oil of the low hydraulic pressure to a first low-pressure line;
• a low-pressure regulator valve connected to the first low-pressure line, recirculating through a first recirculation line a portion of the oil of the low hydraulic pressure supplied through the first low-pressure line directly to the input line to regulate the low hydraulic pressure of the first low-pressure line, and supplying the regulated low hydraulic pressure to a low pressure portion through a second low-pressure line;
• a high-pressure hydraulic pump connected to the first low-pressure line, receiving a portion of the oil of the low hydraulic pressure in the first low-pressure line, increasing the low hydraulic pressure to a high hydraulic pressure, and supplying the oil of the increased hydraulic pressure to a high pressure portion through a high-pressure line;
• and
• a high-pressure regulator valve connected to the high-pressure line, recirculating a portion of the oil of the high hydraulic pressure supplied through the high-pressure line directly to the first low-pressure line through a second recirculation line, selectively recirculating a portion of the oil of the high hydraulic pressure supplied through the high-pressure line to the second recirculation line through a third recirculation line, and supplying the oil of the high hydraulic pressure regulated through the high-pressure regulator valve to the high pressure portion,
• wherein the third recirculation line is selectively connected to the second recirculation line according to a strength of the high hydraulic pressure of the oil in the high-pressure line.