Novel Current-Sharing IC Balances Two Supplies with Ease Pinkesh Sachdev Failure is not an option. That’s the likely motto for the architects of today’s alwaysup electrical infrastructure—think telecommunications networks, the Internet and the electrical grid. The problem is that the bricks of this infrastructure, from the humble capacitor to the brainy blade-servers, have a limited lifetime usually ending at the most Murphy of moments. The usual workaround to the mortality problem is redundancy—backup systems ready to take over whenever a critical component fails. For instance, high availability computer servers typically ship with two similar DC supplies feeding power to each individual board. Each supply is capable of taking on the entire load by itself, with the two supplies diode-ORed together via power diodes to create a single 1 + 1 redundant supply. That is, the higher voltage supply delivers power to the load, while the other supply idly stands by. If the active supply voltage drops or disappears, due to failure or removal, the once lowervoltage supply becomes the higher voltage supply, so it takes over the load. The diodes prevent back-feeding and crossconduction between supplies while protecting the system from a supply failure. 5A 12.2V NC •Supply lifetimes are extended if each takes on half the load, spreading the supply heat and reducing thermal stresses on supply components. A rule of thumb for the lifetime of electronics is that the failure rate of components halves for every 10°C fall in temperature. That’s a significant dependability gain. + 325mV – SUM85N03-06P 39nF* EN1 CPO1 0.1µF The diode-OR is a simple winner-takeall system where the highest voltage supply sources the entire load current. The lower voltage supply remains idle until called into action. Although easy to implement, the 1 + 1 solution is inefficient, wasting resources that could be better used to improve overall operating efficiency and lifetime. It is far better for the supplies to share the load in tandem, offering several advantages: VIN1 GATE1 OUT1 VCC LTC4370 GND RANGE FETON1 2mΩ FETON2 2mΩ OUT2 EN2 CPO2 VIN2 *OPTIONAL, FOR FAST TURN-ON 10A 11.875V GATE2 COMP 39nF* 11.9V 11.875V 0.18µF SUM85N03-06P + 25mV – 5A 20 | July 2012 : LT Journal of Analog Innovation Figure 1. The LTC4370 balancing a 10A load current between two diode-ORed 12V supplies. Sharing is achieved by modulating the MOSFET voltage drops to offset the mismatch in the supply voltages. •Because the lower voltage supply is always operational, there is no surprise when transitioning to a backup supply that might have already silently failed— a possibility in a simple diode-OR system. •It is possible in a load-sharing system to parallel smaller at-hand supplies to build a larger one. •The recovery dynamics on supply failure are smoother and faster, since the supply changes are on the order of less and more, not off and on. •A DC/DC converter formed by two supplies running at half capacity has better overall conversion efficiency than a single supply running near full capacity. METHODS OF CURRENT SHARING Connecting the outputs of multiple power supplies allows them to share a common load current. The division of the load current among the supplies depends on the individual supply output voltages and supply path resistances to the common load. This is known as droop sharing. To prevent back-feeding of a supply and to isolate the system from a faulting supply, diodes can be inserted in series with each supply. Of course, this added diode voltage drop affects the balance of the load sharing. design features The LTC4370 introduces a new paradigm for current sharing, where the contributions from individual supplies are under full active control, but no share bus, with its extra wires, is required. Complete control is as easy to implement as a simple diode-OR droop sharing system, but the traditional passive diodes are replaced with adjustable diodes, with turn-on voltages that can be adjusted to achieve actively balanced current sharing. THE CURRENT SHARING CONTROLLER The LTC4370 features Linear Technology’s proprietary adjustable-diode current sharing technique. It balances the load between two supplies using external N-channel MOSFETs that act as adjustable diodes whose turn-on voltage can be modulated to achieve balanced sharing. Figure 1 shows the LTC4370 sharing a 10A load between two 12V supplies Figure 2 shows the device internals that affect load sharing. Error amplifier EA monitors the differential voltage between the OUT1 and OUT2 pins. It sets the forward regulation voltage VFR of two servo amplifiers (SA1, SA2), one for each supply. The servo amplifier modulates the gate of the external MOSFET (hence its resistance) such that the forward drop across the MOSFET is equal to the forward regulation voltage. The error amplifier sets Figure 2. Load-sharing-related internals of the LTC4370 I1 R1 M1 VSUPPLY1 C1 This article introduces a new method of