It’s often necessary to connect point-of- load (POL) dc/dc power supply modules in parallel to get a sufficiently high-current output for applications such as test instruments, blade servers, and so forth.
Edited by Leland Teschler
Fully integrated POL power modules have a built-in inductor, power MOSFET, PWM controller, and supporting circuitry.
It is important to ensure that current shares evenly when paralleling dc/dc regulator modules. Conceptually, part-to-part variations and output-voltage regulation errors can make the output voltages of two identical POL supplies differ though the supplies are programmed to have the same target value. For example, a 1.5-V output POL can have a ±1% error on its output voltage. When two POL supplies connect in parallel, the output voltage difference between two modules can be as high as 1.5 V x 2% = 30 mV. So assuming a 5-mΩ interconnection impedance, the circulation current can be as high as 30 mV/5 mΩ = 6 A. Without current sharing, a two-phase, 20-A supply with two 10-A POLs in parallel might see one channel providing 16 A and the other only 4 A. The unbalanced phase current can cause serious thermal and reliability issues for the entire system.
A point to note is that the LTM4601 family of μModule POLs has integrated current-mode controllers to handle current sharing. The device is a complete 12-A supply housed in a 15 x 15 x 2.8-mm LGA package with an IC formfactor and size. It operates from a 4.5 to 28-V input voltage range. Multiple LTM4601 modules can easily connect in parallel for high-current applications. The accompanying diagram shows how to connect one LTM4601 (with remote sensing) and three LTM4601-1 (without remote sensing) power modules to devise a 40-A dc/dc step-down supply.
It is possible to parallel multiple LTM4601 μModule regulators with balanced phase current. Inside the μModule, the inductor current is sensed and compared to the control signal, Vcomp. The PWM control adjusts the duty cycle of the dc-dc buck converter to make the inductor current follow Vcomp. And the inductor current increases linearly with Vcomp. Thus the inductor acts like a voltage-controlled current source. Current shares evenly just by connecting all the current control signal pins together. In the LTM4601, the COMP pin is both the output of the op amp that handles voltage-loop compensation and the reference voltage of the inductor current. The chip uses transconductance (gm) op amps, so their outputs (the COMP pins) can be tied together.
The concept becomes clear from a conceptual drawing of two gm op amps paralleled to generate a common current reference voltage, Vcomp. The transconductance error amplifier’s valid input range is 10% of the reference voltage. The Vref is a fixed internal voltage. For LTM4601, it is 0.6 V. Even with part-to-part reference voltage variations of 1%, error amplifiers are not saturated. In addition, connecting feedback pins together lets just one divider resistor set the output voltage. In the drawing, the upper resistor is inside the LTM4601. The lower resistor is external and its value determines the output voltage. And μModules with a phase-locked loop (PLL) can each be synchronized by interleaving clocks to eliminate beat frequency noise at both the input and output terminals.
A thermal image of four LTM4601 modules with 20-V input and 1.5-V output at 40-A load current shows well-balanced temperatures that indicate the modules share load current well.
— Manjing Xie, Henry Zhang,
Linear Technology Corp.
Linear Technology Corp., linear.com
The thermal image of four parallel modules reveals an even distribution of current (Vin = 20 V, Vo = 1.5 V, Io = 40 A).