current sharing, allowing active control of individual supply contributions, but with the simplicity of droop sharing. In this system, the diodes are replaced with adjustable diodes with turn-on voltages that can be adjusted to achieve balanced current sharing. This produces better sharing accuracy than droop sharing and the power spent in the adjustable diodes is the minimum required to achieve sharing, far less than that lost in a traditional diode. Because no sharing bus is required, it offers simpler supplyindependent compensation and portable design. Supplies with difficult or no access to their trim pins and feedback networks are ideal for this technique. VIN1 GATE1 CPO1 + OUT1 CHARGE PUMP1 SA1 – VFR1 + – COMP CC + OUT1 – OUT2 EA SERVO ADJUST gm = 150µS + – 10µA + VIN2 GATE2 CHARGE PUMP2 CPO2 VCC RANGE R3 0.3V DISABLE LOAD SHARE – + – SA2 TO LOAD + VFR2 – Droop sharing is simple but sharing accuracy is poorly controlled, and the series diodes present a voltage and power loss. A more controlled way of current sharing is to monitor the supply current, compare it to an average current required from each supply, then adjust the supply voltage (through its trim pin or feedback network) until the supply current matches the required value. This method requires wires to every supply—a share bus—to signal the current contribution required from each. The current sharing loop compensation is customized to accommodate the power supply loop dynamics. Controlled current sharing requires careful design and access to all of the supplies—not possible in some systems. VCC OUT2 C2 R2 VSUPPLY2 I2 M2 July 2012 : LT Journal of Analog Innovation | 21 NORMALIZED CURRENT I2 VRANGE = 500mV 1 I1 1 = 2RS SLOPE 0.5 I1 I2 0 –500mV 0 500mV SHARING CAPTURE RANGE ±∆VIN(SH) Figure 3. Current sharing characteristic of the LTC4370 method as the supply voltage difference varies. MAXIMUM M2 MOSFET POWER DISSIPATION VIN1 – VIN2 = ∆VIN IL • RS MAXIMUM M1 MOSFET POWER DISSIPATION MOSFET FORWARD DROP VFR(MAX) 525mV VFWD2 VFWD1 VFR(MIN) –500mV 25mV 0 500mV VIN1 – VIN2 = ∆VIN DRAWING IS NOT TO SCALE! the VFR on the lower voltage supply to a minimum value of 25mV. The servo on the higher voltage supply is set to 25mV plus the difference in the two supply voltages. In this way both the OUT pin voltages are equalized. OUT1 = OUT2 implies I1 • R1 = I2 • R2 . Hence, I1 = I2 if R1 = R2 . A simple adjustment to different-valued sense resistors can be used to set up ratiometric sharing, i.e., I1 /I2 = R2 /R1. Note that the load voltage tracks 25mV below the lowest supply voltage. The MOSFET in conjunction with the servo amplifier behaves like a diode whose turn-on voltage is the forward regulation voltage. The MOSFET is turned off when its forward drop falls below the regulation voltage. With increasing MOSFET current, the gate voltage rises to reduce the onresistance to maintain the forward drop at VFR . This happens until the gate voltage rails out at 12V above the source. Further rise in current increases the drop across the MOSFET linearly as IFET • RDS(ON). Given the above, when the error amplifier sets the forward regulation voltage of the servo amplifier, it is functionally equivalent to adjusting the turn-on voltage 22 | July 2012 : LT Journal of Analog Innovation of the (MOSFET-based) diode. The adjustment range runs from a minimum of 25mV to a maximum set by the RANGE pin (see “Design Considerations” below). The controller can load share supplies from 0V to 18V. When both supplies are below 2.9V, an external supply in the range 2.9V to 6V is required at the VCC pin to power the LTC4370. Under reverse current conditions the gate of the MOSFET is turned off within 1µs. The gate is also turned on in under a microsecond for a large forward drop. The fast turn-on, important for low voltage supplies, is achieved with a reservoir capacitor on the integrated charge pump output. It stores charge at device power-up and delivers 1.4A of gate pull-up current during a fast turn-on event. The EN1 and EN2 pins can be used to turn off their respective MOSFETs. Note that current can still flow through the body diode of the MOSFET. When both channels are off, the device current consumption is reduced to 80µ A per supply. The FETON outputs indicate whether the respective MOSFET is on or off. THE CURRENT SHARING CHARACTERISTIC Figure 3 shows the current sharing characteristic of the LTC4370, adjustablediode method. There are two plots, both with the supply voltage difference, ΔVIN = VIN1 – VIN2 , on the x-axis. The top plot shows the two supply currents normalized to the load current; the lower shows the forward voltage drops, VFWDx, across the MOSFETs. When both supply voltages are equal (ΔVIN = 0V), the supply currents are equal, and both forward voltages are at the minimum servo voltage of 25mV. As VIN1 increases above VIN2 (positive ΔVIN), VFWD2 stays at 25mV, while VFWD1 increases exactly with ΔVIN to maintain OUT1 = OUT2. This is turn keeps I1 = I2 = 0.5ILOAD. There is an upper limit to the adjustment on VFWD set by the RANGE pin. For the example in Figure 3, that limit is 525mV, set by the RANGE pin at 500mV. Once VFWD1 hits this limit, sharing becomes imbalanced and any further rise in VIN1 pushes OUT1 above OUT2. The break point is VFR(MAX) – VFR(MIN), where more of the load current comes from the higher voltage supply. When OUT1 – OUT2 = ILOAD • RSENSE , the entire load current transfers over to I1. This is the operating point with the maximum power dissipation in MOSFET M1, since the entire load current flows through it with the maximum forward drop. For example, a 10A load current causes 5.3W (= 10A • 525mV) dissipated in the MOSFET. For any further rise in ΔVIN, the controller ramps down the forward design features The LTC4370’s novel approach to load-sharing power supplies results in easy design, especially with supplies that don’t lend themselves to on-the-fly tweaking. Inherent diode behavior protects supplies from reverse currents and the system from faulting supplies. drop across M1 to the minimum 25mV. This minimizes power dissipation in the MOSFET for large VIN when the load current is not being shared. The behavior is symmetric for negative ΔVIN . The sharing capture range in this example is 500mV and is set by the RANGE pin voltage. With this range the controller can share supplies that have a tolerance of ±250mV. This translates to the following: ±7.5% tolerance on a 3.3V supply, ±5% on a 5V, and ±2% on a 12V supply. DESIGN CONSIDERATIONS These are some of the high level considerations for a load share design. MOSFET Choice — Ideally the MOSFET’s RDS(ON) should be small enough that the controller can servo the minimum forward regulation voltage of 25mV across the MOSFET with half of the load current flowing through it. A higher RDS(ON) prevents the controller from regulating 25mV. In this case, the unregulated drop is 0.5IL • RDS(ON). As this drop rises, the sharing break point (now defined by VFR(MAX) – 0.5IL • RDS(ON)) occurs earlier, shrinking the capture range. Since the MOSFET dissipates power, up to IL • VFR(MAX) as in Figure 3, its package and heat sink should be chosen appropriately. The only way to dissipate less power in the MOSFET is by using more accurate supplies or by forgoing sharing range. RANGE Pin — The RANGE pin sets the shar- ing capture range of the application, which in turn depends on the accuracy of the supplies. For example, a 5V system with ±3% tolerance supplies Figure 4. 5V diode-OR load share with status light. Red LED D1 lights up whenever any MOSFET is off, indicating a break in sharing. SUM85N03-06P VINA 5V 39nF EN1 0.1µF CPO1 VIN1 GATE1 VCC GND LTC4370 OUT1 FETON1 FETON2 D1 820Ω SHARE OFF 2.5mΩ 2.5mΩ OUT 10A RANGE would need a sharing range of OUT2 30.1k 2 • 5V • 3% or 300mV (higher EN2 CPO2 VIN2 GATE2 COMP 0.18µF supply is 5.15V while lower 39nF D1: RED LED LN1251C VINB is 4.85V). The RANGE pin has 5V SUM85N03-06P a precise internal pull-up current of 10µ A. Placing a 30.1k resistor on the RANGE pin sets its voltage to 301mV and CONCLUSION now the controller can compensate for the Balancing load currents between supplies 300mV supply difference (see Figure 4). is a historically difficult problem, conjuring visions of juggling on a tightrope. Leaving the RANGE pin open (as shown When power modules or bricks don’t offer in Figure 1) gives the maximum posbuilt-in support, some designers will spend sible sharing range of 600mV. But when significant time designing a well-controlled servo voltages approach the diode voltsystem (and redesigning it whenever the age, currents can flow through the body supply type changes); others will settle for diode of the MOSFET causing loss of crude resistance-based droop sharing. sharing. Connecting RANGE to VCC disables load share to transform the device The LTC4370 takes a completely differinto a dual ideal-diode controller. ent approach to load-sharing supplies Compensation — The load share loop is compensated by a single capacitor from the COMP pin to ground. This capacitor must be 50× the input (gate) capacitance of the MOSFET, CISS . If fast gate turn-on is not being used (CPO capacitors absent) then the capacitor can be just 10× CISS . Sense Resistors — The sense resistors deter- than any other controller. It eases design, especially with supplies that don’t lend themselves to on-the-fly tweaking, and it can be ported to various types of supplies. Inherent diode behavior protects supplies from reverse currents and the system from faulting supplies. The LTC4370 provides a simple, elegant and compact solution to a complicated problem. n mine the load sharing accuracy. Accuracy improves as resistor voltage drops increase. The maximum error amplifier offset is 2mV. Therefore, a 25mV sense resistor drop yields a 4% sharing error. The resistance can be lowered if power dissipation is more important than accuracy. July 2012 : LT Journal of Analog Innovation | 23